# Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review

Tiffanie Scandolera, Gianluca Teano, Masoud Naderpour, Valérie Geffroy, Stéphanie Pflieger

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## To cite this version:

Tiffanie Scandolera, Gianluca Teano, Masoud Naderpour, Valérie Geffroy, Stéphanie Pflieger. Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review. Environmental and Experimental Botany, 2024, 221, pp.105737. 10.1016/j.envexpbot.2024.105737. hal-04603867

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Submitted on 6 Jun 2024

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# Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review

Tiffanie Scandolera, Gianluca Teano, Masoud Naderpour, Valérie Geffroy, Stéphanie Pflieger

► To cite this version:

Tiffanie Scandolera, Gianluca Teano, Masoud Naderpour, Valérie Geffroy, Stéphanie Pflieger. Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review. Environmental and Experimental Botany, 2024, 221, pp.105737. 10.1016/j.envexpbot.2024.105737. hal-04603867

HAL Id: hal-04603867

https://hal.inrae.fr/hal-04603867v1

Submitted on 6 Jun 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

Review article

# Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review

Tiffanie Scandolera<sup>a, b</sup>, Gianluca Teano<sup>a, b</sup>, Masoud Naderpour<sup>a, b, c</sup>, Valérie Geffroy<sup>a, b</sup>, Stéphanie Pflieger<sup>a, b, *</sup>

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

### Keywords:

Climate change

Elevated atmospheric carbon dioxide

Elevated temperature

Viral disease

Viral symptoms

Molecular defense pathways

## ABSTRACT

Understanding and anticipating the impacts of climate change on plant-pathogen interactions are a major challenge for the agriculture of the 21st century. Prediction models forecast an increase in atmospheric carbon dioxide (CO₂) levels by 2100 that could reach 1045 ppm. Plant physiology is directly affected by an increase of atmospheric CO₂ as plants are living organisms that consume CO₂ through photosynthesis to produce organic matter. Since the early days of agriculture, plant diseases can alter not only quality of plant productions but can also be responsible for important yield losses. Plant viruses are obligate, acellular pathogens that cause serious epidemics in major agricultural crops with annual yield losses of more than $ 30 billion. As elevated concentration of atmospheric CO₂ (eCO₂) modulates plant primary and secondary metabolisms and as viruses are obligate pathogens, it is likely that eCO₂ can modulate plant molecular defenses to viruses. In that context, the present review focuses on the effect of eCO₂ on plant physiological and molecular responses to virus infections. First, we will compare the different experimental methodologies used to study the impact of eCO₂ enrichment on plant-virus interactions and discuss the different designs applied for experiments. We will also present the impact of eCO₂ on virus-infection parameters in infected plants and describe the impact of combined abiotic stresses, including eCO₂ and temperature, on plant-virus interactions.

## 1. Introduction

Climate change is defined as a long-term change in the average weather patterns that have come to define Earth's local, regional and global climates (NOAA 2023: https://gml.noaa.gov/ccgg/trends/). These last ten years, global climate change has become a major concern of modern agriculture (IPCC, 2021) because it affects the normal development of plants and their responses to biotic and abiotic stresses (Chaudhry and Sidhu, 2022; Velásquez et al., 2018). One of the main causes of climate change are human activities (Eyring et al., 2021) that deliver increased greenhouse gases in Earth's atmosphere such as carbon dioxide (CO₂), nitrous oxide, methane, and chlorofluorocarbons (Blunden and Boyer, 2022).

Elevated concentration of atmospheric CO₂ (eCO₂) is one of the main drivers of global climate change (IPCC, 2013). Indeed, CO₂ is one of the most important greenhouse gases in Earth's atmosphere with a content of ~64% (IPCC, 2014). Massive CO₂ emissions are principally due to

human anthropogenic activities such as fossil-based fuel burning, deforestation, and urbanization during the last 200 years (Ahuja et al., 2010; Friedlingstein et al., 2022). Currently, mean atmospheric CO₂ levels have surpassed the 420 µL·L⁻¹ (=ppm, parts per million) in May 2022 at Mauna Loa Observatory in Hawaii (NOAA 2023: https://gml.noaa.gov/ccgg/trends/) compared to 280 ppm at the beginning of the 19th century defined as being the pre-industrial period. The May 2022 level represents the highest concentration of CO₂ ever recorded in the history of humankind and is equivalent to that found 4.1–4.5 million years ago in the Pliocene Epoch. Predictive models have forecast a continuous increase of atmospheric CO₂ concentration in the coming decades with the high-emissions scenario predicting to reach 1045 ppm by the year 2100 (IPCC, 2021).

As CO₂ is a greenhouse gas it traps heat, gradually causing the rise of surface air temperature which has been projected to rise by +2.4 to +4.8°C by the end of the 21st century compared to the recent past (1995–2014) (IPCC, 2021). This phenomenon is commonly called global

<footnote><sup>a</sup> Université Paris-Saclay, CNRS, INRAE, Université Evry, Institute of Plant Sciences Paris-Saclay (IPS2), Gif sur Yvette 91190, France</footnote>

<footnote><sup>b</sup> Université Paris-Cité, CNRS, INRAE, Institute of Plant Sciences Paris-Saclay (IPS2), Gif sur Yvette 91190, France</footnote>

<footnote><sup>c</sup> Seed and Plant Certification and Registration Research Institute, Agricultural Research, Education and Extension Organization, Ministry of Agriculture, Iran</footnote>

<footnote>* Correspondence to: Institute of Plant Sciences Paris-Saclay, Gif sur Yvette 91190, France</footnote>

<footnote>E-mail address: stephanie.pflieger@universite-paris-saclay.fr (S. Pflieger).</footnote>
<figure><img src="image_2.png" /></figure>

Review article

# Insights into the effects of elevated atmospheric carbon dioxide on plant-virus interactions: A literature review

Tiffanie Scandolera<sup>a, b</sup>, Gianluca Teano<sup>a, b</sup>, Masoud Naderpour<sup>a, b, c</sup>, Valérie Geffroy<sup>a, b</sup>, Stéphanie Pflieger<sup>a, b, *,</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Climate change

Elevated atmospheric carbon dioxide

Elevated temperature

Viral disease

Viral symptoms

Molecular defense pathways

## ABSTRACT

Understanding and anticipating the impacts of climate change on plant-pathogen interactions are a major challenge for the agriculture of the 21st century. Prediction models forecast an increase in atmospheric carbon dioxide (CO₂) levels by 2100 that could reach 1045 ppm. Plant physiology is directly affected by an increase of atmospheric CO₂ as plants are living organisms that consume CO₂ through photosynthesis to produce organic matter. Since the early days of agriculture, plant diseases can alter not only quality of plant productions but can also be responsible for important yield losses. Plant viruses are obligate, acellular pathogens that cause serious epidemics in major agricultural crops with annual yield losses of more than $ 30 billion. As elevated concentration of atmospheric CO₂ (eCO₂) modulates plant primary and secondary metabolisms and as viruses are obligate pathogens, it is likely that eCO₂ can modulate plant molecular defenses to viruses. In that context, the present review focuses on the effect of eCO₂ on plant physiological and molecular responses to virus infections. First, we will compare the different experimental methodologies used to study the impact of eCO₂ enrichment on plant-virus interactions and discuss the different designs applied for experiments. We will also present the impact of eCO₂ on virus-infection parameters in infected plants and describe the impact of combined abiotic stresses, including eCO₂ and temperature, on plant-virus interactions.

## 1. Introduction

Climate change is defined as a long-term change in the average weather patterns that have come to define Earth's local, regional and global climates (NOAA 2023: https://gml.noaa.gov/ccgg/trends/). These last ten years, global climate change has become a major concern of modern agriculture (IPCC, 2021) because it affects the normal development of plants and their responses to biotic and abiotic stresses (Chaudhry and Sidhu, 2022; Velásquez et al., 2018). One of the main causes of climate change are human activities (Eyring et al., 2021) that deliver increased greenhouse gases in Earth's atmosphere such as carbon dioxide (CO₂), nitrous oxide, methane, and chlorofluorocarbons (Blunden and Boyer, 2022).

Elevated concentration of atmospheric CO₂ (eCO₂) is one of the main drivers of global climate change (IPCC, 2013). Indeed, CO₂ is one of the most important greenhouse gases in Earth's atmosphere with a content of ~64% (IPCC, 2014). Massive CO₂ emissions are principally due to

human anthropogenic activities such as fossil-based fuel burning, deforestation, and urbanization during the last 200 years (Ahuja et al., 2010; Friedlingstein et al., 2022). Currently, mean atmospheric CO₂ levels have surpassed the 420 µL·L⁻¹ (=ppm, parts per million) in May 2022 at Mauna Loa Observatory in Hawaii (NOAA 2023: https://gml.noaa.gov/ccgg/trends/) compared to 280 ppm at the beginning of the 19th century defined as being the pre-industrial period. The May 2022 level represents the highest concentration of CO₂ ever recorded in the history of humankind and is equivalent to that found 4.1–4.5 million years ago in the Pliocene Epoch. Predictive models have forecast a continuous increase of atmospheric CO₂ concentration in the coming decades with the high-emissions scenario predicting to reach 1045 ppm by the year 2100 (IPCC, 2021).

As CO₂ is a greenhouse gas it traps heat, gradually causing the rise of surface air temperature which has been projected to rise by +2.4 to +4.8°C by the end of the 21st century compared to the recent past (1995–2014) (IPCC, 2021). This phenomenon is commonly called global

<footnote><sup>a</sup> Université Paris-Saclay, CNRS, INRAE, Université Evry, Institute of Plant Sciences Paris-Saclay (IPS2), Gif sur Yvette 91190, France</footnote>

<footnote><sup>b</sup> Université Paris-Cité, CNRS, INRAE, Institute of Plant Sciences Paris-Saclay (IPS2), Gif sur Yvette 91190, France</footnote>

<footnote><sup>c</sup> Seed and Plant Certification and Registration Research Institute, Agricultural Research, Education and Extension Organization, Ministry of Agriculture, Iran</footnote>

<footnote>* Correspondence to: Institute of Plant Sciences Paris-Saclay, Gif sur Yvette 91190, France</footnote>

<footnote>E-mail address: stephanie.pflieger@universite-paris-saclay.fr (S. Pflieger).</footnote>
warming and is defined as the long-term heating of Earth's surface observed since the pre-industrial period due to human activities which increases heat-trapping greenhouse gas levels in Earth's atmosphere (NASA 2023: https://climate.nasa.gov/). Thus, evolution of future climate will be characterized by increase of both atmospheric CO₂ and atmosphere temperature. Global warming is already having dramatic consequences on climate, including increased heat waves, droughts, fires, and floods.

As plants are living organisms that consume CO₂ through photosynthesis to produce organic matter, the first studies of eCO₂ effect on plants focused on monitoring the evolution of growth and yield, further showing a positive effect of eCO₂ especially in C3 plants (Enoch et al., 1976, 1971) (Fig. 1). This positive effect is linked to a reduction in the rate of both stomatal conductance and photorespiration thereby increasing the rate of photosynthesis via Rubisco activity. In addition, eCO₂ has been shown to also enhance plant productivity in the C4 plants, although the fixation of CO₂ is already optimized, and this may or may not be directly related to the rate of photosynthesis. The articles related to these studies have been recently synthesized in an excellent review (Roy and Mathur, 2021) so that we will give here only the main features of plant responses to eCO₂ (Fig. 1). The positive effect of eCO₂ on plant growth and biomass in C3 plants, and to a lesser extent in C4 plants, were shown to be correlated with responses affecting leaf physiology such as the increase of the mesophyll cell size and density along with the increase in the number of chloroplast and volume of starch granules (reviewed in Roy and Mathur, 2021). Altogether, these changes affect leaf quality because of an increased C:N ratio that stimulates accumulation of carbon-containing secondary metabolites such as phenolics (e. g. salicylic acid), polyphenolics (e. g. lignin), and flavonoids, while decreasing the nitrogen-containing compounds (Roy and Mathur, 2021). At the molecular level, genes related to biosynthesis pathways of secondary metabolites, photosynthesis, carbohydrate metabolism, growth and development as well as stress responsive genes, are upregulated (Eastburn et al., 2011), whereas genes involved in photo-respiratory pathways, nitrogen metabolism for example, are down-regulated (Vicente et al., 2018). Reactive oxygen species are key players in biotic and abiotic stress responses. Reduction-oxidation processes play key roles in acclimation to high CO₂, with specific enzymes

acting in compartment-specific signaling (Foyer and Noctor, 2020; Noctor and Mhamdi, 2017). Apart from this, eCO₂ has been shown to modulate accumulation of the main phytohormones: on the one hand, salicylic acid (SA), abscisic acid (ABA), and cytokinins levels are increased, whereas on the other hand, jasmonic acid (JA) and ethylene contents are decreased (Roy and Mathur, 2021) (Fig. 1). Some reports state that “elevated CO₂ reconfigures the phytohormone signaling networks in a manner that affect plant response towards both abiotic and biotic stresses” (Zhang et al., 2015).

Since the early days of agriculture, plant diseases alter not only quality of plant productions but can also be responsible for important yield losses. According to Oerke and Dehne (2004), even with the present crop protection practices, field losses due to plant diseases are estimated around 26%-40% depending on the crop and environmental conditions. The extent to which climate change will affect interactions between plants and pathogens is still an open question. Therefore, understanding and anticipating the impact of climate change on plant-pathogen interactions is a major challenge for the agriculture of tomorrow (AbdElgawad et al., 2016; Kazan, 2018). This knowledge will be critical for developing climate-resilient strategies against plant pathogens, to keep the agriculture sustainable and to derive societal benefits from plants, now and in the future. According to the FAO reports, about 842 million people around the world are suffering from starvation and about 312 million children are stunted, underweighted or wasted from malnutrition (FAO: Raney et al., 2013). A more recent report showed that about 2.3 billion people around the world were in food insecurity in 2021 (SOFA 2022: https://www.fao.org/publications/s/home/fao-flagship-publications/the-state-of-food-and-agriculture/en). Besides, the size of the global population will lie between 9.4 and 10.0 billion in 2050 and between 8.9 and 12.4 billion in 2100 (United Nations Department of Economic and Social Affairs, P.D, 2022) meaning that the global food production should be increased up to 60% to maintain food security (Jones and Naidu, 2019). Thus, minimizing field losses due to diseases in the future is a major challenge for scientists in order to improve crop productivity whilst using environmentally-safe control measures.

Plant viruses are one of the biotic agents that cause serious epidemics in major agricultural crops with annual yield losses of more than $ 30

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** General overview of the impacts of viral pathogens, elevated CO₂ and combination of both on plant physiology and immune responses.</figcaption></figure>
warming and is defined as the long-term heating of Earth's surface observed since the pre-industrial period due to human activities which increases heat-trapping greenhouse gas levels in Earth's atmosphere (NASA 2023: https://climate.nasa.gov/). Thus, evolution of future climate will be characterized by increase of both atmospheric CO₂ and atmosphere temperature. Global warming is already having dramatic consequences on climate, including increased heat waves, droughts, fires, and floods.

As plants are living organisms that consume CO₂ through photosynthesis to produce organic matter, the first studies of eCO₂ effect on plants focused on monitoring the evolution of growth and yield, further showing a positive effect of eCO₂ especially in C3 plants (Enoch et al., 1976, 1971) (Fig. 1). This positive effect is linked to a reduction in the rate of both stomatal conductance and photorespiration thereby increasing the rate of photosynthesis via Rubisco activity. In addition, eCO₂ has been shown to also enhance plant productivity in the C4 plants, although the fixation of CO₂ is already optimized, and this may or may not be directly related to the rate of photosynthesis. The articles related to these studies have been recently synthesized in an excellent review (Roy and Mathur, 2021) so that we will give here only the main features of plant responses to eCO₂ (Fig. 1). The positive effect of eCO₂ on plant growth and biomass in C3 plants, and to a lesser extent in C4 plants, were shown to be correlated with responses affecting leaf physiology such as the increase of the mesophyll cell size and density along with the increase in the number of chloroplast and volume of starch granules (reviewed in Roy and Mathur, 2021). Altogether, these changes affect leaf quality because of an increased C:N ratio that stimulates accumulation of carbon-containing secondary metabolites such as phenolics (e. g. salicylic acid), polyphenolics (e. g. lignin), and flavonoids, while decreasing the nitrogen-containing compounds (Roy and Mathur, 2021). At the molecular level, genes related to biosynthesis pathways of secondary metabolites, photosynthesis, carbohydrate metabolism, growth and development as well as stress responsive genes, are upregulated (Eastburn et al., 2011), whereas genes involved in photosynthetic pathways, nitrogen metabolism for example, are down-regulated (Vicente et al., 2018). Reactive oxygen species are key players in biotic and abiotic stress responses. Reduction-oxidation processes play key roles in acclimation to high CO₂, with specific enzymes

acting in compartment-specific signaling (Foyer and Noctor, 2020; Noctor and Mhamdi, 2017). Apart from this, eCO₂ has been shown to modulate accumulation of the main phytohormones: on the one hand, salicylic acid (SA), abscisic acid (ABA), and cytokinins levels are increased, whereas on the other hand, jasmonic acid (JA) and ethylene contents are decreased (Roy and Mathur, 2021) (Fig. 1). Some reports state that “elevated CO₂ reconfigures the phytohormone signaling networks in a manner that affect plant response towards both abiotic and biotic stresses” (Zhang et al., 2015).

Since the early days of agriculture, plant diseases alter not only quality of plant productions but can also be responsible for important yield losses. According to Oerke and Dehne (2004), even with the present crop protection practices, field losses due to plant diseases are estimated around 26%-40% depending on the crop and environmental conditions. The extent to which climate change will affect interactions between plants and pathogens is still an open question. Therefore, understanding and anticipating the impact of climate change on plant-pathogen interactions is a major challenge for the agriculture of tomorrow (AbdElgawad et al., 2016; Kazan, 2018). This knowledge will be critical for developing climate-resilient strategies against plant pathogens, to keep the agriculture sustainable and to derive societal benefits from plants, now and in the future. According to the FAO reports, about 842 million people around the world are suffering from starvation and about 312 million children are stunted, underweighted or wasted from malnutrition (FAO: Raney et al., 2013). A more recent report showed that about 2.3 billion people around the world were in food insecurity in 2021 (SOFA 2022: https://www.fao.org/publications/s/home/fao-flagship-publications/the-state-of-food-and-agriculture/en). Besides, the size of the global population will lie between 9.4 and 10.0 billion in 2050 and between 8.9 and 12.4 billion in 2100 (United Nations Department of Economic and Social Affairs, P.D, 2022) meaning that the global food production should be increased up to 60% to maintain food security (Jones and Naidu, 2019). Thus, minimizing field losses due to diseases in the future is a major challenge for scientists in order to improve crop productivity whilst using environmentally-safe control measures.

Plant viruses are one of the biotic agents that cause serious epidemics in major agricultural crops with annual yield losses of more than $ 30

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** General overview of the impacts of viral pathogens, elevated CO₂ and combination of both on plant physiology and immune responses.</figcaption></figure>
billion (Sastry and Zitter, 2014). Viruses are obligate, acellular pathogens, that need a host to perform their biological cycle. Among plant viruses, positive sense single-stranded RNA (e. g. messenger-type RNA) viruses are of major economic importance. In nature, plant viruses are transmitted by several means such as seeds, pollens, and insect vectors the vast majority of which having piercing-sucking mouthparts, like aphids for example. Unlike other plant pathogens, no effective pesticide-based control measures have been recognized so far to combat viruses. However, efficient prevention of virus diseases can be achieved by utilizing a combination of approaches including cultural practices, virus-free planting material, pesticides against insect vectors and plant-genetic resistance (Schmitt-Keichinger, 2019).

Plant immune responses against viruses depends on molecular mechanisms induced by virus infection. First, RNA silencing, also called RNA interference or RNAi (Ratcliff et al., 1997), and anti-viral pattern-triggered immunity (PTI) (Kørner et al., 2013; Niehl et al., 2016) are both major mechanisms in antiviral plant defense based on the recognition of viral-derived double-stranded RNAs (Amari and Niehl, 2020; Niehl et al., 2016). Both mechanisms can be suppressed by viral proteins called viral suppressors of RNAi (VSR) and viral effectors, respectively (Csorba et al., 2015; Kong et al., 2018; Nicaise and Candes, 2017; Zvereva et al., 2016). Consequently, plants have evolved Resistance (R)-proteins (dominant resistance genes encoding Nucleotide binding-Leucin rich repeat Receptors (NLRs)) that are able to recognize viral effectors either directly, by physical interaction between the two proteins, or indirectly, by involving a host intermediate called the virulence target (Palukaitis and Yoon, 2020). The resistance mediated by NLR proteins is called effector-triggered immunity (ETI) (Palukaitis and Yoon, 2020). Many R genes against viruses have been identified so far, as well as the corresponding viral-encoded effectors that are subsequently referred to as avirulence (Avr) factors (de Ronde et al., 2014; Gouveia et al., 2017). ETI, also named incompatible interaction, consists in a high specificity of pathogen recognition as previously described by Flor in the 'gene-for-gene' model (Flor, 1955) and in a rapid and strong defense response that shares many components with PTI (Thomma et al., 2011). In some plant-virus interactions, ETI can be associated with hypersensitive reaction (HR), a form of programmed cell death (Kabbage et al., 2017) that induces local necrotic spots corresponding to dead cells at the sites of viral primary infection. It is well known since the 1980s that SA is the major phytohormone involved in plant defense against viruses (Whitham et al., 2006). Moreover, SA mediates the induction of defense-related genes like pathogenesis-related (PR) proteins for example in both compatible (diseased plants) and incompatible (resistant plants) host-virus interactions. Some reports also suggested that JA is involved in plant response to viruses, as JA-accumulation mutants present less virus accumulation (Zarate et al., 2007).

This review, divided into four parts, attempts to shed light on the effect of eCO₂ on plant immune responses to virus infection. First, we will compare the different experimental methodologies used to study the impact of eCO₂ enrichment on plant-virus interactions. We will also discuss how the experimental design (eCO₂ timing of application, chosen level of eCO₂ as well as virus inoculation method) can influence the obtained results. In the second part, we will present the impact of eCO₂ on virus-infected plants both at the phenotypic and molecular level (intensity of viral symptoms and virus titer, impact on plant growth and quality and on plant molecular defense responses). The third part will present the impact of combined abiotic stresses, including eCO₂ and temperature, on plant-virus interactions. Finally, we will discuss possible directions for future research.

## 2. Experimental methodologies used to study the impact of eCO₂ on plant-virus pathosystems

### 2.1. CO₂ enrichment facilities

In order to enrich plant's living atmosphere, several eCO₂

enrichment systems are practically affordable. In a total of 18 articles dealing with plant-virus pathosystems, we recorded three different eCO₂ enrichment facilities: Controlled Environment Chambers (CECs), Open Top Chambers (OTCs), and Free-Air CO₂ Enrichment (FACE) systems. These technologies mainly differ in the way environmental factors are applied, as well as by their features and type of virus/vector income (Fig. 2).

The prevalent technology used to study the effect of eCO₂ on plant-virus interactions is CECs used in 72.2% of the 18 articles (Fig. 2, Table S1). Use of CECs consist of growing plants in a completely controlled chamber that may display different sizes, from a growth cabinet to a several square-meter surface chamber or greenhouse. Indeed, specific controlled conditions can be applied as each environmental parameter (temperature, relative humidity, light intensity, photoperiod and CO₂ concentration) can be individually set in a growth program, offering the advantage of combining different abiotic stresses (e.g. del Toro et al., 2017). Moreover, experiments in CECs were found to be repeatable (Drake et al., 1985). However, even though this technology is easy and reliable, results are difficult to extrapolate to natural conditions since environmental factors usually display constant values (Drake et al., 1985) (Fig. 2). In CECs, the size of the chamber often limits the experiments to small-sized plants grown in pots, limiting the studies to individual-plant scale (Fig. 2). Furthermore, in natural conditions plants are usually submitted to mixed viral infections that can result in synergisms among viruses to cause more sever disease incidence. As this is not the case in CEC, this fact is another disadvantage of CEC.

OTCs are the second most prominent technology used to study the impact of eCO₂ on plant-virus interactions with 22.2% of utilization in the 18 articles (Fig. 2, Table S1). It is worth noting that this percentage represents only four articles with all authors belonging to the same research institute i.e. State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Beijing, China (Fu et al., 2010; Guo et al., 2016; Huang et al., 2012; Ye et al., 2010). So, these studies are related to environmental conditions of a world-specific area. Indeed, OTCs are field-adapted growing surfaces that are made of a plastic enclosure delimited by peripheric walls and an open top, so that the inside air communicates with the outside atmosphere (Fig. 2) (Mauri, 2010). Air enriched in CO₂ enters at the bottom of the OTC and flows out the open top. Different models of OTCs have been engineered to study the impact of aerial pollutants, in this case, atmospheric CO₂, in natural conditions (D'Andrea and Rinaldi, 2010; Uprety et al., 2006). One advantage of OTC is that it can host either potted plants or field-grown crops as well as young trees (D'Andrea and Rinaldi, 2010). However, OTCs do have some disadvantages such as an alteration of the microenvironment like a higher air humidity and temperature, as well as an attenuation of the solar radiation due to the presence of the peripheric walls (D'Andrea and Rinaldi, 2010; Drake et al., 1985; Leadley et al., 1997). Nevertheless, it is an interesting intermediate facility compared to CEC and FACE (see below) as it benefits from natural sunlight and natural environmental conditions (Fig. 2). Moreover, OTC is adapted to crop cultures and allows to perform micro-ecosystem-scale studies in a cost-effective way compared to FACE (Drake et al., 1985; Leadley et al., 1997) (Fig. 2).

As for FACE, this system ranks at the third place with 5.6% of utilization in the 18 articles (Fig. 2, Table S1). Indeed, a single paper reports a FACE four-year study of yellow dwarf viruses incidence in wheat cultures (Trębicki et al., 2017). FACE is constituted of a circular plot delimited by several-meters-high vertical vent pipes that deliver CO₂ gas inside the plot (Machacova, 2010; Mauri, 2010). Having a diameter of several meters each, FACE plots are suitable to study tall field-grown plants like trees for example (Machacova, 2010; Mauri, 2010). In contrast to CECs, FACE is the closest system to field conditions, allowing the study of the impact of eCO₂ on plant-virus pathosystem with the most natural conditions in a crop culture and taking into account the complete natural ecosystem (Drake et al., 1985). The main disadvantage of FACE is the expensive operational cost of this technology estimated to
billion (Sastry and Zitter, 2014). Viruses are obligate, acellular pathogens, that need a host to perform their biological cycle. Among plant viruses, positive sense single-stranded RNA (e. g. messenger-type RNA) viruses are of major economic importance. In nature, plant viruses are transmitted by several means such as seeds, pollens, and insect vectors the vast majority of which having piercing-sucking mouthparts, like aphids for example. Unlike other plant pathogens, no effective pesticide-based control measures have been recognized so far to combat viruses. However, efficient prevention of virus diseases can be achieved by utilizing a combination of approaches including cultural practices, virus-free planting material, pesticides against insect vectors and plant-genetic resistance (Schmitt-Keichinger, 2019).

Plant immune responses against viruses depends on molecular mechanisms induced by virus infection. First, RNA silencing, also called RNA interference or RNAi (Ratcliff et al., 1997), and anti-viral pattern-triggered immunity (PTI) (Kørner et al., 2013; Niehl et al., 2016) are both major mechanisms in antiviral plant defense based on the recognition of viral-derived double-stranded RNAs (Amari and Niehl, 2020; Niehl et al., 2016). Both mechanisms can be suppressed by viral proteins called viral suppressors of RNAi (VSR) and viral effectors, respectively (Csorba et al., 2015; Kong et al., 2018; Nicaise and Candes, 2017; Zvereva et al., 2016). Consequently, plants have evolved Resistance (R)-proteins (dominant resistance genes encoding Nucleotide binding-Leucin rich repeat Receptors (NLRs)) that are able to recognize viral effectors either directly, by physical interaction between the two proteins, or indirectly, by involving a host intermediate called the virulence target (Palukaitis and Yoon, 2020). The resistance mediated by NLR proteins is called effector-triggered immunity (ETI) (Palukaitis and Yoon, 2020). Many R genes against viruses have been identified so far, as well as the corresponding viral-encoded effectors that are subsequently referred to as avirulence (Avr) factors (de Ronde et al., 2014; Gouveia et al., 2017). ETI, also named incompatible interaction, consists in a high specificity of pathogen recognition as previously described by Flor in the 'gene-for-gene' model (Flor, 1955) and in a rapid and strong defense response that shares many components with PTI (Thomma et al., 2011). In some plant-virus interactions, ETI can be associated with hypersensitive reaction (HR), a form of programmed cell death (Kabbage et al., 2017) that induces local necrotic spots corresponding to dead cells at the sites of viral primary infection. It is well known since the 1980s that SA is the major phytohormone involved in plant defense against viruses (Whitham et al., 2006). Moreover, SA mediates the induction of defense-related genes like pathogenesis-related (PR) proteins for example in both compatible (diseased plants) and incompatible (resistant plants) host-virus interactions. Some reports also suggested that JA is involved in plant response to viruses, as JA-accumulation mutants present less virus accumulation (Zarate et al., 2007).

This review, divided into four parts, attempts to shed light on the effect of eCO₂ on plant immune responses to virus infection. First, we will compare the different experimental methodologies used to study the impact of eCO₂ enrichment on plant-virus interactions. We will also discuss how the experimental design (eCO₂ timing of application, chosen level of eCO₂ as well as virus inoculation method) can influence the obtained results. In the second part, we will present the impact of eCO₂ on virus-infected plants both at the phenotypic and molecular level (intensity of viral symptoms and virus titer, impact on plant growth and quality and on plant molecular defense responses). The third part will present the impact of combined abiotic stresses, including eCO₂ and temperature, on plant-virus interactions. Finally, we will discuss possible directions for future research.

## 2. Experimental methodologies used to study the impact of eCO₂ on plant-virus pathosystems

### 2.1. CO₂ enrichment facilities

In order to enrich plant's living atmosphere, several eCO₂

enrichment systems are practically affordable. In a total of 18 articles dealing with plant-virus pathosystems, we recorded three different eCO₂ enrichment facilities: Controlled Environment Chambers (CECs), Open Top Chambers (OTCs), and Free-Air CO₂ Enrichment (FACE) systems. These technologies mainly differ in the way environmental factors are applied, as well as by their features and type of virus/vector income (Fig. 2).

The prevalent technology used to study the effect of eCO₂ on plant-virus interactions is CECs used in 72.2% of the 18 articles (Fig. 2, Table S1). Use of CECs consist of growing plants in a completely controlled chamber that may display different sizes, from a growth cabinet to a several square-meter surface chamber or greenhouse. Indeed, specific controlled conditions can be applied as each environmental parameter (temperature, relative humidity, light intensity, photoperiod and CO₂ concentration) can be individually set in a growth program, offering the advantage of combining different abiotic stresses (e.g. del Toro et al., 2017). Moreover, experiments in CECs were found to be repeatable (Drake et al., 1985). However, even though this technology is easy and reliable, results are difficult to extrapolate to natural conditions since environmental factors usually display constant values (Drake et al., 1985) (Fig. 2). In CECs, the size of the chamber often limits the experiments to small-sized plants grown in pots, limiting the studies to individual-plant scale (Fig. 2). Furthermore, in natural conditions plants are usually submitted to mixed viral infections that can result in synergisms among viruses to cause more sever disease incidence. As this is not the case in CEC, this fact is another disadvantage of CEC.

OTCs are the second most prominent technology used to study the impact of eCO₂ on plant-virus interactions with 22.2% of utilization in the 18 articles (Fig. 2, Table S1). It is worth noting that this percentage represents only four articles with all authors belonging to the same research institute i.e. State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Beijing, China (Fu et al., 2010; Guo et al., 2016; Huang et al., 2012; Ye et al., 2010). So, these studies are related to environmental conditions of a world-specific area. Indeed, OTCs are field-adapted growing surfaces that are made of a plastic enclosure delimited by peripheric walls and an open top, so that the inside air communicates with the outside atmosphere (Fig. 2) (Mauri, 2010). Air enriched in CO₂ enters at the bottom of the OTC and flows out the open top. Different models of OTCs have been engineered to study the impact of aerial pollutants, in this case, atmospheric CO₂, in natural conditions (D'Andrea and Rinaldi, 2010; Uprety et al., 2006). One advantage of OTC is that it can host either potted plants or field-grown crops as well as young trees (D'Andrea and Rinaldi, 2010). However, OTCs do have some disadvantages such as an alteration of the microenvironment like a higher air humidity and temperature, as well as an attenuation of the solar radiation due to the presence of the peripheric walls (D'Andrea and Rinaldi, 2010; Drake et al., 1985; Leadley et al., 1997). Nevertheless, it is an interesting intermediate facility compared to CEC and FACE (see below) as it benefits from natural sunlight and natural environmental conditions (Fig. 2). Moreover, OTC is adapted to crop cultures and allows to perform micro-ecosystem-scale studies in a cost-effective way compared to FACE (Drake et al., 1985; Leadley et al., 1997) (Fig. 2).

As for FACE, this system ranks at the third place with 5.6% of utilization in the 18 articles (Fig. 2, Table S1). Indeed, a single paper reports a FACE four-year study of yellow dwarf viruses incidence in wheat cultures (Trębicki et al., 2017). FACE is constituted of a circular plot delimited by several-meters-high vertical vent pipes that deliver CO₂ gas inside the plot (Machacova, 2010; Mauri, 2010). Having a diameter of several meters each, FACE plots are suitable to study tall field-grown plants like trees for example (Machacova, 2010; Mauri, 2010). In contrast to CECs, FACE is the closest system to field conditions, allowing the study of the impact of eCO₂ on plant-virus pathosystem with the most natural conditions in a crop culture and taking into account the complete natural ecosystem (Drake et al., 1985). The main disadvantage of FACE is the expensive operational cost of this technology estimated to
<table><thead><tr><th colspan="4">CO<sub>2</sub> enrichment facility</th></tr><tr><th></th><th>CEC (72,2%<sup>a</sup>)</th><th>OTC (22,2%<sup>a</sup>)</th><th>FACE (5,6%<sup>a</sup>)</th></tr></thead><tbody><tr><th>General overview</th><td>Growth chamber<br>greenhouse</td><td>Cabinet</td><td></td></tr><tr><th>Environmental factors</th><td>Artificial :<br>tubes, LED, sodium lamps<br>Fixed, constant<br>Fixed, constant<br>No wind</td><td>Natural sunlight<br>(but solar radiation is attenuated)<br>Natural (but plant canopy is warmer inside compared to outside)<br>Natural (but higher inside compared to outside)<br>Reduced wind</td><td>Natural sunlight<br>Natural<br>Natural<br>Natural</td></tr><tr><th>Features</th><td>Small -medium potted plants<br>Individual plant scale study<br>+</td><td>Medium-tall potted or field-grown plants (crops, young trees)<br>Micro-ecosystem scale study<br>++</td><td>Tall field-grown plants<br>(crops or trees)<br>Ecosystem scale study<br>+++</td></tr><tr><th>Virus/vector income</th><td>Artificial inoculation</td><td>Natural epidemics /<br>artificial inoculation</td><td>Natural epidemics</td></tr></tbody></table>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 2.** The different CO₂ enrichment facilities used in 18 published studies on plant-virus interaction: general overview, environmental factors, features and type of virus/vector income <sup>a</sup> Percentage of utilization of each facility in the 18 articles.</figcaption></figure>

be about four times more than a comparable OTC study (Drake et al., 1985; Hendrey et al., 1993). Another limitation of FACE is the spatial and temporal fluctuations in eCO₂ concentrations due to weather conditions such as wind (Drake et al., 1985) since the plot is not delimited by vertical walls. Recent FACE systems are equipped with multiple detectors that adapt CO₂ diffusion in relation with the intensity and

orientation of the wind (Mauri, 2010).

To conclude, among the 18 published articles studying the impact of eCO₂ on plant-virus interactions, most of them are using CECs for CO₂ enrichment. Even if OTC and FACE systems are closer to natural conditions and able to take into account the plant's ecosystem, their use is presently limited by their difficulty of implementation and their high

<figure><img src="image_14.png" /><figcaption>**Fig. 3.** Overview of the CO₂ application timing chosen in 18 enrichment studies on plant-virus interactions <sup>a</sup> Percentage of utilization based on 18 articles.</figcaption></figure>
<table><thead><tr><th colspan="4">CO<sub>2</sub> enrichment facility</th></tr><tr><th></th><th>CEC (72,2%<sup>a</sup>)</th><th>OTC (22,2%<sup>a</sup>)</th><th>FACE (5,6%<sup>a</sup>)</th></tr></thead><tbody><tr><th>General overview</th><td>Growth chamber<br>greenhouse</td><td>Growth chamber<br>cabinet</td><td>Face (5,6%<sup>a</sup>)</td></tr><tr><th>Environmental factors</th><td>Artificial :<br>tubes, LED, sodium lamps<br>Fixed, constant<br>Fixed, constant<br>No wind</td><td>Natural sunlight<br>(but solar radiation is attenuated)<br>Natural (but plant canopy is warmer inside compared to outside)<br>Natural (but higher inside compared to outside)<br>Reduced wind</td><td>Natural sunlight<br>Natural<br>Natural<br>Natural</td></tr><tr><th>Features</th><td>Small -medium potted plants<br>Individual plant scale study<br>+</td><td>Medium-tall potted or field-grown plants (crops, young trees)<br>Micro-ecosystem scale study<br>++</td><td>Tall field-grown plants<br>(crops or trees)<br>Ecosystem scale study<br>+++</td></tr><tr><th>Virus/vector income</th><td>Artificial inoculation</td><td>Natural epidemics /<br>artificial inoculation</td><td>Natural epidemics</td></tr></tbody></table>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 2.** The different CO₂ enrichment facilities used in 18 published studies on plant-virus interaction: general overview, environmental factors, features and type of virus/vector income <sup>a</sup> Percentage of utilization of each facility in the 18 articles.</figcaption></figure>

be about four times more than a comparable OTC study (Drake et al., 1985; Hendrey et al., 1993). Another limitation of FACE is the spatial and temporal fluctuations in eCO₂ concentrations due to weather conditions such as wind (Drake et al., 1985) since the plot is not delimited by vertical walls. Recent FACE systems are equipped with multiple detectors that adapt CO₂ diffusion in relation with the intensity and

orientation of the wind (Mauri, 2010).

To conclude, among the 18 published articles studying the impact of eCO₂ on plant-virus interactions, most of them are using CECs for CO₂ enrichment. Even if OTC and FACE systems are closer to natural conditions and able to take into account the plant's ecosystem, their use is presently limited by their difficulty of implementation and their high

<figure><img src="image_13.png" /><figcaption>**Fig. 3.** Overview of the CO₂ application timing chosen in 18 enrichment studies on plant-virus interactions <sup>a</sup> Percentage of utilization based on 18 articles.</figcaption></figure>
operational cost.

## 2.2. Timing of application of CO₂ enrichment studies

Timing of application of eCO₂ whether on healthy plants before virus inoculation or on virus-infected plants, is a crucial parameter to study the impact of eCO₂ on plant-virus interactions. In our literature survey, we found that most of the eCO₂ studies fit one out of three implementation patterns for eCO₂ application during plant-virus experiments presented hereafter. In these three approaches, once eCO₂ is applied, it is further maintained throughout the whole duration of the experiment. The first approach consists in an early exposure to eCO₂, from the sowing of the seeds (~28%, Fig. 3) until the end of the experiment, thus mimicking a condition of climate change and constant increase of eCO₂ concentration. In the second approach, eCO₂ is applied at variable dates after germination, between sowing and virus infection (~33%, Fig. 3), thus allowing plants to germinate in ambient CO₂ (aCO₂), to be transplanted and to further adapt to the eCO₂ environment before viral challenge. In the third approach, eCO₂ is applied immediately after virus inoculation (~39%, Fig. 3), allowing a normal development of the plant until the viral infection. Thus, to conclude, it is important to notice that in most of the studies, eCO₂ is applied before virus inoculation, which presents the advantage to allow plants to acclimate to eCO₂. As eCO₂ is further maintained after virus inoculation, the virus itself will undergo putative effects of eCO₂.

## 2.3. Chosen values for CO₂ enrichment studies

Current atmospheric CO₂ concentration is around 400 ppm and it has been forecasted to reach about 1000 ppm in 2100 in the worst predictive scenario (IPCC, 2021). Accordingly, the CO₂ concentration in ambient conditions (aCO₂) currently used in most studies ranges from 350 to 450 ppm (100% on the basis of 18 articles, Table 1). Elevated CO₂ conditions ranges from ~550 ppm to ~1000 ppm depending on the year taken into account (2050 or 2100) and on the chosen predictive model (IPCC, 2021; IPCC, 2013), with most studies using ~750 ppm (~32% on the basis of 18 articles, Table 1). Technical limitation can also influence the eCO₂ concentration actually used in the experiments. Indeed, FACE

<caption>**Table 1**</caption>

Overview of the ambient and elevated atmospheric CO₂ values chosen in 18 enrichment studies on plant-virus interactions.

<table><thead><tr><th></th><th>Concentration (ppm)</th><th>Percentage of utilization<sup>a</sup></th><th>Reference</th></tr></thead><tbody><tr><td>aCO<sub>2</sub><sup>b</sup></td><td>350–450</td><td>100%</td><td></td></tr><tr><td>eCO<sub>2</sub><sup>c</sup></td><td>540–580</td><td>~16%</td><td>Rúa et al., (2013)<br>Chung et al., (2017), Trębicki et al., (2017)</td></tr><tr><td></td><td>650</td><td>~26%</td><td>Trębicki et al., (2015), (2016)<br>Vandegeer et al., (2016)<br>Vassiliadis et al., (2016), (2018)</td></tr><tr><td></td><td>700–750</td><td>~32%</td><td>Malmström and Field, (1997)<br>Fu et al., (2010)<br>Ye et al., (2010)<br>Huang et al., (2012)<br>Guo et al., (2016), (2021)</td></tr><tr><td></td><td>800</td><td>~5%</td><td>Zhang et al., (2015)</td></tr><tr><td></td><td>940–1000</td><td>~21%</td><td>Matros et al., (2006)<br>Aguilar et al., (2015)<br>del Toro et al., (2015)<br>Chung et al., (2017)</td></tr></tbody></table>

technology has an intrinsic limitation in the eCO₂ range, as local CO₂ enrichment is limited by the cost of the delivery system. Thus, the applied eCO₂ levels are quite low, 550 ppm (Trębicki et al., 2017). For the OTC technologies, eCO₂ concentration is around 750 ppm. On the other hand, for the growth chambers which are completely closed environments, eCO₂ ranges from ~550–1000 ppm in the published studies, the concentrations depending on each laboratory. So, the overall review of the chosen values for eCO₂ applied to plant-virus interactions shows a high variability, depending on the selected eCO₂ enrichment technology and predictive model. The wide range of eCO₂ concentration illustrates that there is not yet a standard consensus on the eCO₂ value in the plant pathogen community. The selection of a defined set of conditions might lead to more consistent results among different studies.

## 2.4. Virus inoculation methods used in CO₂ enrichment studies

Virus/vector application is a crucial step for experimental studies dealing with the impact of eCO₂ on plant-virus interactions. Whether artificial or natural, different approaches are available to perform viral infection of plants. Mechanical virus inoculation (25% of the 18 articles; Table 2) is technically the easiest approach, when possible. Indeed, this technique requires the capacity of the virus to be transmitted by wounding of the leaf surface. It is thus restricted to certain plant virus species (e.g. *Potyviruses*, *Tobamoviruses*). The basic procedure is to rub the upper surface of a young plant leaf with leaf sap obtained by grinding infected plant material with a pestle using a mortar and a buffer that minimizes virus degradation. An abrasive (e.g. carborundum) is often used to favor wounds on the leaf surface thus enabling virus entry without causing too much damage to the epidermal cells (Hull, 2009).

For plant viruses that are recalcitrant to mechanical inoculation, the use of bacterial or insect (aphids, whiteflies) vectors is necessary for virus delivery into the plant (e.g. *Luteoviruses*, *Begomoviruses*) (Hull, 2009). Agro-inoculation (i.e. inoculation using *Agrobacterium* as a vector for virus delivery) is used in 25% of the articles (Table 2). Agro-inoculation is based on the ability of the T-DNA (transfer-DNA) of *Agrobacterium tumefaciens* to transfer itself into the plant cell (Gleba et al., 2013; Hull, 2009). The construction of infectious clones containing DNA copies of DNA viruses and cDNA copies of RNA viruses incorporated into the T-DNA of a binary plasmid of *Agrobacterium tumefaciens* is a prerequisite. Together with a suitable promoter and after plant injection using a syringe, the DNA/cDNA copies of virus genome in the T-DNA are expressed as RNAs allowing the translation of viral proteins and the production of a functional viral particle, leading to an infection cycle (Gleba et al., 2013). Once viral particles produced, cell-to-cell movement and systemic movement allow the spreading of virus particles in the whole plant.

Viral infection using insect vectors is the technique that better mimics natural infection. Indeed, the majority of plant viruses are transmitted by insect vectors (e.g. aphids). This method can be employed for nonpersistent viruses (i.e. viruses carry externally on vector mouthparts with short acquisition and transmission times), semi-persistent viruses (i.e. viruses that require longer periods to be acquired and transmitted), and persistent viruses (i.e. viruses that require a greater time for acquisition and transmission because they must traverse the gut and reach the salivary glands of the vector) (Gray, 2008). Viral inoculation using insect vectors correspond to 50% of the published studies (Table 2), all of them focusing on B/C/MYDV. Viral inoculation based on insect vectors also allows to study either the effect of a given parameter on the behavior of the vector itself (e.g. aphid abundance per plant, aphid weight, duration of feeding...) (Fu et al., 2010; Guo et al., 2016; Trębicki et al., 2016; Vassiliadis et al., 2018) or the effect of several parameters on the vector population (e.g. percentage of transmission from one plant to another) (Bosquee et al., 2018; Dáder et al., 2016; Peñalver-Cruz et al., 2020).

In conclusion, the viral inoculation method used in the 18 articles

<footnote>a Percentage of utilization based on 18 articles</footnote>

<footnote>b Concentration of ambient atmospheric CO₂ (aCO₂)</footnote>

<footnote>c Concentration of elevated atmospheric CO₂ (eCO₂)</footnote>
operational cost.

## 2.2. Timing of application of CO₂ enrichment studies

Timing of application of eCO₂ whether on healthy plants before virus inoculation or on virus-infected plants, is a crucial parameter to study the impact of eCO₂ on plant-virus interactions. In our literature survey, we found that most of the eCO₂ studies fit one out of three implementation patterns for eCO₂ application during plant-virus experiments presented hereafter. In these three approaches, once eCO₂ is applied, it is further maintained throughout the whole duration of the experiment. The first approach consists in an early exposure to eCO₂, from the sowing of the seeds (~28%, Fig. 3) until the end of the experiment, thus mimicking a condition of climate change and constant increase of eCO₂ concentration. In the second approach, eCO₂ is applied at variable dates after germination, between sowing and virus infection (~33%, Fig. 3), thus allowing plants to germinate in ambient CO₂ (aCO₂), to be transplanted and to further adapt to the eCO₂ environment before viral challenge. In the third approach, eCO₂ is applied immediately after virus inoculation (~39%, Fig. 3), allowing a normal development of the plant until the viral infection. Thus, to conclude, it is important to notice that in most of the studies, eCO₂ is applied before virus inoculation, which presents the advantage to allow plants to acclimate to eCO₂. As eCO₂ is further maintained after virus inoculation, the virus itself will undergo putative effects of eCO₂.

## 2.3. Chosen values for CO₂ enrichment studies

Current atmospheric CO₂ concentration is around 400 ppm and it has been forecasted to reach about 1000 ppm in 2100 in the worst predictive scenario (IPCC, 2021). Accordingly, the CO₂ concentration in ambient conditions (aCO₂) currently used in most studies ranges from 350 to 450 ppm (100% on the basis of 18 articles, Table 1). Elevated CO₂ conditions ranges from ~550 ppm to ~1000 ppm depending on the year taken into account (2050 or 2100) and on the chosen predictive model (IPCC, 2021; IPCC, 2013), with most studies using ~750 ppm (~32% on the basis of 18 articles, Table 1). Technical limitation can also influence the eCO₂ concentration actually used in the experiments. Indeed, FACE

<caption>**Table 1**</caption>

Overview of the ambient and elevated atmospheric CO₂ values chosen in 18 enrichment studies on plant-virus interactions.

<table><thead><tr><th></th><th>Concentration (ppm)</th><th>Percentage of utilization<sup>a</sup></th><th>Reference</th></tr></thead><tbody><tr><td>aCO<sub>2</sub><sup>b</sup></td><td>350–450</td><td>100%</td><td></td></tr><tr><td>eCO<sub>2</sub><sup>c</sup></td><td>540–580</td><td>~16%</td><td>Rúa et al., (2013)<br>Chung et al., (2017), Trębicki et al., (2017)</td></tr><tr><td></td><td>650</td><td>~26%</td><td>Trębicki et al., (2015), (2016)<br>Vandegeer et al., (2016)<br>Vassiliadis et al., (2016), (2018)</td></tr><tr><td></td><td>700–750</td><td>~32%</td><td>Malmström and Field, (1997)<br>Fu et al., (2010)<br>Ye et al., (2010)<br>Huang et al., (2012)<br>Guo et al., (2016), (2021)</td></tr><tr><td></td><td>800</td><td>~5%</td><td>Zhang et al., (2015)</td></tr><tr><td></td><td>940–1000</td><td>~21%</td><td>Matros et al., (2006)<br>Aguilar et al., (2015)<br>del Toro et al., (2015)<br>Chung et al., (2017)</td></tr></tbody></table>

technology has an intrinsic limitation in the eCO₂ range, as local CO₂ enrichment is limited by the cost of the delivery system. Thus, the applied eCO₂ levels are quite low, 550 ppm (Trębicki et al., 2017). For the OTC technologies, eCO₂ concentration is around 750 ppm. On the other hand, for the growth chambers which are completely closed environments, eCO₂ ranges from ~550–1000 ppm in the published studies, the concentrations depending on each laboratory. So, the overall review of the chosen values for eCO₂ applied to plant-virus interactions shows a high variability, depending on the selected eCO₂ enrichment technology and predictive model. The wide range of eCO₂ concentration illustrates that there is not yet a standard consensus on the eCO₂ value in the plant pathogen community. The selection of a defined set of conditions might lead to more consistent results among different studies.

## 2.4. Virus inoculation methods used in CO₂ enrichment studies

Virus/vector application is a crucial step for experimental studies dealing with the impact of eCO₂ on plant-virus interactions. Whether artificial or natural, different approaches are available to perform viral infection of plants. Mechanical virus inoculation (25% of the 18 articles; Table 2) is technically the easiest approach, when possible. Indeed, this technique requires the capacity of the virus to be transmitted by wounding of the leaf surface. It is thus restricted to certain plant virus species (e.g. Potyviruses, Tobamoviruses). The basic procedure is to rub the upper surface of a young plant leaf with leaf sap obtained by grinding infected plant material with a pestle using a mortar and a buffer that minimizes virus degradation. An abrasive (e.g. carborundum) is often used to favor wounds on the leaf surface thus enabling virus entry without causing too much damage to the epidermal cells (Hull, 2009).

For plant viruses that are recalcitrant to mechanical inoculation, the use of bacterial or insect (aphids, whiteflies) vectors is necessary for virus delivery into the plant (e.g. Luteoviruses, Begomoviruses) (Hull, 2009). Agro-inoculation (i.e. inoculation using Agrobacterium as a vector for virus delivery) is used in 25% of the articles (Table 2). Agro-inoculation is based on the ability of the T-DNA (transfer-DNA) of Agrobacterium tumefaciens to transfer itself into the plant cell (Gleba et al., 2013; Hull, 2009). The construction of infectious clones containing DNA copies of DNA viruses and cDNA copies of RNA viruses incorporated into the T-DNA of a binary plasmid of Agrobacterium tumefaciens is a prerequisite. Together with a suitable promoter and after plant injection using a syringe, the DNA/cDNA copies of virus genome in the T-DNA are expressed as RNAs allowing the translation of viral proteins and the production of a functional viral particle, leading to an infection cycle (Gleba et al., 2013). Once viral particles produced, cell-to-cell movement and systemic movement allow the spreading of virus particles in the whole plant.

Viral infection using insect vectors is the technique that better mimics natural infection. Indeed, the majority of plant viruses are transmitted by insect vectors (e.g. aphids). This method can be employed for nonpersistent viruses (i.e. viruses carry externally on vector mouthparts with short acquisition and transmission times), semi-persistent viruses (i.e. viruses that require longer periods to be acquired and transmitted), and persistent viruses (i.e. viruses that require a greater time for acquisition and transmission because they must traverse the gut and reach the salivary glands of the vector) (Gray, 2008). Viral inoculation using insect vectors correspond to 50% of the published studies (Table 2), all of them focusing on B/C/MYDV. Viral inoculation based on insect vectors also allows to study either the effect of a given parameter on the behavior of the vector itself (e.g. aphid abundance per plant, aphid weight, duration of feeding...) (Fu et al., 2010; Guo et al., 2016; Trębicki et al., 2016; Vassiliadis et al., 2018) or the effect of several parameters on the vector population (e.g. percentage of transmission from one plant to another) (Bosquee et al., 2018; Dáder et al., 2016; Peñalver-Cruz et al., 2020).

In conclusion, the viral inoculation method used in the 18 articles

<footnote>a Percentage of utilization based on 18 articles</footnote>

<footnote>b Concentration of ambient atmospheric CO₂ (aCO₂)</footnote>

<footnote>c Concentration of elevated atmospheric CO₂ (eCO₂)</footnote>
<caption>**Table 2**</caption>

Virus inoculation methods used to study the impact of eCO₂ on plant-virus pathosystems (based on 18 articles).

<table><thead><tr><th>Inoculation method</th><th>Percentage of utilization</th><th>Virus</th><th>Plant species</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">Aphid/whitefly</td><td rowspan="10">50%</td><td rowspan="2">B/C/ MYDV<sup>a</sup></td><td>Avena sativa</td><td>Malmström and Field, (1997)</td></tr><tr><td>Avena fatua</td><td>Rúa et al., (2013)</td></tr><tr><td rowspan="2">PLRV</td><td>Bromus hordeaceus</td><td rowspan="2">Trębicki et al., (2015), (2016), Trębicki et al., (2017)</td></tr><tr><td>Triticum aestivum</td></tr><tr><td rowspan="2">TYLCV<sup>b</sup></td><td>Solanum tuberosum</td><td>Vandegeer et al., (2016)</td></tr><tr><td>Solanum lycopersicum</td><td>Vassiliadis et al., (2016), (2018)</td></tr><tr><td rowspan="3">Agro-inoculation</td><td rowspan="3">25%</td><td>PVX</td><td>Nicotiana benthamiana</td><td>Chung et al., (2017)<br/>Guo et al., (2016)<br/>Aguilar et al., (2015)<br/>del Toro et al., (2015)</td></tr><tr><td>TYLCV</td><td>Solanum lycopersicum</td><td>Guo et al., (2016)<br/>Huang et al., (2012)</td></tr><tr><td>CMV</td><td>Nicotiana benthamiana<br/>Nicotiana tabacum</td><td>Guo et al., (2012)</td></tr><tr><td rowspan="4">Mechanical</td><td rowspan="4">25%</td><td>PVY</td><td>Nicotiana benthamiana<br/>Nicotiana tabacum</td><td>del Toro et al., (2015)<br/>Matros et al., (2006)</td></tr><tr><td>CMV</td><td>Nicotiana benthamiana<br/>Nicotiana tabacum</td><td>Ye et al., (2010)<br/>del Toro et al., (2015)<br/>Fu et al., (2010)</td></tr><tr><td>TMV</td><td>Solanum lycopersicum</td><td>Zhang et al., (2015)</td></tr><tr><td></td><td></td><td></td></tr></tbody></table>

studying the impact of eCO₂ on plant-virus interactions is mainly dependent on the studied virus and its internal properties related to the way of infection. The apparent over-representation of aphid transmission (~50% of the studies) is mainly due to the high number of studies using BYDV on wheat.

## 2.5. Virus detection methods

Beyond viral symptoms, virus detection and quantification of virus concentration in plant tissues is a critical step to assess viral responses in either susceptible or resistant plant genotypes. In the literature overview presented here, such experiments have been conducted in 14 out of the 18 articles of the literature survey. Depending on the study, symptom-based evaluations, biochemical assay, immune-based assays or molecular assays were performed and sometimes several methods were used in a combined way (Table 3, Table S2).

For symptom-based evaluations, various techniques were employed, including visual comparisons of symptom intensities (del Toro et al.,

<caption>**Table 3**</caption>

Overview of the virus detection methods used in the literature survey of 14 articles studying the effect of elevated CO₂ concentrations on plant-virus pathosystems.

<table><thead><tr><th>Virus detection method</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">Symptom-based evaluation</td><td>Visual comparisons of symptom intensities</td><td>del Toro et al., (2015)</td></tr><tr><td>Disease incidence</td><td>Huang et al., (2012)<br/>Guo et al., (2016)</td></tr><tr><td>Disease severity</td><td>Vassiliadis et al., (2016)<br/>Vassiliadis et al., (2018)</td></tr><tr><td>Disease index (DI)</td><td>Huang et al., (2012)<br/>Guo et al., (2016)</td></tr><tr><td>Biochemical assay</td><td>Foliar POD activity</td><td>Ye et al., (2010)</td></tr><tr><td rowspan="3">Immuno-based assay</td><td>ELISA</td><td>Matros et al., (2006)<br/>Huang et al., (2012),<br/>Rúa et al., (2013)<br/>del Toro et al., (2015)</td></tr><tr><td>Western blot + densitometric analysis of blotted protein bands</td><td rowspan="2">Trębicki et al., (2017)</td></tr><tr><td>Tissue blot immunoassay (TBIA) + calculation of virus incidence</td></tr><tr><td rowspan="2">Molecular assay</td><td>Semi-quantitative RT-PCR</td><td>Zhang et al., (2015)</td></tr><tr><td>Quantitative RT-PCR</td><td>Aguilar et al., (2015),<br/>Trębicki et al., (2015)<br/>Chung et al., (2017)<br/>Guo et al., (2021)</td></tr></tbody></table>

Abbreviations: RT-PCR: reverse transcription polymerase chain reaction; POD: foliar peroxidase activity; ELISA: enzyme-linked immunoabsorbent assay

2015) or calculations of disease incidence, disease severity, and disease index. Disease incidence is defined as the percentage of plants displaying visible viral symptom expression amongst all the initially inoculated plants (Huang et al., 2012; Guo et al., 2016; Vassiliadis et al., 2016) (Table 3). In studies performed by Vassiliadis et al. (2016, 2018), disease severity is graded as the number of symptomatic leaves divided by the total number of fully developed leaves per plant. Disease index (DI) is based on phenotypic observations of viral symptoms using a quantitative scoring scale translating symptom intensity into defined ranks. DI values were calculated as follows:

$$ (DI) = \frac{\sum Ni \times Ri}{(N \times Rh)} \times 100 $$

where Ni corresponds to the number of plants in the disease symptom ranking, Ri corresponds to the disease symptom rank, N corresponds to the overall number of plants, and Rh represents the highest disease symptom rank (Huang et al., 2012; Guo et al., 2016; Vassiliadis et al., 2016) (Table 3). In addition, Ye et al. (2010) used a biochemical assay to measure variations in foliar peroxidase (POD) activity and suggested its correlation with virus resistance cost.

Immuno-based assays provide relevant measures of relative viral titer in infected plant tissues especially when using monoclonal antibodies often raised again the virus coat protein (CP). In the literature survey of 14 articles, enzyme-linked immunoabsorbent assay (ELISA) (Matros et al., 2006; Huang et al., 2012; Rúa et al., 2013) or western blot coupled to densitometric analysis of blotted protein bands (del Toro et al., 2015). Moreover, in a four-year FACE experiment on wheat, Trębicki et al. (2017) investigated the percentage of virus infection per year using tissue blot immunoassays on 25–30 random plant samples collected from the two CO₂ levels coupled to calculation of the percent of change (Table 3) according to the formula:

$$ [(eCO_2-aCO_2)/aCO_2*100]. $$

Finally, molecular detection techniques such as reverse transcription polymerase chain reaction (RT-PCR) (either semi-quantitative RT-PCR (Zhang et al., 2015) or quantitative RT-PCR (Aguilar et al., 2015; Trębicki et al., 2015; Chung et al., 2017; Guo et al., 2021) were frequently used in the 14 articles of the literature survey (Table 3), mainly because the majority of plant viruses are RNA viruses.

<footnote><sup>a</sup> Some viruses cause barley yellow dwarf disease; BYDV: *Barley yellow dwarf virus*, *Luteovirus*; CYDV: *Cereal yellow dwarf virus*, *Polarovirus*; MYDV: *Maize yellow dwarf virus*, unassigned</footnote>

<footnote><sup>b</sup> TYLCV (*Tomato yellow leaf curl virus*, *Begomovirus*) transmits in nature by whiteflies. Guo et al., (2016) also used both agroinfiltration and *B. tabaci* for infecting plants of their experiments.</footnote>
<caption>**Table 2**</caption>

Virus inoculation methods used to study the impact of eCO₂ on plant-virus pathosystems (based on 18 articles).

<table><thead><tr><th>Inoculation method</th><th>Percentage of utilization</th><th>Virus</th><th>Plant species</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">Aphid/whitefly</td><td rowspan="10">50%</td><td rowspan="2">B/C/ MYDV<sup>a</sup></td><td>Avena sativa</td><td>Malmström and Field, (1997)</td></tr><tr><td>Avena fatua</td><td>Rúa et al., (2013)</td></tr><tr><td rowspan="2">PLRV</td><td>Bromus hordeaceus</td><td rowspan="2">Trębicki et al., (2015), (2016), Trębicki et al., (2017)</td></tr><tr><td>Triticum aestivum</td></tr><tr><td rowspan="2">TYLCV<sup>b</sup></td><td>Solanum tuberosum</td><td>Vandegeer et al., (2016)</td></tr><tr><td>Solanum lycopersicum</td><td>Vassiliadis et al., (2016), (2018)</td></tr><tr><td rowspan="2">PVX</td><td>Nicotiana benthamiana</td><td>Chung et al., (2017)</td></tr><tr><td></td><td>Guo et al., (2016)</td></tr><tr><td rowspan="2">TYLCV</td><td>Solanum lycopersicum</td><td>Aguilar et al., (2015)</td></tr><tr><td></td><td>del Toro et al., (2015)</td></tr><tr><td rowspan="2">CMV</td><td>Nicotiana benthamiana</td><td>Guo et al., (2016)</td></tr><tr><td>Nicotiana tabacum</td><td>Huang et al., (2012)</td></tr><tr><td rowspan="5">Agro-inoculation</td><td rowspan="5">25%</td><td rowspan="2">PVY</td><td>Nicotiana benthamiana</td><td>del Toro et al., (2015)</td></tr><tr><td>Nicotiana tabacum</td><td>Matros et al., (2006)</td></tr><tr><td rowspan="2">CMV</td><td>Nicotiana benthamiana</td><td>Ye et al., (2010)</td></tr><tr><td>Nicotiana tabacum</td><td>del Toro et al., (2015)</td></tr><tr><td rowspan="2">TMV</td><td>Solanum lycopersicum</td><td>Fu et al., (2010)</td></tr><tr><td></td><td>Zhang et al., (2015)</td></tr></tbody></table>

studying the impact of eCO₂ on plant-virus interactions is mainly dependent on the studied virus and its internal properties related to the way of infection. The apparent over-representation of aphid transmission (~50% of the studies) is mainly due to the high number of studies using BYDV on wheat.

## 2.5. Virus detection methods

Beyond viral symptoms, virus detection and quantification of virus concentration in plant tissues is a critical step to assess viral responses in either susceptible or resistant plant genotypes. In the literature overview presented here, such experiments have been conducted in 14 out of the 18 articles of the literature survey. Depending on the study, symptom-based evaluations, biochemical assay, immune-based assays or molecular assays were performed and sometimes several methods were used in a combined way (Table 3, Table S2).

For symptom-based evaluations, various techniques were employed, including visual comparisons of symptom intensities (del Toro et al.,

<caption>**Table 3**</caption>

Overview of the virus detection methods used in the literature survey of 14 articles studying the effect of elevated CO₂ concentrations on plant-virus pathosystems.

<table><thead><tr><th>Virus detection method</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">Symptom-based evaluation</td><td>Visual comparisons of symptom intensities</td><td>del Toro et al., (2015)</td></tr><tr><td>Disease incidence</td><td>Huang et al., (2012)<br>Guo et al., (2016)</td></tr><tr><td>Disease severity</td><td>Vassiliadis et al., (2016)<br>Vassiliadis et al., (2018)</td></tr><tr><td>Disease index (DI)</td><td>Huang et al., (2012)<br>Guo et al., (2016)</td></tr><tr><td>Biochemical assay</td><td>Foliar POD activity</td><td>Ye et al., (2010)</td></tr><tr><td rowspan="3">Immuno-based assay</td><td>ELISA</td><td>Matros et al., (2006)<br>Huang et al., (2012),<br>Rúa et al., (2013)</td></tr><tr><td>Western blot + densitometric analysis of blotted protein bands</td><td>del Toro et al., (2015)</td></tr><tr><td>Tissue blot immunoassay (TBIA) + calculation of virus incidence</td><td>Trębicki et al., (2017)</td></tr><tr><td rowspan="2">Molecular assay</td><td>Semi-quantitative RT-PCR</td><td>Zhang et al., (2015)</td></tr><tr><td>Quantitative RT-PCR</td><td>Aguilar et al., (2015),<br>Trębicki et al., (2015)<br>Chung et al., (2017)</td></tr></tbody></table>

Abbreviations: RT-PCR: reverse transcription polymerase chain reaction; POD: foliar peroxidase activity; ELISA: enzyme-linked immunoabsorbent assay

2015) or calculations of disease incidence, disease severity, and disease index. Disease incidence is defined as the percentage of plants displaying visible viral symptom expression amongst all the initially inoculated plants (Huang et al., 2012; Guo et al., 2016; Vassiliadis et al., 2016) (Table 3). In studies performed by Vassiliadis et al. (2016, 2018), disease severity is graded as the number of symptomatic leaves divided by the total number of fully developed leaves per plant. Disease index (DI) is based on phenotypic observations of viral symptoms using a quantitative scoring scale translating symptom intensity into defined ranks. DI values were calculated as follows:

$$ (DI) = \frac{\sum Ni \times Ri}{(N \times Rh)} \times 100 $$

where Ni corresponds to the number of plants in the disease symptom ranking i, Ri corresponds to the disease symptom rank, N corresponds to the overall number of plants, and Rh represents the highest disease symptom rank (Huang et al., 2012; Guo et al., 2016; Vassiliadis et al., 2016) (Table 3). In addition, Ye et al. (2010) used a biochemical assay to measure variations in foliar peroxidase (POD) activity and suggested its correlation with virus resistance cost.

Immuno-based assays provide relevant measures of relative viral titer in infected plant tissues especially when using monoclonal antibodies often raised again the virus coat protein (CP). In the literature survey of 14 articles, enzyme-linked immunoabsorbent assay (ELISA) (Matros et al., 2006; Huang et al., 2012; Rúa et al., 2013) or western blot coupled to densitometric analysis of blotted protein bands (del Toro et al., 2015). Moreover, in a four-year FACE experiment on wheat, Trębicki et al. (2017) investigated the percentage of virus infection per year using tissue blot immunoassays on 25–30 random plant samples collected from the two CO₂ levels coupled to calculation of the percent of change (Table 3) according to the formula:

$$ [(eCO_2-aCO_2)/aCO_2*100]. $$

Finally, molecular detection techniques such as reverse transcription polymerase chain reaction (RT-PCR) (either semi-quantitative RT-PCR (Zhang et al., 2015) or quantitative RT-PCR (Aguilar et al., 2015; Trębicki et al., 2015; Chung et al., 2017; Guo et al., 2021) were frequently used in the 14 articles of the literature survey (Table 3), mainly because the majority of plant viruses are RNA viruses.

<footnote><sup>a</sup> Some viruses cause barley yellow dwarf disease; BYDV: *Barley yellow dwarf virus*, *Luteovirus*; CYDV: *Cereal yellow dwarf virus*, *Polerovirus*; MYDV: *Maize yellow dwarf virus*, unassigned</footnote>

<footnote><sup>b</sup> TYLCV (*Tomato yellow leaf curl virus*, *Begomovirus*) transmits in nature by whiteflies. Guo et al., (2016) also used both agroinfiltration and *B. tabaci* for infecting plants of their experiments.</footnote>
To conclude, virus infection and impact on their host plants were evaluated in the 18 articles using conventional methods reflecting to some extent the expertise of the laboratory and the availability of technical resources and material, especially for immune-based (antisera) and molecular assays (lightcycler for qRT-PCR).

# 3. Impact of eCO₂ on virus-infected plants

## 3.1. Impact of eCO₂ on the intensity of viral symptoms and virus titer

Two different approaches are mainly used to quantify the impact of eCO₂ on a given plant-virus interaction. The first one relies on the visual scoring of disease severity using a quantitative scale based on the macroscopic viral symptoms. The second approach relies on molecular quantification of virus titer by monitoring viral coating protein production (using ELISA) or by quantifying viral RNAs by RT-qPCR technologies. Among the 18 articles studying the impact of eCO₂ on plant-virus interactions, 14 articles followed the evolution of either viral symptoms or virus titer. In 35% of the studies, viral symptoms or virus titer were reported to be increased suggesting that plants are more susceptible to viral pathogens when grown in eCO₂ atmosphere (Chung et al., 2017; Guo et al., 2016 (for cv. Motelle Mi-1.2); Trębicki et al., 2017; Vassiliadis et al., 2018, 2016; Ye et al., 2010) (Table S2). In contrast, 29% of the studies concluded to a decreased plant susceptibility suggesting a higher level of plant resistance to viruses under eCO₂ atmosphere (Aguilar et al., 2015; Guo et al., 2021, 2016 (for cv. Moneymaker); Huang et al., 2012; Zhang et al., 2015) (Supplemental Table S2). In 24% of the studies, variable results were found between macroscopic observation and virus titer (del Toro et al., 2015) (for CMV and PVX) or depending on the organs of the plant (Matros et al., 2006; Trębicki et al., 2015). Finally, in 12% of the studies, there was no difference in viral infection levels in aCO₂ compared to eCO₂ (Rúa et al., 2013; del Toro et al., 2015) (for PVY-0) (Table S2).

Certain pathosystems display a consistent behaviour, with concordant results between articles. Indeed, for the wheat/BYDV pathosystem, all studies demonstrated an increase in susceptibility through an increase in disease severity (Vassiliadis et al., 2018, 2016), virus incidence (Trębicki et al., 2017) or by virus titer in leaves (Trębicki et al., 2015) (Table S2). These results are confirmed in different eCO₂ enrichment systems (FACE and CECs). In contrast, for the *Nicotiana benthamiana* /PVX pathosystem,(Aguilar et al., 2015) and (del Toro et al., 2015) an increase of resistance was observed as testified by a decreased of viral titer by RT-qPCR (Supplemental Table S2).

On the other hand, the genetic background of cultivars seems to have an effect in certain pathosystems (Guo et al., 2016; Trębicki et al., 2017). Indeed, in a same study on the tomato/TYLCV pathosystem, the disease incidence and disease index showed that plants of the cultivar Moneymaker (whitefly susceptible) are more resistant, whereas plants of the cultivar Motelle Mi-1.2, (whitefly resistant) are more susceptible (Guo et al., 2016) (Table S2). Moreover, in an experiment with 8 wheat cultivars and 4 virus species in 4 years with the FACE system (Trębicki et al., 2017), virus incidence on wheat increased in eCO₂, but for some cultivars like Bolac or Impala, the virus incidence was lower in eCO₂ (Table S2).

Overall, the impact of eCO₂ on the intensity of viral symptoms and virus titer appear to be dependent on the studied plant/virus pathosystem. It should be emphasized that only compatible interactions have been studied up to date. Among them, an increase of viral symptoms or virus titer was more largely reported.

## 3.2. Impact of eCO₂ on plant growth and chemical contents in virus-infected plants

We recorded 10 studies that investigated the variation of plant growth under eCO₂ condition in virus-infected plants (Table S3). Plant growth was mainly evaluated by measuring the dry weight of plant

aboveground parts (stems and leaves). In 8 out of the 10 studies, the aboveground biomass was shown to be increased under eCO₂+virus compared to aCO₂+virus (Fu et al., 2010; Guo et al., 2021; Huang et al., 2012; Malmström and Field, 1997; Trębicki et al., 2015; Trębicki et al., 2016; Vandegeer et al., 2016). For example, when the increase in aboveground biomass was quantified, it ranges from +14,3% in toma-to/TYLCV pathosystem (Huang et al., 2012), +21% in tobacco/CMV pathosystem (Fu et al., 2010), to +44.4% in tobacco/PVYN pathosystem (Ye et al., 2010). Moreover, in 4 out of the 8 studies, the increased biomass was shown to be correlated with an increase of other plant growth parameters like plant height and stem diameter in toma-to/TYLCV pathosystem (Huang et al., 2012), leaf area (Trębicki et al., 2015; Trębicki et al., 2016) and tiller number (Trębicki et al., 2015; Trębicki et al., 2016; Vandegeer et al., 2016) in wheat/BYDV pathosystem (Supplemental Table S3). In addition, Malmström and Field (1997) also demonstrated that the increased biomass was related to an increased photosynthesis and instantaneous water use efficiency in BYDV-infected oat plants (Table S3). Nevertheless, it is worth to highlight that in 2 studies dealing with the wheat/BYDV pathosystem, no significant difference was observed for both dry weight (Vassiliadis et al., 2016) and plant height / tiller number (Vassiliadis et al., 2018, 2016) (Table S3). The authors suggest that this result is mainly related to the early harvest of plants which did not allow the emergence of dwarfing symptoms linked to BYDV infection. Overall, virus-infected plants under eCO₂ display the same kind of response as healthy plants under eCO₂, that is an increased biomass. However, as virus infection often impairs plant growth, it seems that infected plants may be even more boosted by eCO₂ resulting in a biomass comparable to that of healthy plants. To conclude, it appears that eCO₂ compensate the loss of biomass due to virus infection when virus infection and eCO₂ are combined.

Quantitative chemical analyses of primary plant metabolism in virus-infected plants grown under eCO₂ has been documented in 11 articles (Table S3). As the C/N ratio of plants is globally increased in eCO₂ growth condition (Matros et al., 2006; Yang et al., 2011) (Fig. 1), several studies focused on the quantification of carbohydrates versus nitrogen contents. Depending on the pathosystem, nitrogen content was either increased in tobacco/CMV (Fu et al., 2010) or decreased in wheat/BYDV (Trębicki et al., 2016; Vassiliadis et al., 2016) (Table S3). However, no difference in foliar carbohydrates accumulation has been observed in oat/BYDV (Malmström and Field, 1997) and wheat/BYDV (Vassiliadis et al., 2016) (Table S3). More specifically, a great increase in sugar concentration has been reported in wheat+BYDV (glucose +84%, fructose +91%, mannitol +93%, trehalose +95%) (Vassiliadis et al., 2016) (Table S3). Concerning organic acids, a decreased concentration of citric acid (-22%) has been shown in wheat/BYDV (Vassiliadis et al., 2016). Moreover, no discriminatory metabolites were detected in BYDV-infected wheat plants between aCO₂ and eCO₂ (Vassiliadis et al., 2018). Regarding total plant protein content, Fu et al. (2010) and del Toro et al. (2015) reported lower protein amounts in four pathosystems: CMV-infected tobacco and *N. benthamiana* challenged with CMV, PVY or PVX. Thus, regarding these reports, it is difficult to rule out a main effect of combined virus infection + eCO₂ stresses on primary plant metabolism since contrasting results have been observed.

Concerning secondary metabolism, very limited research has been conducted and only one study reported a lower nicotine content in CMV-infected tobacco plants (Fu et al., 2010) (Table S3). Moreover, two studies showed either a decreased level of electrolyte leakage in *Nicotiana benthamiana* /PVX (Aguilar et al., 2015) or an increased foliar Ca²⁺ concentration in CMV-infected tobacco plants (Guo et al., 2021) under eCO₂ than under aCO₂ (Table S3).

## 3.3. Impact of eCO₂ on plant molecular defense responses to viruses

Since eCO₂ modulates primary and secondary metabolisms and since viruses are obligate pathogens, many studies have aimed to study to
To conclude, virus infection and impact on their host plants were evaluated in the 18 articles using conventional methods reflecting to some extent the expertise of the laboratory and the availability of technical resources and material, especially for immune-based (antisera) and molecular assays (lightcycler for qRT-PCR).

# 3. Impact of eCO₂ on virus-infected plants

## 3.1. Impact of eCO₂ on the intensity of viral symptoms and virus titer

Two different approaches are mainly used to quantify the impact of eCO₂ on a given plant-virus interaction. The first one relies on the visual scoring of disease severity using a quantitative scale based on the macroscopic viral symptoms. The second approach relies on molecular quantification of virus titer by monitoring viral coating protein production (using ELISA) or by quantifying viral RNAs by RT-qPCR technologies. Among the 18 articles studying the impact of eCO₂ on plant-virus interactions, 14 articles followed the evolution of either viral symptoms or virus titer. In 35% of the studies, viral symptoms or virus titer were reported to be increased suggesting that plants are more susceptible to viral pathogens when grown in eCO₂ atmosphere (Chung et al., 2017; Guo et al., 2016 (for cv. Motelle Mi-1.2); Trębicki et al., 2017; Vassiliadis et al., 2018, 2016; Ye et al., 2010) (Table S2). In contrast, 29% of the studies concluded to a decreased plant susceptibility suggesting a higher level of plant resistance to viruses under eCO₂ atmosphere (Aguilar et al., 2015; Guo et al., 2021, 2016 (for cv. Moneymaker); Huang et al., 2012; Zhang et al., 2015) (Supplemental Table S2). In 24% of the studies, variable results were found between macroscopic observation and virus titer (del Toro et al., 2015) (for CMV and PVX) or depending on the organs of the plant (Matros et al., 2006; Trębicki et al., 2015). Finally, in 12% of the studies, there was no difference in viral infection levels in aCO₂ compared to eCO₂ (Rúa et al., 2013; del Toro et al., 2015) (for PVY-0) (Table S2).

Certain pathosystems display a consistent behaviour, with concordant results between articles. Indeed, for the wheat/BYDV pathosystem, all studies demonstrated an increase in susceptibility through an increase in disease severity (Vassiliadis et al., 2018, 2016), virus incidence (Trębicki et al., 2017) or by virus titer in leaves (Trębicki et al., 2015) (Table S2). These results are confirmed in different eCO₂ enrichment systems (FACE and CECs). In contrast, for the Nicotiana benthamina/PVX pathosystem,(Aguilar et al., 2015) and (del Toro et al., 2015) an increase of resistance was observed as testified by a decreased of viral titer by RT-qPCR (Supplemental Table S2).

On the other hand, the genetic background of cultivars seems to have an effect in certain pathosystems (Guo et al., 2016; Trębicki et al., 2017). Indeed, in a same study on the tomato/TYLCV pathosystem, the disease incidence and disease index showed that plants of the cultivar Moneymaker (whitefly susceptible) are more resistant, whereas plants of the cultivar Motelle Mi-1.2, (whitefly resistant) are more susceptible (Guo et al., 2016) (Table S2). Moreover, in an experiment with 8 wheat cultivars and 4 virus species in 4 years with the FACE system (Trębicki et al., 2017), virus incidence on wheat increased in eCO₂, but for some cultivars like Bolac or Impala, the virus incidence was lower in eCO₂ (Table S2).

Overall, the impact of eCO₂ on the intensity of viral symptoms and virus titer appear to be dependent on the studied plant/virus pathosystem. It should be emphasized that only compatible interactions have been studied up to date. Among them, an increase of viral symptoms or virus titer was more largely reported.

## 3.2. Impact of eCO₂ on plant growth and chemical contents in virus-infected plants

We recorded 10 studies that investigated the variation of plant growth under eCO₂ condition in virus-infected plants (Table S3). Plant growth was mainly evaluated by measuring the dry weight of plant

aboveground parts (stems and leaves). In 8 out of the 10 studies, the aboveground biomass was shown to be increased under eCO₂+virus compared to aCO₂+virus (Fu et al., 2010; Guo et al., 2021; Huang et al., 2012; Malmström and Field, 1997; Trębicki et al., 2015; Trębicki et al., 2016; Vandegeer et al., 2016). For example, when the increase in aboveground biomass was quantified, it ranges from +14,3% in toma-to/TYLCV pathosystem (Huang et al., 2012), +21% in tobacco/CMV pathosystem (Fu et al., 2010), to +44.4% in tobacco/PVYN pathosystem (Ye et al., 2010). Moreover, in 4 out of the 8 studies, the increased biomass was shown to be correlated with an increase of other plant growth parameters like plant height and stem diameter in toma-to/TYLCV pathosystem (Huang et al., 2012), leaf area (Trębicki et al., 2015; Trębicki et al., 2016) and tiller number (Trębicki et al., 2015; Trębicki et al., 2016; Vandegeer et al., 2016) in wheat/BYDV pathosystem (Supplemental Table S3). In addition, Malmström and Field (1997) also demonstrated that the increased biomass was related to an increased photosynthesis and instantaneous water use efficiency in BYDV-infected oat plants (Table S3). Nevertheless, it is worth to highlight that in 2 studies dealing with the wheat/BYDV pathosystem, no significant difference was observed for both dry weight (Vassiliadis et al., 2016) and plant height / tiller number (Vassiliadis et al., 2018, 2016) (Table S3). The authors suggest that this result is mainly related to the early harvest of plants which did not allow the emergence of dwarfing symptoms linked to BYDV infection. Overall, virus-infected plants under eCO₂ display the same kind of response as healthy plants under eCO₂, that is an increased biomass. However, as virus infection often impairs plant growth, it seems that infected plants may be even more boosted by eCO₂ resulting in a biomass comparable to that of healthy plants. To conclude, it appears that eCO₂ compensate the loss of biomass due to virus infection when virus infection and eCO₂ are combined.

Quantitative chemical analyses of primary plant metabolism in virus-infected plants grown under eCO₂ has been documented in 11 articles (Table S3). As the C/N ratio of plants is globally increased in eCO₂ growth condition (Matros et al., 2006; Yang et al., 2011) (Fig. 1), several studies focused on the quantification of carbohydrates versus nitrogen contents. Depending on the pathosystem, nitrogen content was either increased in tobacco/CMV (Fu et al., 2010) or decreased in wheat/BYDV (Trębicki et al., 2016; Vassiliadis et al., 2016) (Table S3). However, no difference in foliar carbohydrates accumulation has been observed in oat/BYDV (Malmström and Field, 1997) and wheat/BYDV (Vassiliadis et al., 2016) (Table S3). More specifically, a great increase in sugar concentration has been reported in wheat+BYDV (glucose +84%, fructose +91%, mannitol +93%, trehalose +95%) (Vassiliadis et al., 2016) (Table S3). Concerning organic acids, a decreased concentration of citric acid (-22%) has been shown in wheat/BYDV (Vassiliadis et al., 2016). Moreover, no discriminatory metabolites were detected in BYDV-infected wheat plants between aCO₂ and eCO₂ (Vassiliadis et al., 2018). Regarding total plant protein content, Fu et al. (2010) and del Toro et al. (2015) reported lower protein amounts in four pathosystems: CMV-infected tobacco and N. benthiana challenged with CMV, PVY or PVX. Thus, regarding these reports, it is difficult to rule out a main effect of combined virus infection+ eCO₂ stresses on primary plant metabolism since contrasting results have been observed.

Concerning secondary metabolism, very limited research has been conducted and only one study reported a lower nicotine content in CMV-infected tobacco plants (Fu et al., 2010) (Table S3). Moreover, two studies showed either a decreased level of electrolyte leakage in Nicotiana benthiana/PVX (Aguilar et al., 2015) or an increased foliar Ca²⁺ concentration in CMV-infected tobacco plants (Guo et al., 2021) under eCO₂ than under aCO₂ (Table S3).

## 3.3. Impact of eCO₂ on plant molecular defense responses to viruses

Since eCO₂ modulates primary and secondary metabolisms and since viruses are obligate pathogens, many studies have aimed to study to
what extent eCO₂ can modulate plant molecular defenses against vi-
ruses. Here, we attempt to summarize articles from the literature dealing
with the impact of eCO₂ on phytohormone accumulation and expression
of defense genes (i.e. genes encoding enzymes involved in phytohor-
mone biosynthesis pathways, genes encoding defense proteins like
Pathogenesis-Related (PR) proteins, as well as genes related to RNA
silencing).

Elevated CO₂ is known to alter defense signaling pathways mediated by phytohormones especially SA and JA (reviewed in Roy and Mathur, 2021). Indeed, eCO₂ increases the SA signaling pathway but decreases the JA signaling pathway (Li et al., 2016; Sun et al., 2013), thus highlighting antagonism relation between SA and JA. Indeed, SA and other phytohormones have a complex regulation due to cross-talks, synergisms and antagonisms between hormones (Husen and Zhang, 2023). Significant accumulation of SA was observed in Arabidopsis and in three crop species in eCO₂ condition (1000 ppm) (Mhamdi and Noctor, 2016). As most plant viruses activate the SA signaling pathway (Pegadaraju et al., 2007; Thompson and Goggin, 2006), it was expected that over-accumulation of SA under eCO₂ condition could strengthen plant resistance to virus infection. Analysis of data from a total of 4 studies from the literature shows that in 3 of them (75%), either SA accumulates in eCO₂ condition, or expressions of the PAL or ICS genes of the SA pathway are increased (Table S3). In all these studies, the plant-virus interaction is compatible. There is only one exception where a decreased accumulation of SA, a decreased PAL activity and a decreased PAL gene expression was observed (Guo et al., 2016). In this last study, the plant virus interaction is incompatible since the plant species is a tomato cultivar Motelle possessing the Mi-1.2 resistance gene (Guo et al., 2016). For JA, under eCO₂, half of the published studies (based on a total of 4 studies) report an increase in JA accumulation, whereas there is no change in JA levels in the two remaining studies (Table S3). Concerning defense genes, PR gene expression was increased in two pathosystems out of four (Table S3), while in the others, PR1 gene expression is either unchanged (Matros et al., 2006) or decreased (Guo et al., 2016). Finally, Zhang et al. (2015) report that in tomato, NPR1 gene expression is increased and this is the only article that studies NPR1. Huang et al. (2012) report that ABA level is unchanged in TYLCV-infected tomato plants under eCO₂ condition compared to plants placed in aCO₂ condition. To summarize, we recorded only 4 studies focusing on phytohor-mone variation. This limited research work probably illustrates only partially the complex impact of combined biotic plus abiotic stresses on phytohormone-mediated responses.

One study investigated the putative implication of the RNAi pathway on the defense response against viral pathogens in eCO₂ condition (Guo et al., 2021). This pathway is triggered by viral-derived dsRNAs and results in the degradation of these dsRNAs thanks to key cellular proteins such as Dicer-like (DCL), RNA-dependent RNA polymerase (RDR) and Argonaute (AGO) proteins (Baulcombe, 2004; Csorba et al., 2009; Ratcliff et al., 1997). At a macroscopic level, infected plants may recover from virus infection and a lower virus titer as well as decreased symptoms have been documented (Baulcombe, 2004; Wingard, 1928). Accordingly, the modulation of the RNAi-mediated resistance under eCO₂ has been recently evaluated by Guo et al. (2021) on the pathosystem tobacco/CMV. A decreased CMV copy numbers was found in tobacco plants under eCO₂, suggesting that CMV genomic RNAs may be degraded by an enhanced RNAi pathway. The transcription of key genes encoding DCL, RDR and AGO proteins, with functions related to different steps of virus-induced RNA silencing, were investigated. Among these genes DCL2, DCL4, and AGO1 were shown to be down-regulated in CMV-infected tobacco plants under eCO₂, which is in contradiction with the decreased CMV accumulation. To our knowledge, this is the only study that investigates the impact of eCO₂ on the plant antiviral RNA silencing pathway.

Furthermore, three studies investigated the efficiency of VSR to
counteract the plant RNAi pathway in eCO₂ compared to aCO₂ (Aguilar
et al., 2015; Guo et al., 2021; del Toro et al., 2015). Firstly, the

functional VSR 2b protein of CMV was shown to be necessary to suppress systemic RNA silencing in tobacco plants under both CO₂ levels, but suppression was less efficient under eCO₂ compared to aCO₂ (Guo et al., 2021). It was reported before that 2b activity in CMV can be suppressed by rgs-CaM, a calmodulin in Nicotiana benthamiana (Nakahara and Masuta, 2014) and that the free-calcium concentration and downstream rgs-CaM gene expression were increased under eCO₂ in trembling aspen trees (Gupta et al., 2005). Guo et al. (2021) found that both calcium concentration and expression of rgs-CaM were increased under eCO₂ in CMV-infected tobacco plants as well. Indeed, eCO₂ decreased 2b gene expression and subsequently reduce the efficiency of virus infection (Guo et al., 2021). Consequently, the infection efficiency of CMV was decreased. Secondly, higher suppression of silencing by both VSR 2b of CMV and HcPro of PVY was observed in agropatch assays, that means outside the context of viral infection (del Toro et al., 2015). Thirdly, Aguilar et al. (2015) demonstrated that the P25 VSR of PVX displayed comparable suppressor activity under eCO₂ versus aCO₂ in agropatch assays. Altogether these results on VSRs are likely contradictory so that more studies are required in the future to better analyze the effect of eCO₂ on viral VSR efficiency and more largely on the plant RNA silencing pathway.

4. Impact of combined abiotic stresses including eCO₂ on plant-virus interactions

Despite a growing interest of the scientific community over the effect of climate change on plant-virus interaction, only seven studies have addressed the combined impact of eCO₂ and elevated temperature in virus-infected plants (Table S4). In the previous section we have reviewed the impact of eCO₂ over plant-virus interaction, and we refer to several recent reviews for the impact of elevated temperature on plant-virus diseases (Amari et al., 2021; Tsai et al., 2022; Venkatesh and Kang, 2019). Given the forecasted climate change scenario, there is a growing need of experiments combining eCO₂ and high temperatures, as under both abiotic stresses, the biological results could differ from what anticipated by the studying of a single abiotic stress at the time. Here we will focus on the few studies that have combined both eCO₂ and high temperature, thus mimicking climate change conditions (CCC).

To date, most studies have investigated the CCC influence on plant-
viral interaction using the *Nicothiana benthamiana*/PVY pathosystem:
the three available studies using this pathosystem (del Toro et al., 2019,
2017; Makki et al., 2021) found a consistent reduction of viral titer,
regardless of the sampling time 14 dpi (del Toro et al., 2019), 7 dpi or 12
dpi (del Toro et al., 2017) or 10 dpi (Makki et al., 2021) (Table S4). In
addition, this marked reduction in viral titer was associated with a
marked attenuation of PVY symptoms in infected plants under CCC
condition (del Toro et al., 2019, 2017; Makki et al., 2021) (Table S4).

Complex interaction of CCC with both host and pathogen could
indeed be associated to a different isolate-specific response possibly
influenced by the specificity of the different viral isolate due to their
environmental adaptation. While different geographic PVY isolates
display different HCPro sequences (a key multifunctional player in
plant-virus molecular interactions), these changes were not associated to
differences in viral symptoms in a CCC condition (Makki et al., 2021).
Further studies will be needed to better investigate the diversity of
symptoms displayed by several PVY isolates.

Reduced viral infection symptoms under CCC were also observed for
PVX (Potexvirus) (del Toro et al., 2017) but no change neither in viral
titer nor in viral symptoms was observed for CMV (Cucumovirus) (del
Toro et al., 2017) (Table S4), thus leaving an open question on the CCC
influence on Cucumovirus. These results, obtained under CCC condition,
are consistent with the global decrease of viral symptoms and viral titer
observed for eCO₂, while hinting at a pathosystem-specific effect of the
CCC influence on plant-virus interaction.

Contradictory, increased viral symptoms or infection rates were
observed for other pathosystems such as wheat /BYDV-PAV
what extent eCO₂ can modulate plant molecular defenses against vi-
ruses. Here, we attempt to summarize articles from the literature dealing
with the impact of eCO₂ on phytohormone accumulation and expression
of defense genes (i.e. genes encoding enzymes involved in phytohor-
mone biosynthesis pathways, genes encoding defense proteins like
Pathogenesis-Related (PR) proteins, as well as genes related to RNA
silencing).

Elevated CO2 is known to alter defense signaling pathways mediated by phytohormones especially SA and JA (reviewed in Roy and Mathur, 2021). Indeed, eCO2 increases the SA signaling pathway but decreases the JA signaling pathway (Li et al., 2016; Sun et al., 2013), thus highlighting antagonism relation between SA and JA. Indeed, SA and other phytohormones have a complex regulation due to cross-talks, synergisms and antagonisms between hormones (Husen and Zhang, 2023). Significant accumulation of SA was observed in Arabidopsis and in three crop species in eCO2 condition (1000 ppm) (Mhamdi and Noctor, 2016). As most plant viruses activate the SA signaling pathway (Pegadaraju et al., 2007; Thompson and Goggin, 2006), it was expected that over-accumulation of SA under eCO2 condition could strengthen plant resistance to virus infection. Analysis of data from a total of 4 studies from the literature shows that in 3 of them (75%), either SA accumulates in eCO2 condition, or expressions of the PAL or ICS genes of the SA pathway are increased (Table S3). In all these studies, the plant-virus interaction is compatible. There is only one exception where a decreased accumulation of SA, a decreased PAL activity and a decreased PAL gene expression was observed (Guo et al., 2016). In this last study, the plant virus interaction is incompatible since the plant species is a tomato cultivar Motelle possessing the Mi-1.2 resistance gene (Guo et al., 2016). For JA, under eCO2, half of the published studies (based on a total of 4 studies) report an increase in JA accumulation, whereas there is no change in JA levels in the two remaining studies (Table S3). Concerning defense genes, PR gene expression was increased in two pathosystems out of four (Table S3), while in the others, PR1 gene expression is either unchanged (Matros et al., 2006) or decreased (Guo et al., 2016). Finally, Zhang et al. (2015) report that in tomato, NPR1 gene expression is increased and this is the only article that studies NPR1. Huang et al. (2012) report that ABA level is unchanged in TYLCV-infected tomato plants under eCO2 condition compared to plants placed in aCO2 condition. To summarize, we recorded only 4 studies focusing on phytohor-mone variation. This limited research work probably illustrates only partially the complex impact of combined biotic plus abiotic stresses on phytohormone-mediated responses.

One study investigated the putative implication of the RNAi pathway on the defense response against viral pathogens in eCO₂ condition (Guo et al., 2021). This pathway is triggered by viral-derived dsRNAs and results in the degradation of these dsRNAs thanks to key cellular proteins such as Dicer-like (DCL), RNA-dependent RNA polymerase (RDR) and Argonaute (AGO) proteins (Baulcombe, 2004; Csorba et al., 2009; Ratcliff et al., 1997). At a macroscopic level, infected plants may recover from virus infection and a lower virus titer as well as decreased symptoms have been documented (Baulcombe, 2004; Wingard, 1928). Accordingly, the modulation of the RNAi-mediated resistance under eCO₂ has been recently evaluated by Guo et al. (2021) on the pathosystem tobacco/CMV. A decreased CMV copy numbers was found in tobacco plants under eCO₂, suggesting that CMV genomic RNAs may be degraded by an enhanced RNAi pathway. The transcription of key genes encoding DCL, RDR and AGO proteins, with functions related to different steps of virus-induced RNA silencing, were investigated. Among these genes DCL2, DCL4, and AGO1 were shown to be down-regulated in CMV-infected tobacco plants under eCO₂, which is in contradiction with the decreased CMV accumulation. To our knowledge, this is the only study that investigates the impact of eCO₂ on the plant antiviral RNA silencing pathway.

Furthermore, three studies investigated the efficiency of VSR to
counteract the plant RNAi pathway in eCO₂ compared to aCO₂ (Aguilar
et al., 2015; Guo et al., 2021; del Toro et al., 2015). Firstly, the

functional VSR 2b protein of CMV was shown to be necessary to suppress systemic RNA silencing in tobacco plants under both CO2 levels, but suppression was less efficient under eCO2 compared to aCO2 (Guo et al., 2021). It was reported before that 2b activity in CMV can be suppressed by rgs-CaM, a calmodulin in Nicotiana benthamiana (Nakahara and Masuta, 2014) and that the free-calcium concentration and downstream rgs-CaM gene expression were increased under eCO2 in trembling aspen trees (Gupta et al., 2005). Guo et al. (2021) found that both calcium concentration and expression of rgs-CaM were increased under eCO2 in CMV-infected tobacco plants as well. Indeed, eCO2 decreased 2b gene expression and subsequently reduce the efficiency of virus infection (Guo et al., 2021). Consequently, the infection efficiency of CMV was decreased. Secondly, higher suppression of silencing by both VSR 2b of CMV and HcPro of PVY was observed in agropatch assays, that means outside the context of viral infection (del Toro et al., 2015). Thirdly, Aguilar et al. (2015) demonstrated that the P25 VSR of PVX displayed comparable suppressor activity under eCO2 versus aCO2 in agropatch assays. Altogether these results on VSRs are likely contradictory so that more studies are required in the future to better analyze the effect of eCO2 on viral VSR efficiency and more largely on the plant RNA silencing pathway.

4. Impact of combined abiotic stresses including eCO₂ on plant-virus interactions

Despite a growing interest of the scientific community over the effect of climate change on plant-virus interaction, only seven studies have addressed the combined impact of eCO₂ and elevated temperature in virus-infected plants (Table S4). In the previous section we have reviewed the impact of eCO₂ over plant-virus interaction, and we refer to several recent reviews for the impact of elevated temperature on plant-virus diseases (Amari et al., 2021; Tsai et al., 2022; Venkatesh and Kang, 2019). Given the forecasted climate change scenario, there is a growing need of experiments combining eCO₂ and high temperatures, as under both abiotic stresses, the biological results could differ from what anticipated by the studying of a single abiotic stress at the time. Here we will focus on the few studies that have combined both eCO₂ and high temperature, thus mimicking climate change conditions (CCC).

To date, most studies have investigated the CCC influence on plant-
viral interaction using the *Nicothiana benthamiana*/PVY pathosystem:
the three available studies using this pathosystem (del Toro et al., 2019,
2017; Makki et al., 2021) found a consistent reduction of viral titer,
regardless of the sampling time 14 dpi (del Toro et al., 2019), 7 dpi or 12
dpi (del Toro et al., 2017) or 10 dpi (Makki et al., 2021) (Table S4). In
addition, this marked reduction in viral titer was associated with a
marked attenuation of PVY symptoms in infected plants under CCC
condition (del Toro et al., 2019, 2017; Makki et al., 2021) (Table S4).

Complex interaction of CCC with both host and pathogen could
indeed be associated to a different isolate-specific response possibly
influenced by the specificity of the different viral isolate due to their
environmental adaptation. While different geographic PVY isolates
display different HCPro sequences (a key multifunctional player in
plant-virus molecular interactions), these changes were not associated to
differences in viral symptoms in a CCC condition (Makki et al., 2021).
Further studies will be needed to better investigate the diversity of
symptoms displayed by several PVY isolates.

Reduced viral infection symptoms under CCC were also observed for
PVX (Potexvirus) (del Toro et al., 2017) but no change neither in viral
titer nor in viral symptoms was observed for CMV (Cucumovirus) (del
Toro et al., 2017) (Table S4), thus leaving an open question on the CCC
influence on Cucumovirus. These results, obtained under CCC condition,
are consistent with the global decrease of viral symptoms and viral titer
observed for eCO2, while hinting at a pathosystem-specific effect of the
CCC influence on plant-virus interaction.

Contradictory, increased viral symptoms or infection rates were
observed for other pathosystems such as wheat /BYDV-PAV
(Moreno-Delafuente et al., 2020, 2021) and potato/Potato leafroll virus (PLRV) (Chung et al., 2017), respectively (Table S4). These studies also pointed out an increased biomass and an elevated C/N ratio (Moreno-Delafuente et al., 2020, 2021), consistently with the increase of biomass and decrease of N content observed in eCO₂ treatment alone. Such changes on biomass production and N content could have a major impact in shaping plant-feeding aphid populations, as CCC influence on their growth could change their ability to spread the virus (Van Munster, 2020). Interestingly, (Moreno-Delafuente et al., 2020) and colleagues have observed that bird cherry-oat aphid (Rhopalosiphum padi L.) displays a reduced period from birth to adulthood and a decreased fecundity under CCC leading to a possible reduction of virus spreading. At the same time, CCC were associated to a faster appearance of virus symptoms, which could increase visual attraction of *R. padi* and thus indicating a possible compensatory effect (Moreno-Delafuente et al., 2020). Moreover, Moreno-Delafuente et al. (2021) studied a tritrophic plant-parasite-parasitoid system using the wheat-*R. padi*-Aphidius colemani system as a model. They showed that under CCC, even though the developmental time from oviposition to adult emergence of *A. colemani* was significantly decreased on BYDV-PAV-infected plants, parasitism rate, sex ratio and pupal survivorship was unchanged, suggesting that biocontrol of *R. padi* using *A. colemani* may still be efficient under future climate (Moreno-Delafuente et al., 2021).

Interestingly, one study addressed how CCC affect virus-induced resistance to either another pathogen, the bacteria *Pseudomonas syringae*, or to drought in two plant species: *N. benthamiana* (incompatible host) and *Arabidopsis thaliana* (compatible host) (Aguilar et al., 2020). Results obtained in *N. benthamiana* showed that CCC negatively affected resistance to *P. syringae* and to drought induced by virus infection in correlation with decreased viral symptoms as well as virus titer, cancelled H₂O₂ production, and reduced expression of SA-responsive genes (Aguilar et al., 2020) (Table S4).

Overall, these studies highlight the complexity of plant-virus-environment interactions and somehow it is difficult to predict ways in which changes in temperature and CO₂ concentrations can affect plant-virus interactions, and underline the need for continued research in this area to better understand the impacts of climate change on global food security.

## 5. Conclusions and future prospects

Climate change is a natural process, the intensity and speed of which has increased dramatically since the end of the 19th century due to raising levels of atmospheric CO₂ emissions linked to human industrial activity. Enormous collective efforts will be needed to stabilize the changes already underway, and even more to limit them (IPCC 2023, https://www.ipcc.ch/report/ar6/syr/). Humankind will therefore have to learn to live with some of the effects of climate change, particularly those on present agriculture (IPCC 2023, https://www.ipcc.ch/report/ar6/syr/) which aims to ensure food security while developing sustainable agricultural management practices. Indeed, future climate may influence negatively crop resistance to different kinds of pathogens and could thus endanger global food security (Eckardt et al., 2023). Thus, extensive research studies are required to investigate the impact of future climate scenarios on plant resistance to pathogens in general.

This review focuses on the impact of eCO₂ on plant-virus interactions. More specifically, in this review, we analysed the experimental work produced to date to anticipate the potential impact that eCO₂ will have on the resistance level of crops to viral pathogens. Despite the importance of eCO₂, our literature survey revealed that the number of scientific studies published is, to date, relatively limited since we recorded only 18 articles dealing with the impact of eCO₂ on plant-virus interactions. Moreover, the literature survey revealed that studies are limited to only two plant families: Poaceae and Solanaceae (Tables S2 and S3). Even among these plant families, the number of species is very limited (Triticum aestivum, Bromus hordeaceus, and Avena sp. for Poaceae,

tomato, tobacco and *Nicotiana benthamiana* for Solanaceae) providing only a partial picture and not taking into account the wide diversity of crops. It is worth to notice that no species of the *Fabaceae* has been studied yet whereas they play a crucial role in agroecology practices mainly due their capacity to fix atmospheric nitrogen and to release it in soils when used as green fertilizers. Furthermore, scientific researches conducted so far in relation with the impact of elevated CO₂ on plant-virus pathosystems have focused on annual short-cycle species (Tables S2, S3 and S4). Indeed, our literature overview reveals a complete gap of knowledge regarding perennial species, i.e. fruit or forest trees, mainly due to the challenges associated with experiments on perennial plants. Indeed, factors such as time constraints, logistical complexities, and higher costs contribute to the limited study of perennial species. It is worth to notice that most fruit species, mainly exported by developing countries from Africa, South America, and Asia, have received minimal attention beyond superficial characterization (Tatineni and Hein, 2023). Additionally, these developing countries are the most exposed to societal challenges including climate change, rapid human population growth, and food insecurity (Jones and Naidu, 2019). As a matter of fact, they also encompass the most vulnerable producers for whom it has become urgent to address studies concerning viral diseases, or more largely biotic stresses, and climate changes also with perennial plants.

All the studies have considered plant-virus compatible interactions. As crop varieties are usually carrying stacked major resistance genes mainly encoding immune NLR receptors and leading to resistance to specific pathogens, incompatible plant-virus interactions should also be studied in the coming years. A special focus may also be done on resistant genotypes controlled by recessive genes or even QTLs.

In addition, these limited reports concerning the impact of eCO₂ on plant-virus interactions are not yet sufficient to understand the general trend of eCO₂ impact. Surprisingly, opposite trends were reported, with several papers concluding to an increased resistance response while others demonstrating the opposite. As it is critical that we understand how eCO₂ will impact plant resistance in the future, additional studies are needed, possibly focusing on other economically important crops and considering a wider spectrum of viral pathogens. Only the study of a more diverse panel of plant-virus pathosystems will allow the scientific community to get new insights into the mechanisms underlying plant response to viruses under eCO₂.

Furthermore, in nature, plants are being challenged by multiple stresses simultaneously in addition to viral infection (Prasad et al., 2022b). However, most of the studies take into account only a single stress in standardized and controlled conditions such as in CEC. Other technologies such as OTC and FACE, while being difficult to set-up and having higher operational costs, are more likely to reproduce field conditions and might therefore be more easily translated to the field. Thus, OTC and FACE should be developed in the future as the more reliable facilities to study the impact of eCO₂ at the whole ecosystem scale. For example, the AmazonFACE project is an innovative project to understand how the Amazon Rainforest can respond to climate change (https://amazonface.unicamp.br/en/). The aim of the program is to expose an area of mature rainforest in the Amazon to a predicted future eCO₂ concentration at a research station near Manaus, Brazil and to monitor the impacts on plant physiological parameters such as carbon, nutrients, water and biodiversity for instance. Thus, projects such as AmazonFACE constitute large open-air laboratories and will address a critical gap in international climate science research.

Another possible direction for future research is the understanding of the mechanisms underlying impact of eCO₂ on plant pathogen interaction, taking into account not only resistance but also pathogenicity. Up to now, to our knowledge, the molecular responses have been addressed by studying individual genes, chosen as 'candidate genes' based on previous knowledge related to their implication in plant defense responses (SA/JA related or PR protein genes for example) (Tenllado and Canto, 2020; Bazinet et al., 2022). It is now timely to use
(Moreno-Delafuente et al., 2020, 2021) and potato/Potato leafroll virus (PLRV) (Chung et al., 2017), respectively (Table S4). These studies also pointed out an increased biomass and an elevated C/N ratio (Moreno-Delafuente et al., 2020, 2021), consistently with the increase of biomass and decrease of N content observed in eCO₂ treatment alone. Such changes on biomass production and N content could have a major impact in shaping plant-feeding aphid populations, as CCC influence on their growth could change their ability to spread the virus (Van Munster, 2020). Interestingly, (Moreno-Delafuente et al., 2020) and colleagues have observed that bird cherry-oat aphid (Rhopalosiphum padi L.) displays a reduced period from birth to adulthood and a decreased fecundity under CCC leading to a possible reduction of virus spreading. At the same time, CCC were associated to a faster appearance of virus symptoms, which could increase visual attraction of *R. padi* and thus indicating a possible compensatory effect (Moreno-Delafuente et al., 2020). Moreover, Moreno-Delafuente et al. (2021) studied a tritrophic plant-parasite-parasitoid system using the wheat-*R. padi*-Aphidius colemani system as a model. They showed that under CCC, even though the developmental time from oviposition to adult emergence of *A. colemani* was significantly decreased on BYDV-PAV-infected plants, parasitism rate, sex ratio and pupal survivorship was unchanged, suggesting that biocontrol of *R. padi* using *A. colemani* may still be efficient under future climate (Moreno-Delafuente et al., 2021).

Interestingly, one study addressed how CCC affect virus-induced resistance to either another pathogen, the bacteria *Pseudomonas syringae*, or to drought in two plant species: *N. benthamiana* (incompatible host) and *Arabidopsis thaliana* (compatible host) (Aguilar et al., 2020). Results obtained in *N. benthamiana* showed that CCC negatively affected resistance to *P. syringae* and to drought induced by virus infection in correlation with decreased viral symptoms as well as virus titer, cancelled H₂O₂ production, and reduced expression of SA-responsive genes (Aguilar et al., 2020) (Table S4).

Overall, these studies highlight the complexity of plant-virus-environment interactions and somehow it is difficult to predict ways in which changes in temperature and CO₂ concentrations can affect plant-virus interactions, and underline the need for continued research in this area to better understand the impacts of climate change on global food security.

## 5. Conclusions and future prospects

Climate change is a natural process, the intensity and speed of which has increased dramatically since the end of the 19th century due to raising levels of atmospheric CO₂ emissions linked to human industrial activity. Enormous collective efforts will be needed to stabilize the changes already underway, and even more to limit them (IPCC 2023, https://www.ipcc.ch/report/ar6/syr/). Humankind will therefore have to learn to live with some of the effects of climate change, particularly those on present agriculture (IPCC 2023, https://www.ipcc.ch/report/ar6/syr/) which aims to ensure food security while developing sustainable agricultural management practices. Indeed, future climate may influence negatively crop resistance to different kinds of pathogens and could thus endanger global food security (Eckardt et al., 2023). Thus, extensive research studies are required to investigate the impact of future climate scenarios on plant resistance to pathogens in general.

This review focuses on the impact of eCO₂ on plant-virus interactions. More specifically, in this review, we analysed the experimental work produced to date to anticipate the potential impact that eCO₂ will have on the resistance level of crops to viral pathogens. Despite the importance of eCO₂, our literature survey revealed that the number of scientific studies published is, to date, relatively limited since we recorded only 18 articles dealing with the impact of eCO₂ on plant-virus interactions. Moreover, the literature survey revealed that studies are limited to only two plant families: Poaceae and Solanaceae (Tables S2 and S3). Even among these plant families, the number of species is very limited (Triticum aestivum, Bromus hordeaceus, and Avena sp. for Poaceae,

tomato, tobacco and *Nicotiana benthamiana* for Solanaceae) providing only a partial picture and not taking into account the wide diversity of crops. It is worth to notice that no species of the *Fabaceae* has been studied yet whereas they play a crucial role in agroecology practices mainly due their capacity to fix atmospheric nitrogen and to release it in soils when used as green fertilizers. Furthermore, scientific researches conducted so far in relation with the impact of elevated CO₂ on plant-virus pathosystems have focused on annual short-cycle species (Tables S2, S3 and S4). Indeed, our literature overview reveals a complete gap of knowledge regarding perennial species, i.e. fruit or forest trees, mainly due to the challenges associated with experiments on perennial plants. Indeed, factors such as time constraints, logistical complexities, and higher costs contribute to the limited study of perennial species. It is worth to notice that most fruit species, mainly exported by developing countries from Africa, South America, and Asia, have received minimal attention beyond superficial characterization (Tatineni and Hein, 2023). Additionally, these developing countries are the most exposed to societal challenges including climate change, rapid human population growth, and food insecurity (Jones and Naidu, 2019). As a matter of fact, they also encompass the most vulnerable producers for whom it has become urgent to address studies concerning viral diseases, or more largely biotic stresses, and climate changes also with perennial plants.

All the studies have considered plant-virus compatible interactions. As crop varieties are usually carrying stacked major resistance genes mainly encoding immune NLR receptors and leading to resistance to specific pathogens, incompatible plant-virus interactions should also be studied in the coming years. A special focus may also be done on resistant genotypes controlled by recessive genes or even QTLs.

In addition, these limited reports concerning the impact of eCO₂ on plant-virus interactions are not yet sufficient to understand the general trend of eCO₂ impact. Surprisingly, opposite trends were reported, with several papers concluding to an increased resistance response while others demonstrating the opposite. As it is critical that we understand how eCO₂ will impact plant resistance in the future, additional studies are needed, possibly focusing on other economically important crops and considering a wider spectrum of viral pathogens. Only the study of a more diverse panel of plant-virus pathosystems will allow the scientific community to get new insights into the mechanisms underlying plant response to viruses under eCO₂.

Furthermore, in nature, plants are being challenged by multiple stresses simultaneously in addition to viral infection (Prasad et al., 2022b). However, most of the studies take into account only a single stress in standardized and controlled conditions such as in CEC. Other technologies such as OTC and FACE, while being difficult to set-up and having higher operational costs, are more likely to reproduce field conditions and might therefore be more easily translated to the field. Thus, OTC and FACE should be developed in the future as the more reliable facilities to study the impact of eCO₂ at the whole ecosystem scale. For example, the AmazonFACE project is an innovative project to understand how the Amazon Rainforest can respond to climate change (https://amazonface.unicamp.br/en/). The aim of the program is to expose an area of mature rainforest in the Amazon to a predicted future eCO₂ concentration at a research station near Manaus, Brazil and to monitor the impacts on plant physiological parameters such as carbon, nutrients, water and biodiversity for instance. Thus, projects such as AmazonFACE constitute large open-air laboratories and will address a critical gap in international climate science research.

Another possible direction for future research is the understanding of the mechanisms underlying impact of eCO₂ on plant pathogen interaction, taking into account not only resistance but also pathogenicity. Up to now, to our knowledge, the molecular responses have been addressed by studying individual genes, chosen as 'candidate genes' based on previous knowledge related to their implication in plant defense responses (SA/JA related or PR protein genes for example) (Tenllado and Canto, 2020; Bazinet et al., 2022). It is now timely to use
high-throughput technologies such as genome-wide ‘omics’ techniques in order to perform a molecular characterization of the combined effect of viral pathogen and eCO₂ and to study overall changes mediated by eCO₂ at transcriptome, proteome and metabolome levels in an unbiased approach. This will help to have an exhaustive view of which genes/proteins/networks are really impacted by eCO₂. Such knowledge would pave the way to develop climate-resilient cultivars via crop breeding for example (Fig. 4).

Some authors also suggested that climate change will have strong effects on virus molecular features themselves (Prasad Dash et al., 2021) as well as on viral epidemiology (Jones, 2016; Trębicki, 2020). For example, plant viruses enduring climate change conditions may have the potential to increase in range and severity as a result of higher mutation rates coupled to more frequent recombination events (Jones, 2016). To date, little attention has been paid to investigate the direct effects of eCO₂ or temperature or combination of both on these viral parameters. Moreover, as viruses are obligate pathogens, it should be worth studying the impact on environmental stresses on the enzymatic properties of the viral RNA dependent RNA polymerase itself since its activity conditions viral replication. Viral cell to cell movement may also be impacted by environmental stresses such as temperature for example (Amari et al., 2021; Meziadi et al., 2021). On a wider extent, viruses may take advantage of a boosted plant metabolism due to eCO₂ and systemic movement through phloem sieve that may also be accelerated. Deeper understanding of the direct impacts of eCO₂ on virus life-cycles would be of great importance to improve strategies to better manage viral diseases under future climate conditions (Fig. 4). Finally, as many viruses of agricultural importance are transmitted by vectors including aphids, whiteflies or thrips, another important issue that is worth to highlight here is the putative impact of climate change (elevated CO₂, temperature etc... or combined factors) on viral vector behaviors. Results from the literature described mixed effects (reviewed in Jones, 2016; Sun et al., 2016). In the cases in which vector behaviors are promoted by factors related to climate change, their populations may increase in natural and cultivated ecosystems. In the future, this could lead to an increase in the use of chemical control products like insecticides that are

widely used by farmers worldwide to manage viral vectors with high efficiency. Indeed, low vector populations limit the infection by plant viruses in the fields as well as the spreading of the viral disease between plants and at long-distance (Tatineni and Hein, 2023). Consequently, repeated applications and long-term use of chemical pesticides may impair natural ecosystems as well as agroecosystems due to soil contamination and impacts on beneficial insect populations as pollinators for example. Thus, in the case of vector-transmitted plant viruses, the study of the vector partner should be included in scientific studies dealing with the impact of climate change studies on plant-virus interactions and a tripartite interaction should be considered (Fig. 4).

Application of more precise pathogen detection approaches could also improve the experimental data of similar studies in the future. Several pathogen detection methods have been developed and used for plant virus diagnosis. These include symptom-based analysis either in host or indicator plants, biochemical approaches, serological methods relying on viral encoded proteins, and molecular assays based on viral genome sequences. Although these conventional detection methods are commonly used for virus diagnosis, their application feasibility and detection sensitivity are somehow questionable. For instance, low specificity of commercial antibodies for detection of diverse viral strains or isolates (e.g. ELISA, blotting/hybridization), mutations in viral genome sequences and presence of inhibitors within plant extracts that lead to false-positive and false-negative results (PCR and its derivative approaches), and duration of disease symptom appearance and disability of indicator plants in detecting nonvirulent strains (biological indexing) (Lopez et al., 2009; Naderpour, 2018; Venbrux et al., 2023; John et al., 2023) are among disadvantages of traditional diagnosis as says. Moreover, these techniques are dependent on viral proteins and genomic sequences of the known viruses, and therefore, are not able to track unknown or newly-emerged viruses. Additionally, visual assessment of disease symptoms in their hosts or indicator plants are not reliable in many cases (e.g. cryptic viruses, host tolerance, mild strains, host recovery, nonvirulent isolates, etc.).

Although RT-qPCR that was used in some of those 18 articles (e.g. (Trębicki et al., 2015; Aguilar et al., 2015; Zhang et al., 2015; Chung

<figure><img src="image_15.png" /><figcaption>**Fig. 4.** Integrated framework for the development of disease-resistant and climate-resilient crops from the knowledge performed by fundamental and applied research using combined biotic and abiotic stresses.</figcaption></figure>
high-throughput technologies such as genome-wide ‘omics’ techniques in order to perform a molecular characterization of the combined effect of viral pathogen and eCO₂ and to study overall changes mediated by eCO₂ at transcriptome, proteome and metabolome levels in an unbiased approach. This will help to have an exhaustive view of which genes/proteins/networks are really impacted by eCO₂. Such knowledge would pave the way to develop climate-resilient cultivars via crop breeding for example (Fig. 4).

Some authors also suggested that climate change will have strong effects on virus molecular features themselves (Prasad Dash et al., 2021) as well as on viral epidemiology (Jones, 2016; Trębicki, 2020). For example, plant viruses enduring climate change conditions may have the potential to increase in range and severity as a result of higher mutation rates coupled to more frequent recombination events (Jones, 2016). To date, little attention has been paid to investigate the direct effects of eCO₂ or temperature or combination of both on these viral parameters. Moreover, as viruses are obligate pathogens, it should be worth studying the impact on environmental stresses on the enzymatic properties of the viral RNA dependent RNA polymerase itself since its activity conditions viral replication. Viral cell to cell movement may also be impacted by environmental stresses such as temperature for example (Amari et al., 2021; Meziadi et al., 2021). On a wider extent, viruses may take advantage of a boosted plant metabolism due to eCO₂ and systemic movement through phloem sieve that may also be accelerated. Deeper understanding of the direct impacts of eCO₂ on virus life-cycles would be of great importance to improve strategies to better manage viral diseases under future climate conditions (Fig. 4). Finally, as many viruses of agricultural importance are transmitted by vectors including aphids, whiteflies or thrips, another important issue that is worth to highlight here is the putative impact of climate change (elevated CO₂, temperature etc... or combined factors) on viral vector behaviors. Results from the literature described mixed effects (reviewed in Jones, 2016; Sun et al., 2016). In the cases in which vector behaviors are promoted by factors related to climate change, their populations may increase in natural and cultivated ecosystems. In the future, this could lead to an increase in the use of chemical control products like insecticides that are

widely used by farmers worldwide to manage viral vectors with high efficiency. Indeed, low vector populations limit the infection by plant viruses in the fields as well as the spreading of the viral disease between plants and at long-distance (Tatineni and Hein, 2023). Consequently, repeated applications and long-term use of chemical pesticides may impair natural ecosystems as well as agroecosystems due to soil contamination and impacts on beneficial insect populations as pollinators for example. Thus, in the case of vector-transmitted plant viruses, the study of the vector partner should be included in scientific studies dealing with the impact of climate change studies on plant-virus interactions and a tripartite interaction should be considered (Fig. 4).

Application of more precise pathogen detection approaches could also improve the experimental data of similar studies in the future. Several pathogen detection methods have been developed and used for plant virus diagnosis. These include symptom-based analysis either in host or indicator plants, biochemical approaches, serological methods relying on viral encoded proteins, and molecular assays based on viral genome sequences. Although these conventional detection methods are commonly used for virus diagnosis, their application feasibility and detection sensitivity are somehow questionable. For instance, low specificity of commercial antibodies for detection of diverse viral strains or isolates (e.g. ELISA, blotting/hybridization), mutations in viral genome sequences and presence of inhibitors within plant extracts that lead to false-positive and false-negative results (PCR and its derivative approaches), and duration of disease symptom appearance and disability of indicator plants in detecting nonvirulent strains (biological indexing) (Lopez et al., 2009; Naderpour, 2018; Venbrux et al., 2023; John et al., 2023) are among disadvantages of traditional diagnosis says. Moreover, these techniques are dependent on viral proteins and genomic sequences of the known viruses, and therefore, are not able to track unknown or newly-emerged viruses. Additionally, visual assessment of disease symptoms in their hosts or indicator plants are not reliable in many cases (e.g. cryptic viruses, host tolerance, mild strains, host recovery, nonvirulent isolates, etc.).

Although RT-qPCR that was used in some of those 18 articles (e.g. (Trębicki et al., 2015; Aguilar et al., 2015; Zhang et al., 2015; Chung

<figure><img src="image_14.png" /><figcaption>**Fig. 4.** Integrated framework for the development of disease-resistant and climate-resilient crops from the knowledge performed by fundamental and applied research using combined biotic and abiotic stresses.</figcaption></figure>
et al., 2017; Guo et al., 2021) and CRISPR-Cas-based detection techniques (Prasad et al., 2022a) are some of the robust detection methods in terms of accuracy and repeatability, but still mutations in viral strains or isolates could also be a bottle neck. However, the newly developed technique based on next generation or high-throughput sequencing (NGS or HTS) is enough sensitive to track any known and unknown viruses within host plants. In combination with bioinformatics and machine learning for processing and analyzing the resulted sequencing data, HTS could be an absolutely reliable method to track any viruses or other pathogens in host plants. For instance, in eCO₂ experiments on plant-virus interactions, HTS could be useful to monitor natural epidemics in OTC and FACE studies. It should be noted that highly reliable and sensitive virus detection methods will help to advance decision-making by breeders thus accelerating plant breeding programs in order to develop virus resistant cultivars as this is suggested to be the most effective control measure to resist virus diseases.

All things considered, the field of plant virology in relation to climate change is still an emerging-field of research. Additional works need to be done in the coming years. This future knowledge will help researchers devising strategies to develop climate-resilient and virus-resistant crop varieties by breeding or genome engineering approaches and/or addition of beneficial microbes (Eckardt et al., 2023) (Fig. 4).

## Funding

This work was supported by INRAe BAP funding and ANR project 'HiPATH' (ANR-17-CE20-0025). IPS2 benefits from the support of fundings from Saclay Plant Sciences-SPS LabEx (ANR-17-EUR-0007). Université Paris Cité funded the stay of Masoud Naderpour as an invited researcher at IPS2.

## CRediT authorship contribution statement

**Gianluca Teano:** Writing – review & editing, Writing – original draft. **Masoud Naderpour:** Writing – review & editing, Writing – original draft. **Valérie Geffroy:** Writing – review & editing, Funding acquisition. **Tiffanie Scandolera:** Writing – review & editing, Writing – original draft, Visualization, Conceptualization. **Stéphanie Pflieger:** Writing – review & editing, Writing – original draft, Visualization, Supervision, Funding acquisition, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We are grateful to Cécile Desbiez (INRAE Montfavet France), Richard Berthomé (INRAE Castanet-Tolosan, France) and Cécile Raynaud (IPS2, Gif sur Yvette) for stimulating discussions in the context of T. Scandolera's PhD committee supervision. We also thank all members of our research group for helpful discussions.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.envexpbot.2024.105737.

## References

* AbdElgawad, H., Zinta, G., Beemster, G.T.S., Janssens, I.A., Asard, H., 2016. Future Climate CO₂ Levels Mitigate Stress Impact on Plants: Increased Defense or Decreased Challenge? Front. Plant Sci. 7 (556), 7. https://doi.org/10.3389/fpls.2016.00556.

* Aguilar, E., Allende, L., Del Toro, F.J., Chung, B.N., Canto, T., Tenllado, F., 2015. Effects of elevated CO₂ and temperature on pathogenicity determinants and virulence of

potato virus X/Potyvirus-Associated synergism. Mol. Plant-Microbe Interact. 28, 1364-1373. https://doi.org/10.1094/MPMI-08-15-0178-R.

* Aguilar, E., Del Toro, F.J., Figueira-Galán, D.F., Hou, W., Canto, T., Tenllado, F., 2020. Virus infection induces resistance to Pseudomonas syringae and to drought in both compatible and incompatible bacteria-host interactions, which are compromised under conditions of elevated temperature and CO₂ levels. J. Gen. Virol. 101, 122-135. https://doi.org/10.1099/igv.0.001353.

* Ahuja, I., de Vos, R.C.H., Bones, A.M., Hall, R.D., 2010. Plant molecular stress responses face climate change. Trends Plant Sci. 15, 664-674. https://doi.org/10.1016/j.t plants.2010.08.002.

* Amari, K., Niehl, A., 2020. Nucleic acid-mediated PAMP-triggered immunity in plants. Curr. Opin. Virol. 42, 32-39. https://doi.org/10.1016/j.coviro.2020.04.003.

* Amari, K., Huang, C., Heinlein, M., 2021. Potential Impact of Global Warming on Virus Propagation in Infected Plants and Agricultural Productivity. Front. Plant Sci. 12, 1-7. https://doi.org/10.3389/fpls.2021.649768.

* Baulcombe, D., 2004. RNA silencing in plants. Nature 431, 356-363.

* Bazinet, Q., Tang, L., Bede, J.C., 2022. Impact of future elevated carbon dioxide on C3 plant resistance to biotic stresses. MPMI 35 (7), 527-539. https://doi.org/10.1094/MPMI-07-21-0189-FI.

* Blunden, J., Boyer, T., 2022. State of the Climate in 2021. Spec. Suppl. Bull. Am. Meteorol. Soc. Vol. 103 (No. 8), 103.

* Bosquee, E., Boullis, A., Bertaux, M., Francis, F., Verheggen, F.J., 2018. Dispersion of Myzus persicae and transmission of Potato virus Y under elevated CO₂ atmosphere. Entomol. Exp. Appl. 166, 380-385. https://doi.org/10.1111/eea.12661.

* Chaudhry, S., Sidhu, G.P.S., 2022. Climate change regulated abiotic stress mechanisms in plants: a comprehensive review. Plant Cell Rep. 41, 1-31. https://doi.org/10.1007/s00299-021-02759-5.

* Chung, B.N., Koh, S.W., Choi, K.S., Joa, J.H., Kim, C.H., Selvakumar, G., 2017. Temperature and CO₂ level influence potato leafroll virus infection in Solanum tuberosum. Plant Pathol. J. 33, 522-527. https://doi.org/10.5423/PPJ. NT.01.2017.0019.

* Csorba, T., Pantaleo, V., Burgyán, J., 2009. Chapter 2 - RNA Silencing: An Antiviral Mechanism. In: Loebenstein, G., Carr, J.P.B.T.-A., in, V.R. (Eds.), Natural and Engineered Resistance to Plant Viruses, Part I. Academic Press, pp. 35-230. https://doi.org/10.1016/S0065-3527(09)07502-2.

* Csorba, T., Kontra, L., Burgyán, J., 2015. viral silencing suppressors: Tools forged to fine-tune host-pathogen coexistence. Virology 479-480 85-103. https://doi.org/10.1016/j.virol.2015.02.028.

* D'Andrea, L., Rinaldi, M., 2010. Systems to evaluate the effects of atmospheric CO₂ concentration on field crops: A review of open top chambers. Ital. J. Agrometeorol. 4, 23.

* Dáder, B., Fereres, A., Moreno, A., Trebicki, P., 2016. Elevated CO₂ impacts bell pepper growth with consequences to Myzus persicae life history, feeding behaviour and virus transmission ability. Sci. Rep. 6, 1-10. https://doi.org/10.1038/srep19120.

* Drake, B.G., Rogers, H.H., Allen, L.H., 1985. Methods of exposing plants to elevated carbon dioxide. Direct Eff. increasing Carbon Dioxide Veg. 11-32.

* Eastburn, D.M., McElrone, A.J., Bilgin, D.D., 2011. Influence of atmospheric and climatic change on plant-pathogen interactions. Plant Pathol. 60, 54-69. https://doi.org/10.1111/j.1365-3059.2010.02402.x.

* Eckardt, N.A., Ainsworth, E.A., Bahuguna, R.N., Broadley, M.R., Busch, W., Carpita, N.C., Castrillo, G., Chory, J., Dehaan, L.R., Duarte, C.M., Henry, A., Jagadish, S.V.K., Langdale, J.A., Leakey, A.D.B., Liao, J.C., Lu, K.J., McCann, M.C., McKay, J.K., Odeny, D.A., Jorge De Oliveira, E., Platten, J.D., Rabbi, I., Rim, E.Y., Ronald, P.C., Salt, D.E., Shigenaga, A.M., Wang, E., Wolfe, M., Zhang, X., 2023. Climate change challenges, plant science solutions. Plant Cell 35, 24-66. https://doi.org/10.1093/plcell/koac303.

* Enoch, H.Z., Zieslin, N., Biran, Y., Halevy, A.H., Schwarz, M., Kesler, B., Shimsi, D., 1971. Principles of CO₂ nutrition research. : Symp. Greenh. Clim.: Eval. Res. Methods (32), 97-118.

* Enoch, H.Z., Rylski, I., Spigelman, M., 1976. CO₂ enrichment of strawberry and cucumber plants grown in unheated greenhouses in Israel. Sci. Hortic. (Amst. J. 5), 33-41. https://doi.org/10.1016/0304-4238(76)90020-0.

* Eyring, V., Gillett, N.P., Achuta Rao, K.M., Barimalala, R., Barreiro Parrillo, M., Bellouin, N., Cassou, C., Durack, P.J., Kosaka, Y., McGregor, S., Min, S., Morgenstern, O., Sun, Y., 2021. Human Influence on the Climate System, in: Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S.L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M.I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J.B.R., Maycock, T.K., Waterfield, T., Yelekçi, O., Yu, R., Zhou, B. (Eds.), Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 423-552. https://doi.org/10.1017/9781009157896.005.

* Flor, H.H., 1955. Current statut of the gene-for-gene concept. Annu. Rev. Phytopathol. 1971, 275-296.

* Foyer, C.H., Noctor, G., 2020. Redox Homeostasis and Signaling in a Higher-CO₂ World. Annu. Rev. Plant Biol. 71, 157-182. https://doi.org/10.1146/annurev-arplant-050718-095955.

* Friedlingstein, P., Jones, M.W., O'Sullivan, M., Andrew, R.M., Bakker, D.C.E., Hauck, J., Le Quéré, C., Peters, G.P., Peters, W., Pongratz, J., Sitch, S., Canadell, J.G., Ciais, P., Jackson, R.B., Alin, S.R., Anthoni, P., Bates, N.R., Becker, M., Bellouin, N., Bopp, L., Chau, T.T.T., Chevallier, F., Chini, L.P., Cronin, M., Currie, K.I., Decharme, B., Djeutchouang, L.M., Dou, X., Evans, W., Feely, R.A., Feng, L., Gasser, T., Gilfillan, D., Gkritzalis, T., Grassi, G., Gregor, L., Gruber, N., Gürses, Ö., Harris, I., Houghton, R. A., Hurtt, G.C., Iida, Y., Ilyina, T., Luijkx, I.T., Jain, A., Jones, S.D., Kato, E., Kennedy, D., Klein Goldewijk, K., Knauer, J., Korsbakken, J.I., Körtzinger, A., Landschützer, P., Lauvset, S.K., Lefevre, N., Lienert, S., Liu, J., Marland, G.,
et al., 2017; Guo et al., 2021) and CRISPR-Cas-based detection techniques (Prasad et al., 2022a) are some of the robust detection methods in terms of accuracy and repeatability, but still mutations in viral strains or isolates could also be a bottle neck. However, the newly developed technique based on next generation or high-throughput sequencing (NGS or HTS) is enough sensitive to track any known and unknown viruses within host plants. In combination with bioinformatics and machine learning for processing and analyzing the resulted sequencing data, HTS could be an absolutely reliable method to track any viruses or other pathogens in host plants. For instance, in eCO₂ experiments on plant-virus interactions, HTS could be useful to monitor natural epidemics in OTC and FACE studies. It should be noted that highly reliable and sensitive virus detection methods will help to advance decision-making by breeders thus accelerating plant breeding programs in order to develop virus resistant cultivars as this is suggested to be the most effective control measure to resist virus diseases.

All things considered, the field of plant virology in relation to climate change is still an emerging-field of research. Additional works need to be done in the coming years. This future knowledge will help researchers devising strategies to develop climate-resilient and virus-resistant crop varieties by breeding or genome engineering approaches and/or addition of beneficial microbes (Eckardt et al., 2023) (Fig. 4).

## Funding

This work was supported by INRAe BAP funding and ANR project 'HiPATH' (ANR-17-CE20-0025). IPS2 benefits from the support of fundings from Saclay Plant Sciences-SPS LabEx (ANR-17-EUR-0007). Université Paris Cité funded the stay of Masoud Naderpour as an invited researcher at IPS2.

## CRediT authorship contribution statement

**Gianluca Teano:** Writing – review & editing, Writing – original draft. **Masoud Naderpour:** Writing – review & editing, Writing – original draft. **Valérie Geffroy:** Writing – review & editing, Funding acquisition. **Tiffanie Scandolera:** Writing – review & editing, Writing – original draft, Visualization, Conceptualization. **Stéphanie Pflieger:** Writing – review & editing, Writing – original draft, Visualization, Supervision, Funding acquisition, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We are grateful to Cécile Desbiez (INRAE Montfavet France), Richard Berthomé (INRAE Castanet-Tolosan, France) and Cécile Raynaud (IPS2, Gif sur Yvette) for stimulating discussions in the context of T. Scandolera's PhD committee supervision. We also thank all members of our research group for helpful discussions.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.envexpbot.2024.105737.

## References

* AbdElgawad, H., Zinta, G., Beemster, G.T.S., Janssens, I.A., Asard, H., 2016. Future Climate CO₂ Levels Mitigate Stress Impact on Plants: Increased Defense or Decreased Challenge? Front. Plant Sci. 7 (556), 7. https://doi.org/10.3389/fpls.2016.00556.

* Aguilar, E., Allende, L., Del Toro, F.J., Chung, B.N., Canto, T., Tenllado, F., 2015. Effects of elevated CO₂ and temperature on pathogenicity determinants and virulence of

potato virus X/Potyvirus-Associated synergism. Mol. Plant-Microbe Interact. 28, 1364-1373. https://doi.org/10.1094/MPMI-08-15-0178-R.

* Aguilar, E., Del Toro, F.J., Figueira-Galán, D.F., Hou, W., Canto, T., Tenllado, F., 2020. Virus infection induces resistance to Pseudomonas syringae and to drought in both compatible and incompatible bacteria-host interactions, which are compromised under conditions of elevated temperature and CO₂ levels. J. Gen. Virol. 101, 122-135. https://doi.org/10.1099/igv.0.001353.

* Ahuja, I., de Vos, R.C.H., Bones, A.M., Hall, R.D., 2010. Plant molecular stress responses face climate change. Trends Plant Sci. 15, 664-674. https://doi.org/10.1016/j.t plants.2010.08.002.

* Amari, K., Niehl, A., 2020. Nucleic acid-mediated PAMP-triggered immunity in plants. Curr. Opin. Virol. 42, 32-39. https://doi.org/10.1016/j.coviro.2020.04.003.

* Amari, K., Huang, C., Heinlein, M., 2021. Potential Impact of Global Warming on Virus Propagation in Infected Plants and Agricultural Productivity. Front. Plant Sci. 12, 1-7. https://doi.org/10.3389/fpls.2021.649768.

* Baulcombe, D., 2004. RNA silencing in plants. Nature 431, 356-363.

* Bazinet, Q., Tang, L., Bede, J.C., 2022. Impact of future elevated carbon dioxide on C3 plant resistance to biotic stresses. MPMI 35 (7), 527-539. https://doi.org/10.1094/MPMI-07-21-0189-FI.

* Blunden, J., Boyer, T., 2022. State of the Climate in 2021. Spec. Suppl. Bull. Am. Meteorol. Soc. Vol. 103 (No. 8), 103.

* Bosquee, E., Boullis, A., Bertaux, M., Francis, F., Verheggen, F.J., 2018. Dispersion of Myzus persicae and transmission of Potato virus Y under elevated CO₂ atmosphere. Entomol. Exp. Appl. 166, 380-385. https://doi.org/10.1111/eea.12661.

* Chaudhry, S., Sidhu, G.P.S., 2022. Climate change regulated abiotic stress mechanisms in plants: a comprehensive review. Plant Cell Rep. 41, 1-31. https://doi.org/10.1007/s00299-021-02759-5.

* Chung, B.N., Koh, S.W., Choi, K.S., Joa, J.H., Kim, C.H., Selvakumar, G., 2017. Temperature and CO₂ level influence potato leafroll virus infection in Solanum tuberosum. Plant Pathol. J. 33, 522-527. https://doi.org/10.5423/PPJ. NT.01.2017.0019.

* Csorba, T., Pantaleo, V., Burgyán, J., 2009. Chapter 2 - RNA Silencing: An Antiviral Mechanism. In: Loebenstein, G., Carr, J.P.B.T.-A., in, V.R. (Eds.), Natural and Engineered Resistance to Plant Viruses, Part I. Academic Press, pp. 35-230. https://doi.org/10.1016/S0065-3527(09)07502-2.

* Csorba, T., Kontra, L., Burgyán, J., 2015. viral silencing suppressors: Tools forged to fine-tune host-pathogen coexistence. Virology 479-480 85-103. https://doi.org/10.1016/j.virol.2015.02.028.

* D'Andrea, L., Rinaldi, M., 2010. Systems to evaluate the effects of atmospheric CO₂ concentration on field crops: A review of open top chambers. Ital. J. Agrometeorol. 4, 23.

* Dáder, B., Fereres, A., Moreno, A., Trebicki, P., 2016. Elevated CO₂ impacts bell pepper growth with consequences to Myzus persicae life history, feeding behaviour and virus transmission ability. Sci. Rep. 6, 1-10. https://doi.org/10.1038/srep19120.

* Drake, B.G., Rogers, H.H., Allen, L.H., 1985. Methods of exposing plants to elevated carbon dioxide. Direct Eff. increasing Carbon Dioxide Veg. 11-32.

* Eastburn, D.M., McElrone, A.J., Bilgin, D.D., 2011. Influence of atmospheric and climatic change on plant-pathogen interactions. Plant Pathol. 60, 54-69. https://doi.org/10.1111/j.1365-3059.2010.02402.x.

* Eckardt, N.A., Ainsworth, E.A., Bahuguna, R.N., Broadley, M.R., Busch, W., Carpita, N.C., Castrillo, G., Chory, J., Dehaan, L.R., Duarte, C.M., Henry, A., Jagadish, S.V.K., Langdale, J.A., Leakey, A.D.B., Liao, J.C., Lu, K.J., McCann, M.C., McKay, J.K., Odeny, D.A., Jorge De Oliveira, E., Platten, J.D., Rabbi, I., Rim, E.Y., Ronald, P.C., Salt, D.E., Shigenaga, A.M., Wang, E., Wolfe, M., Zhang, X., 2023. Climate change challenges, plant science solutions. Plant Cell 35, 24-66. https://doi.org/10.1093/plcell/koac303.

* Enoch, H.Z., Zieslin, N., Biran, Y., Halevy, A.H., Schwarz, M., Kesler, B., Shimsi, D., 1971. Principles of CO₂ nutrition research. : Symp. Greenh. Clim.: Eval. Res. Methods (32), 97-118.

* Enoch, H.Z., Rylski, I., Spigelman, M., 1976. CO₂ enrichment of strawberry and cucumber plants grown in unheated greenhouses in Israel. Sci. Hortic. (Amst. J. 5), 33-41. https://doi.org/10.1016/0304-4238(76)90020-0.

* Eyring, V., Gillett, N.P., Achuta Rao, K.M., Barimalala, R., Barreiro Parrillo, M., Bellouin, N., Cassou, C., Durack, P.J., Kosaka, Y., McGregor, S., Min, S., Morgenstern, O., Sun, Y., 2021. Human Influence on the Climate System, in: Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S.L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M.I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J.B.R., Maycock, T.K., Waterfield, T., Yelekçi, O., Yu, R., Zhou, B. (Eds.), Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 423-552. https://doi.org/10.1017/9781009157896.005.

* Flor, H.H., 1955. Current statut of the gene-for-gene concept. Annu. Rev. Phytopathol. 1971, 275-296.

* Foyer, C.H., Noctor, G., 2020. Redox Homeostasis and Signaling in a Higher-CO₂ World. Annu. Rev. Plant Biol. 71, 157-182. https://doi.org/10.1146/annurev-arplant-050718-095955.

* Friedlingstein, P., Jones, M.W., O'Sullivan, M., Andrew, R.M., Bakker, D.C.E., Hauck, J., Le Quéré, C., Peters, G.P., Peters, W., Pongratz, J., Sitch, S., Canadell, J.G., Ciais, P., Jackson, R.B., Alin, S.R., Anthoni, P., Bates, N.R., Becker, M., Bellouin, N., Bopp, L., Chau, T.T.T., Chevallier, F., Chini, L.P., Cronin, M., Currie, K.I., Decharme, B., Djeutchouang, L.M., Dou, X., Evans, W., Feely, R.A., Feng, L., Gasser, T., Gilfillan, D., Gkritzalis, T., Grassi, G., Gregor, L., Gruber, N., Gürses, Ö., Harris, I., Houghton, R. A., Hurtt, G.C., Iida, Y., Ilyina, T., Luijkx, I.T., Jain, A., Jones, S.D., Kato, E., Kennedy, D., Klein Goldewijk, K., Knauer, J., Korsbakken, J.I., Körtzinger, A., Landschützer, P., Lauvset, S.K., Lefevre, N., Lienert, S., Liu, J., Marland, G.,
McGuire, P.C., Melton, J.R., Munro, D.R., Nabel, J.E.M.S., Nakaoka, S.-I., Niya, Y., Ono, T., Pierrot, D., Poulter, B., Rehder, G., Resplandy, L., Robertson, E., Rödenbeck, C., Rosan, T.M., Schwinger, J., Schwingenschackl, C., Séférían, R., Sutton, A.J., Sweeney, C., Tanhua, T., Tans, P.P., Tian, H., Tilbrook, B., Tubiello, F., van der Werf, G.R., Viichard, N., Wada, C., Wanninkhof, R., Watson, A.J., Willis, D., Wiltshire, A.J., Yuan, W., Yue, C., Yue, X., Zaehle, S., Zeng, J., 2022. Global Carbon Budget 2021. Earth Syst. Sci. Data 14, 1917–2005. https://doi.org/10.5194/essd-14-1917-2022.

Fu, X., Ye, L., Kang, L., Ge, F., 2010. Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid. Plant, Cell Environ. 33, 2056–2064. https://doi.org/10.1111/j.1365-3040.2010.02205.x.

Gleba, Y.Y., Tusé, D., Giritch, A., 2013. Plant Viral Vectors for Delivery by Agrobacterium. : Ann. Rheum. Dis. 155–192. https://doi.org/10.1007/82_2013_352.
Gouveia, B.C., Calil, I.P., Machado, J.P.B., Santos, A.A., Fontes, E.P.B., 2017. Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants. Front. Microbiol.

Gray, S.M., 2008. Aphid transmission of plant viruses. Curr. Protoc. Microbiol. 1–10. https://doi.org/10.1002/9780471729259.mc16b01s10.

Guo, H., Ge, P., Tong, J., Zhang, Y., Peng, X., Zhao, Z., Ge, F., Sun, Y., 2021. Elevated carbon dioxide levels decreases cucumber mosaic virus accumulation in correlation with greater accumulation of rgs-cam, an inhibitor of a viral suppressor of rna1. Plants 10, 1–13. https://doi.org/10.3390/plants10010059.

Guo, Huijuan, Huang, L., Sun, Y., Guo, Honggang, Ge, F., 2016. The contrasting effects of elevated CO2 on TYLCV infection of tomato genotypes with and without the resistance gene, Mi-1.2. Front. Plant Sci. 7, 1–14. https://doi.org/10.3389/fpls.2016.01680.

Gupta, P., Duplessis, S., White, H., Karnosky, D.F., Martin, F., Podila, G.K., 2005. Gene expression patterns of trembling aspen trees following long-term exposure to interacting elevated CO2 and tropospheric O3. N. Phytol. 167, 129–142. https://doi.org/10.1111/j.1469-8137.2005.01422.x.

Hendrey, G.R., Lewin, K.F., Nagy, J., 1993. Free air carbon dioxide enrichment: development, progress, results. Vegetatio 104–105, 17–31. https://doi.org/10.1007/BF00048142.

Huang, L., Ren, Q., Sun, Y., Ye, L., Cao, H., Ge, F., 2012. Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato. Plant Biol. 14, 905–913. https://doi.org/10.1111/j.1438-8677.2012.00582.x.

Hull, R., 2009. Mechanical inoculation of plant viruses. Curr. Protoc. Microbiol. 6–9. https://doi.org/10.1002/9780471729259.mc16b06s13.

Husen, A., Zhang, W., 2023. Hormonal Cross-Talk, Plant Defense and Development: Plant Biology, Sustainability and Climate Change, Plant Biology, sustainability and climate change. Elsevier Science.

IPCC, 2013, Stocker, T.F., Qin, D., Plattner, G.-K., Alexander, L. V, Allen, S.K., Bindoff, N. L., Bréon, F.-M., Church, J.A., Cubasch, U., Emori, S., others, 2013. Technical summary, in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, pp. 33–115.

IPCC, 2014, Pachauri, R.K., Allen, M.R., Barros, V.R., Broome, J., Cramer, W., Christ, R., Church, J.A., Clarke, L., Dahe, Q., Dasgupta, P., Others, 2014. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change. Ipcc.

IPCC, 2021. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. https://doi.org/10.1017/9781009157896.

John, M.A., Bankole, I., Ajayi-Moses, O., Ijila, T., Jeje, T., Lalit, P., 2023. Relevance of advanced plant disease detection techniques in disease and pest management for ensuring food security and their implication: a review. Am. J. Plant Sci. 14, 1260–1295. https://doi.org/10.4236/ajps.2023.1411086.

Jones, R.A.C., 2016. Future Scenarios for Plant Virus Pathogens as Climate Change Progresses. Adv. Virus Res. 95, 87–147. https://doi.org/10.1016/bs.aivir.2016.02.004.

Jones, R.A.C., Naidu, R.A., 2019. Global Dimensions of Plant Virus Diseases: Current Status and Future Perspectives. Annu. Rev. Virol. 6, 387–409. https://doi.org/10.1146/annurev-virusology-092818-015066.

Kabbage, M., Kessens, R., Bartholomay, L.C., Williams, B., 2017. The Life and Death of a Plant Cell. Annu. Rev. Plant Biol. 68, 375–404. https://doi.org/10.1146/annurev-arplant-043015-111655.

Kazan, K., 2018. Plant-biotech interactions under elevated CO2: A molecular perspective. Environ. Exp. Bot. 153, 249–261. https://doi.org/10.1016/j.envexpbot.2018.06.005.

Kong, A., Thorleifsson, G., Frigge, M.L., Vilhjalmsson, B.J., Young, A.I., Thorgeirsson, T. E., Benonisdottir, S., Oddsson, A., Halldorsson, B.V., Masson, G., Gudbjartsson, D.F., Helgason, A., Bjornsdottir, J., Thorsteinsdottir, U., Stefansson, K., 2018. The nature of nurture: Effects of parental genotypes. Science 359 (80), 424–428. https://doi.org/10.1126/science.aan6877.

Kørner, C.J., Klauser, D., Niehl, A., Domínguez-Ferreras, A., Chinchilla, D., Boller, T., Heinlein, M., Hann, D.R., 2013. The Immunity Regulator BAK1 Contributes to Resistance Against Diverse RNA Viruses. Mol. Plant-Microbe Interact. 26, 1271–1280. https://doi.org/10.1094/MPMI-06-13-0179-R.

Leadley, P.W., Niklaus, P., Stocker, R., Korner, C., 1997. Screen-aided CO2 control (SACC): A middle ground between FACE and open-top chambers. Acta Oecologica 18, 207–219. https://doi.org/10.1016/S1146-609X(97)80007-0.

Li, X., Ahammed, G.J., Li, Z., Tang, M., Yan, P., Han, W., 2016. Decreased Biosynthesis of Jasmonic Acid via Lipoxygenase Pathway Compromised Caffeine-Induced Resistance to Colletotrichum gloeosporioides Under Elevated CO2 in Tea Seedlings. Phytopathology® 106, 1270–1277. https://doi.org/10.1094/PHYTO-12-15-0336-R.

Lopez, M.M., Llop, P., Olmos, A., Marco-Noales, E., Cambra, M., Bertolini, E., 2009. Are molecular tools solving the challenges posed by detection of plant pathogenic bacteria and viruses? Curr. Issues Mol. Biol. 11, 13–46. https://doi.org/10.21775/cimb.011.013.

Machacova, K., 2010. Open top chamber and free air CO2 enrichment - approaches to investigate tree responses to elevated CO2. iForest - Biogeosciences 3, 102–105. https://doi.org/10.3832/ifor0544-003.

Makki, M., Toro, F.J., Del, Necira, K., Tenllado, F., Djilani-Khouadja, F., Canto, T., 2021. Differences in virulence among pvy isolates of different geographical origins when infecting an experimental host under two growing environments are not determined by hcpro. Plants 10. https://doi.org/10.3390/plants10061086.

Malmström, C.M., Field, C.B., 1997. Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell Environ. 20, 178–188. https://doi.org/10.1046/j.1365-3040.1997.doi-63.x.

Matros, A., Amme, S., Kettig, B., Buck-Sorlin, G.H., Sonnewald, U., Mock, H.P., 2006. Growth at elevated CO2 concentrations leads to modified profiles of secondary metabolites in tobacco cv. SamsunNN and to increased resistance against infection with potato virus Y. Plant, Cell Environ. 29, 126–137. https://doi.org/10.1111/j.1365-3040.2005.01406.x.

Mauri, A., 2010. Field experiments using CO2 Enrichment: A comparison of two main methods. iForest 3, 109–112. https://doi.org/10.3832/ifor0545-003.

Meziadi, C., Lintz, J., Naderpour, M., Gautier, C., Blanchet, S., Noly, A., Gratias-Weill, A., Geffroy, V., Pflieger, S., 2021. R-bpmv-mediated resistance to bean pod mottle virus in Phaseolus vulgaris l. Is heat-stable but elevated temperatures boost viral infection in susceptible genotypes. Viruses 13. https://doi.org/10.3390/v13071239.

Mhamdi, A., Noctor, G., 2016. High CO2 primes plant biotic stress defences through redox-linked pathways. Plant Physiol. 172, 929–942. https://doi.org/10.1104/pp.16.01129.

Moreno-Delafuente, A., Viñuela, E., Fereres, A., Medina, P., Trébicki, P., 2020. Simultaneous increase in co2 and temperature alters wheat growth and aphid performance differently depending on virus infection. Insects 11, 1–20. https://doi.org/10.3390/Insects11080459.

Moreno-Delafuente, A., Viñuela, E., Fereres, A., Medina, P., Trébicki, P., 2021. Combined effects of elevated CO2 and temperature on multitrophic interactions involving a parasitoid of plant virus vectors. BioControl 66, 307–319. https://doi.org/10.1007/s10526-020-10069-0.

Naderpour, M., 2018. Challenges facing the detection of systemic pathogens in certification programs. Acta Hortic. (1269). 89–95. https://doi.org/10.17660/ActaHortic.2020.1269.12.

Nakahara, K.S., Masuta, C., 2014. Interaction between viral RNA silencing suppressors and host factors in plant immunity. Curr. Opin. Plant Biol. 20, 88–95. https://doi.org/10.1016/j.jpsi.2014.05.004.

Nicaise, V., Candesre, T., 2017. Plum pox virus capsid protein suppresses plant pathogen-associated molecular pattern (PAMP)-triggered immunity. Mol. Plant Pathol. 18, 878–886. https://doi.org/10.1111/mpp.12247.

Niehl, A., Wyrsch, I., Boller, T., Heinlein, M., 2016. Double-stranded RNAs induce a pattern-triggered immune signaling pathway in plants. N. Phytol. 211, 1008–1019. https://doi.org/10.1111/nph.13944.

Noctor, G., Mhamdi, A., 2017. Climate Change, CO2 and Defense: The Metabolic, Redox, and Signaling Perspectives. Trends Plant Sci. 22, 857–870. https://doi.org/10.1016/j.tplants.2017.07.007.

Oerke, E.-C., Dehne, H.-W., 2004. Safeguarding production—losses in major crops and the role of crop protection. Crop Prot. 23, 275–285. https://doi.org/10.1016/j.croprot.2003.10.001.

Palukaitis, P., Yoon, J.Y., 2020. R gene mediated defense against viruses. Curr. Opin. Virol. 45, 1–7. https://doi.org/10.1016/j.coviro.2020.04.001.

Pegadaraju, V., Louis, J., Singh, V., Reese, J.C., Bautor, J., Feys, B.J., Cook, G., Parker, J. E., Shah, J., 2007. Phloem-based resistance in green peach aphid is controlled by Arabidopsis PHYTOALEXIN DEFICIENT4 without its signaling partner ENHANCED DISEASE SUSCEPTIBILITY1. Plant J. 52, 332–341. https://doi.org/10.1111/j.1365-313X.2007.03241.x.

Peñalver-Cruz, A., Garzo, E., Prieto-Ruiz, I., Díaz-Carro, M., Winters, A., Moreno, A., Fereres, A., 2020. Feeding behavior, life history, and virus transmission ability of Bemisia tabaci Mediterranean species (Hemiptera: Aleyrodidae) under elevated CO2. Insect Sci. 27, 558–570. https://doi.org/10.1111/1744-7917.12661.

Prasad, A., Sett, S., Prasad, M., 2022b. Plant-virus-abiotic stress interactions; a complex interplay. Environ. Exp. Bot. 199, 104869 https://doi.org/10.1016/j.envexpbot.2022.104869.

Prasad, D., Mani, N.K., Pandey, D.M., 2022a. CRISPR/Cas technology: opportunities for phytopathogenic viruses detection. J. Biotechnol. 360, 211–217. https://doi.org/10.1016/j.jbiotec.2022.11.010.

Prasad Dash, S., Dipankar, P., Burange, P.S., Rouse, B.T., Sarangi, P.P., 2021. Climate change: how it impacts the emergence, transmission, resistance and consequences of viral infections in animals and plants. Crit. Rev. Microbiol. 47 (3), 307–322. https://doi.org/10.1080/1040841X.2021.1879006.

Raney, T., Croppenstedt, A., Garrett, J., Lowder, S., 2013. The State of Food and Agriculture 2013: Food Systems for Better Nutrition.

Ratcliff, F., Harrison, B.D., Baulcombe, D.C., 1997. A Similarity Between Viral Defense and Gene Silencing in Plants. Science 276 (80), 1558–1560. https://doi.org/10.1126/science.276.5318.1558.

de Ronde, B., Butterbach, P., Kormelink, R., 2014. Dominant resistance against plant viruses. Front. Plant Sci. 5, 307. https://doi.org/10.3389/fpls.2014.00307.

Roy, S., Mathur, P., 2021. Delineating the mechanisms of elevated CO2 mediated growth, stress tolerance and phytohormonal regulation in plants. Plant Cell Rep. 40, 1345–1365. https://doi.org/10.1007/s00299-021-02738-w.
McGuire, P.C., Melton, J.R., Munro, D.R., Nabel, J.E.M.S., Nakaoka, S.-I., Niya, Y., Ono, T., Pierrot, D., Poulter, B., Rehder, G., Resplandy, L., Robertson, E., Rösenbeck, C., Rosan, T.M., Schwinger, J., Schwingenschackl, C., Séférían, R., Sutton, A.J., Sweeney, C., Tanhua, T., Tans, P.P., Tian, H., Tilbrook, B., Tubiello, F., van der Werf, G.R., Viichard, N., Wada, C., Wanninkhof, R., Watson, A.J., Willis, D., Wiltshire, A.J., Yuan, W., Yue, C., Yue, X., Zaehle, S., Zeng, J., 2022. Global Carbon Budget 2021. Earth Syst. Sci. Data 14, 1917–2005. https://doi.org/10.5194/essd-14-1917-2022.

Fu, X., Ye, L., Kang, L., Ge, F., 2010. Elevated CO2 shifts the focus of tobacco plant defences from cucumber mosaic virus to the green peach aphid. Plant, Cell Environ. 33, 2056–2064. https://doi.org/10.1111/j.1365-3040.2010.02205.x.

Gleba, Y.Y., Tusé, D., Giritch, A., 2013. Plant Viral Vectors for Delivery by Agrobacterium. : Ann. Rheum. Dis. 155–192. https://doi.org/10.1007/82_2013_352.
Gouveia, B.C., Calil, I.P., Machado, J.P.B., Santos, A.A., Fontes, E.P.B., 2017. Immune Receptors and Co-receptors in Antiviral Innate Immunity in Plants. Front. Microbiol.

Gray, S.M., 2008. Aphid transmission of plant viruses. Curr. Protoc. Microbiol. 1–10. https://doi.org/10.1002/9780471729259.mc16b01s10.

Guo, H., Ge, P., Tong, J., Zhang, Y., Peng, X., Zhao, Z., Ge, F., Sun, Y., 2021. Elevated carbon dioxide levels decreases cucumber mosaic virus accumulation in correlation with greater accumulation of rgs-cam, an inhibitor of a viral suppressor of rna1. Plants 10, 1–13. https://doi.org/10.3390/plants10010059.

Guo, Huijuan, Huang, L., Sun, Y., Guo, Honggang, Ge, F., 2016. The contrasting effects of elevated CO2 on TYLCV infection of tomato genotypes with and without the resistance gene, Mi-1.2. Front. Plant Sci. 7, 1–14. https://doi.org/10.3389/fpls.2016.01680.

Gupta, P., Duplessis, S., White, H., Karnosky, D.F., Martin, F., Podila, G.K., 2005. Gene expression patterns of trembling aspen trees following long-term exposure to interacting elevated CO2 and tropospheric O3. N. Phytol. 167, 129–142. https://doi.org/10.1111/j.1469-8137.2005.01422.x.

Hendrey, G.R., Lewin, K.F., Nagy, J., 1993. Free air carbon dioxide enrichment: development, progress, results. Vegetatio 104–105, 17–31. https://doi.org/10.1007/BF00048142.

Huang, L., Ren, Q., Sun, Y., Ye, L., Cao, H., Ge, F., 2012. Lower incidence and severity of tomato virus in elevated CO2 is accompanied by modulated plant induced defence in tomato. Plant Biol. 14, 905–913. https://doi.org/10.1111/j.1438-8677.2012.00582.x.

Hull, R., 2009. Mechanical inoculation of plant viruses. Curr. Protoc. Microbiol. 6–9. https://doi.org/10.1002/9780471729259.mc16b06s13.

Husen, A., Zhang, W., 2023. Hormonal Cross-Talk, Plant Defense and Development: Plant Biology, Sustainability and Climate Change, Plant Biology, sustainability and climate change. Elsevier Science.

IPCC, 2013, Stocker, T.F., Qin, D., Plattner, G.-K., Alexander, L. V, Allen, S.K., Bindoff, N. L., Bréon, F.-M., Church, J.A., Cubasch, U., Emori, S., others, 2013. Technical summary, in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, pp. 33–115.

IPCC, 2014, Pachauri, R.K., Allen, M.R., Barros, V.R., Broome, J., Cramer, W., Christ, R., Church, J.A., Clarke, L., Dahe, Q., Dasgupta, P., Others, 2014. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change. Ipcc.

IPCC, 2021. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. https://doi.org/10.1017/9781009157896.

John, M.A., Bankole, I., Ajayi-Moses, O., Ijila, T., Jeje, T., Lalit, P., 2023. Relevance of advanced plant disease detection techniques in disease and pest management for ensuring food security and their implication: a review. Am. J. Plant Sci. 14, 1260–1295. https://doi.org/10.4236/ajps.2023.1411086.

Jones, R.A.C., 2016. Future Scenarios for Plant Virus Pathogens as Climate Change Progresses. Adv. Virus Res. 95, 87–147. https://doi.org/10.1016/bs.aivir.2016.02.004.

Jones, R.A.C., Naidu, R.A., 2019. Global Dimensions of Plant Virus Diseases: Current Status and Future Perspectives. Annu. Rev. Virol. 6, 387–409. https://doi.org/10.1146/annurev-virology-092818-015066.

Kabbage, M., Kessens, R., Bartholomay, L.C., Williams, B., 2017. The Life and Death of a Plant Cell. Annu. Rev. Plant Biol. 68, 375–404. https://doi.org/10.1146/annurev-arplant-043015-111655.

Kazan, K., 2018. Plant-biotech interactions under elevated CO2: A molecular perspective. Environ. Exp. Bot. 153, 249–261. https://doi.org/10.1016/j.envexpbot.2018.06.005.

Kong, A., Thorleifsson, G., Frigge, M.L., Vilhjalmsson, B.J., Young, A.I., Thorgeirsson, T. E., Benonisdottir, S., Oddsson, A., Halldorsson, B.V., Masson, G., Gudbjartsson, D.F., Helgason, A., Bjornsdottir, J., Thorsteinsdottir, U., Stefansson, K., 2018. The nature of nurture: Effects of parental genotypes. Science 359 (80), 424–428. https://doi.org/10.1126/science.aan6877.

Kørner, C.J., Klauser, D., Niehl, A., Domínguez-Ferreras, A., Chinchilla, D., Boller, T., Heinlein, M., Hann, D.R., 2013. The Immunity Regulator BAK1 Contributes to Resistance Against Diverse RNA Viruses. Mol. Plant-Microbe Interact. 26, 1271–1280. https://doi.org/10.1094/MPMI-06-13-0179-R.

Leadley, P.W., Niklaus, P., Stocker, R., Korner, C., 1997. Screen-aided CO2 control (SACC): A middle ground between FACE and open-top chambers. Acta Oecologica 18, 207–219. https://doi.org/10.1016/S1146-609X(97)80007-0.

Li, X., Ahammed, G.J., Li, Z., Tang, M., Yan, P., Han, W., 2016. Decreased Biosynthesis of Jasmonic Acid via Lipoxygenase Pathway Compromised Caffeine-Induced Resistance to Colletotrichum gloeosporioides Under Elevated CO2 in Tea Seedlings. Phytopathology® 106, 1270–1277. https://doi.org/10.1094/PHYTO-12-15-0336-R.

Lopez, M.M., Llop, P., Olmos, A., Marco-Noales, E., Cambra, M., Bertolini, E., 2009. Are molecular tools solving the challenges posed by detection of plant pathogenic bacteria and viruses? Curr. Issues Mol. Biol. 11, 13–46. https://doi.org/10.21775/cimb.011.013.

Machacova, K., 2010. Open top chamber and free air CO2 enrichment - approaches to investigate tree responses to elevated CO2. iForest - Biogeosciences 3, 102–105. https://doi.org/10.3832/ifor0544-003.

Makki, M., Toro, F.J., Del, Necira, K., Tenllado, F., Djilani-Khouadja, F., Canto, T., 2021. Differences in virulence among pvy isolates of different geographical origins when infecting an experimental host under two growing environments are not determined by hcpro. Plants 10. https://doi.org/10.3390/plants10061086.

Malmström, C.M., Field, C.B., 1997. Virus-induced differences in the response of oat plants to elevated carbon dioxide. Plant, Cell Environ. 20, 178–188. https://doi.org/10.1046/j.1365-3040.1997.doi-63.x.

Matros, A., Amme, S., Kettig, B., Buck-Sorlin, G.H., Sonnewald, U., Mock, H.P., 2006. Growth at elevated CO2 concentrations leads to modified profiles of secondary metabolites in tobacco cv. SamsunNN and to increased resistance against infection with potato virus Y. Plant, Cell Environ. 29, 126–137. https://doi.org/10.1111/j.1365-3040.2005.01406.x.

Mauri, A., 2010. Field experiments using CO2 Enrichment: A comparison of two main methods. iForest 3, 109–112. https://doi.org/10.3832/ifor0545-003.

Meziadi, C., Lintz, J., Naderpour, M., Gautier, C., Blanchet, S., Noly, A., Gratias-Weill, A., Gefroy, V., Pflieger, S., 2021. R-bpmv-mediated resistance to bean pod mottle virus in Phaseolus vulgaris l. Is heat-stable but elevated temperatures boost viral infection in susceptible genotypes. Viruses 13. https://doi.org/10.3390/v13071239.

Mhamdi, A., Noctor, G., 2016. High CO2 primes plant biotic stress defences through redox-linked pathways. Plant Physiol. 172, 929–942. https://doi.org/10.1104/pp.16.01129.

Moreno-Delafuente, A., Viñuela, E., Fereres, A., Medina, P., Trébicki, P., 2020. Simultaneous increase in co2 and temperature alters wheat growth and aphid performance differently depending on virus infection. Insects 11, 1–20. https://doi.org/10.3390/Insects11080459.

Moreno-Delafuente, A., Viñuela, E., Fereres, A., Medina, P., Trébicki, P., 2021. Combined effects of elevated CO2 and temperature on multitrophic interactions involving a parasitoid of plant virus vectors. BioControl 66, 307–319. https://doi.org/10.1007/s10526-020-10069-0.

Naderpour, M., 2018. Challenges facing the detection of systemic pathogens in certification programs. Acta Hortic. (1269). 89–95. https://doi.org/10.17660/ActaHortic.2020.1269.12.

Nakahara, K.S., Masuta, C., 2014. Interaction between viral RNA silencing suppressors and host factors in plant immunity. Curr. Opin. Plant Biol. 20, 88–95. https://doi.org/10.1016/j.jpsi.2014.05.004.

Nicaise, V., Candesre, T., 2017. Plum pox virus capsid protein suppresses plant pathogen-associated molecular pattern (PAMP)-triggered immunity. Mol. Plant Pathol. 18, 878–886. https://doi.org/10.1111/mpp.12247.

Niehl, A., Wyrsch, I., Boller, T., Heinlein, M., 2016. Double-stranded RNAs induce a pattern-triggered immune signaling pathway in plants. N. Phytol. 211, 1008–1019. https://doi.org/10.1111/nph.13944.

Noctor, G., Mhamdi, A., 2017. Climate Change, CO2 and Defense: The Metabolic, Redox, and Signaling Perspectives. Trends Plant Sci. 22, 857–870. https://doi.org/10.1016/j.tplants.2017.07.007.

Oerke, E.-C., Dehne, H.-W., 2004. Safeguarding production—losses in major crops and the role of crop protection. Crop Prot. 23, 275–285. https://doi.org/10.1016/j.croprot.2003.10.001.

Palukaitis, P., Yoon, J.Y., 2020. R gene mediated defense against viruses. Curr. Opin. Virol. 45, 1–7. https://doi.org/10.1016/j.coviro.2020.04.001.

Pegadaraju, V., Louis, J., Singh, V., Reese, J.C., Bautor, J., Feys, B.J., Cook, G., Parker, J. E., Shah, J., 2007. Phloem-based resistance in green peach aphid is controlled by Arabidopsis PHYTOALEXIN DEFICIENT4 without its signaling partner ENHANCED DISEASE SUSCEPTIBILITY1. Plant J. 52, 332–341. https://doi.org/10.1111/j.1365-313X.2007.03241.x.

Peñalver-Cruz, A., Garzo, E., Prieto-Ruiz, I., Díaz-Carro, M., Winters, A., Moreno, A., Fereres, A., 2020. Feeding behavior, life history, and virus transmission ability of Bemisia tabaci Mediterranean species (Hemiptera: Aleyrodidae) under elevated CO 2. Insect Sci. 27, 558–570. https://doi.org/10.1111/1744-7917.12661.

Prasad, A., Sett, S., Prasad, M., 2022b. Plant-virus-abiotic stress interactions; a complex interplay. Environ. Exp. Bot. 199, 104869 https://doi.org/10.1016/j.envexpbot.2022.104869.

Prasad, D., Mani, N.K., Pandey, D.M., 2022a. CRISPR/Cas technology: opportunities for phytopathogenic viruses detection. J. Biotechnol. 360, 211–217. https://doi.org/10.1016/j.jbiotec.2022.11.010.

Prasad Dash, S., Dipankar, P., Burange, P.S., Rouse, B.T., Sarangi, P.P., 2021. Climate change: how it impacts the emergence, transmission, resistance and consequences of viral infections in animals and plants. Crit. Rev. Microbiol. 47 (3), 307–322. https://doi.org/10.1080/1040841X.2021.1879006.

Raney, T., Croppenstedt, A., Garrett, J., Lowder, S., 2013. The State of Food and Agriculture 2013: Food Systems for Better Nutrition.

Ratcliff, F., Harrison, B.D., Baulcombe, D.C., 1997. A Similarity Between Viral Defense and Gene Silencing in Plants. Science 276 (80), 1558–1560. https://doi.org/10.1126/science.276.5318.1558.

de Ronde, B., Butterbach, P., Kormelink, R., 2014. Dominant resistance against plant viruses. Front. Plant Sci. 5, 307. https://doi.org/10.3389/fpls.2014.00307.

Roy, S., Mathur, P., 2021. Delineating the mechanisms of elevated CO2 mediated growth, stress tolerance and phytohormonal regulation in plants. Plant Cell Rep. 40, 1345–1365. https://doi.org/10.1007/s00299-021-02738-w.
Rúa, M.A., Umbanhowar, J., Hu, S., Burkey, K.O., Mitchell, C.E., 2013. Elevated CO2 spurs reciprocal positive effects between a plant virus and an arbuscular mycorrhizal fungus. N. Phytol. 199, 541–549. https://doi.org/10.1111/nph.12273.

Sastry, K.S., Zitter, T.A., 2014. Management of Virus and Viroid Diseases of Crops in the Tropics BT - Plant Virus and Viroid Diseases in the Tropics: Volume 2: Epidemiology and Management, in: Sastry, K.S., A. Zitter, T. (Eds.), Springer Netherlands, Dordrecht, pp. 149–480. https://doi.org/10.1007/978-94-007-7820-7.2.

Schmitt-Keichinger, C., 2019. Manipulating Cellular Factors to Combat Viruses: A Case Study From the Plant Eukaryotic Translation Initiation Factors eIF4. Front. Microbiol. 10, 1–8. https://doi.org/10.3389/fmicb.2019.00017.

Sun, Y., Guo, H., Zhu-Salzman, K., Ge, F., 2013. Elevated CO2 increases the abundance of the peach aphid on Arabidopsis by reducing jasmonic acid defenses. Plant Sci. 210, 128–140. https://doi.org/10.1016/j.plantsci.2013.05.014.

Sun, Y., Guo, H., Ge, F., 2016. Plant-aphid interactions under elevated CO2: some cues from aphid feeding behavior. Front. Plant Sci. 7, 502.

Tatineni, S., Hein, G.L., 2023. Plant viruses of agricultural importance; current and future perspectives of virus disease management strategies. Phytopathology 113, 117–141.

Tenllado, F., Canto, T., 2020. Effects of a changing environment on the defense of plants to viruses. Curr. Opin. Virology 42, 40–46. https://doi.org/10.1016/j.coviro.2020.04.007.

Thomma, B.P.H.J., Nürnberger, T., Joosten, M.H.A.J., 2011. Of PAMPs and effectors: The blurred PTI-ETI dichotomy. Plant Cell 23, 4–15. https://doi.org/10.1105/tpc.110.082602.

Thompson, G.A., Goggin, F.L., 2006. Transcriptomics and functional genomics of plant defence induction by phloem-feeding insects. J. Exp. Bot. 57, 755–766. https://doi.org/10.1093/jxb/erj135.

del Toro, F.J., Del Aguilar, E., Hernández-Waliaς, F.J., Tenllado, F., Chung, B.N., Canto, T., 2015. High temperature, high ambient CO2 affect the interactions between three positive-sense RNA viruses and a compatible host differentially, but not their silencing suppression efficiencies. PLoS One 10, 1–18. https://doi.org/10.1371/journal.pone.0136062.

del Toro, F.J., Rakhshandehroo, F., Larruy, B., Aguilar, E., Tenllado, F., Canto, T., 2017. Effects of simultaneously elevated temperature and CO2 levels on Nicotiana benthamiana and its infection by different positive-sense RNA viruses are cumulative and virus type-specific. Virology 511, 184–192. https://doi.org/10.1016/j.virol.2017.08.015.

del Toro, F.J., Choi, K.S., Rakhshandehroo, F., Aguilar, E., Tenllado, F., Canto, T., 2019. Ambient conditions of elevated temperature and CO2 levels are detrimental to the probabilities of transmission by insects of a Potato virus Y isolate and to its simulated prevalence in the environment. Virology 530, 1–10. https://doi.org/10.1016/j.virol.2019.02.001.

Trębicki, P., Vandegeer, R.K., Bosque-Pérez, N.A., Powell, K.S., Dader, B., Freeman, A.J., Yen, A.L., Fitzgerald, G.J., Luck, J.E., 2016. Virus infection mediates the effects of elevated CO2 on plants and vectors. Sci. Rep. 6, 1–11. https://doi.org/10.1038/srep22785.

Trębicki, P., 2020. Climate change and plant virus epidemiology. Virus Res. 286, 198059. https://doi.org/10.1016/j.virusres.2020.198059.

Trębicki, P., Nancarrow, N., Cole, E., Bosque-Pérez, N.A., Constable, F.E., Freeman, A.J., Rodoni, B., Yen, A.L., Luck, J.E., Fitzgerald, G.J., 2015. Virus disease in wheat predicted to increase with a changing climate. Glob. Chang. Biol. 21, 3511–3519. https://doi.org/10.1111/gcb.12941.

Trębicki, P., Nancarrow, N., Bosque-Pérez, N.A., Rodoni, B., Aftab, M., Freeman, A., Yen, A., Fitzgerald, G.J., 2017. Virus incidence in wheat increases under elevated CO2: A 4-year study of yellow dwarf viruses from a free air carbon dioxide facility. Virus Res 241, 137–144. https://doi.org/10.1016/j.virusres.2017.06.027.

Tsai, W.A., Brosnan, C.A., Mitter, N., Dietzgen, R.G., 2022. Perspectives on plant virus diseases in a climate change scenario of elevated temperatures. Stress Biol. 2 https://doi.org/10.1007/s44154-022-00058-x.

United Nations Department of Economic and Social Affairs, P.D. (2022). W.P.P. 2022: S. of R., 2022. World Population Prospects 2022, United Nation.

Uprety, D.C., Garg, S.C., Bisht, B.S., Maini, H.K., Dwivedi, N., Paswan, G., Raj, A., Saxena, D.C., 2006. Carbon dioxide enrichment technologies for crop response studies. J. Sci. Ind. Res. (India). 65 859–866.

Van Munster, M., 2020. Impact of abiotic stresses on plant virus transmission by aphids. Viruses 12, 216. https://doi.org/10.3390/v12020216.

Vandegeer, R.K., Powell, K.S., Tausz, M., 2016. Barley yellow dwarf virus infection and elevated CO2 alter the antioxidants ascorbate and glutathione in wheat. J. Plant Physiol. 199, 96–99. https://doi.org/10.1016/j.jplph.2016.05.007.

Vassiliadis, S., Plummer, K.M., Powell, K.S., Trębicki, P., Luck, J.E., Rochfort, S.J., 2016. The effect of elevated CO2 and virus infection on the primary metabolism of wheat. Funct. Plant Biol. 43, 892–902. https://doi.org/10.1071/FP15242.

Vassiliadis, S., Plummer, K.M., Powell, K.S., Rochfort, S.J., 2018. Elevated CO2 and virus infection impacts wheat and aphid metabolism. Metabolomics 14, 0. https://doi.org/10.1007/s11306-018-1425-x.

Velásquez, A.C., Castroverde, C.D.M., He, S.Y., 2018. Plant–Pathogen Warfare under Changing Climate Conditions. Curr. Biol. 28, R619–R634. https://doi.org/10.1016/j.cub.2018.03.054.

Venbrux, M., Crauwels, S., Rediers, H., 2023. Current and emerging trends in techniques for plant pathogen detection. Front. Plant Sci. 14, 1120968 https://doi.org/10.3389/fplts.2023.1120968.

Venkatesh, J., Kang, B.C., 2019. Current views on temperature-modulated R gene-mediated plant defense responses and tradeoffs between plant growth and immunity. Curr. Opin. Plant Biol. 50, 9–17. https://doi.org/10.1016/j.pbi.2019.02.002.

Vicente, R., Martínez-Carrasco, R., Pérez, P., Morcuende, R., 2018. New insights into the impacts of elevated CO2, nitrogen, and temperature levels on the regulation of C and N metabolism in durum wheat using network analysis. N. Biotechnol. 40, 192–199. https://doi.org/10.1016/j.nbt.2017.08.003.

Whitham, S.A., Yang, C., Goodin, M.M., 2006. Global impact: Elucidating plant responses to viral infection. Mol. Plant-Microbe Interact. 19, 1207–1215. https://doi.org/10.1094/PMMI-19-1207.

Wingard, S.A., 1928. Hosts and symptoms of ring spot. a Virus Dis. Plants 127–153 (USDA).

Yang, Y., Luo, Y., Lu, M., Schädel, C., Han, W., 2011. Terrestrial C:N stoichiometry in response to elevated CO2 and N addition: a synthesis of two meta-analyses. Plant Soil 343, 393–400. https://doi.org/10.1007/s11104-011-0736-8.

Ye, L., Fu, X., Ge, F., 2010. Elevated CO2 alleviates damage from Potato virus Y infection in tobacco plants. Plant Sci. 179, 219–224. https://doi.org/10.1016/j.plantsci.2010.05.004.

Zarate, S.I., Kempema, L.A., Walling, L.L., 2007. Silverleaf Whitefly Induces Salicylic Acid Defenses and Suppresses Effectual Jasmonic Acid Defenses. Plant Physiol. 143, 866–875. https://doi.org/10.1104/pp.106.090035.

Zhang, S., Li, X., Sun, Z., Shao, S., Hu, L., Ye, M., Zhou, Y., Xia, X., Yu, J., Shi, K., 2015. Antagonism between phytohormone signalling underlies the variation in disease susceptibility of tomato plants under elevated CO2. J. Exp. Bot. 66, 1951–1963. https://doi.org/10.1093/jxb/eru538.

Zvereva, A.S., Golyaev, V., Turco, S., Gubaeva, E.G., Rajeswaran, R., Schepetilnikov, M. V., Srour, O., Ryabova, L.A., Boller, T., Pooggin, M.M., 2016. Viral protein suppresses oxidative burst and salicylic acid-dependent autophagy and facilitates bacterial growth on virus-infected plants. N. Phytol. 211, 1020–1034. https://doi.org/10.1111/nph.13967.
Rúa, M.A., Umbanhowar, J., Hu, S., Burkey, K.O., Mitchell, C.E., 2013. Elevated CO2 spurs reciprocal positive effects between a plant virus and an arbuscular mycorrhizal fungus. N. Phytol. 199, 541–549. https://doi.org/10.1111/nph.12273.

Sastry, K.S., Zitter, T.A., 2014. Management of Virus and Viroid Diseases of Crops in the Tropics BT - Plant Virus and Viroid Diseases in the Tropics: Volume 2: Epidemiology and Management, in: Sastry, K.S., A. Zitter, T. (Eds.), Springer Netherlands, Dordrecht, pp. 149–480. https://doi.org/10.1007/978-94-007-7820-7.2.

Schmitt-Keichinger, C., 2019. Manipulating Cellular Factors to Combat Viruses: A Case Study From the Plant Eukaryotic Translation Initiation Factors eIF4. Front. Microbiol. 10, 1–8. https://doi.org/10.3389/fmicb.2019.00017.

Sun, Y., Guo, H., Zhu-Salzman, K., Ge, F., 2013. Elevated CO2 increases the abundance of the peach aphid on Arabidopsis by reducing jasmonic acid defenses. Plant Sci. 210, 128–140. https://doi.org/10.1016/j.plantsci.2013.05.014.

Sun, Y., Guo, H., Ge, F., 2016. Plant-aphid interactions under elevated CO2: some cues from aphid feeding behavior. Front. Plant Sci. 7, 502.

Tatineni, S., Hein, G.L., 2023. Plant viruses of agricultural importance; current and future perspectives of virus disease management strategies. Phytopathology 113, 117–141.

Tenllado, F., Canto, T., 2020. Effects of a changing environment on the defense of plants to viruses. Curr. Opin. Virology 42, 40–46. https://doi.org/10.1016/j.coviro.2020.04.007.

Thomma, B.P.H.J., Nürnberger, T., Joosten, M.H.A.J., 2011. Of PAMPs and effectors: The blurred PTI-ETI dichotomy. Plant Cell 23, 4–15. https://doi.org/10.1105/tpc.110.082602.

Thompson, G.A., Goggin, F.L., 2006. Transcriptomics and functional genomics of plant defence induction by phloem-feeding insects. J. Exp. Bot. 57, 755–766. https://doi.org/10.1093/jxb/erj135.

del Toro, F.J., Del Aguilar, E., Hernández-Waliaς, F.J., Tenllado, F., Chung, B.N., Canto, T., 2015. High temperature, high ambient CO2 affect the interactions between three positive-sense RNA viruses and a compatible host differentially, but not their silencing suppression efficiencies. PLoS One 10, 1–18. https://doi.org/10.1371/journal.pone.0136062.

del Toro, F.J., Rakhshandehroo, F., Larruy, B., Aguilar, E., Tenllado, F., Canto, T., 2017. Effects of simultaneously elevated temperature and CO2 levels on Nicotiana benthamiana and its infection by different positive-sense RNA viruses are cumulative and virus type-specific. Virology 511, 184–192. https://doi.org/10.1016/j.virol.2017.08.015.

del Toro, F.J., Choi, K.S., Rakhshandehroo, F., Aguilar, E., Tenllado, F., Canto, T., 2019. Ambient conditions of elevated temperature and CO2 levels are detrimental to the probabilities of transmission by insects of a Potato virus Y isolate and to its simulated prevalence in the environment. Virology 530, 1–10. https://doi.org/10.1016/j.virol.2019.02.001.

Trębicki, P., Vandegeer, R.K., Bosque-Pérez, N.A., Powell, K.S., Dader, B., Freeman, A.J., Yen, A.L., Fitzgerald, G.J., Luck, J.E., 2016. Virus infection mediates the effects of elevated CO2 on plants and vectors. Sci. Rep. 6, 1–11. https://doi.org/10.1038/srep22785.

Trębicki, P., 2020. Climate change and plant virus epidemiology. Virus Res. 286, 198059. https://doi.org/10.1016/j.virusres.2020.198059.

Trębicki, P., Nancarrow, N., Cole, E., Bosque-Pérez, N.A., Constable, F.E., Freeman, A.J., Rodoni, B., Yen, A.L., Luck, J.E., Fitzgerald, G.J., 2015. Virus disease in wheat predicted to increase with a changing climate. Glob. Chang. Biol. 21, 3511–3519. https://doi.org/10.1111/gcb.12941.

Trębicki, P., Nancarrow, N., Bosque-Pérez, N.A., Rodoni, B., Aftab, M., Freeman, A., Yen, A., Fitzgerald, G.J., 2017. Virus incidence in wheat increases under elevated CO2: A 4-year study of yellow dwarf viruses from a free air carbon dioxide facility. Virus Res 241, 137–144. https://doi.org/10.1016/j.virusres.2017.06.027.

Tsai, W.A., Brosnan, C.A., Mitter, N., Dietzgen, R.G., 2022. Perspectives on plant virus diseases in a climate change scenario of elevated temperatures. Stress Biol. 2 https://doi.org/10.1007/s44154-022-00058-x.

United Nations Department of Economic and Social Affairs, P.D. (2022). W.P.P. 2022: S. of R., 2022. World Population Prospects 2022, United Nation.

Uprety, D.C., Garg, S.C., Bisht, B.S., Maini, H.K., Dwivedi, N., Paswan, G., Raj, A., Saxena, D.C., 2006. Carbon dioxide enrichment technologies for crop response studies. J. Sci. Ind. Res. (India). 65 859–866.

Van Munster, M., 2020. Impact of abiotic stresses on plant virus transmission by aphids. Viruses 12, 216. https://doi.org/10.3390/v12020216.

Vandegeer, R.K., Powell, K.S., Tausz, M., 2016. Barley yellow dwarf virus infection and elevated CO2 alter the antioxidants ascorbate and glutathione in wheat. J. Plant Physiol. 199, 96–99. https://doi.org/10.1016/j.jplph.2016.05.007.

Vassiliadis, S., Plummer, K.M., Powell, K.S., Trębicki, P., Luck, J.E., Rochfort, S.J., 2016. The effect of elevated CO2 and virus infection on the primary metabolism of wheat. Funct. Plant Biol. 43, 892–902. https://doi.org/10.1071/FP15242.

Vassiliadis, S., Plummer, K.M., Powell, K.S., Rochfort, S.J., 2018. Elevated CO2 and virus infection impacts wheat and aphid metabolism. Metabolomics 14, 0. https://doi.org/10.1007/s11306-018-1425-x.

Velásquez, A.C., Castroverde, C.D.M., He, S.Y., 2018. Plant–Pathogen Warfare under Changing Climate Conditions. Curr. Biol. 28, R619–R634. https://doi.org/10.1016/j.cub.2018.03.054.

Venbrux, M., Crauwels, S., Rediers, H., 2023. Current and emerging trends in techniques for plant pathogen detection. Front. Plant Sci. 14, 1120968 https://doi.org/10.3389/fplts.2023.1120968.

Venkatesh, J., Kang, B.C., 2019. Current views on temperature-modulated R gene-mediated plant defense responses and tradeoffs between plant growth and immunity. Curr. Opin. Plant Biol. 50, 9–17. https://doi.org/10.1016/j.pbi.2019.02.002.

Vicente, R., Martínez-Carrasco, R., Pérez, P., Morcuende, R., 2018. New insights into the impacts of elevated CO2, nitrogen, and temperature levels on the regulation of C and N metabolism in durum wheat using network analysis. N. Biotechnol. 40, 192–199. https://doi.org/10.1016/j.nbt.2017.08.003.

Whitham, S.A., Yang, C., Goodin, M.M., 2006. Global impact: Elucidating plant responses to viral infection. Mol. Plant-Microbe Interact. 19, 1207–1215. https://doi.org/10.1094/PMMI-19-1207.

Wingard, S.A., 1928. Hosts and symptoms of ring spot. a Virus Dis. Plants 127–153 (USDA).

Yang, Y., Luo, Y., Lu, M., Schädel, C., Han, W., 2011. Terrestrial C:N stoichiometry in response to elevated CO2 and N addition: a synthesis of two meta-analyses. Plant Soil 343, 393–400. https://doi.org/10.1007/s11104-011-0736-8.

Ye, L., Fu, X., Ge, F., 2010. Elevated CO2 alleviates damage from Potato virus Y infection in tobacco plants. Plant Sci. 179, 219–224. https://doi.org/10.1016/j.plantsci.2010.05.004.

Zarate, S.I., Kempema, L.A., Walling, L.L., 2007. Silverleaf Whitefly Induces Salicylic Acid Defenses and Suppresses Effectual Jasmonic Acid Defenses. Plant Physiol. 143, 866–875. https://doi.org/10.1104/pp.106.090035.

Zhang, S., Li, X., Sun, Z., Shao, S., Hu, L., Ye, M., Zhou, Y., Xia, X., Yu, J., Shi, K., 2015. Antagonism between phytohormone signalling underlies the variation in disease susceptibility of tomato plants under elevated CO2. J. Exp. Bot. 66, 1951–1963. https://doi.org/10.1093/jxb/eru538.

Zvereva, A.S., Golyaev, V., Turco, S., Gubaeva, E.G., Rajeswaran, R., Schepetilnikov, M. V., Srour, O., Ryabova, L.A., Boller, T., Pooggin, M.M., 2016. Viral protein suppresses oxidative burst and salicylic acid-dependent autophagy and facilitates bacterial growth on virus-infected plants. N. Phytol. 211, 1020–1034. https://doi.org/10.1111/nph.13967.