Interactions between cotton plants mediated by volatile organic compounds : prospects for pest control ?
Authors:   , Grandi Luca 
Published: September 2023

Cotton (Gossypium spp) is one of the most studied plants in the world due to its great economic importance. This highly valuable crop is cultivated worldwide primarily to produce textile fibers and, to a lesser extent, for animal forage and cooking oil production. As for all plants, cotton is involved in several beneficial and antagonistic interactions and in nature it has evolved several direct and indirect defenses to ward off antagonists and to facilitate beneficial interactions. To protect and enhance the performance of cultivated cotton, various pest-management strategies are currently applied. For instance, pesticides, herbicides and fertilizers are intensively used in cotton cultivation. In addition, genetically modified varieties have been created in order to tolerate herbicides or enhance resistance to pest insects. Although these techniques result in short term economic returns, development of resistance in pests, weakening of natural plant defenses, environment pollution with agrochemicals, and unpredictable effects of introducing genetic modified crops, have increased the need for alternative, more environmentally friendly pest control strategies. This need is further enhanced by human activities that facilitate the introduction and dispersal of non-native invasive organisms. To fight these pests, bio-control and integrated pest management (IPM) strategies have been proposed, but much knowledge is lacking to achieve the full potential of these strategies. For instance, a comprehensive understanding of the plant’s direct and indirect inducible defenses against insects and pathogens under natural conditions is essential. Studies on indirect plants defenses have shown that volatile organic compounds (VOCs) emitted by herbivore-attacked plants, are able to attract the natural enemies of pests and may repel pests. Moreover, these herbivore-induced plant volatiles (HIPVs) may also serve as signals that alert undamaged leaves and neighboring plants to incoming attack and may prime them for enhanced defense induction. Recent studies suggest that this priming of neighbors has potential for application, by enhancing pest resistance in cotton, but the underlying mechanisms are still poorly understood. The aim of my thesis was to contribute to a better understanding of VOC-mediated signaling among cotton plants by elucidating the biochemical and molecular mechanisms that are involved in priming and/or induction. With a number of controlled plant exposure experiments I could show that cotton HIPVs change the physiology of receiver plants by means of defense-related gene activation, changes in plant hormones levels, as well as increases in levels of secondary metabolites. These changes resulted in increased VOC emissions by receiver plants and reduced palatability of the plants to caterpillar pests. These changes were found to be plant genotype-specific and seem to be mainly driven by VOCs that are emitted in the later stage of the caterpillar infestations (inducible volatiles). This notion was based on an experiment where plants were exposed to the volatiles of relatively freshly damaged plants or of plants with older damage. We further tested the importance of the different classes of volatiles by exposing plants to different sets of authentic, pure versions of VOCs. Unfortunately, due to high variation and difficulties in controlling dispenser emission rates, we could not determine single or group of compounds responsible for defense induction in neighboring plants. We hypothesized that intact plants may require a full natural blend, a specific ratio or other non-tested compounds in order to activate defense responses. Furthermore, we also analyzed field-collected leaves from plots with and without mechanically damaged (i.e. topped) plants and we could confirm that damage-induced volatiles can enhance the production of defense compounds in the plant itself and in neighboring plants. This thesis highlights the potential using volatile-mediated interactions among cotton plants for the development of a novel IPM strategy that enhances cotton resistance to pests. Still, further studies are required in order to identify potent volatiles compounds that may eventually be applied for cotton protection.