Frontiers in plant–microbe interactions - part 1
Authors: Xia Li , Yuan Zhilin , Plett Jonathan 
Published: June 2023

We present an exciting two-part webinar highlighting the latest research in plant–microbe interactions, hosted by Francis Martin.

Following the recent publication of our collection on the ‘[Impact of global change on the plant microbiome ](https://nph.onlinelibrary.wiley.com/toc/14698137/2022/234/6)’, and ahead of the forthcoming special issue on '[Mycorrhizal research now: from micro-to macro scale](https://www.newphytologist.org/news/view/326)’, these webinars will explore the latest research on the fascinating and complex relationships between plants and microbes. Leading researchers will share their insights into the molecular mechanisms that underlie the symbiotic relationships between plants and different types of microbes, including nodulating bacteria, endophytes, and mycorrhizal fungi.

Part 2 will be held on 15 June

This will be held on Zoom Webinar you can access the webinar here on Cassyni, or register on [Eventbrite](https://www.eventbrite.com/e/new-phytologist-now-frontiers-in-plantmicrobe-interactions-part-1-registration-634728288397?aff=odcleoeventsincollection keep_tld=1) to get the zoom link via email.

Regulatory network coordinating nodulation and autoregulation of nodulation

Upon perception of rhizobial signal by Nod factor receptors, NF signaling is activated to initiate rhizobial infection and nodule organogenesis. In soybean, we have identified a core molecular module GmNINa-miR172c-NNC1 that coordinates these processes during nodulation. The essential nodulation regulator GmNINa is induced by rhizobia, and then activates miR172c transcription. miR172c cleaves the mRNA of its target gene NNC1, which encodes a nodulation repressor of the nodulation genes, such as *ENOD40*, resulting in rhizobial infection and organogenesis. In addition, we show that this module also activates autoregulation of nodulation (AON) through a double secured mechanism to ensure nodulation is systemically controlled. Furthermore, we show that AON suppresses further nodulation by attenuating the function of GmNINa-miR172c-NNC1 module through a B-type cytokinin response factor GmRR11d. GmRR11d is induced by rhizobia and represses GmNINa by directly binding to GmNIN promoter and inhibiting transcriptional activity of GmNSP1a on GmNINa in a GmNARK-dependent manner. Thus, we have constructed a regulatory network involved in nodulation in soybean.

Physiological and genomic remodeling of endophytic fungi in response to their environment

Root endophytic fungi (EF) play a crucial role in root guilds and have gathered increasing attention due to their diverse ecological functions and flexible lifestyles. The establishment of mutualistic symbioses with fungal endophytes across various lineages often enhances plant fitness, including growth and adaptation. The ‘symbiosis triangle’ hypothesis emphasizes the significance of environmental factors in regulating the outcomes of these symbiotic interactions. In this seminar, I will present prime examples that illustrate how soil nutrients and salt stress influence the behavior of endophytes at both the physiological and genomic scales.

Studies have demonstrated that variations in external nutrient levels or forms can lead to extensive physiological remodeling in fungi. These adaptations encompass changes in nutrient transporters, lipid metabolism, secondary metabolite biosynthesis, and plant-cell-wall-degradation enzymes. These changes contribute, at least in part, to the maintenance and erosion of symbiotic relationships. Furthermore, recent findings indicate that certain populations of EF undergo population divergence arising from positive selection of genomic islands and chromosome polymorphisms, including ploidy changes, in response to natural soil saline conditions. We propose that hybridization accompanied by doubling ploidy allows EF to generate phenotypic novelty and drive genome evolution for saline adaptation. Notably, allodiploid EF strains exhibit increased vigor under a range of abiotic stresses, which suggests the potential of using allodiploid EF isolates to improve plant salt tolerance.

By gaining a better understanding on how environment conditions impact EF physiology and genome evolution, we can establish a new framework to enhance the symbiotic efficiency of EF, and ultimately benefiting host plants and their fitness.

Molecular mechanisms driving the development and functioning of ectomycorrhizal symbioses

In marginal, nutrient poor forest soils, the roots of dominant tree species enter into mutually beneficial, nutrient acquiring interactions with ectomycorrhizal (ECM) fungi. Despite the inherent benefit to the plant, the molecular events that underpin these interactions remain poorly understood. One of the main aspects studied to date regarding the controls of these symbioses include how plant immunity is modulated during the ingrowth of fungi into plant roots. While effector-like proteins and microRNAs (miRNAs) are well known to regulate pathogenic plant-microbe interactions, significantly fewer signalling molecules used to foster mutualistic symbioses by ectomycorrhizal (ECM) fungi have been characterized. ECM fungi, on an evolutionary timescale, are a relatively recent class of symbiotic organisms having evolved from saprotrophic fungi  270 MYA. As their ancestral lifestyle did not interact with live plants, convergent evolution may have led to mechanisms in ECM fungi that enable them to enter into symbiosis with plants in a similar manner to pathogenic microbes. Using the model interaction between the forest tree *Eucalyptus grandis* and the mutualistic ECM fungus *Pisolithus microcarpus* we have been investigating the role of these two classes of signature signals during the establishment of symbiosis. I will cover our recent characterization of several effector-like ‘Mycorrhiza-Induced Small Secreted Proteins’ (MiSSPs) and fungal miRNAs during the mutualistic interaction with *E. grandis*. Using sequencing, *in situ* localization, and mutagenic work we demonstrate that both classes of microbial signals have the capacity to directly alter host metabolism and hormone perception, pathways integral to the plant immune system during microbial challenge. Altogether, our work is providing new evidence of cross-kingdom communication between ECM fungi and plant roots that was previously undescribed, shedding light on how this relatively recent lifestyle has convergently evolved pathways to pathogenic microbes during host colonization.