Effects of soil rhizobia and drought on plant-vector-pathogen interactions on a legume host

Symbiosis between rhizobia and legumes increases plant tolerance to herbivores and pathogens and mitigates harm from water stress. Yet, few studies have assessed how soil microbes like rhizobia affect plants facing biotic and abiotic stress at the same time. Here we assessed whether rhizobia mediated interactions between a legume host (Pisum sativum), a vector herbivore (pea aphid, Acrythosiphon pisum), a plant virus (Pea enation mosaic virus, PEMV), and abiotic stress in the form of varying water availability. We show rhizobia promoted plant growth and mitigated osmotic stress from reduced watering. Rhizobia also increased plant tolerance to PEMV under full water conditions, but had no effect on PEMV when plants were grown with reduced water. To assess the mechanisms that mediated these complex interactions observed, we measured gene transcripts related to phytohormone signalling. We found relative expression of gene transcripts associated with salicylic acid, jasmonic acid, abscicic acid, and ethylene signaling were affected by interactions between rhizobia and water availability; each of these pathways affects PEMV transmission. Our study shows beneficial effects of rhizobia on legumes are impacted by biotic and abiotic stress and benefits of rhizobia will likely be context-dependent in field settings.