Characterization of the tripartite interaction between tomato, arbuscular mycorrhizal fungi and Cucumber mosaic virus

The mutualistic association established between arbuscular mycorrhizal (AM) fungi and the majority of land plants is considered a natural instrument to improve plant health and productivity; mycorrhizal plants, beside an improved mineral nutrition, often show higher tolerance to biotic and abiotic stresses (1). The AM symbiosis can influence the outcome of plant-pathogen interactions; however, the impact of the AM symbiosis on the infection by viral pathogens is still largely uncertain and little explored (2,3). In this study tomato (Solanum lycopersicum, Moneymaker cv) plants were grown under controlled conditions and inoculated with the AM fungus Funneliformis mosseae. Once the colonization had developed, plants were inoculated with Cucumber mosaic virus (CMV), which causes one of the most serious viral diseases of tomatoes worldwide (4). Four biological conditions were set up: control plants (C), CMV-infected plants (V), mycorrhizal plants (M) and CMV-infected mycorrhizal plants (MV). At the time of analysis (28 dpi - days post infection), the mycorrhization level was not significantly modified by virus infection, indicating that the AM colonization was unaffected by the presence and spread of the virus. On the contrary, in MV plants viral symptoms were milder than in V plants at 14 dpi while no difference was found at 28 dpi. At the two time points the concentration of viral RNA was similar in shoots of V and MV plants. Physiological analyses showed that in virus-infected plants the AM symbiosis improved plant performances in terms of stomatal conductance and photosynthetic CO2 assimilation pointing to a positive role in water use efficiency of the host plant. Genome-wide transcriptomic data obtained by the RNASeq technique are currently under analysis. In their whole the results highlight a systemic effect of the AM symbiosis which alters the responses to the viral infection.