Hormonal changes associated with arbuscular mycorrhizal fungi indicate defense‐like alterations in virus‐stressed grapevine

Grapevine is an economically important crop, affected by major production losses due to high virus prevalence. Arbuscular mycorrhizal fungi (AMF) can reduce the impact of plant biotic stresses. However, hormonal response to the simultaneous presence of viruses and AMF remains largely unknown. In this study, we explored the potential of AMF to modify the grapevine's defense response to compatible virus infections. We used GRSPaV, GLRaV-3, and GPGV as infectious viral agents, separately or in different combinations. Two AMF inoculums were tested for their bioprotective abilities, RHIZ (Rhizophagus irregularis) and MIX (R. irregularis, Funneliformis mosseae, F. caledonium). Generally, MIX induced stronger physiological responses than RHIZ inoculum, especially during the earlier phase of symbiosis. The main findings were connected to the hormonal profile of the grapevine infected by all three viruses and inoculated with MIX. In particular, salicylic acid (SA) and abscisic acid (ABA) concentrations were induced five and fifteen months post AMF inoculation, respectively. Expressions of VvNCED1 and VvBG1 were up-regulated in uninoculated grapevines, indicating slower induction of stress response mechanisms. Parameters related to plant vigour and growth were induced in grapevine at both time points, regardless of the virus combination. In conclusion, the defense-like response induced by AMF in grapevines infected with multiple viruses is characterized by the induction of ABA and SA, accompanied by a consistent enhancement of vigor parameters. This study confirms AMF symbiosis as a potentially promising additional tool for combating viral diseases in vineyards.