P-145 - Benzoxazinoids as key factors in maize colonization and growth promotion by the endophytic fungus Colletotrichum tofieldiae

Fungal endophytes have emerged as key tools for enhancing plant productivity and resilience, yet their mechanisms for plant colonization remain poorly understood. Colletotrichum tofieldiae (Ct) is a fungal endophyte that promotes maize growth and yield. To investigate maize-Ct interaction, we analyzed RNA-seq data from Ct-inoculated maize seedlings at early stages. Our results highlight maize secondary metabolite pathways linked to defense responses. While terpenoid phytoalexin pathways are induced, benzoxazinoid (BX) biosynthesis is notably repressed in Ct-inoculated roots at 7 days post-inoculation (dpi), coinciding with the maximum fungal biomass. BXs are indole-derived defenses unique to grasses, with a biosynthesis pathway interconnected with other indole-related compounds, such as the auxin phytohormone indole-3-acetic acid (IAA). BX-deficient bx1 mutants showed higher Ct colonization than wild-type plants, yet this increase did not trigger pathogenic effects. Instead, Ct's growth-promoting benefits were enhanced in bx1 mutants, with greater shoot and root biomass gains. These findings reveal BXs as inhibitors of Ct colonization and growth promotion. This study offers insights into grass-specific defense pathways and may have implications for the effective application of FE-based biostimulant strategies for maize and other grass crops.