Infection of maize by <i>Ustilago maydis</i> remodels the phyllosphere microbiome and requires the activity of antimicrobial effectors

Abstract Plant-associated microbial communities play a critical role in plant health and disease resistance, but the mechanisms which reshape these communities during pathogen infection are poorly understood. In this study, we investigated how infection of maize by the smut fungus Ustilago maydis is functionally linked with the bacterial phyllosphere microbiome and explored the role of an antimicrobial effector GH25 in fungal infection. Using a combination of culture-dependent and culture-independent approaches, we compared the leaf microbiomes of infected and uninfected plants. We observed a significant increase in microbial abundance and pronounced shifts in community composition and identified distinct health-associated (HCom) and disease-associated (DCom) bacterial communities. To assess whether U. maydis directly manipulates the microbiome, we tested the antimicrobial activity of the antimicrobial effector GH25 against isolated strains. Notably, all HCom bacteria were sensitive to GH25 and co-inoculation of HCom bacteria with a U. maydis Δ gh25 knockout mutant significantly reduced fungal virulence. In contrast, DCom exhibited minimal sensitivity to U. maydis and did not affect the virulence of U. maydis Δ gh25 . Functional profiling revealed infection-associated shifts in predicted metabolic potential, consistent with U. maydis induced leaf tumors being strong sink tissues. Together, the data shows that U. maydis infection reshapes the maize phyllosphere microbiome through a combination of effector-mediated antimicrobial activity and host metabolic reprogramming.