P-139 - A recombinant smut hybrid provides insights into gene regulation, pathogenesis and tumorigenesis

Smut fungi infect around 1500 plant species, primarily Poaceae, encompassing economically significant cereals and crops. The closely related maize smut fungi, Ustilago maydis and Sporisorium reilianum, have analogous genomes in terms of size and synteny, rendering them ideal organisms to study orthologous effector genes. S. reilianum spreads systemically in maize, causing symptoms in inflorescences, whereas U. maydis induces localized tumors at infection sites. To investigate the molecular basis of these phenotypic differences, we generated an interspecific hybrid (rUSH) between U. maydis and S. reilianum, with the mating type system of S. reilianum. rUSH exhibited an S. reilianum-like phenotype throughout development, except teliospore formation. Subsequent transcriptomic analysis revealed up‑regulation of pathogenicity-related effector orthologs in rUSH, but not in a wild-type hybrid control. We identified 253 differentially expressed effector orthologs, with distinct regulatory patterns, including cis-, trans-, and rUSH-specific regulation. Three new virulence factors were identified among the rUSH-specific regulated effector genes. Ultimately, overexpressing U. maydis transcription factors in rUSH revealed UmHdp2 as a key regulator of tumorigenesis, controlling 41 effector genes linked to plant tumor induction. These results demonstrate the potential of interspecific hybrids to elucidate the molecular mechanisms underlying pathogenic variation in closely related fungi.