P-165 - Functional adaptation of a transcriptional activator effector family in smut fungi and beyond

Phytopathogens secrete effectors that inhibit the plant immunity and modulate the host’s transcriptome for their infection. In a previous study, we functionally characterized Ustilago maydis Sts2, which functions as a transactivator effector (TAE) to activate the expression of maize leaf developmental regulators for hyperplastic tumor formation. Interestingly, Sts2 orthologs from U. maydis and Sporisorium reilianum, its closest pathogenic relative, exhibit diverse virulence functions.  Based on all the identified Sts2 orthologs in smut, we computed the ancestral sequence and evaluated their transcriptional activation activities and virulence functions in U. maydis. Our results suggest that Sts2 orthologs represent a functional TAE family in smut fungi. During the speciation of U. maydis, UmSts2 underwent neo-functionalization, which modulate its unique function for tumor induction. In addition, AlphaFold based 3D structure analysis identified two structurally variable domains in Sts2. Complementation assay using chimeric mutants carrying different variable domains validated that they determine the functional divergence of Sts2 in U. maydis and S. reilianum. Moreover, we discovered novel, functional TAEs in the phytopathogens Magnaporthe oryzae and Bipolaris maydis. Heterologous expression of these effectors in U. maydis modulate its virulence, demonstrating distinct functions of TAEs in different plant pathogens.