Homeobox transcription factor BmHox7 is required for sexual reproduction, but not for appressorial development and pathogenicity in Bipolaris maydis

Bipolaris maydis, the causal agent of southern corn leaf blight, possesses 10 putative homeobox genes in its genome. In this study, we disrupted and characterized a homeobox gene BmHox7 in B. maydis. While BmHox7 orthologues are required for appressorium formation in Sordariomycetous plant pathogenic fungi (Magnaporthe oryzae, Colletotrichum orbiculare, and C. scovillei), ΔBmHox7 strains formed appressoria normally and exhibited virulence comparable to the wild-type strain. However, in crossing test, ΔBmHox7 strains produced small, immature pseudothecia lacking beaks. Most pseudothecia of BmHox7 mutants contained no asci, indicating that BmHox7 is crucial for sexual reproduction. These findings demonstrate that in B. maydis, BmHox7 is crucial for sexual reproduction but dispensable for appressorial formation and pathogenicity.