MAPKK-encoding gene Ste7 in Bipolaris maydis is required for development and morphogenesis

• We identified and characterized a MAPKK-encoding gene, Ste7, in Bipolaris maydis.
• Ste7 had critical roles in conidiation and conidia germination.
• Ste7 was essential for plant infection, mainly in appressorium formation.
• Ste7 also regulated protothecium formation in sexual reproduction.
• Ste7 MAPKK was a component of the Chk1 MAPK cascade, regulating various developmental and morphogenetic processes.

In this study, we identified a gene, Ste7, in the genome of Bipolaris maydis encoding a mitogen-activated protein kinase (MAPK) kinase homologous to yeast Ste7. A Δste7 strain was defective in conidiation and conidial germination, and was severely impaired in lesion formation, mainly due to the loss of appressoria, whereas the effect on infectious hyphal development was not significant. The Δste7 strain did not have a maternal ability for sexual reproduction, mainly in protothecium formation. These phenotypes of Δste7 were also observed in Δchk1 and Δste11, null mutant strains of MAPK and MAPK kinase kinase, respectively. These observations strongly suggested that Ste7 is a component of the Chk1 MAPK cascade, and is involved in the regulation of various developmental and morphogenetic processes in the asexual and sexual lifecycles of B. maydis.