Dual regulation of appressorium development in the maize pathogen <i>Bipolaris maydis</i>: BmOPY2-mediated hydrophobic surface recognition and pectin recognition

The CHK1 MAPK pathway is crucial in appressorium formation and is highly conserved among plant pathogenic fungi. Here, we investigated a putative upstream regulator of this pathway, BmOPY2, in the maize pathogen Bipolaris maydis. Yeast two-hybrid analysis confirmed the interaction between BmOPY2 and BmSTE50, suggesting that BmOPY2 functions as an upstream regulator of the CHK1 MAPK pathway. To investigate the role of BmOPY2 in appressorium formation, we generated BmOPY2-disrupted (∆BmOPY2) mutants. These mutants formed appressoria normally on maize leaves, but did not form them on plastic Petri dishes. This suggests that BmOPY2 regulates appressorium formation via hydrophobic surface recognition but not via recognition of host-derived chemicals. Plant waxes or cutin monomers are recognized by other fungal pathogens, but these substances failed to restore appressorium formation in ∆BmOPY2 mutants. In comparison with the wild type, the ∆BmOPY2 mutants showed increased appressorium formation on intercellular spaces of maize leaves, suggesting that pectin―a component of these spaces―may promote this process. The addition of pectin restored appressorium formation by ∆BmOPY2 mutants on plastic surfaces. These findings reveal a novel dual regulation of appressorium formation in B. maydis, involving both BmOPY2-mediated hydrophobic surface recognition and a distinct pectin-dependent pathway.