Plant pathogenic fungi Colletotrichum and Magnaporthe share a common G ₁ phase monitoring strategy for proper appressorium development

To breach the plant cuticle, many plant pathogenic fungi differentiate specialized infection structures (appressoria). In Colletotrichum orbiculare (cucumber anthracnose fungus), this differentiation requires unique proper G ₁ /S phase progression, regulated by two-component GTPase activating protein CoBub2/CoBfa1 and GTPase CoTem1. Since their homologues regulate mitotic exit, cytokinesis, or septum formation from yeasts to mammals, we asked whether the BUB2 function in G ₁ /S progression is specific to plant pathogenic fungi. Colletotrichum higginsianum and Magnaporthe oryzae were genetically analyzed to investigate conservation of BUB2 roles in cell cycle regulation, septum formation, and virulence. Expression profile of cobub2Δ was analyzed using a custom microarray. In bub2 mutants of both fungi, S phase initiation was earlier, and septum formation coordinated with a septation initiation network protein and contractile actin ring was impaired. Earlier G ₁ /S transition in cobub2Δ results in especially high expression of DNA replication genes and differing regulation of virulence-associated genes that encode proteins such as carbohydrate-active enzymes and small secreted proteins. The virulence of chbub2Δ and mobub2Δ was significantly reduced. Our evidence shows that BUB2 regulation of G ₁ /S transition and septum formation supports its specific requirement for appressorium development in plant pathogenic fungi.