Morphogenesis and adaptive strategies for infection in plant pathogenic fungi

The unique role of cell cycle regulators in Colletotrichum orbiculareColletotrichum orbiculare is the causal agent of cucumber anthracnose disease.The infection process is initiated through the recognition of specific surface cues by aseptate conidia.Following germination of the conidia, the emerging germ tubes differentiate into dome-shaped appressoria, which subsequently undergo nuclear division (Kubo and Takano 2013; Takano et al. 2001).From random geneinsertional mutagenesis in C. orbiculare, the CoBUB2 gene was identified as responsible for the pathogenicity-deficient phenotype on cucumber (Fukada and Kubo 2015).CoBub2 harbours a Rab GTPase-activating protein (GAP) domain, with highly conserved homologous sequences observed across yeast and filamentous fungi.In Saccharomyces cerevisiae, Bub2 forms a two-component GAP complex with Bfa1, which targets the GTPase Tem1.The Bub2-Bfa1 GAP complex in S. cerevisiae plays a critical role in monitoring cell cycle checkpoints, ensuring accurate distribution of replicated genomic material and preventing premature mitotic exit (Hu and Elledge 2002).By analysing the timing of entry into the DNA replication phase (S phase), we found that the C. orbiculare cobub2 and cobfa1 mutants displayed deficiencies in monitoring the G1 to S phase transition during conidial germination and appressorium formation.Thus, we conclude that the CoBub2-CoBfa1 GAP complex plays a distinct role in regulating G1/S phase progression in C. orbiculare, differing from its function in homologs in yeasts.To assess whether the regulation of G1/S progression by CoBub2/CoBfa1 is conserved across other plant pathogenic fungi, we investigated the role of Bub2 in G1/S progression in C. higginsianum and Magnaporthe oryzae, other representative species that penetrate host plants via appressoria.Our findings demonstrate that BUB2 is a crucial negative regulator of G1/S progression and initiates septum formation in these fungi (Fukada et al. 2019).