Homeobox transcription factor CoHox3 is essential for appressorium formation in the plant pathogenic fungus Colletotrichum orbiculare

Abstract The plant pathogenic fungus Colletotrichum orbiculare , which is the causal agent of cucumber anthracnose disease, forms dome-shaped, melanized appressoria as a host invasion structure. We found 10 conserved homeobox transcription factors based on genome analysis. In this study, we functionally analyzed one of the C. orbiculare homeobox gene named CoHox3 ( Colletotrichum orbiculare Homeobox transcription factor 3) during the infection process. CoHox3 knockout mutants were non-pathogenic on host leaves (inoculated with 1.0 × 10 4 conidia/mL). Conidia of CoHox3 mutants germinated and/or formed lightly pigmented structures, but did not form mature appressoria on glass slides, artificial membranes, or host leaves. Only 0.3–0.6% of germinated conidia of CoHox3 mutants formed infection hyphae on host leaves. Green fluorescent protein-based expression experiments showed that germinated conidia of the CoHox3 mutant did not express CoGAS1 gene, an appressorium-specific gene, although wild-type strain 104-T expressed the CoGAS1 gene during appressorium formation. When conidia of the CoHox3 mutant was incubated on glass slides, nuclear division was significantly later than that of wild-type, which formed appressorium. Thus, homeobox transcription factor CoHox3 is required to form normal appressoria.