The homeobox transcription factor CoHox1 is required for the morphogenesis of infection hyphae in host plants and pathogenicity in Colletotrichum orbiculare

The hemibiotrophic fungus Colletotrichum orbiculare is the causal agent of anthracnose of cucumber. During host plant infection, appressoria penetrate into the host cells, and biotrophic infection hyphae elongate inside the host cells, then switch to necrotrophic phase. We found that the homeobox transcription factor CoHox1 is a novel type of pathogenesis-related gene. CoHox1 disruption mutants of C. orbiculare were non-pathogenic on host cucumber cotyledons and formed no lesions at wound sites on leaves. On artificial surfaces (glass and cellulose membrane), CoHox1 disruption mutants formed normal appressoria and infection hyphae similar to a wild-type strain. On the other hand, microscope observation showed that CoHox1 disruption mutants has a decreased rate of penetration of host leaves, and the infection hyphae of CoHox1 disruption mutants did not extend from the penetrated host cell into neighboring cells. On heat-shocked host leaves, CoHox1 disruption mutants also formed no lesions. These results showed that homeobox transcription factor CoHox1 may be required for pathogenicity, especially the morphogenesis of infection hyphae in host cells by C. orbiculare.