Homeobox transcriptional factor CoHox4 regulates hyphal growth, conidial morphology and appressorial formation in Colletotrichum orbiculare

A homeobox encodes a transcription factor containing a DNA-binding motif consisting of 180 bp termed the homeodomain and it is known to play an important role in regulating morphogenesis in fungi and other eukaryotes. The 10 genes encoding homeobox transcription factors were identified from the Colletotrichum orbiculare genome. In this study, we functionally analyzed CoHox4, one of the 10 homeobox genes conserved in C. orbiculare. CoHox4-knockout mutants exhibited a marked decrease in vegetative hyphal growth compared with the wild type. Observation of hyphal growth in the early stage of culture revealed increased hyphal branching of CoHox4 knockout mutants compared with the wild type. Conidia formed by CoHox4 knockout mutants were 1.35-times longer than those formed by the wild-type. The elongated conidia formed abnormal appressoria on glass and host leaves. Furthermore, virulence of the CoHox4-knockout mutants was markedly reduced on intact and injured cucumber cotyledons. In addition, they had low sensitivity to calcofluor white. These results suggest that CoHox4 regulates morphological differentiation such as vegetative hyphal growth, conidial morphology and appressorial formation and is required for full virulence of this fungus.