Identification of virulence genes in the crucifer anthracnose fungus Colletotrichum higginsianum by insertional mutagenesis.

To investigate the molecular and genetic mechanisms underlying virulence of Colletotrichum higginsianum on Arabidopsis thaliana, a T-DNA insertion mutant library of C. higginsianum, the causal agent of crucifer anthracnose, was established using Agrobacterium tumefaciens-mediated transformation. Among 875 transformants tested for virulence on Arabidopsis, six mutants with altered virulence, including an appressorial melanin-deficient mutant T734, two mutants defective in penetration, T45 and B30, and three mutants, T679, T732 and T801, that cause hypersensitive reactions on host Arabidopsis, were obtained. Southern blot analysis indicated that the mutants T732 and T734 harbored single-site T-DNA integrations, while B30 harbored two T-DNA insertions. Border flanking sequences of T-DNAs from these mutants were recovered by inverse polymerase chain reaction (PCR) and thermal asymmetric interlaced PCR. Sequence analyses revealed that single T-DNA insertions in mutant T734 targeted the coding region of a gene with unknown function, and in mutant T732 targeted a gene encoding a copper amine oxidase. The two T-DNA insertion sites in mutant B30 were found in the coding region of a gene encoding an exosome component and in the upstream region of a DUF221-domain gene. None of these genes have previously been implicated in virulence of the phytopathogenic fungi. Among these avirulent mutants, T734 showed altered color in colony growth and produced melanin-deficient, albino appressoria. The T-DNA insert in T734 was detected in the coding region of a gene named C. higginsianum melanin-deficiency gene (Ch-MEL1), which is highly similar to a gene encoding a hypothetical protein in Colletotrichum gloeosporioides (GenBank ELA33048). To validate whether the Ch-MEL1 gene was associated with virulence of the mutant T734, a targeted gene disruption and complementation approach was used. The appressoria of ▵Ch-mel1 null mutants were defective in melanization and failed to penetrate the host epidermal cells. When inoculated onto the wounded leaf tissues, the ▵Ch-mel1 mutants grew on host tissues but failed to cause lesions beyond the wound site. In contrast, both the complement C▵Ch-mel1-2 and the wild type produced melanized appressoria and caused necrosis on leaves of Arabidopsis. Ch-MEL1 is required for both appressorial melanin production in C. higginsianum and post-invasive growth in host tissues. Together with identification of other avirulent mutants and their associated genes, this study provides novel insights into molecular mechanisms underlying virulence of the hemibiotroph, C. higginsianum.