A conserved Colletotrichum effector that modulates the host immune response was acquired by horizontal gene transfer from plants

Colletotrichum graminicola is an ascomycete fungus that causes maize anthracnose, one of the most devastating maize diseases worldwide. Previously, our group discovered the a subtilisin-like serine protease encoding gene named Colletotrichum plant-like subtilisin (CPLS) that was acquired by Colletotrichum spp. from plants through horizontal gene transfer (HGT). An updated phylogenetic analysis revealed that CPLS homologs are present in all members of the genus Colletotrichum and in only two other fungal genera, Diaporthe and Rhynchosporium. In vivo CPLS expression analyses using qRT-PCR and transcriptional fusions with the CPLS promoter and GFP showed that CPLS is expressed during the early stages of infection, reaching a maximum gene expression at 48 hours post-infection. To understand the role of the protein in virulence, we prepared CPLS null mutants by gene replacement. The null mutants showed similar growth rate, sporulation rate and colony morphology as the wild-type strain. To test the effect of the absence of CPLS in the maize anthracnose process, leaf pathogenicity assays were performed and resulted in a hypervirulent phenotype, supporting a role of CPLS in fungal virulence during plant-fungus interaction and suggesting a possible linkage of CPLS to plant cell death processes. In contrast, no differences between the CPLS null mutant and wild-type strain were seen during root infection assays indicating that the role of CPLS in virulence is tissue specific. We also performed a genome wide transcriptional analysis to understand the plant responses to CPLS during the colonization of leaves. We found that CPLS suppresses the expression of flavonoid and phenylpropanoid biosynthesis and several other immune related genes. Thus, we confirmed that CPLS is a conserved effector in Colletotrichum spp. and in C. graminicola it contributes to virulence by modulating host defense responses to promote plant susceptibility.