Small RNA-Based Antiviral Defense in the Phytopathogenic Fungus Colletotrichum higginsianum

Members of the Colletotrichum genus cause disease in over 3,000 species of plants, including nearly every crop grown. C. higginsianum infects many members of the Brassicaceae, including Arabidopsis, offering the opportunity to study and manipulate both host and pathogen. Components of the RNAi machinery were identified in C. higginsianum and knock-out mutants were created. Sequencing of mRNA and small RNA populations from each mutant genotype, as well as small RNA populations from an AGO1 immunoprecipitation, was used to identify small RNA-producing loci in C. higginsianum mycelial tissue. Analyses revealed the presence of an uncharacterized dsRNA virus within C. higginsianum that was de-regulated in the ΔDCL1 and ΔAGO1 strains.

A number of mycoviruses have been described with a wide range of effects on fungal phenotypes, although primarily, these infections are asymptomatic.  In C. higginsianum no effect was observed on vegetative growth in the RNAi mutants when grown on complex or minimal media. However conidia production was negatively impacted; ΔDCL1 and ΔAGO1 produced six- and three-fold fewer conidia than the control strains, respectively. As the initial biotrophic phase of infection by C. higginsianum requires an interaction between a spore and the host surface, decreasing the number of available spores has a negative effect on fungal fitness with respect to infection initiation. Thus RNA silencing in C. higginsianum has a crucial role in antiviral defense that directly impacts conidation, a crucial step in its lifecycle, both as a free-living organism and as a plant-pathogen.