Genome-wide characterisation of pathogenicity-related proteins in Mycena citricolor, the causal agent of the American Leaf Spot in coffee

Abstract

Mycena citricolor is a fungus that causes the American Leaf Spot (ALS) disease in multiple hosts, including coffee and avocado. This hemibiotroph penetrates the plant through damage induced by oxalic acid. This can cause 20-90% crop losses in coffee depending on the environmental and production conditions. M. citricolor is the only known pathogenic species in the Mycena genus, a large group of saprophytic mushrooms. Comparing the saprophytic and pathogenic genomes can allow us to identify genetic machinery associated with the pathogens genome-wide functional acquisitions to cause disease. To identify pathogenicity-related genes in M. citricolor, we analysed protein family copy-number variation, secretome prediction, and homology to known virulence factors in two M. citricolor assemblies, including a newly assembled and annotated long-read genome. We found that the pathogenic M. citricolor had a higher proportion of secreted genes expanded in copy-number, and expanded gene copies homologous to known virulence factors than the saprophytic Mycena. We shortlisted over 300 candidate genes in each M. citricolor assembly. Focusing on genes strongly regulated during plant interaction, we found over 100 candidates, primarily from multiple copies (up to 4-3 times) of 42 well-known virulence factors (e.g. MFS1, CUTA, NoxA/B, OLE1, NorA), plus a few clade-specific uncharacterised genes. M. citricolor transition to a pathogenic lifestyle reflected genome-wide functional changes. M. citricolor seems to primarily depend on well-known virulence factors in large copy numbers, suggesting the molecular plant-interaction processes involved are like those of better-studied fungi. Hypothetically, the development of ALS resistance could mirror studied responses to these virulence factors.