Victorin, the host-selective cyclic peptide toxin from the oat pathogen Cochliobolus victoriae , is ribosomally encoded

Significance Studies of the 1940s Victoria blight of oats epidemic discovered that some fungal pathogens secrete HSTs responsible for symptom development and specificity of the associated disease. The causal necrotrophic pathogen of Victoria blight,C. victoriae , secretes the peptide HST victorin, which was, subsequently, shown to constitute a novel class of effectors that exploit host immunity pathways aimed at repelling biotrophic pathogens. Although these discoveries have broadened our mechanistic understanding of plant-pathogen interactions, the genetic and biochemical origins of victorin have remained elusive. Here, we solve this decades-old mystery by demonstrating that victorin is produced ribosomally, not, as assumed, by nonribosomal peptide synthetase. Furthermore, we identify a CAO enzyme responsible for converting victorin to its active form.