What is it about?
Popular oat varieties with resistance to the important crown rust disease in the 1940s became vulnerable to a new oat disease called Victoria Blight, which led to an outbreak in oat-growing regions in United States. The culprit, a fungus Cochliobolus victoriae, produces a peptide host-selective toxin, victorin, that target specifically these crown rust-resistant oat varieties. Victorin was shown to exploit the oat's resistance mechanism against crown rust for disease susceptibility. The complex structure of victorin suggested it was synthesized by nonribosomal peptide synthetase (NRPS) pathway. However, for many years the genes that encode the peptide was not found despite the genome of the fungal pathogen C. victoriae has been sequenced. Here, we show that victorin is a Ribosomally synthesized and Post-translational modified Peptide (RiPP). The genes encoding victorin biosynthesis are buried among gene-sparse repeat-rich region of the genomes unveiled by long-read sequencing. Importantly, we also uncovered a key enzyme, a copper amine oxidase, that converts the victorin peptide into the active form.
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Why is it important?
This work sets the foundation for further study of victorin biosynthesis, in which many intriguing questions remain. Importantly, the discovery of homologous victorin-like gene clusters in two distantly related plant-associating sordariomycete fungi open up new questions about the evolutionary origin of victorin biosynthetic genes. The study of how host-selective toxins are acquired and evolved will improve our understanding of how new crop diseases emerged and could help prevent future new disease outbreaks. Given that victorin binds and inhibits thioredoxin, a promising target for anticancer drug development, the chemistry and biology of this subgroup of victorin-like RiPPs warrants further investigation.
Perspectives
It was a great pleasure to collaborate with Prof Gillian Turgeon who is an expert in Cochliobolus molecular plant pathology and genetics and has deep knowledge about the history of the Victoria Blight disease and its related scientific development. It is also always fun to collaborate with Dr Megan McDonald and Prof Peter Solomon. Dr McDonald's long-read sequencing data was key to the broader discovery of the victorin biosynthetic genes and their genomic context. It was great to have the opportunity to interact with Prof Thomas Wolpert who discovered the mode of action of victorin during this work. His generosity in sharing the victorin standard and insights in handling victorin made it possible for us to verify our discoveries. This work is mainly driven by the PhD student Simon Kessler at UWA along with Dr Xianghui Zhang who was a visiting fellow at Cornell and has been 3 years in the making. It is great to see that it is finally published in PNAS and solving this long-standing puzzle related to a "textbook example" host-selective toxin that I learned when I was a young graduate student is highly satisfying. I hope that it will contribute to our understanding of how new crop disease emerge and also led to a stronger appreciation of this underexplored class of peptide metabolites in fungi.
Yit-Heng Chooi
University of Western Australia
Read the Original
This page is a summary of: Victorin, the host-selective cyclic peptide toxin from the oat pathogen
Cochliobolus victoriae
, is ribosomally encoded, Proceedings of the National Academy of Sciences, September 2020, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2010573117.
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