A guardian of grasses: Specific origin and conservation of a unique disease-resistance gene in the grass lineage

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Abstract
The maize Hm1 gene provides protection against a lethal leaf blight and ear mold disease caused by Cochliobolus carbonum race 1 (CCR1). Although it was the first disease-resistance (DR) gene to be cloned, it remains a novelty because, instead of participating in the plant recognition and response system as most DR genes do, Hm1 disarms the pathogen directly. It does so by encoding an NADPH-dependent reductase, whose function is to inactivate Helminthosporium carbonum (HC) toxin, an epoxide-containing cyclic tetrapeptide, which the pathogen produces as a key virulence factor to colonize maize. Although CCR1 is strictly a pathogen of maize, orthologs of Hm1 and the HC-toxin reductase activity are present in the grass family, suggesting an ancient and evolutionarily conserved role of this DR trait in plants. Here, we provide proof for such a role by demonstrating its involvement in nonhost resistance of barley to CCR1. Barley leaves in which expression of the Hm1 homologue was silenced became susceptible to infection by CCR1, but only if the pathogen was able to produce HC toxin. Phylogenetic analysis indicated that Hm1 evolved exclusively and early in the grass lineage. Given the devastating ability of CCR1 to kill maize, these findings imply that the evolution and/or geographical distribution of grasses may have been constrained if Hm1 did not emerge.