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A major class of disease-resistance (R) genes in plants encode nucleotide-binding site/leucine-rich repeat (LRR) proteins. The LRR domains mediate recognition of pathogen-derived elicitors. Here we describe a random in vitro mutation analysis illustrating how mutations in an R protein (Rx) LRR domain generate disease-resistance specificity. The original Rx protein confers resistance only against a subset of potato virus X (PVX) strains, whereas selected mutants were effective against an additional strain of PVX and against the distantly related poplar mosaic virus. These effects of LRR mutations indicate that in vitro evolution of R genes could be exploited for enhancement of disease resistance in crop plants. Our results also illustrate how short-term evolution of disease resistance in wild populations might be toward broader spectrum resistance against multiple strains of the pathogen. The breadth of the disease-resistance phenotype from a natural R gene may be influenced by the tradeoff between the costs and benefits of broad-spectrum disease resistance.
Farnham et al. (Wed,) studied this question.