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A broad range of genetic mechanisms has been shown to influence the evolution of disease resistance (R) genes in plants. Based on data from a limited number of species, particularly lettuce, it is apparent that these mechanisms have been important at different times and influence different parts of the resistance protein and that R genes within the same cluster can exhibit heterogeneous rates of evolution. The different rates of evolution may be indicative of different types of pathogen ligands detected. These analyses need to be extended to other crop species to test the generality of these findings.

The proposed parallel studies on several plants and pathogens will characterize allelic and orthologous variation for plant R genes, bacterial avirulence/effector genes, as well as plant genes encoding the targets of the effectors. The will determine whether variation in whether disease occurs or not is due to differences at R genes or effector target genes.

This project is focused on understanding the evolution of resistance genes and the plant targets of bacterial effector proteins using a comparative approach to dissect plants’ abilities to detect effector proteins from Pseudomonas syringae. We will analyze sequence diversity in R genes from multiple plant species to test for heterogeneous rates of evolution. In parallel, we will screen Arabidopsis and diverse crop species for their ability to recognize effector proteins. We will analyze Arabidopsis, tomato and lettuce in depth to assess allelic variation to libraries of effectors from four strains of P. syringae. We will also analyze five other crop species in less detail to gain an understanding of orthologous variation. We will then identify potential effector targets and R-genes recognizing effectors and characterize allelic variation of effector targets. Finally, we will study potential interactions between ‘guard’ and effector target proteins. We are well positioned to make rapid progress on this project. We have considerable experience with Arabidopsis, lettuce and tomato, NBS-LRR genes, avirulence/effector proteins of Pseudomonas syringae and the molecular approaches involved.

Understanding the molecular determinants of disease resistance has practical as well as fundamental importance. Lettuce and tomato are two of the top ten most valuable crops in the US. Disease resistance is one of the most agriculturally important traits targeted in crop improvement programs. Understanding the molecular basis of specificity in plant-pathogen interactions and the events resulting in resistance will provide new possibilities for developing more durable disease resistance in plants.
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