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
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.