Functional dissection of Arabidopsis thaliana resistance gene RPS2

This study identified loci that function with the Arabidopsis resistance gene RPS2 to mediate gene-for-gene resistance against Pseudomonas syringae (Psg) expressing the avirulence gene avrRpt2. Genetic and molecular analyses of progeny from crosses between the resistant ecotype Col-0 and the susceptible ecotype Po-1 indicated that susceptibility to Psg avrRpt2 is due to variation at both RPS2 and at other loci. The Po-1 allele of RPS2 was found to be functional, but only when present in a partial Col-0 genetic background, while the functional Col-0 RPS2 allele was sometimes prevented from signaling for resistance in a partial Po-1 genetic background. Expression of avrRpt2-specific resistance thus appears to be modified by allele-specific interactions among RPS2 alleles and alleles of other loci that function in the RPS2-pathway. The leucine rich repeat domains of RPS2 were identified as a structural determinant of allele-specific interactions that affect the RPS2-mediated resistance response. Quantitative trait analysis of marker data from Po-1 x Co-0 progeny localized significant contributions to RPS2-phenotype to two genetic intervals, in addition to the RPS2 locus, on Arabidopsis chromosome 4.; Po-1 is also susceptible to P. syringae expressing the avirulence gene avrRpm1. As was the case with the RPS2-avrRpt2 pathway, susceptibility was found to be due to variation at both the resistance gene RPM1 on chromosome 3 and a genetic interval on chromosome 4. There is overlap between the chromosome 4 interval that contributes to RPM1-mediated disease resistance phenotype and one of the genetic intervals with significant contribution to RPS2-mediated disease resistance phenotype. This common genetic interval might encode signaling components that are shared between RPS2- and RPM1-resistance pathways.; In the second part of this study the role of the nucleotide-binding site (NBS) of RPS2 in mediating resistance was investigated. Mutant alleles of RPS2 with mutations that would result in a single amino acid change in the NBS domain were stably transformed into a rps2/rps2 genetic background and evaluated for their ability to mediate a resistance response against P. syringae expressing avrRpt2. The lack of a resistance response in six of the nine RPS2 P-Loop mutant lines suggests that a functional P-Loop is critical for RPS2 resistance function.; In summary, this study identifies loci that interact with RPS2 and RPM1 and domains within RPS2 that modify expression and strength of the resistance response. Further characterization and cloning of the RPS2- and RPM1-interacting loci, conjoined with functional analysis of RPS2 protein domains such as the LRRs and the NBS, should provide valuable insights into the structure, molecular interactions and function of resistance genes involved in gene-for-gene resistance.