| My research has focused on the pbs3 mutant of Arabidopsis, which is compromised in defense responses triggered by many different resistance (R) genes. Genetic analyses revealed that this mutant contains mutations in at least two genes that contribute to its enhanced susceptibility to bacterial pathogens. I used a map-based cloning approach to identify both of these genes. One of the genes is SID2, which encodes an enzyme, isochorismate synthase, that catalyzes a key step in salicylic acid (SA) biosynthesis. SA is a plant hormone that induces multiple defense genes. Consistent with this discovery, pbs3 mutant plants were found to have very low levels of SA compared to wild-type plants. The second gene mutated in pbs3 plants is At5g13320, and is similar to the auxin-induced gene GH3 in soybean, and to 18 other Arabidopsis genes. Several members of this family have been shown to encode enzymes that catalyze addition of adenosine monophosphate onto plant hormones. Together, these findings point to a key role for SA and possibly other plant hormones in regulating R-gene dependent defense responses.; I have also conducted a bioinformatics analysis of R genes in Arabidopsis in relationship to segmental duplication events. Arabidopsis has experienced at least three whole genome duplication events. In order to understand the evolutionary relationships among Arabidopsis R genes, of which there are ∼150 in a single genome, and these whole genome duplication events, I identified segmental-duplication events and combined these data with a phylogenetic analysis of R genes. Based on the frequency of silent mutations in the duplicated genes, my analyses suggest that the expansion and diversification of R genes occurred after the most recent whole genome duplication event (∼90 Million years ago). This study will facilitate identification of syntenic R gene clusters among Arabidopsis and crop species. |
