Understanding the mechanisms of cir1 disease resistance in Arabidopsis thaliana

Includes abstract.Includes bibliographical references.Plants have evolved an elaborate and very effective defence system to curb disease caused by pathogen infections. To gain insight into the defence signalling network and defence responses deployed by plants for resistance to pathogens, the defence-related Arabidopsis thaliana mutant cir1 (constitutively induced resistance 1) was further investigated. It was previously shown that cir1 constitutively expresses salicylic acid-, jasmonic acid- and ethylene-dependent defence-related genes and exhibits increased resistance to the virulent bacterial pathogen Pseudomonas syringae pv. tomato and the virulent oomycete pathogen Hyaloperonospora parasitica Noco2. Through first–pass mapping experiments, it was formerly determined that the CIR1 locus is located on the lower arm of chromosome IV. With the aim of identifying the CIR1 gene, comprehensive genomic mapping of cir1 was conducted in this study. Upon the generation of a suitable mapping population, PCR-based markers were employed to narrow down CIR1 location to 309.10 kb. This region was included in six genomic DNA clones which were tested for complementation of the cir1 mutant. A small region in which CIR1 resides was identified and possible candidate genes within it were investigated. It was established that CIR1 is one of eight annotated genes. This study also assessed which known components of the defence signalling network play a role in cir1-mediated resistance, to establish a possible function of CIR1 in the Arabidopsis defence network. Epistasis analyses were performed between cir1 and the eds1 (enhanced disease susceptibility 1) and pad4 (phytoalexin deficient 4) mutants which regulate the salicylic acid signalling pathway, as well as the coi1 (coronatine-insensitive 1) mutant which functions in the jasmonic acid signalling pathway. The disease resistance profiles of cir1 eds1, cir1 pad4 and cir1 coi1 double mutants to infection by virulent P. syringae and virulent H. parasitica established that coi1, pad4 and eds1 are epistatic to cir1, suggesting that CIR1 is located upstream in the defence signalling network. Through defence-related gene expression profiling, it was found that cir1 simultaneously activates multiple signalling pathways, resulting in the induced expression of many defence-related genes and the increased expression of some of these genes was correlated to cir1’s enhanced resistance to virulent pathogens. Therefore, it appears that CIR1 functions as a negative regulator of the disease resistance signalling network. Furthermore, EDS1 protein accumulation may play a role in cir1-mediated resistance as it was found that cir1 has a stabilizing effect on the EDS1 protein