Identification of key Arabidopsis genes required for resistance against Botrytis cinerea

Despite the significant progress achieved in molecular biology in the last few years, our knowledge about the mechanisms of plant resistance against necrotrophic and non-host pathogens is still rudimentary. Here, we report the isolation and characterization of three Arabidopsis mutants selected for altered resistance against B. cinerea. Mutations in the Increased Botrytis Resistance (IBRJ) gene resulted in significant resistance against B. cinerea and A. brassicicola, another necrotrophic fungus. Interestingly, ibri plants also exhibited enhanced susceptibility to the host and non-host bacterial pathogens P. syringae pv. tomato and P. fluorescens, respectively. Conversely, resistance against B. gram mis f. sp. tritici and E. cichoracearum was not affected, suggesting JBRJ is required for resistance against some, but not all pathogens. By TAIL-PCR IBRJ gene was found to be allelic to asymetric leaves 1 (as]). AS] belongs to the R2R3 subfamily of the MYB-domain and has been extensively studied because of its central function in leaf development. Interestingly, neither as2 or erecta mutants or the conditional line KNJ are affected in resistance against B. cinerea or PstDC3 000. These results suggest that as]-mediated disease resistance and susceptibility is independent from the AS] -dependent pathway regulating leaf development. Moreover, we show that AS] function in disease resistance & susceptibility is conserved in at least three evolutionary divergent plant species, suggesting AS] function in disease resistance is relatively ancient. The analysis of a substantial series of as] double mutants in Arab idopsis revealed that both the production and perception of jasmonate (JA) and ethylene (ET) are required for as]-mediated disease resistance against necrotrophic pathogens. Moreover, the expression of JA/ET-dependent defence genes was shown to be accelerated in as] plants in response to B. cinerea challenge. While as] resistance against necrotrophic pathogens seems to be associated with enhanced expression of JA/ET dependent genes, susceptibility against the different strains of host and non-host bacterial pathogens occurred in the presence of normal PR-] expression and salicylic acid accumulation, suggesting susceptibility towards bacterial pathogens is not associated with defects in the activation of SA signalling pathway. In sum, our findings indicate that AS] is a negative regulator of resistance against necrotrophic fungi and a positive regulator of nonhost resistance and basal protection against bacterial pathogens in both Arabidopsis and other plant species. In contrast to ibr], the ibr2 and ebsl (Enhanced Botrytis susceptible]) mutants seem to specifically affect resistance against B. cinerea. The ibr2 was mapped to a short interval on chromosome five. Despite ibr2 cloning is not accomplished, we speculated that the mutation is in an alpha-tubulin gene because ibr2 plant morphology resembles the phenotype of the previously characterized lefty] and lefty2 mutants, which also encode alpha-tubulins. Our speculation is supported by the presence of two alpha-tubulins in the mapped region on chromosome 5, close to the predicted location of ibr2. The Botrytis susceptible line ebs] co-segregates with basta resistance, indicating that the mutated gene is tagged. However, the T-DNA inserted contains additional sequences on its left border that prevented successful cloning. The ebs] plants show impaired resistance against B. cinerea, but unaffected resistance against virulent and avirulent strains of PstDC3 000, suggetsing EBS] plays a more specific role in resistance against necrotrophic pathogens. Further characterization of ibr2 and ebs] is required in order to elucidate their role in plant disease resistance.