Potenciation of plant defense responses during biocontrol bacteria-mediated induced systemic resistance in Arabidopsis

Among beneficial soil microorganisms, other well-known group are Plant Growth Promoting Rhizobacteria. In addition to their positive effect on plant growth, some root-colonising rhizobacteria are able to trigger in the plant a broad-spectrum induced disease resistance, commonly referred to as rhizobacteria-mediated induced systemic resistance (ISR; Van Loon et al., 1998). At Utrecht University, the Phytopathology group is studying the mechanisms involved in the ISR triggered by selected strains of fluorescent Pseudomonas spp. My contribution to the research aimed to elucidate the role of jasmonic acid (JA)-dependent defence responses in plants during Induced Systemic Resistance. Some pathogen responsive genes are primed to respond more strongly upon pathogen attack in ISR expressing plants. Most of the primed genes were predicted to be regulated by JA- and/or ET- signalling pathways. Transcriptional analysis of the whole Arabidopsis genome revealed that an important subset of JA responsive genes, including genes coding for signalling and defence-related proteins, are induced faster and to a higher level in ISR expressing plants after JA treatment.

We hypothesized that primed tissues would be able to trigger defence mechanisms more efficiently in response to the pathogen via potentiation of the JA signalling pathway. Different regulation patterns of gene expression were identified by Adaptive Clustering Analysis, and a detailed study of the promoter sequences of the genes forming the main regulons, using a bootstraping approach, resulted in the identification of a transcription factor, AtMYC2, as putative responsible of the priming effect during ISR. Phenotypic and molecular analysis of plants overexpressing or lacking a functional AtMYC2 protein has confirmed the role of this transcription factor in priming defense responses: the loss of function mutants were unable to develop the state of ISR after colonization by the rhizobacteria, and the overexpressors showed a very significant reduction of the disease after pathogen attack in the absence of the inducing rhizobacteria. The results confirm the adequacy of the genomic approach to understand, at system level, plant defense mechanisms and predict key regulatory elements with potential to boost plant defences and stimulate plant immunity.