Salicylic acid (SA) is a crucial signal to activate defense genes in response to different environmental stressful conditions. Several proteins in the SA-signal pathway, such as TGA transcription factors and the co-activator NPR1, have been identified. Nevertheless, the mechanism of gene activation by SA is not clearly understood yet. According to their activation kinetics, SA-responsive genes are classified in early and late genes. It has been suggested that differential requirement of the co-activator NPR1 and cis-elements could exist for the activation of these groups of genes. Using transcriptome analysis in response to SA we identified 229 genes whose expression increases after 2,5h of treatment. Most of them (199) required NPR1 for their activation, while 30 are also activated in the npr1-1 mutant. We made an in silico search for over-represented promoter sequences and for expression patterns under stress conditions, by using the MotifSampler software and the GeneVestigator database, respectively. For a group of selected genes we confirmed the kinetics of induction by SA, the NPR1 dependence, we evaluated requirement of de novo protein synthesis and response to pathogen infection. Furthermore, we also evaluated for these genes the effect of SA on the mRNA stability, by using transcription inhibitors. All this information allowed us to distinguish at least two different pathways for gene activation mediated by SA.
This work was supported by research grants 1060494 from Fondecyt-CONICYT and 2005-7-186 from CONICYT/SECyT Cooperation Program.
