Signal Transduction and Transcriptional Regulation of Plant Defence Responses

Plant defence responses are regulated by a complex network of signal molecules and transcriptional regulators. Resistance (R) genes play a role in specific recognition of a pathogen and initiate defence responses. Three key signal molecules, namely salicylic acid (SA), jasmonate (JA) and ethylene (ET) mediate expression of both specific (R gene-mediated) as well as basal defence responses. The signalling pathways involving these molecules have been studied using mutants showing enhanced susceptibility or resistance to pathogens, or those showing insensitivity to exogenously applied SA, JA or ET. Crosstalk between the different signalling pathways leads to fine-tuning of the expression of defence-related genes. Complex regulatory networks modulated by at least eight different families of transcription factors play a role in defence gene expression. The transcription factors bind to cis-elements present in the promoters of the defence genes and are specific for different transcription factors. Several cis-elements often lie within the same promoter indicating that different transcription factors may lead to induction of the same defence gene. The redundancy observed in defence signalling and gene expression enables a plant to successfully combat a wide range of pathogens