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Signal Transduction and Transcriptional Regulation of Plant Defence Responses

Signal Transduction and Transcriptional Regulation of Plant Defence Responses

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Review Article Department of Botany, University of Pune, Pune, India Signal Transduction and Transcriptional Regulation of Plant Defence Responses B. L. B. L. Jalali Jalali 1 , S. S. Bhargava Bhargava 2 and A. A. Kamble Kamble 2 Authors Õ addresses: 1 Department of Plant Pathology, C.C.S. Haryana Agricultural University, Hisar 125004; 2 Department of Botany, University of Pune, Pune 411007, India (correspondence to S. Bhargava. E-mail: [email protected]) Received October 8, 2005; accepted November 7, 2005 Keywords: defence signalling, salicylic acid, jasmonate, ethylene, transcription factors Abstract Plant defence responses are regulated by a complex network of signal molecules and transcriptional regula- tors. Resistance ( R ) genes play a role in specific recog- nition 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 show- ing enhanced susceptibility or resistance to pathogens, or those showing insensitivity to exogenously applied SA, JA or ET. Crosstalk between the different signal- ling pathways leads to fine-tuning of the expression of defence-related genes. Complex regulatory networks modulated by at least eight different families of tran- scription 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 dif- ferent 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. Introduction Plants cope with pathogen attack through complex adaptive responses. Some of the responses are consti- tutive and pathogen non-specific, but a majority of them are induced after recognition of some feature of the pathogen. Depending on the severity of the responses, a plant may succumb to the pathogen (com- patible interactions), or prevent the pathogen from col- onizing it (incompatible interactions). Specific recognition of a pathogen-induced product of the avirulence ( avr ) gene by a resistance ( R ) gene product of the plant initiates the defence responses. Several alterations in cellular metabolism occur as an outcome of R avr binding. Calcium influx, generation of active oxygen species and activation of protein kinases are some of the earlier events. Later, new transcripts accumulate which code for either signalling molecules or molecules associated with defence execution or metabolism (Eulgem et al., 2004). Diverse R avr signalling pathways are known to converge and often lead to the expression of common sets of target genes. The defence responses are expressed through accumulation of downstream signalling molecules-like
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