Salicylic Acid-Mediated Defence Signalling in Respect to Its Perception, Alteration and Transduction

Abstract

Salicylic acid (SA) is a small phenolic molecule, well known for its remedial properties in the case of human health. In plants, as a phytohormone, it regulates many physiological aspects like adventitious root initiation, germination of seeds, stomatal closure, flowering, senescence and thermogenesis. As a signalling molecule, defence responses mediated via SA against a variety of biotic and abiotic stresses are already well reported. It can be synthesized from cinnamic acid via phenylalanine ammonia-lyase into the cytosol while in the chloroplasts from isochorismic acid by isochorismate synthase; however, differences occur on the basis of growth conditions, developmental stage and plant species. Since long research is in progress to study its signal transduction pathways triggered via plant stress responses, and a major work has been done in exploring SA signalling with special attention to its receptors identification. The use of several biochemical methods and high-throughput screen development makes the way easy to hunt for SA receptors and to explore SA signal perception by identifying various SA-binding proteins. However, genetic evidences are less that confirms that these binding proteins are the true receptors of SA. Huge expression data confirms the connection and coordination between SA and other signalling networks, and the components of this convoluted network are universal under biotic and abiotic stresses. Fascinatingly, NPR1 (non-expressor of pathogenesis-related genes 1) was anticipated to be a SA receptor in this signalling pathway. Both NPR1-independent and NPR1-dependent signalling pathways have been reported, but very less is known about the intermediates of this NPR1-independent signalling pathway. Lots of work is going on to explore SA-overexpressing mutants for studying the downstream targets and regulation of SA biosynthesis. Here we focused on the SA level regulation mechanism with special attention on the perception and transduction of a signal from SA-induced defence response. Studying SA-mediated plant defence will facilitate the advancement of disease-resistant crops involving genetic manipulation of the SA signalling pathway components in a positive direction.