Role of Pathogenesis-Related (PR) Proteins in Plant Defense Mechanism

Plant growth and development is often challenged by several abiotic and biotic stresses, such as drought, cold, salinity, wounding, heavy metals, and pathogen attacks, respectively. A plant responds to these threats by activating a cascade of genes, encoding different effectors, receptors, and signaling and protective molecules. Among all, the induction and accumulation of pathogenesis-related (PR) proteins in plants in response to these adverse conditions is very important as PR proteins are an indispensible component of innate immune responses in plants under biotic or abiotic stress conditions. The PR proteins protect the plants from further infection by not only accumulating locally in the infected and surrounding tissues but also in remote uninfected tissues. Induction of PRs has been reported from many plant species belonging to different families suggesting a general role for these proteins in adaptation to biotic or abiotic stress conditions. PR proteins are also involved in hypersensitive response (HR) or systemic acquired resistance (SAR) against infection. Thus, PR proteins have been defined as “proteins encoded by the host plant but induced only in pathological or related situations,” the latter inferring situations of nonpathogenic origin. In this chapter, structure, biochemistry, source, regulation of gene expression, and role in defense mechanism of various pathogenesis-related proteins will be discussed.