Pathogenesis related proteins: milestones in five decades of research

Plants inducible defense responses cause a variety of alterations to the physiology of plant cells, such as the lignification and strengthening of the cell wall, the induction of an oxidative burst, the accumulation of phytoalexins, pathogenesis-related (PR) proteins, and other antimicrobial metabolites. It also causes cell death due to hypertensive reactions. Plants use PR proteins as one of their most vital defense mechanisms against external damage, pathogen attacks, and harsh climatic conditions. Further, these proteins support various phases of plant development, including seed germination. Since 1970, PR proteins have been recognized as an inducer of disease resistance. However, during the last two decades an increasing number of studies in many different crop plants, shed light on the importance of PR proteins in plant defence mechanism. The main goal of the current review is to provide a comprehensive understanding of PR proteins in plant defense mechanisms. To that end, we have covered all the 19 classes of PR proteins reported so far, their characteristics, how they respond to various stressors, and the pathways that either up- or down-regulate PR proteins in response to various abiotic and biotic stressors. It has also been studied how PR proteins contribute to cross-stress resistance, how SAR develops, how PR genes evolve molecularly over time, and how stress-related memory is passed down through generations to progenies. There has been discussion on the importance of priming and sensitization as well as the function of PR proteins in the development of several allergies in humans and other species. Several PR proteins have been identified, cloned, and related genes expressed to protect the plants against specific diseases and providing resistance to afflicted plants. Modern DNA technology has made it possible to construct transgenic plants to confer resistance. Of late, the improved resistance of the transformed plants to multiple diseases have been attributed to the transgenic expression of different groups of PR proteins. Novel approaches or breakthroughs in PR protein transgenic technology are needed to improve plant health globally.