Application of Chitosan in Plant Defense Responses to Biotic and Abiotic Stresses

Chitosan, a copolymer of N-acetyl-D-glucosamine and D-glucosamine, which possesses properties that make it useful in various fields, is produced by the deacetylation of chitin derivatives. It is used in agriculture as a biostimulant for plant growth and protection, it also induces several responsive genes, proteins, and secondary metabolites in plants. Chitosan elicits a signal transduction pathway and transduces secondary molecules such as hydrogen peroxide and nitric oxide. Under biotic stress, chitosan can stimulate phytoalexins, pathogenesis-related proteins, and proteinase inhibitors. Pretreatment of chitosan before exposure to abiotic stresses (drought, salt, and heat) induces plant growth, production of antioxidant enzymes, secondary metabolites, and abscisic acid (ABA). It also causes changes in physiology, biochemistry, and molecular biology of the plant cells. However, plant responses depend on different chitosan-based structures, concentrations, species, and developmental stages. This review collects updated information on chitosan applications, particularly in plant defense responses to biotic and abiotic stress conditions.