The role of salicylic acid in plant defense responses against biotic stresses

Phytohormones are of great significance in plant growth, development, immunity, and stress responses. As one of the promising phytohormones, salicylic acid (SA) is a natural phenolic compound synthesized from chorismate through two distinct pathways mediated by isochorismate synthase and phenylalanine ammonia-lyase, respectively. It plays crucial roles in regulating various biological processes related to plant growth and development, as well as in response to diverse biotic and abiotic stresses. Pathogens and herbivores are the most confronting biotic factors that pose severe threats to plant fitness and crop yield. The significance of SA in defense responses against these biotic stresses has been well addressed in the past decades. This review pays attention to the biosynthesis, metabolism, and signaling pathways of SA, and SA-mediated defense responses against pathogens and herbivores, thereby providing theoretical perspectives on the regulatory mechanism of SA-mediated immunity under biotic stress and improving the plant fitness and resistance to diseases caused by biotic factors.