Salicylic Acid in Plant Immunity and Beyond.

As the most widely used herbal medicine in human history and a major defense hormone in plants against a broad spectrum of pathogens and abiotic stresses, salicylic acid (SA) has attracted major research interest. With applications of modern technologies over the past 30 years, studies of the effects of SA on plant growth, development, and defense have revealed many new research frontiers, and continue to deliver surprises. In this review, we provide an update on recent advances in our understanding of SA metabolism, perception, and signal transduction mechanisms in plant immunity. An overarching theme emerges that SA executes its many functions through intricate regulation at multiple steps: SA biosynthesis is regulated both locally and systemically, while its perception occurs through multiple cellular targets, including metabolic enzymes, redox regulators, transcription cofactors, and most recently, an RNA-binding protein. Moreover, SA orchestrates a complex series of posttranslational modifications of downstream signaling components and promotes the formation of biomolecular condensates that function as cellular signaling hubs. SA also impacts wider cellular functions through crosstalk with other plant hormones. Looking into the future, we propose new areas for exploration on SA functions, which will undoubtedly uncover more surprises for many years to come.