How does salicylic acid activate plant immunity?

Salicylic acid (SA) is a phytohormone central to plant immune responses, orchestrating both local defense and systemic acquired resistance (SAR), a broad-spectrum and long-lasting immune state. Upon pathogen attack, SA accumulates not only at the infection site but also in distal, uninfected tissues, priming the whole plant for enhanced resistance. Over the past three decades, extensive research, particularly in the model species Arabidopsis thaliana, has firmly established SA as a key regulator of plant immunity, yet several fundamental questions have only recently begun to be resolved. This review synthesizes recent advances in our understanding of SA biosynthesis and the mechanisms underlying its perception and signaling in plants. We summarize the now well-defined enzymatic routes of SA biosynthesis and emerging structural insights into SA recognition and downstream signaling. In addition, we highlight recent breakthroughs in identifying mobile immune signals that trigger SAR. By integrating findings from both model and non-model species, we outline conserved mechanisms, highlight emerging lineage-specific features in SA-activated plant immunity, and emphasize key open questions that remain to be addressed.