Salicylic acid (SA) is an essential plant defense hormone that promotes immunity against biotrophic and semibiotrophic pathogens. It plays crucial roles in basal defense and the amplification of local immune responses, as well as the establishment of systemic acquired resistance. During the past three decades, immense progress has been made in understanding the biosynthesis, homeostasis, perception, and functions of SA. This review summarizes the current knowledge regarding SA in plant immunity and other biological processes. We highlight recent breakthroughs that substantially advanced our understanding of how SA is biosynthesized from isochorismate, how it is perceived, and how SA receptors regulate different aspects of plant immunity. Some key questions in SA biosynthesis and signaling, such as how SA is produced via another intermediate, benzoic acid, and how SA affects the activities of its receptors in the transcriptional regulation of defense genes, remain to be addressed.

Spontaneous degradation in hormone synthesis: The phytohormone salicylic acid (SA) helps plants respond to biological and physical stresses. Rekhter et al. identified the biosynthetic pathway that produces SA in response to pathogens. A precursor, isochorismic acid, is formed in the chloroplast and then exported to the cytosol. There, enzymatically produced isochorismate-9-glutamate spontaneously decomposes to release SA plus a by-product. The results clarify key steps in the mechanisms involved in synthesizing this important regulator of plant immunity. Science , this issue p. 498

Plants use diverse immune receptors to sense pathogen attacks. Recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors localized on the plasma membrane leads to PAMP-triggered immunity (PTI). Detection of pathogen effectors by intracellular or plasma membrane–localized immune receptors results in effector-triggered immunity (ETI). Despite the large variations in the magnitude and duration of immune responses triggered by different PAMPs or pathogen effectors during PTI and ETI, plasma membrane–localized immune receptors activate similar downstream molecular events such as mitogen-activated protein kinase activation, oxidative burst, ion influx, and increased biosynthesis of plant defense hormones, indicating that defense signals initiated at the plasma membrane converge at later points. On the other hand, activation of ETI by immune receptors localized to the nucleus appears to be more directly associated with transcriptional regulation of defense gene expression. Here, we review recent progress in signal transductions downstream of different groups of plant immune receptors, highlighting the converging and diverging molecular events.