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