Isochorismate-derived biosynthesis of the plant stress hormone salicylic acid

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 Pathogen-induced biosynthesis of salicylic acid occurs by combined enzymatic and spontaneous steps in the plant cell cytosol. The phytohormone salicylic acid (SA) controls biotic and abiotic plant stress responses. Plastid-produced chorismate is a branch-point metabolite for SA biosynthesis. Most pathogen-induced SA derives from isochorismate, which is generated from chorismate by the catalytic activity of ISOCHORISMATE SYNTHASE1. Here, we ask how and in which cellular compartment isochorismate is converted to SA. We show that in Arabidopsis, the pathway downstream of isochorismate requires only two additional proteins: ENHANCED DISEASE SUSCEPTIBILITY5, which exports isochorismate from the plastid to the cytosol, and the cytosolic amidotransferase avrPphB SUSCEPTIBLE3 (PBS3). PBS3 catalyzes the conjugation of glutamate to isochorismate to produce isochorismate-9-glutamate, which spontaneously decomposes into SA and 2-hydroxy-acryloyl-N-glutamate. The minimal requirement of three compartmentalized proteins controlling unidirectional forward flux may protect the pathway against evolutionary forces and pathogen perturbations.