Salicylic acid (SA) is a major defense signal in plants. In Arabidopsis thaliana, the main source of SA biosynthesis during abiotic stress (such as UV-C) or pathogen infections is a chloroplast-localized pathway. The enzyme isochorismate synthase 1 (ICS1) uses chorismate to produce isochorismate, which is subsequently converted to SA by an as yet unknown enzyme in higher plants.
To identify additional proteins that play roles in SA production, proteins that co-immunoprecipitated with ICS1 were analyzed by mass spectrometry. A large number of peptides from PROHIBITIN 3 (PHB3) were identified as well as peptides from other PHB family members. This family of proteins has diverse biological functions that include acting as scaffolds for protein complex formation and stabilization. PHB3 was previously localized to mitochondria. Fractionation and live imaging established that a pool of PHB3 also localizes to chloroplasts, where ICS1 resides. Compared to wild-type plants, phb3-3 mutant plants without or with a PHB3 transgene accumulated lower and higher UV-C-induced levels of SA, respectively. This occurs also with PR1 protein levels, which is strongly upregulated by SA. Supporting regulation at post-transcriptional level, concomitant changes in the levels of ICS1 protein, but not in its transcript, were observed. The phb3-3 mutant was hypersusceptible to avirulent strains of Pseudomonas syringae and accumulated less SA and PR1 in response to infection relative to wild-type.
A plausible model is that formation of PHB3-ICS1 complexes stabilizes and possibly activates ICS1 to promote SA production in response to stress.