Two-step biosynthesis of benzyl alcohol from benzoyl-CoA within the phenylalanine-derived salicylic acid biosynthetic pathway in plants

Salicylic acid (SA) is a well-known medicinal compound and an essential phytohormone that regulates plant growth and stress responses. SA biosynthesis in plants occurs through two primary pathways: the isochorismate synthase (ICS) pathway and the phenylalanine ammonia-lyase (PAL) pathway. The ICS pathway is predominantly present in Brassicaceae species, whereas the PAL pathway is more broadly distributed across most seed plants. Although the fundamental steps of the PAL pathway have been delineated, the biosynthesis of its crucial intermediate, benzyl alcohol, remains unknown. In this study, we reveal a two-step pathway for benzyl alcohol biosynthesis from benzoyl-CoA (BA-CoA) in rice. We demonstrate that two uncharacterized peroxisomal enzymes, rice cinnamoyl-CoA reductase 21 (OsCCR21) and cinnamyl alcohol dehydrogenase 6 (OsCAD6), catalyze successive reduction steps in this biosynthetic process. Specifically, OsCCR21, rather than the putative benzaldehyde synthase (BS) heterodimers OsBSα/β, functions as benzoyl-CoA reductase (BCR) to convert BA-CoA into benzaldehyde. OsCAD6 subsequently acts as benzyl alcohol dehydrogenase (BAD) to reduce benzaldehyde to benzyl alcohol. Cross-species comparative genomic analysis combined with in vitro enzymatic assays suggests that peroxisomal OsCCR21 orthologs are specifically conserved in the Poaceae, whereas PhBSα/β and OsCAD6 orthologs are broadly distributed across most angiosperms. Interestingly, PhBSα/β orthologs in Poaceae exhibit slight BS activity, whereas they have evolved strong BS activity in non-Poaceae angiosperms. These findings reveal that two alternative sets of enzymes, BCR-BAD and BSα/β-BAD, are employed for benzyl alcohol biosynthesis from BA-CoA in Poaceae or non-Poaceae angiosperms, providing important insights into phenylalanine-derived SA biosynthesis and plant secondary metabolism.