Salicylic Acid Biosynthesis in Plants

Abstract

Salicylic acid (SA) is an important plant hormone that is best known for mediating host responses upon pathogen infection. Its role in plant defense activation is well established, but its biosynthesis in plants is not fully understood. SA is considered to be derived from two possible pathways; the ICS and PAL pathway, both starting from chorismate. The importance of both pathways for biosynthesis differs between plant species, rendering it hard to make generalizations about SA production that cover the entire plant kingdom. Yet, understanding SA biosynthesis is important to gain insight into how plant pathogen responses function and how pathogens can interfere with them. In this review, we have taken a closer look at how SA is synthesized and the importance of both biosynthesis pathways in different plant species.

Keywords: isochorismate synthase; pathogen infection; phenylalanine ammonia-lyase; plant defense; salicylic acid biosynthesis.

FIGURE 1

Possible biosynthesis routes for SA in plants. Full lines are conversion steps, dotted lines are transport from chloroplast to cytosol, the dashed line is an alternative, unknown biosynthesis route. The question mark indicates an unidentified protein. It is unclear whether the steps leading up to phenylalanine are performed in the chloroplast or cytosol, or in both simultaneously, as there are chloroplastic and cytosolic CMs. Enzymes are indicated in blue and are abbreviated as follows: ICS, isochorismate synthase; CM, chorismate mutase; PAL, phenylalanine ammonia-lyase; AIM1,abnormal inflorescence meristem1; BA2H, benzoic acid 2-hydroxylase; EDS5, ENHANCED DISEASE SUSCEPTIBILITY 5; PBS3, avrPphB SUSCEPTIBLE3; EPS1, ENHANCED PSEUDOMONAS SUSCEPTIBILITY 1. In Arabidopsis, sid1 mutants are loss-of-function eds5 mutants, while sid2 mutants are loss-of-function ics1 mutants.