Hormone Signaling: Current Perspectives on the Roles of Salicylic Acid and Its Derivatives in Plants

Salicylic acid (SA) is an important plant hormone with a wide range of effects on plant growth and metabolism. Plants lacking SA exhibit enhanced susceptibility to pathogens. SA plays important signaling roles in resistance against biotrophic and hemi-biotrophic phytopathogens. It is synthesized in plastids along two pathways, one involving phenylalanine ammonia lyase (PAL) and the other isochorismate synthase (ICS). In Arabidopsis, during immune response most SA is synthesized through the ICS-dependent pathway, but clearly an ICS-independent pathway also exists. Several SA effector proteins have been identified and characterized which mediate downstream SA signaling. This includes SABP, a catalase, SABP2, a methyl salicylate esterase, SABP3, a carbonic anhydrase, NPR1 (non-expressor of pathogenesis-related 1), NPR3 (a NPR1 paralog), and NPR4 (another NPR1 paralog). NPR3 and NPR4 regulate the turnover of NPR1, a process which plays a key role in activating defense gene expression. The role of SA in abiotic stress signaling is gradually becoming clearer. Various components of SA signaling in biotic stress also appear to impact abiotic stress signaling.