A fatty acid desaturase modulates the activation of defense signaling pathways in plants

Salicylic acid (SA) plays an important rote in activating various plant def ense responses, including expression of the pathogenesis-related (PR) genes and systemic acquired resistance. A critical positive regulator of the SA signaling pathway in Arabidopsis is encoded by the NPR1 gene. However, ther e is growing evidence that NPR1-independent pathways can also activate PR e xpression and disease resistance. To elucidate the components associated wi th NPR1-independent defense signaling, we isolated a suppresser of the npr1 -5 allele, designated ssi2. The recessive ssi2 mutation confers constitutiv e PR gene expression, spontaneous lesion formation, and enhanced resistance to Peronospora parasitica. In contrast, a subset of defense responses regu lated by the jasmonic acid (JA) signaling pathway, including expression of the defensin gene PDF1.2 and resistance to Botrytis cinerea, is impaired in ssi2 plants. With the use of a map-based approach, the SSI2 gene was Boned and shown to encode a stearoyl-ACP desaturase (S-ACP DES). S-ACP DES is an archetypical member of a family of soluble fatty acid (FA) desaturases; th ese enzymes play an important role in regulating the overall level of desat urated FAs in the cell. The activity of mutant S-ACP DES enzyme was reduced 10-fold, resulting in elevation of the 18:0 FA content in ssi2 plants. Bec ause reduced S-ACP DES activity leads to the induction of certain defense r esponses and the inhibition of others, we propose that a FA-derived signal modulates crosstalk between different defense signaling pathways.