Roles of plastid ε-3 fatty acid desaturases in defense response of higher plants

In higher plants, the membrane lipids contain a high proportion of trienoic fatty acids. It has been suggested that these fatty acids, especially linolenic acid, play an important role as a precursor to a defense-related signal molecule, jasmonate. In Arabidopsis, three genes encoding the ε-3 fatty acid desaturase, namely, FAD3, FAD7 and FAD8, are responsible for the production of trienoic fatty acids. TheFAD3 enzyme is localized in microsomes, while theFAD7 and theFAD8 enzymes are localized in plastid membranes. Environmental stimuli, such as wounding, salt stress and pathogen invasion, which lead to a rapid increase in jasmonate production, significantly induce expression of theFAD7 andFAD8 genes. Recent findings have supported the view that plastids are involved in jasmonate production. We have been able to clarify the regulatory mechanism of a plastid ε-3 desaturase gene by analyzing the ArabidopsisFAD7 promoter with respect to tissue-specific, light-responsive and wound-induced expression. In particular, this promoter provides a unique model for studying the mechanism of transcriptional activation through wound signal transduction pathways.