The glycerolipids that make up the thylakoid bilayer contain exceptionally high levels of polyunsaturated fatty acids. These fatty acids are very susceptible to oxidation, and the activated oxygen species generated as biproducts of photosynthesis will accelerate the initiation of peroxidation. Fortunately, the chloroplast is well protected from damage caused by fatty acid oxidation (as well as other oxidation reactions) by several antioxi-dant systems. Despite these systems, chemical peroxidation does occur --- particularly after tissue damage from wounding or pathogen infection. The oxylipin compounds produced, including reactive electrophile species (RES), contribute to the induction of defense-gene expression and also act directly in defense against insects and microbes. Plants have evolved enzymatic pathways to facilitate the synthesis of particular oxylipin products, including several that are not synthesized by the chemical peroxidation reactions. The best known of these is the defense hormone, jasmonate, which acts through a specific signaling pathway to regulate plant responses. Jasmonate has additional roles in plant development and metabolic regulation. The recent discovery of a family of repressor proteins, the JAZ proteins that are targets of jasmonate signaling provide new tools to understand the mechanism of jasmonate action.
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