Cyclopentenone signals for plant defense: Remodeling the jasmonic acid response

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Abstract
Compounds derived from the oxidative metabolism of polyunsaturated fatty acids are synthesized in response to external stimuli and serve important roles in controlling diverse processes in both plants and animals. Members of the eicosanoid family of lipid mediators have been studied extensively with respect to their biosynthesis from C20 fatty acids and their function in the regulation of cell differentiation, immune responses, and homeostasis in animal systems (1). In plants, oxygenated derivatives of C18 and C16 fatty acids participate in the regulation of many defense-related and developmental processes. Research on fatty acid-based signaling systems in plants has focused mainly on the hormonally active compound, jasmonic acid (JA). A rapidly growing body of literature indicates that plant defense responses against insect herbivores (Fig. 1) and some microbial pathogens are orchestrated by signaling pathways involving the biosynthesis and subsequent action of JA. In this issue of PNAS, Stintzi et al. (2) used an elegant biochemical genetic approach to determine whether the cyclopentenone precursor of JA, 12-oxo-phytodienoic acid (OPDA), is also a physiological signal for defense. Their demonstration that OPDA confers broad-spectrum resistance in the absence of JA marks a major advance in our understanding of jasmonate-signaled responses. Jasmonic acid and related compounds regulate plant defense responses against hostile invaders. This photograph shows a tobacco hornworm (Manduca sexta) attacking a tomato plant. JA is a terminal product of the octadecanoid pathway (ref. 3; Fig. 2). This series of reactions is initiated by lipoxygenase, which adds molecular oxygen to linolenic acid. The resulting 13-hydroperoxide is converted in the chloroplast to a specific stereoisomer of OPDA (9S,13S-OPDA) by the sequential action of allene oxide synthase and allene oxide cyclase. The next step in the pathway involves reduction of the cyclopentenone ring of OPDA by OPDA reductase (OPR). …

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