Oxylipin signaling in plant stress responses.

Oxidized fatty acids, termed oxylipins, are an important class of signaling molecule in plants, especially related to plant stress responses and innate immunity. The bestcharacterized oxylipins are jasmonic acid (JA) and its immediate precursor 12-oxophytodienoic acid (OPDA), which are formed enzymatically and accumulate in response to various stresses, in particular wounding and pathogen infection (Block et al., 2005). In addition, a number of biologically active oxylipins are formed nonenzymatically via the action of reactive oxygen species (ROS), which also accumulate in response to pathogen infection, heavy metal uptake, and other stresses. There is growing evidence that nonenzymatically formed oxylipins, including hydroxy fatty acids and phytoprostanes, play important signaling roles in plant stress responses (Sattler et al., 2006). Recent work with JA and OPDA has shown that both are active as signaling molecules and induce the expression of overlapping but distinct sets of genes. JA signaling leads to the interaction of the F-box ubiquitin ligase CORONATINE-INSENSITIVE1 (COI1) with JAZ transcriptional repressors, mediating degradation of these repressors of downstream JA-induced genes, many of which are dependent on the key transcription factor MYC2/JIN1 (Chini et al., 2007; reviewed in Santner and Estelle, 2007). Whereas JA induces a set of COI1-dependent genes, OPDA has been found to induce a set of largely COI1independent genes (Stintzi et al., 2001; Taki et al., 2005). Phytoprostanes have also been shown to activate the expression of stress response genes, leading to enhanced protection from subsequent oxidative stress (Thoma et al., 2003; Loeffler et al., 2005), but little is known about phytoprostane signal transduction. Phytoprostanes are grouped into a number of classes, depending on the ring and side chain structures, and plants are known to produce a number of different types (Thoma et al., 2004). The A1-type (PPA1) and the deoxy-J1-type phytoprostanes, as well as OPDA, belong to a subgroup of oxylipins known as reactive electrophile species (RES) because they contain a reactive a,b-unsaturated carbonyl structure that may contribute to their biological activity. RES, for example, have the ability to modify proteins directly by binding to free thiol groups, a property that is not shared by weak electrophilic oxylipins, including JA and the B1-type phytoprostanes (PPB1). It has been suggested that RES, including those produced nonenzymatically, induce a common set of defense genes (Almeras et al., 2003; Weber et al., 2004; Farmer and Davoine, 2007). In this issue of The Plant Cell, Mueller et al. (pages 768–785) examine the effects of OPDA and phytoprostanes on gene expression and plant growth and development in Arabidopsis and show that these oxylipins share similar biological activity that appears to differ considerably from that of JA. In addition, phytoprostanes might have unique activities and targets not shared by OPDA or JA. The authors employ whole-genome microarray analysis, liquid chromatography–tandem mass spectrometry analysis, and enzymatic assays to investigate the biological activity of phytoprostanes in relation to OPDA and JA. Differences in gene induction between JA and OPDA/phytoprostanes suggest the existence of multiple oxylipin signal transduction pathways. Interestingly, many of the genes induced by phytoprostanes and OPDA were found to be dependent on the TGA transcription factors TGA2, TGA5, and TGA6, demonstrating that these factors play an important role in oxylipin signaling (see figure). In addition, it was shown that some of the genes induced by OPDA and phytoprostanes encode enzymes involved in their metabolism (conjugation and/or detoxification). This work shows that nonenzymatically produced oxylipins likely constitute an important component of oxylipin signaling in plant stress responses. PHYTOPROSTANES AND OPDA HAVE OVERLAPPING AND INDEPENDENT FUNCTIONS IN ARABIDOPSIS