Jasmonate Biosynthesis in Arabidopsis
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Jasmonate Biosynthesis in Arabidopsis
JAs (Jasmonates) are potent Lipid Regulators of Defense gene expression and act in development where they are necessary for fertility. The Jasmonate Pathway performs critical roles in Plant Defense, Development, and Metabolism, and it has received its name from MJ (Methyl Jasmonate), a fragrance from scented Jasmine flowers long used in the perfume industry. JA is derived from the Unsaturated Fatty Acid LA (Linolenic Acid) (18:3), an Octadecanoid that is highly abundant in membranes of Higher Plants. Apparently JAs can also be biosynthesized from Hexadecatrienoic Acid (16:3) in many plants, including Arabidopsis. A Phospholipase-A, DAD1 (Defective Anther Dehiscence-1) is responsible for release of LA from membrane lipids. DAD1 has an N-terminal Chloroplast transit peptide, and accumulates in Chloroplasts. DAD1 is required for JA biosynthesis in Stamen development. Another Chloroplast-localized, PLDA (Phospholipase-D-Alpha) is required for Wound-induced JA formation in Arabidopsis (Ref.1).

The formation of JA is complex and requires the Oxygenation of LA (18:3) and Hexadecatrienoic Acid (16:3) to form 13-HPOTrE (13-Hydroperoxylinolenic acid) and 11-HPHTrE (11(S)-Hydroperoxy-7(Z),9(E),13(Z)-Hexadecatrienoic Acid) respectively. Loxs (Lipoxygenases) catalyze the oxygenation of fatty acids to their hydroperoxy derivatives. Those involved in JA biosynthesis include a 13-Lox that produces 13-HPOTrE (13-Hydroperoxy-Octadecatrienoic Acid), a substrate for several enzymes. An Arabidopsis Stroma-localized plastid 13-Lox2 is required for wound-induced JA formation, but is not required for JA-dependent pollen and stamen development. 13-HPOTrE and 11-HPHTrE are then dehydrated by AOS (Allene Oxide Synthase) to form 12,13-EOTr (Linolenic Acid 12-Allene Oxide) and 10,11-EHTr (Hexadecenoic Acid 11-Allene Oxide) respectively, which are then cyclized by AOC (Allene Oxide Cyclase) to form the OPDA (Cyclopentenone 12-Oxophytodienoic Acid, 18 carbons) and its 16-carbon homolog dnOPDA (dinor-Oxo-Phytodienoic Acid) (Ref.2). The biosynthesis of OPDA from LA occurs in the Chloroplast, which contains an abundance of LA esterified in Glycerolipids and Phospholipids. Arabidopsis OPR3 (OPDA Reductase-3) catalyses the reduction of OPDA to OPC-8:0 (3-Oxo-2-(2(Z)-Pentenyl)-Cyclopentane-1 Octanoic Acid). Although Arabidopsis contains at least two other OPR genes, named OPR1 and OPR2, and the transcription of these is wound induced, their protein products do not catalyze the reduction of OPDA (Ref.3). OPC-8:0 undergoes three rounds of Beta-Oxidation to form JA via OPC-6:0 and OPC-4:0. JA itself can then be conjugated to Amino Acids, Methylated to form the volatile derivative MJ, or metabolized to a second volatile, Z-Jasmone. MJ is the methyl ester of JA, a twelve-Carbon Fatty Acid derivative. The Methylation of JA to MJ is catalyzed by an S-Adenosyl-L-Methionine: JMT (JA Carboxyl Methyltransferase) from Arabidopsis. Another route of JA modification is hydroxylation and subsequent sulfonation. Finally, several Phytoprostanes are Cyclopentenones that have OPDA-like structures and there is no evidence that they function as JAs. Gene products for Beta-Oxidation are targeted to the Peroxisome, and gene products that modify JA are presumably Cytoplasmic. The genes for JA biosynthesis are induced at the site of JA formation. Developmentally regulated JA biosynthesis in Arabidopsis is controlled through activation of a JA biosynthetic pathway that differs from, but overlaps with, the biosynthetic pathway that regulates wound-induced JA biosynthesis. Microarray analysis reveals that five out of 41 genes responding to JA are JA biosynthesis genes, indicating the existence of a positive feedback regulatory system for JA biosynthesis. This confirms the findings that JAs induce transcription of DAD1, Lox2, AOS, OPR3, and JMT. Significantly, Wounding and other stresses that elicit JA responses also induce these transcripts. Moreover, transcriptional activation of these genes occurs at the site of JA biosynthesis. JAs therefore appear to be synthesized locally in response to Stress Cues and Developmental Cues, and the products of this pathway provide a feedback loop for amplification of the signal (Ref.4 and Ref.5).