Enzymes of Octadecanoid Biosynthesis in Plants

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Abstract
Octadecanoids, potent cyclic plant signaling molecules derived from α‐linolenic acid, are involved in the regulation of a multitude of physiological processes such as senescence, herbivore and pathogen defense, mechanoperception and morphogenesis. The first cyclic intermediate in the Vick‐Zimmerman pathway of octadecanoid biosynthesis is 12‐oxo‐phytodienoic acid. Its conversion to the end product of the pathway, jasmonic acid, a C12 compound, first proceeds through reduction to 3‐oxo‐2‐(pent‐2′‐enyl)‐cyclopentane‐1‐octanoic acid, which is then converted to jasmonic acid by three cycles of β‐oxidation. The first of these conversions is a decisive point in the biosynthetic sequence, in that it channels the octadecanoid into the pathway of β‐oxidation. 12‐Oxo‐phytodienoate reductase was purified to apparent homogeneity from a cell suspension culture of Corydalis sempervirens. The enzyme is soluble and a monomer of apparent molecular mass 41 kDa which prefers NADPH over NADH to reduce the 10,11‐double bond of 12‐oxo‐phytodienoic acid. The structure of the reaction product was proved by derivatization, GC/MS and NMR analysis. The enzyme accepts both the cis and the trans isomer of 12‐oxo‐phydodienoic acid, with a preference for the cis‐isomer (6:1). 12‐Oxo‐phytodienoate reductase will also convert the synthetic substrate 2‐cyclohexenone to cyclohexanone, but the enzyme did not reduce some other cyclic α,β‐unsaturated ketones tested (the plant hormone abscisic acid or the steroids testosterone and progesterone). Characteristic parameters of 12‐oxo‐phytodienoate reductase were determined.