Plant Hormones

"F. THE ROLES OF HORMONES IN DEFENSE AGAINST INSECTS AND DISEASE (p. 646-647)

F1. Jasmonates

Gregg A. Howe
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA. E-mail: howeg@msu.edu

INTRODUCTION

Jasmonic acid (JA1) and its volatile methyl ester, MeJA, are fatty acidderived cyclopentanones that occur ubiquitously in the plant kingdom. Since the discovery of jasmonates (JAs) in plants over 40 years ago, our understanding of the biosynthesis and physiological function of these compounds has been marked by several major developments. Experiments performed in the 1980s elucidated the JA biosynthetic pathway and demonstrated that exogenous JAs exert effects on a wide range of physiological processes.

The discovery in the early 1990s that JAs act as potent signals for the expression of defensive proteinase inhibitors (PIs) aroused intense interest in the function of hormonally active JAs in plant defense. Research in the past decade has led to several key developments, including identification of genes encoding most of the JA biosynthetic enzymes and discovery of novel biologically active JAs. Identification of a large collection of JA biosynthesis and response mutants has provided important tools to assess the role of JAs in plant developmental and defenserelated processes.

The widespread occurrence of JAs in plants and some lower eukaryotes, together with their capacity to regulate physiological processes in animals (e.g., insects), reinforces the notion that JAs are of general biological interest. JASMONATE BIOSYNTHESIS Oxylipin Metabolism in Plants and Animals Jasmonates belong to the family of oxygenated fatty acid derivatives, collectively called oxylipins, which are produced via the oxidative metabolism of polyunsaturated fatty acids.

In animals, members of the eicosanoid (C20) group of lipid mediators are synthesized from arachidonic acid and function as regulators of cell differentiation, immune responses, and homeostasis. In plants, oxygenated compounds derived mainly from C18 α- linolenic acid (18:3) control a similarly broad spectrum of developmental and defense-related processes (4, 13, 19, 30, 47, 48). The biochemical logic underlying the synthesis of oxylipins in plants and animals is remarkably similar.

Species in both kingdoms utilize cytochromes P450, lipoxygenase (LOX), and cyclooxygenase or cyclooxygenase-like (e.g., plant α-dioxygenase) activities to oxidize polyunsaturated fatty acid substrates (3). The resulting oxygenated fatty acids are further metabolized by various enzymatic and non-enzymatic systems to an array of intermediates and end products. These compounds are typically synthesized de novo in specific cell types upon activation of lipases that release fatty acids from membrane lipids."