Generating and maintaining jasmonic acid in Arabidopsis

Jasmonic acid (JA) is a lipid-derived plant hormone that mediates diverse biological phenomena. It is one of major goals in JA research field to elucidate the regulatory mechanism of JA level. Recently we have demonstrated cooperative and differentiated roles of two chloroplast localized galactolipases, DGL (DONGLE) and DAD1 (DEFECTIVE IN ANTHER DEHISCENCE 1), for the regulation of JA content. The DGL maintains a basal level of JA in unwounded vegetative tissues, while the DAD1 is involved in JA production in floral tissues. The JA in vegetative tissues regulates cell expansion while the JA produced in flowers regulates pollen maturation. After wounding, the cooperative function of DGL and DAD1 causes drastic increase of JA. The analysis of induction kinetics showed that the two enzymes have temporally separated roles in wound response; DGL in early phase and DAD1 in late phase of JA production. In this addendum, we discuss the implications of our recent findings and extend our working model for JA homeostasis in plants.Key words: jasmonic acid, dongle, defective in anther dehiscence 1, galactolipase, phospholipase, plant growth, wound responseJasmonic acid (JA) and its derivatives, collectively referred to as jasmonates, are lipid-derived plant hormones that are ubiquitous in plant kingdom. These compounds play pivotal roles in diverse plant biological processes, such as seed maturation, viable pollen production, root growth, tendril coiling and defense response to biotic and abiotic stresses.¹ Biosynthesis of JA is known to be carried out in two sub-cellular organelles, chloroplast and peroxisome, and enzymes involved in this biosynthetic pathway have been characterized by various studies.² However, initiation and triggering of JA biosynthesis are long-lasted open questions in JA research field. Through the characterization of activation tagging mutant dongle-D (dgl-D), we have firstly demonstrated that chloroplast localized galactolipase DGL catalyzes an initial step of JA biosynthesis in Arabidopsis.³ The dgl-D, DGL overexpressor mutant, showed dwarf phenotype caused by ectopic increase of JA, and the mutant also exhibited constitutive expression of JA responsive genes and increased resistance to fungal pathogen A. brassicicola. While database analysis revealed that DGL shows structural similarity with DEFECTIVE IN ANTHER DEHISCENCE1 (DAD1), a previously reported JA biosynthetic phospholipase A₁, DGL and DAD1 exhibit different spatial expression patterns in normal unwounded condition. As a result, the basal level of JA in leaves is regulated by DGL whereas the JA in flowers is regulated by DAD1. Consistently, the RNAi induced knock-down allele, dgl-i, showed decreased JA level in leaves and larger leaf cell size, suggesting that the specific role of DGL is to regulate vegetative organ growth via maintenance of endogenous JA level in vivo. On the other hand, JA in flowers maintained by DAD1 is shown to synchronize pollen maturation, anther dehiscence and flower opening in Arabidopsis.⁴