New weapons and a rapid response against insect attack.

Jasmonates (JAs) comprise a class of related oxylipin signaling molecules that have overlapping roles in regulating both stress responses and development in plants. Stress responses that depend on JA signaling include not only defense against insects but also defense against microbial pathogens, as well as responses to UV radiation, ozone, and other abiotic stresses (Glazebrook, 2005; Wasternack, 2007; Balbi and Devoto, 2008; Howe and Jander, 2008). In healthy, nonstressed plant tissue, JAs are involved in carbon partitioning, mechanotransduction, senescence, and reproductive development (Browse, 2005). The JA-dependent responses are associated with large-scale reprogramming of gene expression; hundreds of downstream JA-regulated and JA-coregulated genes have been identified (Reymond et al., 2000; Schenk et al., 2000; Devoto et al., 2005; Mandaokar et al., 2006). A substantial proportion of our knowledge about the synthesis and function of JAs has come from studies of plants that are defective in the biosynthesis of the hormone. These include mutants blocked in synthesis of linolenic acid (18:3), the fatty acid precursor of JA (McConn and Browse, 1996; Li et al., 2003) and mutants deficient in enzymes of the JA synthesis pathway (Schaller et al., 2005). These mutants have been particularly useful for investigating the role of JA in regulating the outcome of plant-insect interactions because application of exogenous JA typically restores wild-type function (McConn et al., 1997; Halitschke and Baldwin, 2003; Schilmiller et al., 2007). Other mutants are defective in JA perception and have also been instrumental for the study of induced resistance to herbivory (Stintzi et al., 2001; Li et al., 2004; Reymond et al., 2004; Chen et al., 2005; Mewis et al., 2005; Paschold et al., 2007). The coi1 mutant of Arabidopsis (Arabidopsis thaliana) was isolated on the basis of its resistance to the phytotoxin coronatine (Fig. 1), which induces many of the same responses as JA in plants (Feys et al., 1994). The contributions of coi1 and other mutants to understanding the mechanism of JA signaling are discussed below. The ability of methyl-JA (MeJA) to induce antiinsect proteinase inhibitors (PIs) in tomato (Solanum lycopersicum) was first reported in 1990 (Farmer and Ryan, 1990); articles describing the induction of some pathogen-defense genes followed (Glazebrook, 2005). The critical requirement for JA to induce defense against insects was established through investigations of JA synthesis mutants in tomato (Howe et al., 1996; Li et al., 2005) and Arabidopsis (McConn et al., 1997). Similarly, mutant analysis was central to establishing the importance of JA in defense against pathogens— particularly necrotrophic fungi and oomycetes (Vijayan et al., 1998; Glazebrook, 2005)—and to the recognition that JA is essential during the final stages of flower development in Arabidopsis (McConn and Browse, 1996) and tomato (Li et al., 2004). In this Update, we will focus on the role of JAs in plant defense against arthropod herbivores, but the other processes controlled by the hormone will be discussed to the extent that they have provided new insight into the molecular mechanism of JA signaling.