JA, its structurally related metabolites, and the JA precursor
Mutants of Arabidopsis
thaliana (Arabidopsis) defective in JA biosynthesis
or signaling revealed important roles of JAs in
defense against nectrotrophic pathogens and herbivorous
insects (Browse 2009; Van der Ent et al. 2009b). ALLENE
OXIDE SYNTHASE (AOS) emerged as a key enzyme in
the JA biosynthesis pathway, since mutation of the single
AOS gene in Arabidopsis leads to a complete elimination of
JA production (Park et al. 2002; Von Malek et al. 2002).
Upon synthesis, JA can be readily metabolized to the
volatile methyl jasmonate (MeJA) through the activity of
JA carboxyl methyltransferase (JMT) (Seo et al. 2001).
In addition JA can be conjugated to amino acids such as
isoleucine via the activity of the JA conjugate synthase
JAR1 (Staswick and Tiryaki 2004), resulting in the biologically
highly active form (?)-7-iso-jasmonoyl-L-isoleucine
(JA-Ile) (Fonseca et al. 2009). The F-box protein
COI1 (CORONATINE INSENSITIVE 1 ) is a key regulator
of the JA signaling pathway (Chung et al. 2009).
It functions as a JA-Ile receptor in the E3 ubiquitin-ligase
Skip-Cullin-F-box complex SCFCOI1 (Yan et al. 2009).
Binding of JA-Ile to COI1 leads to degradation of JASMONATE
ZIM-domain (JAZ) transcriptional repressor
proteins via the proteasome (Chini et al. 2007; Thines et al.
2007). In resting cells, JAZ proteins act as transcriptional
repressors of JA signaling by binding to positive transcriptional
regulators, such as MYC2 (Chini et al. 2007).
Hence, JAZ protein degradation results in de-repression of
the JA signaling pathway and the activation of a large
number of JA-responsive genes (Memelink 2009).