Characterization of key transcriptional regulators in jasmonate signaling

Jasmonates (JAs) are a group of oxylipin phytohormones that regulate plant defense and developmental processes. A JA-insensitive mutant coronatine insensitive 1 (coi1) has been the focus of intensive research efforts for more than a decade. COI1 encodes an F-box protein, a component of SCFCOI1 E3 ubiquitin ligase and is required for all or most JA responses, suggesting that protein degradation mediated by the SCFCOI1 complex plays a central role in JA responses. These key components, targets of SCFCOI1, were discovered in our transcription profiling experiments on stamens of the JA biosynthetic mutant opr3; they are the JASMONATE ZIM-DOMAIN (JAZ) proteins. Experiments with plants expressing JAZ-GUS fusion proteins demonstrate that these proteins are destabilized by SCFCOI1 in response to JA. Deletion of a C-terminal conserved domain, the Jas domain, stabilized JAZ proteins and conferred a JA-insensitive phenotype. These results indicate that JAZ proteins are repressors targeted by SCFCOI1 and that the Jas domain is critical for their function. Since JAZ proteins do not contain any recognizable DNA binding domain, they are hypothesized to regulate gene expression through interaction with DNA-binding transcription factors. The basic helixloop helix (bHLH) protein, MYC2, is the only transcription factor known to directly interact with JAZ proteins and regulate JA responses. Using a yeast two-hybrid screen with JAZ1 protein as the bait, I identified two bHLH transcription factors, MYC3 and MYC4, which share high sequence similarity with MYC2. Overexpression of these two transcription factors induced expression of JA-responsive genes and caused anthocyanin accumulation, one of the JA responses in Arabidopsis. Furthermore, MYC3 overexpression plants were hypersensitive to JA in terms of root growth inhibition. Taken together, these data suggest that MYC3 and MYC4 are additional JAZ-regulated transcriptional activators in JA signaling. JAZ-interacting protein 1(JIP1) was also identified in the two-hybrid screen. JIP1 possesses repression activity to reporter gene expression in transient expression assays, suggesting that it is a candidate corepressor for facilitating JAZ repression. Characterizing these JAZ-interacting proteins will help to understand the molecular mechanism by which JAZs repress the expression of JA-responsive genes and refine the current model of JA signaling