All That JAZ

The intoxicating scent of jasmine depends in part on jasmonates, fatty acid-derived hormones implicated in plant defense and development. Jasmonate signaling in plants depends on COI1 (CORONATINE INSENSITIVE1), which is part of the Skp-Cullin-F-box complex SCF COI1 , an E 3 ubiquitin ligase that ubiquitinates proteins and targets them for destruction (see Farmer). Two groups, Thines et al . and Chini et al ., have elucidated the pathway linking jasmonates to SCF COI1 and the mechanism through which proteolytic degradation of ubiquitinated proteins leads to activation of jasmonate target genes. Thines et al . found that eight genes of unknown function, each containing a ZIM (ZO-1 interaction motif) domain, were transcriptionally activated in an Arabidopsis jasmonate-synthesis mutant shortly after treatment with exogenous jasmonate. Jasmonate transcriptionally activated these JAZ (for JASMONATE-ZIM-DOMAIN ) genes in wild-type plants, and fluorescently labeled JAZ1 and JAZ6 fusion proteins localized to the nucleus. When expressed in wild-type plants, a C-terminal deletion mutant of JAZ1 (JAZ1Δ3A) elicited a phenotype similar to that of jasmonate-synthesis mutants or plants lacking COI1, suggesting that it blocked jasmonate signaling. Jasmonate stimulated the COI1- and proteasome-dependent degradation of a fusion protein encoding JAZ1 or JAZ6 linked to β-glucuronidase (JAZ1-GUS or JAZ6-GUS) but not that of JAZ1Δ3A-GUS. Yeast two-hybrid and in vitro pull-down assays indicated that jasmonyl-isoleucine stimulated the direct interaction of Arabidopsis JAZ1 and COI1 as well as that of related tomato proteins. Chini et al ., who previously isolated a jasmonate-insensitive Arabidopsis mutant, determined that the mutated gene ( JAI3 , for JASMONATE-INSENSITIVE 3 ) encoded a member of a family of ZIM-domain-containing proteins (JAZ proteins). Like Thines et al ., they showed that constitutive expression of a JAI3 mutant lacking the C-terminal domain ( JAI3ΔC ) rendered transgenic plants insensitive to jasmonate and that JAI3 interacted with COI1. Jasmonate stimulated the COI1- and proteasome-dependent degradation of a JAI3-green-fluorescent-protein (JAI3-GFP)-fusion protein, but not that of JAI3ΔC-GFP, and JAI3 degradation was impaired in a mutant unable to conjugate jasmonate with amino acids. JAI3 bound MYC2, a transcriptional activator downstream of jasmonate, and microarray analyses indicated that expression of MYC2 target genes was decreased in plants with a dominant-negative JAI3 mutant and that MYC2 regulated expression of JAZ genes. Thus, both studies indicate that jasmonyl-isoleucine stimulates the interaction of COI1 with JAZ-family transcriptional repressors, thereby enabling JAZ protein degradation and the transcriptional activation of jasmonate target genes (including the JAZ proteins themselves). A third study by Yan et al . also identified JAZ proteins as repressors of jasmonate signaling. B. Thines, L. Katsir, M. Melotto, Y. Niu, A. Mandaokar, G. Liu, K. Nomura, S. Y. He, G. A. Howe, J. Browse, JAZ repressor proteins are targets of the SCF COI1 complex during jasmonate signalling. Nature 448 , 661-665 (2007). [PubMed] A. Chini, S. Fonseca, G. Fernández, B. Adie, J. M. Chico, O. Lorenzo, G. García-Casado, I. López-Vidriero, F. M. Lozano, M. R. Ponce, J. L. Micol, R. Solano, The JAZ family of repressors is the missing link in jasmonate signalling. Nature 448 , 666-671 (2007). [PubMed] E. E. Farmer, Jasmonate perception machines. Nature 448 , 659-660 (2007). [PubMed] Y. Yan, S. Stolz, A. Chételat, P. Reymond, M. Pagni, L. Dubugnon, E. E. Farmer, A downstream mediator in the growth repression limb of the jasmonate pathway. Plant Cell , E-pub ahead of print, 3 August (2007). [PubMed]