These results indicate that additional transcription factors are also controlled by JA hormone signaling. Recently, JAM1 (AIB, bHLH17) (Nakata et al., 2013) and other bHLH proteins of the IIId subfamily (Song et al., 2013) have been identified as JAZ-interacting proteins that act as negative regulators of JA signaling by (weakly) competing with MYC2 and other positive effectors for their binding sites in the promoters of JA-response genes. JAZ proteins also bind to MYB21 and MYB24, two transcription factors encoded by genes strongly induced by JA in flower tissues that control stamen and pollen maturation in Arabidopsis (Mandaokar et al., 2006 Song et al., 2011). The JAZ proteins are also able to interact with other bHLH and R2R3 MYB transcription factors, including TT8, G元, EG元, MYB75 and GL1, to regulate JA-mediated anthocyanin accumulation and trichome initiation (Qi et al., 2011). Interestingly, MYC2, MYC3 and MYC4 all induce the expression of several of the 12 known JAZ genes in Arabidopsis, suggesting that rapid resynthesis of JAZ proteins is one mechanism for attenuating the JA signal following its initiation (Chini et al., 2007 Thines et al., 2007 Figueroa and Browse, 2012). Degradation of the JAZ repressors allows the MYC2, MYC3 and MYC4 transcription factors to induce expression of the early genes of JA responses (Lorenzo et al., 2004 Dombrecht et al., 2007 Fernandez-Calvo et al., 2011). In the presence of JA-Ile, JAZ proteins are recruited to the Skip/Cullin/F-box ubiquitin ligase, SCF COI1, and are targeted for degradation in the 26S proteasome (Chini et al., 2007 Thines et al., 2007 Sheard et al., 2010). The COI1 F-box protein acts as a co-receptor with a JAZ protein for the active form of the JA hormone, JA-Ile. JA signaling operates through Jasmonate-ZIM domain proteins (JAZ) that bind to and inhibit the activities of transcription factors, most notably MYC2, MYC3 and MYC4, which are basic helix-loop-helix (bHLH) proteins in the IIIe subfamily that induce the expression of JA-responsive genes (Lorenzo et al., 2004 Dombrecht et al., 2007 Cheng et al., 2011 Fernandez-Calvo et al., 2011 Niu et al., 2011). Jasmonate (JA) is a lipid-derived hormone that induces and controls defense against insects and pathogens, and also has roles in plant growth, development and fertility (Browse, 2009 Kazan and Manners, 2011). Taken together, these results indicate that MYC5, probably together with other, redundant transcription factors, may be activated by JA signaling to induce the expression of MYB21 and components required for male fertility. Importantly, expression of MYB21 and other transcription factors required for stamen and pollen maturation was strongly reduced in stamens of MYC5-SRDX plants relative to the wild type. In particular, MYC5-SRDX plants were male-sterile, with defects in stamen filament elongation, anther dehiscence and pollen viability. Two allelic myc5 mutants exhibited no overt phenotype however, transgenic lines expressing MYC5 fused to an SRDX (SUPERMAN repressive domain X) motif phenocopied mutants defective in JA signaling. A G-box sequence in the JAZ2 promoter was necessary and sufficient for induction by MYC5 (as it is for MYC2, MYC3 and MYC4), and induction of JAZ genes was repressed by co-expression of a stabilized, JAZ1ΔJas repressor. We found that a closely related transcription factor, MYC5 (bHLH28), was able to induce JAZ promoters that control some of the early JA-responsive genes in a Daucus carota (carrot) protoplast expression system. MYC2, MYC3 and MYC4 are JAZ-interacting bHLH transcription factors that play a major role in controlling JA responses in vegetative tissue, but are not likely to play a role in reproductive tissue. Arabidopsis thaliana plants deficient in JA-biosynthesis or -signaling are male-sterile, with defects in stamen and pollen development. Jasmonate hormone (JA) plays critical roles in both plant defense and reproductive development.
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