Jasmonoyl-isoleucine (JA-Ile), the active form of the plant hormone jasmonate (JA), is sensed by the F-box protein CORONATINE INSENSITIVE 1 (COI1), a component of a functional Skp-Cullin-F-box E3 ubiquitin ligase complex. Sensing of JA-Ile by COI1 rapidly triggers genome-wide transcriptional changes that are largely regulated by the basic helix-loop-helix transcription factor MYC2. However, it remains unclear how the JA-Ile receptor protein COI1 relays hormone-specific regulatory signals to the RNA polymerase II general transcriptional machinery. Here, we report that the plant transcriptional coactivator complex Mediator directly links COI1 to the promoters of MYC2 target genes. MED25, a subunit of the Mediator complex, brings COI1 to MYC2 target promoters and facilitates COI1-dependent degradation of jasmonate-ZIM domain (JAZ) transcriptional repressors. MED25 and COI1 influence each other's enrichment on MYC2 target promoters. Furthermore, MED25 physically and functionally interacts with HISTONE ACETYLTRANSFERASE1 (HAC1), which plays an important role in JA signaling by selectively regulating histone (H) 3 lysine (K) 9 (H3K9) acetylation of MYC2 target promoters. Moreover, the enrichment and function of HAC1 on MYC2 target promoters depend on COI1 and MED25. Therefore, the MED25 interface of Mediator links COI1 with HAC1-dependent H3K9 acetylation to activate MYC2-regulated transcription of JAresponsive genes. This study exemplifies how a single Mediator subunit integrates the actions of both genetic and epigenetic regulators into a concerted transcriptional program.is an oxylipin-derived plant hormone that regulates diverse aspects of plant immunity and development (1, 2). Decades of studies in the model plant Arabidopsis thaliana have revealed a core JA signaling module consisting of the F-box protein CORONATINE INSENSITIVE 1 (COI1) (3), a group of jasmonate-ZIM domain (JAZ) proteins (4-6), and the basic helixloop-helix transcription factor MYC2 (7, 8). COI1 forms a functional Skp-Cullin-F-box (SCF) E3 ubiquitin ligase SCF COI1 along with Cullin1 and Skp1-like1 (ASK1) (9, 10), MYC2 acts as a master transcription factor that differentially regulates diverse aspects of JA responses (11-13), and the JAZ proteins are substrates of SCF COI1 and serve as transcriptional repressors of MYC2 (4, 5, 14). The identification of jasmonoyl-isoleucine (JA-Ile) as the receptor-active form of the hormone, along with the discovery that sensing of JA-Ile involves formation of the SCF COI1 -JAZs coreceptor complex (4, 15-17), represented a breakthrough in our mechanistic understanding of JA signaling. In the absence of the hormone, JAZ repressors interact with and repress the activity of MYC2. In response to internal or external cues that trigger JAIle synthesis, elevated JA-Ile levels promote SCF COI1 -dependent degradation of JAZ repressors, and thereby activate (de-repress) the MYC2-directed transcriptional program. These discoveries imply that sensing of the active hormone is tightly linked to transcription of JA-responsive...

Groucho/Thymidine uptake 1 (Gro/Tup1) family proteins are evolutionarily conserved transcriptional coregulators in eukaryotic cells. Despite their prominent function in transcriptional repression, little is known about their role in transcriptional activation and the underlying mechanism. Here, we report that the plant Gro/Tup1 family protein LEUNIG_HOMOLOG (LUH) activates MYELOCYTOMATOSIS2 (MYC2)-directed transcription of JAZ2 and LOX2 via the Mediator complex coactivator and the histone acetyltransferase HAC1. We show that the Mediator subunit MED25 physically recruits LUH to MYC2 target promoters that then links MYC2 with HAC1-dependent acetylation of Lys-9 of histone H3 (H3K9ac) to activate JAZ2 and LOX2. Moreover, LUH promotes hormone-dependent enhancement of protein interactions between MYC2 and its coactivators MED25 and HAC1. Our results demonstrate that LUH interacts with MED25 and HAC1 through its distinct domains, thus imposing a selective advantage by acting as a scaffold for MYC2 activation. Therefore, the function of LUH in regulating jasmonate signaling is distinct from the function of TOPLESS, another member of the Gro/Tup1 family that represses MYC2-dependent gene expression in the resting stage.