Jasmonates (JAs) trigger an important transcriptional reprogramming of plant cells to modulate both basal development and stress responses. In spite of the importance of transcriptional regulation, only one transcription factor (TF), the Arabidopsis thaliana basic helix-loop-helix MYC2, has been described so far as a direct target of JAZ repressors. By means of yeast two-hybrid screening and tandem affinity purification strategies, we identified two previously unknown targets of JAZ repressors, the TFs MYC3 and MYC4, phylogenetically closely related to MYC2. We show that MYC3 and MYC4 interact in vitro and in vivo with JAZ repressors and also form homo- and heterodimers with MYC2 and among themselves. They both are nuclear proteins that bind DNA with sequence specificity similar to that of MYC2. Loss-of-function mutations in any of these two TFs impair full responsiveness to JA and enhance the JA insensitivity of myc2 mutants. Moreover, the triple mutant myc2 myc3 myc4 is as impaired as coi1-1 in the activation of several, but not all, JA-mediated responses such as the defense against bacterial pathogens and insect herbivory. Our results show that MYC3 and MYC4 are activators of JA-regulated programs that act additively with MYC2 to regulate specifically different subsets of the JA-dependent transcriptional response.

The plant hormone jasmonic acid (JA) regulates growth, development and defence responses against pathogens. Recently, a family of proteins called JAZ that repress JA responses were identified, but the mechanism by which they repress gene expression was unknown. Pauwels et al. identify an adaptor protein, designated Novel Interactor of JAZ (NINJA), that recruits co-repressor proteins, TOPLESS (TPL) and TPL-related proteins (TPRs), which are known to mediate auxin-responsive gene expression. These results suggest that TPL and related proteins are part of general repression complexes, which are recruited to multiple signalling pathways. In plants, the hormone jasmonoyl-isoleucine (JA-Ile) regulates growth, development and defence against pathogens. Proteins of the JAZ family repress JA-Ile-dependent gene expression, but the mechanism has been unclear. Here, an adaptor protein, NINJA, has been identified, which recruits co-repressor proteins that are known to mediate auxin-responsive gene expression as well. Hence these co-repressors are part of general repression complexes that are recruited to several different signalling pathways. Jasmonoyl-isoleucine (JA-Ile) is a plant hormone that regulates a broad array of plant defence and developmental processes1,2,3,4,5. JA-Ile-responsive gene expression is regulated by the transcriptional activator MYC2 that interacts physically with the jasmonate ZIM-domain (JAZ) repressor proteins. On perception of JA-Ile, JAZ proteins are degraded and JA-Ile-dependent gene expression is activated6,7. The molecular mechanisms by which JAZ proteins repress gene expression remain unknown. Here we show that the Arabidopsis JAZ proteins recruit the Groucho/Tup1-type co-repressor TOPLESS (TPL)8 and TPL-related proteins (TPRs) through a previously uncharacterized adaptor protein, designated Novel Interactor of JAZ (NINJA). NINJA acts as a transcriptional repressor whose activity is mediated by a functional TPL-binding EAR repression motif. Accordingly, both NINJA and TPL proteins function as negative regulators of jasmonate responses. Our results point to TPL proteins as general co-repressors that affect multiple signalling pathways through the interaction with specific adaptor proteins. This new insight reveals how stress-related and growth-related signalling cascades use common molecular mechanisms to regulate gene expression in plants.

Mix of metabolites tunes root microbiota: Uncharacterized biosynthetic genes in plant genomes suggest that plants make a plethora of specialized metabolites. Huang et al. reconstructed three biosynthetic networks from the small mustard plant Arabidopsis thaliana . Promiscuous acyltransferases and dehydrogenases contributed to metabolite diversification. The plant may use these specialized metabolites to modulate the microbiota surrounding its roots. Disruption of the pathways and intervention with purified compounds caused changes in the root microbiota. Science , this issue p. eaau6389

Jasmonates are phytohormones that regulate many aspects of plant growth, development, and defense. Within the signaling cascades that are triggered by jasmonates, the JASMONATE-ZIM DOMAIN (JAZ) repressor proteins play a central role. The endogenous bioactive JA-Ile conjugate mediates the binding of JAZ proteins to the F-box protein CORONATINE INSENSITIVE1 (COI1), part of the Skp1/Cullin/F-box SCF(COI1) ubiquitin E3 ligase complex. Upon the subsequent destruction of the JAZ proteins by the 26S proteasome, multiple transcription factors are relieved from JAZ-mediated repression, allowing them to activate their respective downstream responses. However, many questions remain regarding the targets, specificity, function, and regulation of the different JAZ proteins. Here, we review recent studies on the model plant Arabidopsis thaliana that provided essential and novel insights. JAZ proteins have been demonstrated to interact with a broad array of transcription factors that each control specific downstream processes. Recruitment of the corepressor TOPLESS unveiled a mechanism for JAZ-mediated gene repression. Finally, the presence of JAZ proteins was also found to be regulated by alternative splicing and interactions with proteins from other hormonal signaling pathways. Overall, these contemporary findings underscore the value of protein-protein interaction studies to acquire fundamental insight into molecular signaling pathways.