NINJA connects the co-repressor TOPLESS to jasmonate signalling

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Abstract
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.