Class I TCP Transcription Factors TCP14 and TCP15 Promote Axillary Branching in Arabidopsis by Counteracting the Action of Class II TCP BRANCHED1

Shoot branching is determined by a balance between factors that promote axillary bud dormancy and factors that release buds from the quiescent state. The TCP family of transcription factors is classified into two classes, Class I and Class II, which usually play different roles. While the role of the Class II TCP BRANCHED1 (BRC1) in suppressing axillary bud development in Arabidopsis thaliana has been widely explored, the function of Class I TCPs in this process remains unknown. We analyzed the role of Class I TCP14 and TCP15 in axillary branch development in Arabidopsis through a series of genetic and molecular studies. In contrast to the increased branch number shown by brc1 mutants, tcp14 tcp15 plants exhibit a reduced number of branches compared with wild-type. Our findings provide evidence that TCP14 and TCP15 act by counteracting BRC1 function through two distinct mechanisms. First, they indirectly reduce BRC1 expression levels. Additionally, TCP15 directly interacts with BRC1 decoying it from chromatin and thereby preventing the transcriptional activation of a set of BRC1-dependent genes. We describe a molecular mechanism by which Class I TCPs physically antagonize the action of the Class II TCP BRC1, aligning with their opposite roles in axillary bud development.