Arabidopsis Transcription Factor TCP5 Controls Plant Thermomorphogenesis by Positively Regulating PIF4 Activity

Plants display thermomorphogenesis in response to high temperature (HT). PHYTOCHROME INTERACTING FACTOR 4 (PIF4) is a central integrator regulated by numerous negative regulators. However, the mechanisms underpinning PIF4 positive regulation are largely unknown. Here, we find that TEOSINTE BRANCHED 1/CYCLOIDEA/PCF 5 (TCP5), TCP13, and TCP17 transcription factors promote the activity of PIF4 at transcriptional and post-transcriptional levels. TCP5 is rapidly induced by HT treatment, and TCP5 protein stability increases under HT. The overexpression of TCP5 causes constitutive thermomorphogenic phenotypes, whereas the tcp5 tcp13 tcp17 triple mutant exhibits aberrant thermomorphogenesis. We demonstrate that TCP5 not only physically interacts with PIF4 to enhance its activity but also directly binds to the promoter of PIF4 to increase its transcript. TCP5 and PIF4 share common downstream targets. The tcp5 tcp13 tcp17 mutant partially restores the long hypocotyls caused by PIF4 overexpression. Our findings provide a layer of understanding about the fine-scale regulation of PIF4 and plant thermomorphogenesis.