The transcriptional repressor TIE2 dampens cytokinin response during root development

Cytokinin plays critical roles in root development. Cytokinin signaling depends on activation of the key transcription factor type B Arabidopsis Response Regulators (ARRs). However, the mechanisms underlying the finely tuned regulation of type B ARR activity remain to be clarified. In this study, we demonstrate that the ERF-associated amphiphilic repression (EAR) motif-containing protein TCP INTERACTOR CONTAINING EAR MOTIF PROTEIN2 (TIE2) forms a negative feedback loop to finely tune the activity of type B ARRs during root development. Disruption of TIE2 and its close homolog TIE1 causes severely shortened roots. TIE2 interacts with type B ARR1 and represses transcription of ARR1 targets. The cytokinin response is correspondingly enhanced in tie1-1 tie2-1. We further show that ARR1 positively regulates TIE1 and TIE2 by directly binding to their promoters. Our findings demonstrate that TIE2 plays key roles in controlling root morphology, and reveal an important negative feedback regulation mechanism for cytokinin signaling.