ABI5-BINDING PROTEIN2 Coordinates CONSTANS to Delay Flowering by Recruiting the Transcriptional Corepressor TPR2

Abstract

ABI5-BINDING PROTEIN2 (AFP2) negatively regulates the abscisic acid signal by accelerating ABI5 degradation during seed germination in Arabidopsis (<i>Arabidopsis thaliana</i>). The abscisic acid signal is reported to delay flowering by up-regulating <i>Flowering Locus C</i> expression, but the role of AFP2 in regulating flowering time is unknown. Here, we found that flowering time was markedly delayed and <i>CONSTANS</i> (<i>CO</i>) expression was reduced in a transgenic Arabidopsis line overexpressing <i>AFP2</i> under LD conditions. Conversely, the loss-of-function <i>afp2</i> mutant showed slightly earlier flowering, with higher <i>CO</i> expression. These data suggest that <i>AFP2</i> negatively regulates photoperiod-dependent flowering time by modulating the CO signal. We then found that <i>AFP2</i> exhibited circadian expression rhythms that peaked during the night. Furthermore, the C-terminus of AFP2 interacted with CO, while its N-terminal ethylene response factor-associated amphiphilic repression motif interacted with the transcriptional corepressor TOPLESS-related protein2 (TPR2). Thus, AFP2 bridges CO and TPR2 to form the CO-AFP2-TPR2 complex. Biochemical and genetic analyses showed that AFP2 mediated CO degradation during the night. AFP2 also recruited histone deacetylase activity at <i>Flowering Locus T</i> chromatin through its interaction with TPR2. Taken together, our results reveal an elaborate mechanism by which AFP2 modulates flowering time through coordinating the activity and stability of CO.