The Chromatin-Remodeling Factor PICKLE Antagonizes Polycomb Repression of <i>FT</i> to Promote Flowering

Abstract

Changing daylength (or photoperiod) is a seasonal cue used by many plants to adjust the timing of their floral transition to ensure reproductive success. An inductive long-day photoperiod triggers the expression of <i>FLOWERING LOCUS T</i> (<i>FT</i>), which promotes flowering. <i>FT</i>, encoding a major component of florigen, is induced in leaf veins specifically at dusk through the photoperiod pathway; however, the modulation of <i>FT</i> expression in response to photoperiod cues remains poorly understood. Here, we report that the balance between Polycomb group (PcG) and Trithorax group (TrxG) proteins sets appropriate <i>FT</i> expression in long days in Arabidopsis (<i>Arabidopsis thaliana</i>). In PcG mutant lines, <i>FT</i> was highly derepressed, but <i>FT</i> expression was decreased to an almost wild-type level and pattern upon the additional disruption of chromatin-remodeling factors PICKLE (PKL) and ARABIDOPSIS HOMOLOG OF TRITHORAX1 (ATX1), but not by disruption of photoperiod pathway components. PKL interacts with ATX1 to mediate trimethylation of histone H3 on lysine-4 at the <i>FT</i> locus, leading to antagonistic effects of PKL and ATX1 on PcG proteins in the regulation of <i>FT</i> expression. Therefore, the TrxG-like protein PKL prevents PcG-mediated silencing to ensure specific and appropriate expression of <i>FT</i>, thereby determining the proper flowering response.