The B3-Domain Transcription Factor VAL1 Regulates the Floral Transition by Repressing <i>FLOWERING LOCUS T</i>

Abstract

Many plants monitor changes in day length (or photoperiod) and adjust the timing of the floral transition accordingly to ensure reproductive success. In long-day plants, a long-day photoperiod triggers the production of florigen, which promotes the floral transition. <i>FLOWERING LOCUS T</i> (<i>FT</i>) in Arabidopsis (<i>Arabidopsis thaliana</i>) encodes a major component of florigen, and <i>FT</i> expression is activated in leaf veins specifically at dusk through the photoperiod pathway. Repression of <i>FT</i> mediated by Polycomb group (PcG) proteins prevents precocious flowering and adds another layer to <i>FT</i> regulation. Here, we identified high-level trimethylation of histone H3 at Lys 27 (H3K27me3) in the high trimethylation region (HTR) of the <i>FT</i> locus from the second intron to the 3' untranslated region. The HTR contains a cis-regulatory DNA element required for H3K27me3 enrichment that is recognized by the transcriptional repressor VIVIPAROUS1/ABSCISIC ACID INSENSITIVE3-LIKE1 (VAL1). VAL1 directly represses <i>FT</i> expression before dusk and at night, coinciding with the high abundance of both <i>VAL1</i> mRNA and VAL1 homodimer. Furthermore, VAL1 recruits LIKE HETEROCHROMATIN PROTEIN1 and MULTICOPY SUPRESSOR OF IRA1 to <i>FT</i> chromatin, leading to an H3K27me3 peak at the HTR of <i>FT</i> These findings reveal a mechanism for PcG repression of <i>FT</i> mediated by an intronic cis-silencing element and suggest a possible role for VAL1 in modulating PcG repression of <i>FT</i> during the flowering response.