A pair of readers of histone H3K4 methylation recruit Polycomb repressive complex 2 to regulate photoperiodic flowering.

The transition from vegetative growth to reproduction in flowering plants is often timed by seasonal changes in day length (photoperiod). In the long-day (LD) plant Arabidopsis thaliana, the photoperiod pathway induces a daily rhythmic activation of the florigen gene FLOWERING LOCUS T (FT) to promote the floral transition. Under inductive LDs, FT expression is activated around dusk, but to be repressed overnight and into the early afternoon the next day. Here, we report that AtING1 and AtING2, Arabidopsis homologs of the mammalian Inhibitor of Growth (ING) proteins, read di- and tri-methylated histone-3 lysine 4 (H3K4me2/me3) on FT chromatin and further recruit Polycomb-repressive complex 2 (PRC2) to repress FT expression at night and into the early afternoon the next day, following FT activation at dusk. This prevents precocious flowering under inductive LDs. Our study reveals that the H3K4me2/me3-ING1/2-PRC2 module timely represses FT expression following the daily rhythmic FT activation, to prevent excessive FT expression and thus precisely control flowering time, in response to inductive photoperiodic signals.