Abstract
In facultative photoperiodic flowering plants, noninductive photoperiods result in a delay in flowering, but such plants eventually flower, illustrating plasticity in an important developmental transition, flowering. The model plant, Arabidopsis, has a facultative photoperiod response. Although the inductive flowering promotion pathway has been extensively studied, the pathway to flowering in noninductive photoperiods is not well understood. Here, we show that a Plant Homeo Domain finger-containing protein, VIN3-LIKE 2 (VIL2), is necessary to maintain the epigenetically repressed state of MAF5 and permit more rapid flowering in noninductive photoperiods in Arabidopsis. Levels of both VIL2 mRNA and protein are under diurnal fluctuation and maintain the repressed state at MAF5 chromatin in a photoperiod-specific manner. VIL2 binds preferentially to dimethylated histone H3 Lys-9 (H3K9me2) peptides in vitro and VIL2 is required for the maintenance of H3K9me2 at MAF5 chromatin in vivo. Furthermore, VIL2 is required for the maintenance of trimethylated histone H3 Lys-27 at MAF5 through the physical association with a component of polycomb repression complex 2. Thus, the repression of MAF5 by VIL2 provides a mechanism to promote flowering in noninductive photoperiods, which contributes to the facultative nature of the Arabidopsis photoperiodic response.