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.
In Arabidopsis, expression of FLC and FLC-related genes (collectively called FLC clade) contributes to flowering time in response to environmental changes, such as day length and temperature, by acting as floral repressors. VIN3 is required for vernalization-mediated FLC repression and a VIN3 related protein, VIN3-LIKE 1/VERNALIZATION 5 (VIL1/VRN5), acts to regulate FLC and FLM in response to vernalization. VIN3 also exists as a small family of PHD finger proteins in Arabidopsis, including VIL1/VRN5, VIL2/VEL1, VIL3/VEL2, and VIL4/VEL3. We showed that the PHD finger protein, VIL2, is required for proper repression of MAF5, an FLC clade member, to accelerate flowering under non-inductive photoperiods. VIL2 acts together with POLYCOMB REPRESSIVE COMPLEX 2 (PRC2) to repress MAF5 in a photoperiod dependent manner.
VERNALIZATION INSENSITIVE3 (VIN3) induction by vernalization is one of the earliest events in the vernalization response of Arabidopsis (Arabidopsis thaliana). However, the mechanism responsible for vernalization-mediated VIN3 induction is poorly understood. Here, we show that the constitutive repression of VIN3 in the absence of the cold is due to multiple repressive components, including a transposable element-derived sequence, LIKE-HETEROCHROMATIN PROTEIN1 and POLYCOMB REPRESSION COMPLEX2. Furthermore, the full extent of VIN3 induction by vernalization requires activating complex components, including EARLY FLOWERING7 and EARLY FLOWERING IN SHORT DAYS. In addition, we observed dynamic changes in the histone modifications present at VIN3 chromatin during the course of vernalization. Our results show that the induction of VIN3 includes dynamic changes at the level of chromatin triggered by long-term cold exposure.