Mutations affecting the Arabidopsis SWC6 gene encoding a putative orthologue of a component of the SWR1 chromatin remodelling complex in plants have been characterized. swc6 mutations cause early flowering, shortened inflorescence internodes, and altered leaf and flower development. These phenotypic defects resemble those of the photoperiod independent early flowering 1 (pie1) and early in short days 1 (esd1) mutants, also affected in homologues of the SWR1 complex subunits. SWC6 is a ubiquitously expressed nuclear HIT-Zn finger-containing protein, with the highest levels found in pollen. Double mutant analyses suggest that swc6 abolishes the FLC-mediated late-flowering phenotype of plants carrying active alleles of FRI and of mutants of the autonomous pathway. It was found that SWC6 is required for the expression of the FLC repressor to levels that inhibit flowering. However, the effect of swc6 in an flc null background and the down-regulation of other FLC-like/MAF genes in swc6 mutants suggest that flowering inhibition mediated by SWC6 occurs through both FLC- and FLC-like gene-dependent pathways. Both genetic and physical interactions between SWC6 and ESD1 have been demonstrated, suggesting that both proteins act in the same complex. Using chromatin immunoprecipitation, it has been determined that SWC6, as previously shown for ESD1, is required for both histone H3 acetylation and H3K4 trimethylation of the FLC chromatin. Altogether, these results suggest that SWC6 and ESD1 are part of an Arabidopsis SWR1 chromatin remodelling complex involved in the regulation of diverse aspects of plant development, including floral repression through the activation of FLC and FLC-like genes.

The interplay among histone modifications modulates the expression of master regulatory genes in development. Chromatin effector proteins bind histone modifications and translate the epigenetic status into gene expression patterns that control development. Here, we show that two Arabidopsis thaliana paralogs encoding plant-specific proteins with a plant homeodomain (PHD) motif, SHORT LIFE (SHL) and EARLY BOLTING IN SHORT DAYS (EBS), function in the chromatin-mediated repression of floral initiation and play independent roles in the control of genes regulating flowering. Previous results showed that repression of the floral integrator FLOWERING LOCUS T (FT) requires EBS. We establish that SHL is necessary to negatively regulate the expression of SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), another floral integrator. SHL and EBS recognize di-and trimethylated histone H3 at lysine 4 and bind regulatory regions of SOC1 and FT, respectively. These PHD proteins maintain an inactive chromatin conformation in SOC1 and FT by preventing high levels of H3 acetylation, bind HISTONE DEACETYLASE6, and play a central role in regulating flowering time. SHL and EBS are widely conserved in plants but are absent in other eukaryotes, suggesting that the regulatory module mediated by these proteins could represent a distinct mechanism for gene expression control in plants.