Brahma is required for proper expression of the floral repressor FLC in Arabidopsis

Background

BRAHMA (BRM) is a member of a family of ATPases of the SWI/SNF chromatin remodeling complexes from Arabidopsis. BRM has been previously shown to be crucial for vegetative and reproductive development.

Methodology/Principal Findings

Here we carry out a detailed analysis of the flowering phenotype of brm mutant plants which reveals that, in addition to repressing the flowering promoting genes CONSTANS (CO), FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), BRM also represses expression of the general flowering repressor FLOWERING LOCUS C (FLC). Thus, in brm mutant plants FLC expression is elevated, and FLC chromatin exhibits increased levels of histone H3 lysine 4 tri-methylation and decreased levels of H3 lysine 27 tri-methylation, indicating that BRM imposes a repressive chromatin configuration at the FLC locus. However, brm mutants display a normal vernalization response, indicating that BRM is not involved in vernalization-mediated FLC repression. Analysis of double mutants suggests that BRM is partially redundant with the autonomous pathway. Analysis of genetic interactions between BRM and the histone H2A.Z deposition machinery demonstrates that brm mutations overcome a requirement of H2A.Z for FLC activation suggesting that in the absence of BRM, a constitutively open chromatin conformation renders H2A.Z dispensable.

Conclusions/Significance

BRM is critical for phase transition in Arabidopsis. Thus, BRM represses expression of the flowering promoting genes CO, FT and SOC1 and of the flowering repressor FLC. Our results indicate that BRM controls expression of FLC by creating a repressive chromatin configuration of the locus.     

Interaction ofFLC and late-flowering mutations inArabidopsis thaliana

The phenotype caused by mutations that affect the timing of flowering inArabidopsis thaliana has been most extensively analyzed in the Landsbergerecta (Ler) genetic background. In Ler, the late-flowering phenotype ofFRIGIDA and mutations inLUMINIDEPENDENS is suppressed by the Ler allele ofFLC. In this study, the interactions of nine mutations conferring late flowering with theFLC allele of the Columbia ecotype (FLC-Col), which does not suppress late flowering, were examined. The effect on flowering time of combining six of the mutations withFLC-Col was additive; plants containingFLC-Col withfd, gi, fwa, fha, ft, andfe flowered slightly later than plants containing these mutations with theLer allele ofFLC. In contrast, a synergistic effect was observed betweenFLC-Col and three mutations;fca, fpa, andfve plants became extremely late flowering when combined withFLC-Col. Maximum delay in flowering for the majority of the mutant strains requiredFLC-Col in a homozygous state, although forfpa andfe a single copy ofFLC-Col allowed maximum lateness. In addition, thefd andfe mutations became more dominant in the presence ofFLC-Col.     

ARABIDOPSIS TRITHORAX-RELATED7 Is Required for Methylation of Lysine 4 of Histone H3 and for Transcriptional Activation of FLOWERING LOCUS C

In the winter-annual accessions of Arabidopsis thaliana, presence of an active allele of FRIGIDA (FRI) elevates expression of FLOWERING LOCUS C (FLC), a repressor of flowering, and thus confers a vernalization requirement. FLC activation by FRI involves methylation of Lys 4 of histone H3 (H3K4) at FLC chromatin. Many multicellular organisms that have been examined contain two classes of H3K4 methylases, a yeast (Saccharomyces cerevisiae) Set1 class and a class related to Drosophila melanogaster Trithorax. In this work, we demonstrate that ARABIDOPSIS TRITHORAX-RELATED7 (ATXR7), a putative Set1 class H3K4 methylase, is required for proper FLC expression. The atxr7 mutation partially suppresses the delayed flowering of a FRI-containing line. The rapid flowering of atxr7 is associated with reduced FLC expression and is accompanied by decreased H3K4 methylation and increased H3K27 methylation at FLC. Thus, ATXR7 is required for the proper levels of these histone modifications that set the level of FLC expression to create a vernalization requirement in winter-annual accessions. Previously, it has been reported that lesions in ATX1, which encodes a Trithorax class H3K4 methylase, partially suppress the delayed flowering of winter-annual Arabidopsis. We show that the flowering phenotype of atx1 atxr7 double mutants is additive relative to those of single mutants. Therefore, both classes of H3K4 methylases appear to be required for proper regulation of FLC expression.