Brahma Is Required for Proper Expression of the Floral Repressor FLC in Arabidopsis

Sara Farrona,Richard M Amasino,Franziska Turck,Robert J Schmitz,José C Reyes,Lidia Hurtado,Francisco J Florencio,Rosana March-Díaz

doi:10.1371/journal.pone.0017997

Abstract

BackgroundBRAHMA (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 FindingsHere 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/SignificanceBRM 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.

Highlights

In eukaryotic cells DNA is wrapped around an octamer of histones to form the nucleosome fiber, the basic component of chromatin
In order to determine whether overexpression of CO and FLOWERING LOCUS T (FT) in the absence of BRM was restricted to the same tissue or whether, in contrast, both genes were ectopically overexpressed, we performed b-glucoronidase (GUS) staining of plants expressing pFT::GUS and gCO::GUS [21] in BRM-silenced plants
These results suggest that BRM is affecting transcriptional repression of both CO and FT, which is consistent with the early-flowering phenotype of the BRM-silenced and the brm mutant plants [5,8]

Summary

Introduction

In eukaryotic cells DNA is wrapped around an octamer of histones to form the nucleosome fiber, the basic component of chromatin. All ATP-dependent CRMs share the presence of a DNAdependent ATPase of the SWI2/SNF2 family, which works as the enzymatic subunit of the complex. The proteins of this family have two conserved catalytic domains, a SNF2_N and a HelicC domain. The N-terminal region of BRM interacts with the Arabidopsis SWI3C and SWI3B proteins [5,8] These proteins are orthologues of the yeast SWI3 protein, another component of the SWI/SNF complex [9]. 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

Methods

Results

Conclusion