Repression of Flowering inArabidopsisRequires Activation ofFLOWERING LOCUS CExpression by the Histone Variant H2A.Z

Abstract

The histone variant H2A.Z has been implicated in numerous chromatin-mediated processes, including transcriptional activation, euchromatin maintenance, and heterochromatin formation. In yeast and humans, H2A.Z is deposited into chromatin by a conserved protein complex known as SWR1 and SRCAP, respectively. Here, we show that mutations in the Arabidopsis thaliana homologs of two components of this complex, ACTIN-RELATED PROTEIN6 (ARP6) and PHOTO-PERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1), produce similar developmental phenotypes and result in the misregulation of a common set of genes. Using H2A.Z-specific antibodies, we demonstrate that ARP6 and PIE1 are required for the deposition of H2A.Z at multiple loci, including the FLOWERING LOCUS C (FLC) gene, a central repressor of the transition to flowering. Loss of H2A.Z from chromatin in arp6 and pie1 mutants results in reduced FLC expression and premature flowering, indicating that this histone variant is required for high-level expression of FLC. In addition to defining a novel mechanism for the regulation of FLC expression, these results support the existence of a SWR1-like complex in Arabidopsis and show that H2A.Z can potentiate transcriptional activation in plants. The finding that H2A.Z remains associated with chromatin throughout mitosis suggests that it may serve an epigenetic memory function by marking active genes and poising silenced genes for reactivation.

Phosphorylation of Histone H2A at Serine 95: A Plant-Specific Mark Involved in Flowering Time Regulation and H2A.Z Deposition

“…These subunits interact with three Arabidopsis homologs of H2A.Z proteins, HTA8, HTA9, and HTA11 (Choi et al, 2007), indicating that the SWR1 complex is involved in H2A.Z deposition. The loss-of-function pie1 mutant flowers early because PIE1 is required for the deposition of H2A.Z at FLC, MAF4, and MAF5 (Deal et al, 2007). NuA4, the only essential histone acetyltransferase complex in S. cerevisiae, acetylates the N-terminal tails of histones H4 and H2A, which directly stimulates the incorporation of H2A.Z by the SWR1 complex (Altaf et al, 2010).…”

The BAF60 Subunit of the SWI/SNF Chromatin-Remodeling Complex Directly Controls the Formation of a Gene Loop at FLOWERING LOCUS C in Arabidopsis

“…In order to determine whether this global increase was due to an enhanced expression of H2A.Z variants, quantitative analysis of the expression of genes encoding H2A.Z was performed. The Arabidopsis genome encodes 13 H2A histones, 3 of which are H2A.Z variants (Deal et al, 2007). Quantitative PCR showed that in Arabidopsis, H2A.Z mRNA was mainly transcribed from the HTA9 gene (Supplemental Figure 6) and that no increase in the expression of this gene was detected in BAF60 RNAi lines, suggesting that H2A.Z accumulation is regulated at the protein level.…”

“…Among genes known to be regulated via H2A.Z incorporation into chromatin, several are involved in the control of flowering time (Deal et al, 2007;Farrona et al, 2011). Hence, we compared the flowering time of BAF60 RNAi lines and Wassilewskija control plants in two independent experiments.…”

“…This complex is involved in the substitution of histone H2A by the histone variant H2A.Z (March-Díaz and Reyes, 2009). Indeed, mutations in PIE1, ARP6 (also known as SUF3 and ESD1 in Arabidopsis), and SEF/ AtSW6C show early flowering and reduced FLC expression (Noh and Amasino, 2003;Choi et al, 2005Choi et al, , 2007Deal et al, 2005;Martin-Trillo et al, 2006;March-Díaz et al, 2007;Lázaro et al, 2008), and ARP6 and PIE1 were found to be required for H2A.Z deposition at FLC (Deal et al, 2007). Finally, it has been shown that disruption of BRM results in high FLC expression and that the brm mutation restores high accumulation of FLC mRNA in pie and sef mutants, suggesting that SWR1/SCRAP and SWI/SNF complexes play antagonistic roles in FLC regulation (Farrona et al, 2011).…”

The FRIGIDA Complex Activates Transcription of FLC, a Strong Flowering Repressor in Arabidopsis, by Recruiting Chromatin Modification Factors

“…The full activation of FLC by FRI requires chromatin modifiers, including proteins catalyzing histone H2B ubiquitination/deubiquitination, histone H3 lysine 4 (H3K4) methylation, H3K36 methylation, and the PAF1 (RNA polymerase associated factor 1) complex (Kim et al, 2009). In addition, the orthologous components of the yeast ATP-dependent SWR1 chromatin remodeling complex (SWR1-C), such as PHOTOPERIOD INDEPENDENT EARLY FLOWERING1 (PIE1), ACTIN-RELATED PROTEIN6 (ARP6)/SUPPRESSOR OF FRIGIDA3 (SUF3), and SWC6, are also required for incorporation of H2A.Z into FLC chromatin (Noh and Amasino, 2003;Choi et al, 2007;Deal et al, 2007). These chromatin-modifying proteins are well conserved among eukaryotes and are required for the epigenetic marks that are used for active transcription (Li et al, 2007).…”