The Nuclear Actin-Related Protein ARP6 Is a Pleiotropic Developmental Regulator Required for the Maintenance ofFLOWERING LOCUS CExpression and Repression of Flowering inArabidopsis w⃞

Abstract

Actin-related proteins (ARPs) are found in the nuclei of all eukaryotic cells, but their functions are generally understood only in the context of their presence in various yeast and animal chromatin-modifying complexes. Arabidopsis thaliana ARP6 is a clear homolog of other eukaryotic ARP6s, including Saccharomyces cerevisiae ARP6, which was identified as a component of the SWR1 chromatin remodeling complex. We examined the subcellular localization, expression patterns, and loss-offunction phenotypes for this protein and found that Arabidopsis ARP6 is localized to the nucleus during interphase but dispersed away from the chromosomes during cell division. ARP6 expression was observed in all vegetative tissues as well as in a subset of reproductive tissues. Null mutations in ARP6 caused numerous defects, including altered development of the leaf, inflorescence, and flower as well as reduced female fertility and early flowering in both long-and short-day photoperiods. The early flowering of arp6 mutants was associated with reduced expression of the central floral repressor gene FLOWERING LOCUS C (FLC) as well as MADS AFFECTING FLOWERING 4 (MAF4) and MAF5. In addition, arp6 mutations suppress the FLC-mediated late flowering of a FRIGIDA-expressing line, indicating that ARP6 is required for the activation of FLC expression to levels that inhibit flowering. These results indicate that ARP6 acts in the nucleus to regulate plant development, and we propose that it does so through modulation of chromatin structure and the control of gene expression.

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

“…In the case of flowering time control, both ARP6 and PIE1 were shown to be required for high-level expression of the floral repressor gene FLC (Noh and Amasino, 2003;Choi et al, 2005;Deal et al, 2005;Martin-Trillo et al, 2006), indicating that these proteins were likely involved in transcriptional regulation. Concurrent with the elucidation of ARP6 and PIE1 function in Arabidopsis, several groups discovered that in Saccharomyces cerevisiae, ARP6 was a component of the SWR1 chromatinremodeling complex (Krogan et al, 2003;Kobor et al, 2004;Mizuguchi et al, 2004).…”

“…Previous work in Arabidopsis showed that loss-of-function mutations in two genes encoding putative homologs of components of the SWR1/SRCAP complex, ACTIN-RELATED PROTEIN6 (ARP6) and the Snf2 protein PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1), led to premature flowering as a result of reduced FLC expression (Noh and Amasino, 2003;Choi et al, 2005;Deal et al, 2005;Martin-Trillo et al, 2006). However, the mechanism by which these proteins regulate FLC expression has not been addressed.…”

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

“…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).…”

INO80 subfamily of chromatin remodeling complexes

“…Taken together, these findings suggest that HP1 and H2AZ, which is deposited by a SWR1 complex that contains Arp6, function together to play an important role in heterochromatin formation. Arabidopsis arp6 null mutants display numerous defects, including altered development of the leaf, inflorescence, and reduced female fertility [78]. Subsequently, it was found that Arabidopsis arp6 mutants have reduced expression of the central floral repressor gene Flowering Locus C (FLC) [78], and that Arp6 is required for histone H3 acetylation and methylation in FLC chromatin locus [72].…”

“…Arabidopsis arp6 null mutants display numerous defects, including altered development of the leaf, inflorescence, and reduced female fertility [78]. Subsequently, it was found that Arabidopsis arp6 mutants have reduced expression of the central floral repressor gene Flowering Locus C (FLC) [78], and that Arp6 is required for histone H3 acetylation and methylation in FLC chromatin locus [72]. However, it remains uncertain whether the Arp6 proteins in these studies function by themselves or as components of complexes similar to the SWR1 chromatin remodeling complex in budding yeast.…”