FVE, an Arabidopsis Homologue of the Retinoblastoma-Associated Protein That Regulates Flowering Time and Cold Response, Binds to Chromatin as a Large Multiprotein Complex

Some genetic studies indicate that plant homologues of proteins involved in chromatin modification and remodeling in other organisms may regulate plant development. Previously, we described an Arabidopsis mutant with altered cold-responsive gene expression (acg1) displaying a late flowering phenotype, a null allele of fve. FVE is a homologue of the mammalian retinoblastoma-associated protein (RbAp), one component of a histone deacetylase (HDAC) complex involved in transcriptional repression, and has been shown to be involved in the deacetylation of the FLOWERING LOCUS C (FLC) chromatin encoding for a repressor of flowering. In an effort to gain insight into the biochemical functions of FVE, we overexpressed FVE tagged with the hemagglutinin (HA) and FLAG epitope at the N-terminus in acg1 mutants. The results of physiological and molecular analyses demonstrated that FVE overexpression in acg1 rescued the mutant phenotypes, including late flowering and alterations in floral pathway gene expression such as FLC, SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), and FLOWERING LOCUS T (FT), and also super-induced cold-responsive reporter gene expression. The chromatin immunoprecipitation experiments revealed the amplification of specific DNA regions of FLC and COLD-REGULATED 15A (COR15A), indicating that FVE may bind to the FLC and COR15A chromatin. Gel-filtration chromatography and the immunoprecipitation of putative FVE complexes showed that FVE forms a protein complex of approximately 1.0 MDa. These results demonstrate that FVE may exist as a multiprotein complex, similar to the mammalian HDAC complex harboring RbAp, to regulate flowering time and cold response by associating with the FLC and COR chromatin.