Control of the transition to flowering by chromatin modifications

The timing of floral transition is critical to reproductive success in angiosperms and is genetically controlled by a network of flowering genes. In Arabidopsis, expression of certain flowering genes is regulated by various chromatin modifications, among which are two central regulators of flowering, namely FLOWERING LOCUS C (FLC) and FLOWERING LOCUS T (FT). Recent studies have revealed that a number of chromatin-modifying components are involved in activation or repression of FLC expression. Activation of FLC expression is associated with various 'active' chromatin modifications including acetylation of core histone tails, histone H3 lysine-4 (H3K4) methylation, H2B monoubiquitination, H3 lysine-36 (H3K36) di- and tri-methylation and deposition of the histone variant H2A.Z, whereas various 'repressive' histone modifications are associated with FLC repression, including histone deacetylation, H3K4 demethylation, histone H3 lysine-9 (H3K9) and H3 lysine-27 (H3K27) methylation, and histone arginine methylation. In addition, recent studies have revealed that Polycomb group gene-mediated transcriptional-silencing mechanism not only represses FLC expression, but also directly represses FT expression. Regulation of FLC expression provides a paradigm for control of the expression of other developmental genes in plants through chromatin mechanisms.