Involvement of the histone acetyltransferase AtHAC1 in the regulation of flowering time via repression of FLOWERING LOCUS C in Arabidopsis

Histone acetylation is an important posttranslational modification correlated with gene activation. In Arabidopsis (Arabidopsis thaliana), the histone acetyltransferase AtHAC1 is homologous to animal p300/CREB (cAMP-responsive element-binding protein)-binding proteins, which are the main histone acetyltransferases participating in many physiological processes, including proliferation, differentiation, and apoptosis. The functions of p300/CREB-binding proteins in animals are well characterized, whereas little is known about the roles of AtHAC1 in developmental control in Arabidopsis. Lesions in AtHAC1 caused pleiotropic developmental defects, including delayed flowering, a shortened primary root, and partially reduced fertility. Analysis of the molecular basis of late flowering in hac1 mutants showed that the hac1 plants respond normally to day length, gibberellic acid treatment, and vernalization. Furthermore, the expression level of the flowering repressor FLOWERING LOCUS C (FLC) is increased in hac1 mutants, indicating that the late-flowering phenotype of hac1 mutants is mediated by FLC. Since histone acetylation is usually associated with the activation of gene expression, histone modifications of FLC chromatin are not affected by mutations in HAC1 and expression levels of all known autonomous pathway genes are unchanged in hac1 plants, we propose that HAC1 affects flowering time by epigenetic modification of factors upstream of FLC.     

Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb repressive complex 2 components

Polycomb group (PcG) proteins are evolutionarily conserved in animals and plants, and play critical roles in the regulation of developmental gene expression. Here we show that the Arabidopsis Polycomb repressive complex 2 (PRC2) subunits CURLY LEAF (CLF), EMBRYONIC FLOWER 2 (EMF2) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE) repress the expression of FLOWERING LOCUS C (FLC), a central repressor of the floral transition in Arabidopsis and FLC relatives. In addition, CLF directly interacts with and mediates the deposition of repressive histone H3 lysine 27 trimethylation (H3K27me3) into FLC and FLC relatives, which suppresses active histone H3 lysine 4 trimethylation (H3K4me3) in these loci. Furthermore, we show that during vegetative development CLF and FIE strongly repress the expression of FLOWERING LOCUS T (FT), a key flowering-time integrator, and that CLF also directly interacts with and mediates the deposition of H3K27me3 into FT chromatin. Our results suggest that PRC2-like complexes containing CLF, EMF2 and FIE, directly interact with and deposit into FT, FLC and FLC relatives repressive trimethyl H3K27 leading to the suppression of active H3K4me3 in these loci, and thus repress the expression of these flowering genes. Given the central roles of FLC and FT in flowering-time regulation in Arabidopsis, these findings suggest that the CLF-containing PRC2-like complexes play a significant role in control of flowering in Arabidopsis.