Regulation of arabidopsis flowering by the histone mark readers MRG1/2 via interaction with CONSTANS to modulate FT expression.

Day-length is important for regulating the transition to reproductive development (flowering) in plants. In the model plant Arabidopsis thaliana, the transcription factor CONSTANS (CO) promotes expression of the florigen FLOWERING LOCUS T (FT), constituting a key flowering pathway under long-day photoperiods. Recent studies have revealed that FT expression is regulated by changes of histone modification marks of the FT chromatin, but the epigenetic regulators that directly interact with the CO protein have not been identified. Here, we show that the Arabidopsis Morf Related Gene (MRG) group proteins MRG1 and MRG2 act as H3K4me3/H3K36me3 readers and physically interact with CO to activate FT expression. In vitro binding analyses indicated that the chromodomains of MRG1 and MRG2 preferentially bind H3K4me3/H3K36me3 peptides. The mrg1 mrg2 double mutant exhibits reduced mRNA levels of FT, but not of CO, and shows a late-flowering phenotype under the long-day but not short-day photoperiod growth conditions. MRG2 associates with the chromatin of FT promoter in a way dependent of both CO and H3K4me3/H3K36me3. Vice versa, loss of MRG1 and MRG2 also impairs CO binding at the FT promoter. Crystal structure analyses of MRG2 bound with H3K4me3/H3K36me3 peptides together with mutagenesis analysis in planta further demonstrated that MRG2 function relies on its H3K4me3/H3K36me3-binding activity. Collectively, our results unravel a novel chromatin regulatory mechanism, linking functions of MRG1 and MRG2 proteins, H3K4/H3K36 methylations, and CO in FT activation in the photoperiodic regulation of flowering time in plants.

Polycomb-group histone methyltransferase CLF is required for proper somatic recombination in Arabidopsis.

Homologous recombination (HR) is a key process during meiosis in reproductive cells and the DNA damage repair process in somatic cells. Although chromatin structure is thought to be crucial for HR, only a small number of chromatin modifiers have been studied in HR regulation so far. Here, we investigated the function of CURLY LEAF (CLF), a Polycomb-group (PcG) gene responsible for histone3 lysine 27 trimethylation (H3K27me3), in somatic and meiotic HR in Arabidopsis thaliana. Although fluorescent protein reporter assays in pollen and seeds showed that the frequency of meiotic cross-over in the loss-of-function mutant clf-29 was not significantly different from that in wild type, there was a lower frequency of HR in clf-29 than in wild type under normal conditions and under bleomycin treatment. The DNA damage levels were comparable between clf-29 and wild type, even though several DNA damage repair genes (e.g. ATM, BRCA2a, RAD50, RAD51, RAD54, and PARP2) were expressed at lower levels in clf-29. Under bleomycin treatment, the expression levels of DNA repair genes were similar in clf-29 and wild type, thus CLF may also regulate HR via other mechanisms. These findings expand the current knowledge of PcG function and contribute to general interests of epigenetic regulation in genome stability regulation.

SDG714 regulates specific gene expression and consequently affects plant growth via H3K9 dimethylation.

Histone lysine methylation is known to be involved in the epigenetic regulation of gene expression in all eukaryotes including plants. Here we show that the rice SDG714 is primarily responsible for dimethylation but not trimethylation on histone H3K9 in vivo. Overexpression of YFP-SDG714 in Arabidopsis significantly inhibits plant growth and this inhibition is associated with an enhanced level of H3K9 dimethylation. Our microarray results show that many genes essential for the plant growth and development were downregulated in transgenic Arabidopsis plants overexpressing YFP-SDG714. By chromatin immunoprecipitation analysis, we show that YFP-SDG714 is targeted to specific chromatin regions and dimethylate the H3K9, which is linked with heterochromatinization and the downregulation of genes. Most interestingly, when YFP-SDG714 production is stopped, the inhibited plants can partially restore their growth, suggesting that the perturbation of gene expression caused by YFP-SDG714 is revertible. Taken together, our results point to an important role of SDG714 in H3K9 dimethylation, suppression of gene expression and plant growth, and provide a potential method to regulate gene expression and plant development by an on-off switch of SDG714 expression.

The Arabidopsis transcription factor AtTCP15 regulates endoreduplication by modulating expression of key cell-cycle genes.

Plant cells frequently undergo endoreduplication, a modified cell cycle in which genome is repeatedly replicated without cytokinesis. As the key step to achieve final size and function for cells, endoreduplication is prevalent during plant development. However, mechanisms to control the balance between endoreduplication and mitotic cell division are still poorly understood. Here, we show that the Arabidopsis TCP (CINCINNATA-like TEOSINTE BRANCHED1-CYCLOIDEA-PCF)-family transcription factor gene AtTCP15 is expressed in trichomes, as well as in rapidly dividing and vascular tissues. Expression of AtTCP15SRDX, AtTCP15 fused with a SRDX repressor domain, induces extra endoreduplication in trichomes and cotyledon cells in transgenic Arabidopsis. On the contrary, overexpression of AtTCP15 suppresses endoreduplication in trichomes and other examined cells. Misregulation of AtTCP15 affects the expression of several important genes involved in cell-cycle regulation. AtTCP15 protein binds directly to the promoter regions of CYCA2;3 and RETINOBLASTOMA-RELATED (RBR) genes, which play key roles in endoreduplication. Taken together, AtTCP15 plays an important role in regulating endoreduplication during Arabidopsis development.

A truncated Arabidopsis NUCLEOSOME ASSEMBLY PROTEIN 1, AtNAP1;3T, alters plant growth responses to abscisic acid and salt in the Atnap1;3-2 mutant.

Chromatin remodeling is thought to have crucial roles in plant adaptive response to environmental stimulus. Here, we report that, in Arabidopsis, the evolutionarily conserved histone chaperone, NUCLEOSOME ASSEMBLY PROTEIN 1 (NAP1), is involved in plant response to abscisic acid (ABA), a phytohormone important in stress adaptation. We show that simultaneous loss-of-function of AtNAP1;1, AtNAP1;2, and AtNAP1;3 (the triple mutant m123-1) caused a slight hypersensitive response to ABA in seedling growth. Strikingly, the other triple mutant m123-2 containing a different mutant allele of AtNAP1;3, the Atnap1;3-2 allele, showed a hyposensitive response to ABA and a decreased tolerance to salt stress. This ABA-hyposensitive and salt response phenotype specifically associated with the Atnap1;3-2 mutant allele. We show that this mutant allele produced a truncated protein, AtNAP1;3T, which lacks 34 amino acids at the C-terminus compared to the wild-type protein AtNAP1;3. We further show that the heterozygous plants containing the Atnap1;3-2 mutant allele as well as transgenic plants overexpressing AtNAP1;3T exhibit ABA-hyposensitive phenotype. It thus indicates that AtNAP1;3T functions as a dominant negative factor in ABA response. The expression of some ABA-responsive genes, including genes encoding protein kinases and transcription regulators, was found perturbed in the mutant and in the AtNAP1;3T transgenic plants. Taken together, our study uncovered AtNAP1 proteins as positive regulators and AtNAP1;3T as a negative regulator in ABA signaling pathways, providing a novel link of chromatin remodeling to hormonal and stress responses.