Arabidopsis TERMINAL FLOWER 2 gene encodes a heterochromatin protein 1 homolog and represses both FLOWERING LOCUS T to regulate flowering time and several floral homeotic genes

Molecular characterization of the tobacco SET domain protein NtSET1 unravels its role in histone methylation, chromatin binding, and segregation

Plants contain a great number of genes encoding a distinctive class of SET domain proteins which harbor a plant-specific N-terminal part together with a C-terminal part showing highest sequence similarity to the catalytic domain of the yeast CLR4, the human SUV39H1 and G9a histone-methyltransferases (HMTases). Here we show that NtSET1, a representative member of this class from tobacco, methylated both K9 and K27 of histone H3 in vitro. Ectopic expression of NtSET1, by an inducible promoter, increased the amount of dimethylated H3K9 and induced chromosome-segregation defects in tobacco BY2 cells. Deletion analyses show that the HMTase activity, the association with specific chromatin regions and with condensed chromosomes, and the cellular effects largely depended on the C-terminal region including the SET domain of the protein. Nevertheless, the N-terminal part of NtSET1 was capable of targeting the green fluorescent protein to interphase chromatin. Finally, we show that NtSET1 bound LHP1, the Arabidopsis homolog of animal heterochromatin protein 1 (HP1), and that LHP1 co-localized with heterochromatin containing high amounts of dimethylated H3K9, suggesting a role for NtSET1 in heterochromatic function. Taken together, our results provide new insights into the molecular and global chromatin-binding activities of this particular class member of plant SET domain proteins.

Dual histone H3 methylation marks at lysines 9 and 27 required for interaction with CHROMOMETHYLASE3

Both DNA methylation and post-translational histone modifications contribute to gene silencing, but the mechanistic relationship between these epigenetic marks is unclear. Mutations in two Arabidopsis genes, the KRYPTONITE (KYP) histone H3 lysine 9 (H3K9) methyltransferase and the CHROMOMETHYLASE3 (CMT3) DNA methyltransferase, cause a reduction of CNG DNA methylation, suggesting that H3K9 methylation controls CNG DNA methylation. Here we show that the chromodomain of CMT3 can directly interact with the N-terminal tail of histone H3, but only when it is simultaneously methylated at both the H3K9 and H3K27 positions. Furthermore, using chromatin immunoprecipitation analysis and immunohistolocalization experiments, we found that H3K27 methylation colocalizes with H3K9 methylation at CMT3-controlled loci. The H3K27 methylation present at heterochromatin was not affected by mutations in KYP or in several Arabidopsis PcG related genes including the Enhancer of Zeste homologs, suggesting that a novel pathway controls heterochromatic H3K27 methylation. Our results suggest a model in which H3K9 methylation by KYP, and H3K27 methylation by an unknown enzyme provide a combinatorial histone code for the recruitment of CMT3 to silent loci.

The Arabidopsis thaliana SNF2 homolog AtBRM controls shoot development and flowering

Chromatin remodeling is essential for the reprogramming of transcription associated with development and cell differentiation. The SWI/SNF complex was the first chromatin remodeling complex characterized in yeast and Drosophila. In this work we have characterized an Arabidopsis thaliana homolog of Brahma, the ATPase of the Drosophila SWI/SNF complex. As its Drosophila counterpart, Arabidopsis thaliana BRAHMA (AtBRM) is a nuclear protein present in a high molecular mass complex. Furthermore, the N terminus of AtBRM interacts, in the two-hybrid system, with CHB4 (AtSWI3C), an Arabidopsis homolog of the yeast SWI/SNF complex subunit SWI3. The AtBRM gene is primarily expressed in meristems, organ primordia and tissues with active cell division. Silencing of the expression of the AtBRM gene by RNA interference demonstrated that AtBRM is required for vegetative and reproductive development. The AtBRM silenced plants exhibited a reduction in overall plant size with small and curled leafs, as well as a reduction in the size of the inflorescence meristem. In the absence of AtBRM, Arabidopsis flowers have small petals and stamens, immature anthers, homeotic transformations and reduced fertility. The AtBRM silenced plants flower earlier than wild-type plants both under inductive and non-inductive photoperiods. Furthermore, levels of CO, FT and SOC1 transcripts were up-regulated under non-inductive conditions suggesting that AtBRM is a repressor of the photoperiod-dependent flowering pathway.

Control ofArabidopsisflowering: the chill before the bloom

The timing of the floral transition has significant consequences for reproductive success in plants. Plants gauge both environmental and endogenous signals before switching to reproductive development. Many temperate species only flower after they have experienced a prolonged period of cold, a process known as vernalization, which aligns flowering with the favourable conditions of spring. Considerable progress has been made in understanding the molecular basis of vernalization in Arabidopsis. A central player in this process is FLC, which blocks flowering by inhibiting genes required to switch the meristem from vegetative to floral development. Recent data shows that many regulators of FLC alter chromatin structure or are involved in RNA processing.

CONSTANS acts in the phloem to regulate a systemic signal that induces photoperiodic flowering of Arabidopsis

Flower development at the shoot apex is initiated in response to environmental cues. Day length is one of the most important of these and is perceived in the leaves. A systemic signal, called the floral stimulus or florigen, is then transmitted from the leaves through the phloem and induces floral development at the shoot apex. Genetic analysis in Arabidopsis identified a pathway of genes required for the initiation of flowering in response to day length. The nuclear zinc-finger protein CONSTANS (CO) plays a central role in this pathway, and in response to long days activates the transcription of FT, which encodes a RAF-kinase-inhibitor-like protein. We show using grafting approaches that CO acts non-cell autonomously to trigger flowering. Although CO is expressed widely, its misexpression from phloem-specific promoters, but not from meristem-specific promoters, is sufficient to induce early flowering and complement the co mutation. The mechanism by which CO triggers flowering from the phloem involves the cell-autonomous activation of FT expression. Genetic approaches indicate that CO activates flowering through both FT-dependent and FT-independent processes, whereas FT acts both in the phloem and the meristem to trigger flowering. We propose that, partly through the activation of FT, CO regulates the synthesis or transport of a systemic flowering signal, thereby positioning this signal within the established hierarchy of regulatory proteins that controls flowering.

It's time to flower: the genetic control of flowering time

In plants, successful sexual reproduction and the ensuing development of seeds and fruits depend on flowering at the right time. This involves coordinating flowering with the appropriate season and with the developmental history of the plant. Genetic and molecular analysis in the small cruciform weed, Arabidopsis, has revealed distinct but linked pathways that are responsible for detecting the major seasonal cues of day length and cold temperature, as well as other local environmental and internal signals. The balance of signals from these pathways is integrated by a common set of genes to determine when flowering occurs. Excitingly, it has been discovered that many of these same genes regulate flowering in other plants, such as rice. This review focuses on recent advances in how three of the signalling pathways (the day-length, vernalisation and autonomous pathways) function to control flowering.

Vernalization and epigenetics: how plants remember winter

One of the remarkable aspects of the promotion of flowering by vernalization is that plants have evolved the ability to measure a complete winter season of cold and to 'remember' this prior cold exposure in the spring. Recent work in Arabidopsis demonstrates the molecular basis of this memory of winter: vernalization causes changes in the chromatin structure of a flowering repressor gene, FLOWERING LOCUS C (FLC), that switch this gene into a repressed state that is mitotically stable. A key component of the vernalization pathway, VERNALIZATION INSENSITIVE3 (VIN3), which is a PHD-domain-containing protein, is induced only after a prolonged period of cold. VIN3 is involved in initiating the modification of FLC chromatin structure. The stable silencing of FLC also requires the DNA-binding protein VERNALIZATION1 (VRN1) and the polycomb-group protein VRN2.

Epigenetic control of plant development: new layers of complexity

Important aspects of plant development are under epigenetic control, that is, under the control of heritable changes in gene expression that are not associated with alterations in DNA sequence. It is becoming clear that RNA molecules play a key role in epigenetic gene regulation by providing sequence specificity for the targeting of developmentally important genes. RNA-based control of gene expression can be exerted posttranscriptionally by interfering with transcript stability or translation. Moreover, RNA molecules also appear to direct developmentally relevant gene regulation at the transcriptional level by modifying chromatin structure and/or DNA methylation.

Genetic regulation of time to flower in Arabidopsis thaliana

In Arabidopsis thaliana, the initiation of flowering is carried out by four genetic pathways: gibberellin, autonomous, vernalization, and light-dependent pathways. These processes are integrated by the function of the genes FD, FE, FWA, PDF2, SOC1, and FT at the integration pathway. The integrated signal of the floral induction is transmitted to the floral meristem identity genes LFY and AP1, and floral morphogenesis is performed.