George Coupland

FT Protein Movement Contributes to Long-Distance Signaling in Floral Induction of Arabidopsis

In plants, seasonal changes in day length are perceived in leaves, which initiate long-distance signaling that induces flowering at the shoot apex. The identity of the long-distance signal has yet to be determined. In Arabidopsis, activation of FLOWERING LOCUS T (FT) transcription in leaf vascular tissue (phloem) induces flowering. We found that FT messenger RNA is required only transiently in the leaf. In addition, FT fusion proteins expressed specifically in phloem cells move to the apex and move long distances between grafted plants. Finally, we provide evidence that FT does not activate an intermediate messenger in leaves. We conclude that FT protein acts as a long-distance signal that induces Arabidopsis flowering.

The CONSTANS gene of arabidopsis promotes flowering and encodes a protein showing similarities to zinc finger transcription factors

The vegetative and reproductive (flowering) phases of Arabidopsis development are clearly separated. The onset of flowering is promoted by long photoperiods, but the constans (co) mutant flowers later than wild type under these conditions. The CO gene was isolated, and two zinc fingers that show a similar spacing of cysteines, but little direct homology, to members of the GATA1 family were identified in the amino acid sequence. co mutations were shown to affect amino acids that are conserved in both fingers. Some transgenic plants containing extra copies of CO flowered earlier than wild type, suggesting that CO activity is limiting on flowering time. Double mutants were constructed containing co and mutations affecting gibberellic acid responses, meristem identity, or phytochrome function, and their phenotypes suggested a model for the role of CO in promoting flowering.

CONSTANS mediates between the circadian clock and the control of flowering in Arabidopsis

Flowering is often triggered by exposing plants to appropriate day lengths. This response requires an endogenous timer called the circadian clock to measure the duration of the day or night. This timer also controls daily rhythms in gene expression and behavioural patterns such as leaf movements. Several Arabidopsis mutations affect both circadian processes and flowering time; but how the effect of these mutations on the circadian clock is related to their influence on flowering remains unknown. Here we show that expression of CONSTANS (CO), a gene that accelerates flowering in response to long days, is modulated by the circadian clock and day length. Expression of a CO target gene, called FLOWERING LOCUS T (FT), is restricted to a similar time of day as expression of CO. Three mutations that affect circadian rhythms and flowering time alter CO and FT expression in ways that are consistent with their effects on flowering. In addition, the late flowering phenotype of such mutants is corrected by overexpressing CO. Thus, CO acts between the circadian clock and the control of flowering, suggesting mechanisms by which day length regulates flowering time.

The late elongated hypocotyl Mutation of Arabidopsis Disrupts Circadian Rhythms and the Photoperiodic Control of Flowering

The dominant late elongated hypocotyl (lhy) mutation of Arabidopsis disrupted circadian clock regulation of gene expression and leaf movements and caused flowering to occur independently of photoperiod. LHY was shown to encode a MYB DNA-binding protein. In wild-type plants, the LHY mRNA showed a circadian pattern of expression with a peak around dawn but in the mutant was expressed constantly at high levels. Increased LHY expression from a transgene caused the endogenous gene to be expressed at a constant level, suggesting that LHY was part of a feedback circuit that regulated its own expression. Thus, constant expression of LHY disrupts several distinct circadian rhythms in Arabidopsis, and LHY may be closely associated with the central oscillator of the circadian clock.

Regulation and identity of florigen: FLOWERING LOCUS T moves center stage.

The transition from vegetative to reproductive growth is controlled by day length in many plant species. Day length is perceived in leaves and induces a systemic signal, called florigen, that moves through the phloem to the shoot apex. At the shoot apical meristem (SAM), florigen causes changes in gene expression that reprogram the SAM to form flowers instead of leaves. Analysis of flowering of Arabidopsis thaliana placed the CONSTANS/FLOWERING LOCUS T (CO/FT) module at the core of a pathway that promotes flowering in response to changes in day length. We describe progress in defining the molecular mechanisms that activate this module in response to changing day length and the increasing evidence that FT protein is a major component of florigen. Finally, we discuss conservation of FT function in other species and how variation in its regulation could generate different flowering behaviors.

GIGANTEA: a circadian clock-controlled gene that regulates photoperiodic flowering in Arabidopsis and encodes a protein with several possible membrane-spanning domains

Flowering of Arabidopsis is promoted by long days and delayed by short days. Mutations in the GIGANTEA (GI) gene delay flowering under long days but have little or no effect under short days. We have now isolated the GI gene and show that it encodes a novel, putative membrane protein. By comparing the sequence of the Arabidopsis gene with that of a likely rice orthologue and by sequencing mutant alleles, we identify regions of the GI protein that are likely to be important for its function. We show that GI expression is regulated by the circadian clock with a peak in transcript levels 8–10 h after dawn. The timing, height and duration of this peak are influenced by daylength. We analysed the interactions between GI and the LHY, CCA1 and ELF3 genes, previously shown to affect daylength responses; we show that the rhythmic pattern of GI expression is altered in the elf3, CCA1‐OX and lhy genotypes, and that CCA1 and LHY expression are reduced by gi mutations. Our results are consistent with the idea that GI plays an important role in regulating the expression of flowering time genes during the promotion of flowering by photoperiod.

The genetic basis of flowering responses to seasonal cues

Plants respond to the changing seasons to initiate developmental programmes precisely at particular times of year. Flowering is the best characterized of these seasonal responses, and in temperate climates it often occurs in spring. Genetic approaches in Arabidopsis thaliana have shown how the underlying responses to changes in day length (photoperiod) or winter temperature (vernalization) are conferred and how these converge to create a robust seasonal response. Recent advances in plant genome analysis have demonstrated the diversity in these regulatory systems in many plant species, including several crops and perennials, such as poplar trees. Here, we report progress in defining the diverse genetic mechanisms that enable plants to recognize winter, spring and autumn to initiate flower development.

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.

LHY and CCA1 are partially redundant genes required to maintain circadian rhythms in Arabidopsis

Several genes are known to regulate circadian rhythms in Arabidopsis, but the identity of the central oscillator has not been established. LHY and CCA1 are related MYB-like transcription factors proposed to be closely involved. Here we demonstrate that, as shown previously for CCA1, inactivation of LHY shortens the period of circadian rhythms in gene expression and leaf movements. By constructing lhy cca1-1 double mutants, we show that LHY and CCA1 are partially redundant and essential for the maintenance of circadian rhythms in constant light. Under light/dark cycles the lhy cca1-1 plants show dramatically earlier phases of expression of GI and TOC1, genes associated with the generation of circadian rhythms and the promotion of LHY and CCA1 expression. We conclude that LHY and CCA1 appear to be negative regulatory elements required for central oscillator function.

Arabidopsis TFL2/LHP1 Specifically Associates with Genes Marked by Trimethylation of Histone H3 Lysine 27

TERMINAL FLOWER 2/LIKE HETEROCHROMATIN PROTEIN 1 (TFL2/LHP1) is the only Arabidopsis protein with overall sequence similarity to the HETEROCHROMATIN PROTEIN 1 (HP1) family of metazoans and S. pombe. TFL2/LHP1 represses transcription of numerous genes, including the flowering-time genes FLOWERING LOCUS T (FT) and FLOWERING LOCUS C (FLC), as well as the floral organ identity genes AGAMOUS (AG) and APETALA 3 (AP3). These genes are also regulated by proteins of the Polycomb repressive complex 2 (PRC2), and it has been proposed that TFL2/LHP1 represents a potential stabilizing factor of PRC2 activity. Here we show by chromatin immunoprecipitation and hybridization to an Arabidopsis Chromosome 4 tiling array (ChIP-chip) that TFL2/LHP1 associates with hundreds of small domains, almost all of which correspond to genes located within euchromatin. We investigated the chromatin marks to which TFL2/LHP1 binds and show that, in vitro, TFL2/LHP1 binds to histone H3 di- or tri-methylated at lysine 9 (H3K9me2 or H3K9me3), the marks recognized by HP1, and to histone H3 trimethylated at lysine 27 (H3K27me3), the mark deposited by PRC2. However, in vivo TFL2/LHP1 association with chromatin occurs almost exclusively and co-extensively with domains marked by H3K27me3, but not H3K9me2 or -3. Moreover, the distribution of H3K27me3 is unaffected in lhp1 mutant plants, indicating that unlike PRC2 components, TFL2/LHP1 is not involved in the deposition of this mark. Rather, our data suggest that TFL2/LHP1 recognizes specifically H3K27me3 in vivo as part of a mechanism that represses the expression of many genes targeted by PRC2.