HUA2 is required for the expression of floral repressors in Arabidopsis thaliana

The HUA2 gene acts as a repressor of floral transition. Lesions in hua2 were identified through a study of natural variation and through two mutant screens. An allele of HUA2 from Landsberg erecta (Ler) contains a premature stop codon and acts as an enhancer of early flowering 4 (elf4) mutants. hua2 single mutants, in the absence of the elf4 lesion, flower earlier than wild type under short days. hua2 mutations partially suppress late flowering in FRIGIDA (FRI )-containing lines, autonomous pathway mutants, and a photoperiod pathway mutant. hua2 mutations suppress late flowering by reducing the expression of several MADS genes that act as floral repressors including FLOWERING LOCUS C (FLC ) and FLOWERING LOCUS M (FLM ).     

Double loss-of-function mutation in EARLY FLOWERING 3 and CRYPTOCHROME 2 genes delays flowering under continuous light but accelerates it under long days and short days: an important role for Arabidopsis CRY2 to accelerate flowering time in continuous light

The photoperiodic response is one of the adaptation mechanisms to seasonal changes of lengths of day and night. The circadian clock plays pivotal roles in this process. In Arabidopsis, LHY, CCA1, ELF3, and other clock proteins play major roles in maintaining circadian rhythms. lhy;cca1 double mutants with severe defects in circadian rhythms showed accelerated flowering under short days (SDs), but delayed flowering under continuous light (LL). The protein level of the floral repressor SVP increased in lhy;cca1 mutants under LL, and the late-flowering phenotype of lhy;cca1 mutants was partially suppressed by svp, flc, or elf3. ELF3 interacted with both CCA1 and SVP, and elf3 suppressed the SVP accumulation in lhy;cca1 under LL. These results suggest that the unique mechanism of the inversion of the flowering response of lhy;cca1 under LL may involve both the ELF3-SVP/FLC-dependent and -independent pathways. In this work, elf3-1 seeds were mutagenized with heavy-ion beams and used to identify mutation(s) that delayed flowering under LL but not long days (LDs) or SDs even without ELF3. In this screening, seven candidate lines named suppressor of elf3 1 (self1), sel3, sel5, sel7, sel14, sel15, and sel20 were identified. Genetic analysis indicated that sel20 was a new deletion allele of a mutation in the blue light receptor, CRY2. A late-flowering phenotype and decrease of FT expression in the elf3;sel20 double mutant was obvious under LL but not under SDs or LDs. These results indicated that the late-flowering phenotype in the double mutant elf3;sel20 as well as in lhy;cca1 was affected by the presence of darkness. The results suggest that CRY2 may play more essential roles in the acceleration of flowering under LL than LDs or SDs.     

CCA1 and ELF3 Interact in the control of hypocotyl length and flowering time in Arabidopsis

The circadian clock is an endogenous oscillator with a period of approximately 24 h that allows organisms to anticipate, and respond to, changes in the environment. In Arabidopsis (Arabidopsis thaliana), the circadian clock regulates a wide variety of physiological processes, including hypocotyl elongation and flowering time. CIRCADIAN CLOCK ASSOCIATED1 (CCA1) is a central clock component, and CCA1 overexpression causes circadian dysfunction, elongated hypocotyls, and late flowering. EARLY FLOWERING3 (ELF3) modulates light input to the clock and is also postulated to be part of the clock mechanism. elf3 mutations cause light-dependent arrhythmicity, elongated hypocotyls, and early flowering. Although both genes affect similar processes, their relationship is not clear. Here, we show that CCA1 represses ELF3 by associating with its promoter, completing a CCA1-ELF3 negative feedback loop that places ELF3 within the oscillator. We also show that ELF3 acts downstream of CCA1, mediating the repression of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5 in the control of hypocotyl elongation. In the regulation of flowering, our findings show that ELF3 and CCA1 either cooperate or act in parallel through the CONSTANS/FLOWERING LOCUS T pathway. In addition, we show that CCA1 represses GIGANTEA and SUPPRESSOR OF CONSTANS1 by direct interaction with their promoters, revealing additional connections between the circadian clock and the flowering pathways.     

A complementary role for ELF3 and TFL1 in the regulation of flowering time by ambient temperature

Plants regulate their time to flowering by gathering information from the environment. Photoperiod and temperature are among the most important environmental variables. Sub-optimal, but not near-freezing, temperatures regulate flowering through the thermosensory pathway, which overlaps with the autonomous pathway. Here we show that ambient temperature regulates flowering by two genetically distinguishable pathways, one requiring TFL1 and another requiring ELF3 . The delay in flowering time observed at lower temperatures was partially suppressed in single elf3 and tfl1 mutants, whereas double elf3 tfl1 mutants were insensitive to temperature. tfl1 mutations abolished the temperature response in cryptochrome mutants that are deficient in photoperiod perception, but not in phyB mutants, which have a constitutive photoperiodic response. In contrast to tfl1 , elf3 mutations were able to suppress the temperature response in phyB mutants, but not in cryptochrome mutants. Gene expression profiles revealed that the tfl1 and elf3 effects are due to the activation of different sets of genes, and identified CCA1 and SOC1/AGL20 as being important cross-talk points. Finally, genome-wide gene expression analysis strongly suggests a general and complementary role for ELF3 and TFL1 in temperature signalling.     

Establishment of the vernalization-responsive, winter-annual habit in Arabidopsis requires a putative histone H3 methyl transferase

Winter-annual accessions of Arabidopsis thaliana are often characterized by a requirement for exposure to the cold of winter to initiate flowering in the spring. The block to flowering prior to cold exposure is due to high levels of the flowering repressor FLOWERING LOCUS C (FLC). Exposure to cold promotes flowering through a process known as vernalization that epigenetically represses FLC expression. Rapid-cycling accessions typically have low levels of FLC expression and therefore do not require vernalization. A screen for mutants in which a winter-annual Arabidopsis is converted to a rapid-cycling type has identified a putative histone H3 methyl transferase that is required for FLC expression. Lesions in this methyl transferase, EARLY FLOWERING IN SHORT DAYS (EFS), result in reduced levels of histone H3 Lys 4 trimethylation in FLC chromatin. EFS is also required for expression of other genes in the FLC clade, such as MADS AFFECTING FLOWERING2 and FLOWERING LOCUS M. The requirement for EFS to permit expression of several FLC clade genes accounts for the ability of efs lesions to suppress delayed flowering due to the presence of FRIGIDA, autonomous pathway mutations, or growth in noninductive photoperiods. efs mutants exhibit pleiotropic phenotypes, indicating that the role of EFS is not limited to the regulation of flowering time.     

The FLX gene of Arabidopsis is required for FRI-dependent activation of FLC expression

The Arabidopsis FLOWERING LOCUS C (FLC) gene encodes a MADS box protein that acts as a dose-dependent repressor of flowering. Mutants and ecotypes with elevated expression of FLC are late flowering and vernalization responsive. In this study we describe an early flowering mutant in the C24 ecotype, flc expressor (flx), that has reduced expression of FLC. FLX encodes a protein of unknown function with putative leucine zipper domains. FLX is required for FRIGIDA (FRI)-mediated activation of FLC but not for activation of FLC in autonomous pathway mutants. FLX is also required for expression of the FLC paralogs MADS AFFECTING FLOWERING 1 (MAF1) and MAF2.     

The nuclear actin-related protein ARP6 is a pleiotropic developmental regulator required for the maintenance of FLOWERING LOCUS C expression and repression of flowering in Arabidopsis

Actin-related proteins (ARPs) are found in the nuclei of all eukaryotic cells, but their functions are generally understood only in the context of their presence in various yeast and animal chromatin-modifying complexes. Arabidopsis thaliana ARP6 is a clear homolog of other eukaryotic ARP6s, including Saccharomyces cerevisiae ARP6, which was identified as a component of the SWR1 chromatin remodeling complex. We examined the subcellular localization, expression patterns, and loss-of-function phenotypes for this protein and found that Arabidopsis ARP6 is localized to the nucleus during interphase but dispersed away from the chromosomes during cell division. ARP6 expression was observed in all vegetative tissues as well as in a subset of reproductive tissues. Null mutations in ARP6 caused numerous defects, including altered development of the leaf, inflorescence, and flower as well as reduced female fertility and early flowering in both long- and short-day photoperiods. The early flowering of arp6 mutants was associated with reduced expression of the central floral repressor gene FLOWERING LOCUS C (FLC) as well as MADS AFFECTING FLOWERING 4 (MAF4) and MAF5. In addition, arp6 mutations suppress the FLC-mediated late flowering of a FRIGIDA-expressing line, indicating that ARP6 is required for the activation of FLC expression to levels that inhibit flowering. These results indicate that ARP6 acts in the nucleus to regulate plant development, and we propose that it does so through modulation of chromatin structure and the control of gene expression.     

GIGANTEA and EARLY FLOWERING 4 in Arabidopsis exhibit differential phase-specific genetic influences over a diurnal cycle

The endogenous circadian clock regulates many physiological processes related to plant survival and adaptability. GIGANTEA (GI), a clock-associated protein, contributes to the maintenance of circadian period length and amplitude, and also regulates flowering time and hypocotyl growth in response to day length. Similarly, EARLY FLOWERING 4 (ELF4), another clock regulator, also contributes to these processes. However, little is known about either the genetic or molecular interactions between GI and ELF4 in Arabidopsis. In this study, we investigated the genetic interactions between GI and ELF4 in the regulation of circadian clock-controlled outputs. Our mutant analysis shows that GI is epistatic to ELF4 in flowering time determination, while ELF4 is epistatic to GI in hypocotyl growth regulation. Moreover, GI and ELF4 have a synergistic or additive effect on endogenous clock regulation. Gene expression profiling of gi, elf4, and gi elf4 mutants further established that GI and ELF4 have differentially dominant influences on circadian physiological outputs at dusk and dawn, respectively. This phasing of GI and ELF4 influences provides a potential means to achieve diversity in the regulation of circadian physiological outputs, including flowering time and hypocotyl growth.     

PIE1, an ISWI family gene,is required for FLC activation and floral repression in Arabidopsis

Proper control of the floral transition is critical for reproductive success in flowering plants. In Arabidopsis, FLOWERING LOCUS C (FLC) is a floral repressor upon which multiple floral regulatory pathways converge. Mutations in PHOTOPERIOD-INDEPENDENT EARLY FLOWERING1 (PIE1) suppress the FLC-mediated delay of flowering as a result of the presence of FRIGIDA or of mutations in autonomous pathway genes. PIE1 is required for high levels of FLC expression in the shoot apex, but it is not required for FLC expression in roots. PIE1 is similar to ATP-dependent, chromatin-remodeling proteins of the ISWI and SWI2/SNF2 family. The role of PIE1 as an activator of FLC is consistent with the general role of ISWI and SWI2/SNF2 family genes as activators of gene expression. The pie1 mutation also causes early flowering in noninductive photoperiods independently of FLC; thus, PIE1 appears to be involved in multiple flowering pathways. PIE1 also plays a role in petal development, as revealed by the suppression of petal defects of the curly leaf mutant by the pie1 mutation.     

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.     

FLOWERING LOCUS C encodes a novel MADS domain protein that acts as a repressor of flowering.

Winter-annual ecotypes of Arabidopsis are relatively late flowering, unless the flowering of these ecotypes is promoted by exposure to cold (vernalization). This vernalization-suppressible, late-flowering phenotype results from the presence of dominant, late-flowering alleles at two loci, FRIGIDA (FRI) and FLOWERING LOCUS C (FLC). In this study, we report that flc null mutations result in early flowering, demonstrating that the role of active FLC alleles is to repress flowering. FLC was isolated by positional cloning and found to encode a novel MADS domain protein. The levels of FLC mRNA are regulated positively by FRI and negatively by LUMINIDEPENDENS. FLC is also negatively regulated by vernalization. Overexpression of FLC from a heterologous promoter is sufficient to delay flowering in the absence of an active FRI allele. We propose that the level of FLC activity acts through a rheostat-like mechanism to control flowering time in Arabidopsis and that modulation of FLC expression is a component of the vernalization response.