Distinct photoperiodic responses are conferred by the same genetic pathway in Arabidopsis and in rice.
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Acclimatization↗
Biomedical subjects
George Coupland
Publications and source records attributed to George Coupland.
Shedding light on the circadian clock and the photoperiodic control of flowering.
Recently, notable progress has been made towards understanding the genetic interactions that underlie the function of the circadian clock in plants, and how these functions are related to the seasonal control of flowering time. The LHY/CCA1 and TOC1 genes have been proposed to participate in a negative feedback loop that is part of the central oscillator of the circadian clock. Furthermore, analysis of a flowering-time pathway has suggested how transcriptional regulation by the circadian clock, combined with post-transcriptional regulation by light, could activate proteins that control flowering time in response to appropriate daylengths.
Arabidopsis↗
Mutation of a family 8 glycosyltransferase gene alters cell wall carbohydrate composition and causes a humidity-sensitive semi-sterile dwarf phenotype in Arabidopsis.
The genome of Arabidopsis thaliana contains about 400 genes coding for glycosyltransferases, many of which are predicted to be involved in the synthesis and remodelling of cell wall components. We describe the isolation of a transposon-tagged mutant, parvus, which under low humidity conditions exhibits a severely dwarfed growth phenotype and failure of anther dehiscence resulting in semi-sterility. All aspects of the mutant phenotype were partially rescued by growth under high-humidity conditions, but not by the application of growth hormones or jasmonic acid. The mutation is caused by insertion of a maize Dissociation (Ds) element in a gene coding for a putative Golgi-localized glycosyltransferase belonging to family 8. Members of this family, originally identified on the basis of similarity to bacterial lipooligosaccharide glycosyltransferases, include enzymes known to be involved in the synthesis of bacterial and plant cell walls. Cell-wall carbohydrate analyses of the parvus mutant indicated reduced levels of rhamnogalacturonan I branching and alterations in the abundance of some xyloglucan linkages that may, however, be indirect consequences of the mutation.
Amino Acid Sequence↗
The evolution of CONSTANS-like gene families in barley, rice, and Arabidopsis.
The CO (CONSTANS) gene of Arabidopsis has an important role in the regulation of flowering by photoperiod. CO is part of a gene family with 17 members that are subdivided into three classes, termed Group I to III here. All members of the family have a CCT (CO, CO-like, TOC1) domain near the carboxy terminus. Group I genes, which include CO, have two zinc finger B-boxes near the amino terminus. Group II genes have one B-box, and Group III genes have one B-box and a second diverged zinc finger. Analysis of rice (Oryza sativa) genomic sequence identified 16 genes (OsA-OsP) that were also divided into these three groups, showing that their evolution predates monocot/dicot divergence. Eight Group I genes (HvCO1-HvCO8) were isolated from barley (Hordeum vulgare), of which two (HvCO1 and HvCO2) were highly CO like. HvCO3 and its rice counterpart (OsB) had one B-box that was distantly related to Group II genes and was probably derived by internal deletion of a two B-box Group I gene. Sequence homology and comparative mapping showed that HvCO1 was the counterpart of OsA (Hd1), a major determinant of photoperiod sensitivity in rice. Major genes determining photoperiod response have been mapped in barley and wheat (Triticum aestivum), but none corresponded to CO-like genes. Thus, selection for variation in photoperiod response has affected different genes in rice and temperate cereals. The peptides of HvCO1, HvCO2 (barley), and Hd1 (rice) show significant structural differences from CO, particularly amino acid changes that are predicted to abolish B-box2 function, suggesting an evolutionary trend toward a one-B-box structure in the most CO-like cereal genes.
Amino Acid Motifs↗
EARLY BOLTING IN SHORT DAYS is related to chromatin remodeling factors and regulates flowering in Arabidopsis by repressing FT.
The timing of flowering initiation depends on the balanced expression of a complex network of genes that are regulated by both endogenous and environmental factors. We showed previously that mutations at the EARLY BOLTING IN SHORT DAYS (EBS) locus of Arabidopsis result in an acceleration of flowering, especially in noninductive photoperiods (short days), and other phenotypic anomalies. We have identified the EBS gene and demonstrate that it encodes a nuclear protein that contains a bromoadjacent homology domain and a plant homeodomain Zn finger. Both types of motif are thought to mediate protein-protein interactions and occur in transcriptional regulators involved in chromatin remodeling, suggesting that EBS is part of a transcriptional repressor complex that modulates chromatin structure and is required to repress the initiation of flowering in short days. Overexpression of EBS has phenotypic effects similar to those of recessive ebs mutations, suggesting that both might disrupt the formation of protein complexes that contain EBS. Analysis of the expression of flowering-time genes in ebs mutants and in EBS-overexpressing plants indicates that EBS participates in the regulation of flowering time by specifically repressing the expression of FT, a key gene in the integration of floral promotion pathways in Arabidopsis.
Amino Acid Sequence↗
Antagonistic regulation of flowering-time gene SOC1 by CONSTANS and FLC via separate promoter motifs.
Flowering in Arabidopsis is controlled by endogenous and environmental signals relayed by distinct genetic pathways. The MADS-box flowering-time gene SOC1 is regulated by several pathways and is proposed to co-ordinate responses to environmental signals. SOC1 is directly activated by CONSTANS (CO) in long photoperiods and is repressed by FLC, a component of the vernalization (low-temperature) pathway. We show that in transgenic plants overexpressing CO and FLC, these proteins regulate flowering time antagonistically and FLC blocks transcriptional activation of SOC1 by CO. A series of SOC1::GUS reporter genes identified a 351 bp promoter sequence that mediates activation by CO and repression by FLC. A CArG box (MADS-domain protein binding element) within this sequence was recognized specifically by FLC in vitro and mediated repression by FLC in vivo, suggesting that FLC binds directly to the SOC1 promoter. We propose that CO is recruited to a separate promoter element by a DNA-binding factor and that activation by CO is impaired when FLC is bound to an adjacent CArG motif.
Arabidopsis↗
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.
Alleles↗
Control of flowering time: interacting pathways as a basis for diversity.
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Arabidopsis↗
Arabidopsis A BOUT DE SOUFFLE, which is homologous with mammalian carnitine acyl carrier, is required for postembryonic growth in the light.
The degradation of storage compounds just after germination is essential to plant development, providing energy and molecules necessary for the building of a photosynthetic apparatus and allowing autotrophic growth. We identified à bout de souffle (bou), a new Arabidopsis mutation. Mutant plants stopped developing after germination and degraded storage lipids, but they did not proceed to autotrophic growth. Neither leaves nor roots developed in the mutant. However, externally added sugar or germination in the dark could bypass this developmental block and allowed mutant plants to develop. The mutated gene was cloned using the transposon Dissociation as a molecular tag. The gene coding sequence showed similarity to those of the mitochondrial carnitine acyl carriers (CACs) or CAC-like proteins. In animals and yeast, these transmembrane proteins are involved in the transport of lipid-derived molecules across mitochondrial membranes for energy and carbon supply. The data presented here suggest that BOU identifies a novel mitochondrial pathway that is necessary to seedling development in the light. The BOU pathway would be an alternative to the well-known glyoxylate pathway.
2,4-Dichlorophenoxyacetic Acid↗
early in short days 4, a mutation in Arabidopsis that causes early flowering and reduces the mRNA abundance of the floral repressor FLC.
The plant shoot is derived from the apical meristem, a group of stem cells formed during embryogenesis. Lateral organs form on the shoot of an adult plant from primordia that arise on the flanks of the shoot apical meristem. Environmental stimuli such as light, temperature and nutrient availability often influence the shape and identity of the organs that develop from these primordia. In particular, the transition from forming vegetative lateral organs to producing flowers often occurs in response to environmental cues. This transition requires increased expression in primordia of genes that confer floral identity, such as the Arabidopsis gene LEAFY. We describe a novel mutant, early in short days 4 (esd4), that dramatically accelerates the transition from vegetative growth to flowering in Arabidopsis: The effect of the mutation is strongest under short photoperiods, which delay flowering of Arabidopsis: The mutant has additional phenotypes, including premature termination of the shoot and an alteration of phyllotaxy along the stem, suggesting that ESD4 has a broader role in plant development. Genetic analysis indicates that ESD4 is most closely associated with the autonomous floral promotion pathway, one of the well-characterized pathways proposed to promote flowering of Arabidopsis: Furthermore, mRNA levels of a floral repressor (FLC), which acts within this pathway, are reduced by esd4, and the expression of flowering-time genes repressed by FLC is increased in the presence of the esd4 mutation. Although the reduction in FLC mRNA abundance is likely to contribute to the esd4 phenotype, our data suggest that esd4 also promotes flowering independently of FLC. The role of ESD4 in the regulation of flowering is discussed with reference to current models on the regulation of flowering in Arabidopsis.
AGAMOUS Protein, Arabidopsis↗