拟南芥春化作用相关基因FLC的表达调控及天然早花突变体的研究进展

植物开花是从营养生长到生殖状态的重要发育转变,是多种内在因子和环境因素共同作用的结果。在拟南芥开花调控网络中,开花抑制基因FLC处于枢纽地位。FLC的表达受许多来自环境和生长发育的信号调控,主要包括：PAF1复合体、SWR1复合体成员,FRI依赖途径、自主途径和春化作用途径基因。本文主要综述了影响FLC表达的春化相关基因及天然早花突变体的研究进展,并根据最新的研究成果提出该研究领域的研究方向和重点。     

Does CK2 affect flowering time by modulating the autonomous pathway in Arabidopsis?

CK2 (Casein kinase II), a ubiquitous Ser/Thr kinase, affects multiple developmental and stress response pathways in Arabidopsis, including flowering time under both long- and short-day conditions through the photoperiod and autonomous pathways. CK2 phosphorylates central clock components, CCA1 and LHY, to modulate circadian clock that regulates flowering time through the photoperiod pathway. However, how CK2 regulates flowering time through the autonomous pathway is still unknown. Analyses of phosphorylation sites using several prediction softwares show that most of the autonomous pathway components have multiple CK2 phosphorylation sites. CK2 might phosphorylate any or all of these components to modulate their activity/stability resulting in altered expression of FLC that drives flowering time through the autonomous pathway.     

Conservation and divergence of autonomous pathway genes in the flowering regulatory network of Beta vulgaris

The transition from vegetative growth to reproductive development is a complex process that requires an integrated response to multiple environmental cues and endogenous signals. In Arabidopsis thaliana, which has a facultative requirement for vernalization and long days, the genes of the autonomous pathway function as floral promoters by repressing the central repressor and vernalization-regulatory gene FLC. Environmental regulation by seasonal changes in daylength is under control of the photoperiod pathway and its key gene CO. The root and leaf crop species Beta vulgaris in the caryophyllid clade of core eudicots, which is only very distantly related to Arabidopsis, is an obligate long-day plant and includes forms with or without vernalization requirement. FLC and CO homologues with related functions in beet have been identified, but the presence of autonomous pathway genes which function in parallel to the vernalization and photoperiod pathways has not yet been reported. Here, this begins to be addressed by the identification and genetic mapping of full-length homologues of the RNA-regulatory gene FLK and the chromatin-regulatory genes FVE, LD, and LDL1. When overexpressed in A. thaliana, BvFLK accelerates bolting in the Col-0 background and fully complements the late-bolting phenotype of an flk mutant through repression of FLC. In contrast, complementation analysis of BvFVE1 and the presence of a putative paralogue in beet suggest evolutionary divergence of FVE homologues. It is further shown that BvFVE1, unlike FVE in Arabidopsis, is under circadian clock control. Together, the data provide first evidence for evolutionary conservation of components of the autonomous pathway in B. vulgaris, while also suggesting divergence or subfunctionalization of one gene. The results are likely to be of broader relevance because B. vulgaris expands the spectrum of evolutionarily diverse species which are subject to differential developmental and/or environmental regulation of floral transition.     

The Arabidopsis FLC protein interacts directly in vivo with SOC1 and FT chromatin and is part of a high-molecular-weight protein complex

The Arabidopsis Flowering Locus C (FLC) protein is a repressor of flowering regulated by genes in the autonomous and vernalization pathways. Previous genetic and transgenic data have suggested that FLC acts by repressing expression of the floral integrator genes SOC1 and FT. We have taken an in vivo approach to determine whether the FLC protein interacts directly with potential DNA targets. Using chromatin immunoprecipitation, we have shown that FLC binds to a region of the first intron of FT that contains a putative CArG box, and have confirmed that FLC binds to a CArG box in the promoter of the SOC1 gene. MADS box proteins are thought to bind their DNA targets as dimers or higher-order multimers. We have shown that FLC is a component of a multimeric protein complex in vivo and that more than one FLC polypeptides can be present in the complex.     

OsFY, a Homolog of AtFY, Encodes a Protein that Can Interact with OsFCA-γ in Rice （Oryza sativa L.）

FCA and FY are flowering time related genes involved in the autonomous flowering pathwayin Arabidopsis.FCA interacts with FY to regulate the alternative processing of FCA pre-mRNA.The FCA/FY interaction is also required for the regulation of FLC expression,a major floral repressor in Arabidopsis.However,it is not clear if the regulation of this autonomous flowering pathway is also present in monocotplants,such as rice.Recently,alternative RNA processing of OsFCA was observed in rice,which stronglysuggested the existence of an autonomous flowering pathway in rice.In this work,we cloned the cDNA ofthe autonomous flowering pathway gene OsFY from rice.The predicted OsFY protein contained a conserved7 WD-repeat region and at least two Pro-Pro-Leu-Pro motifs compared to Arabidopsis FY.The protein-protein interaction between OsFY and OsFCA-γ,the key feature of their gene function,was also demon-strated using the yeast two-hybrid system.The GenBank database search provided evidence of expressionfor other autonomous pathway gene homologs in rice.These results indicate that the autonomous floweringpathway is present in monocots,and the regulation through FY and FCA interaction is conserved betweenmonocots and dicots.     

The VERNALIZATION INDEPENDENCE 4 gene encodes a novel regulator of FLOWERING LOCUS C

The late-flowering, vernalization-responsive habit of many Arabidopsis ecotypes is mediated predominantly through repression of the floral programme by the FLOWERING LOCUS C (FLC) gene. To better understand this repressive mechanism, we have taken a genetic approach to identify novel genes that positively regulate FLC expression. We identified recessive mutations in a gene designated VERNALIZATION INDEPENDENCE 4 (VIP4), that confer early flowering and loss of FLC expression in the absence of cold. We cloned the VIP4 gene and found that it encodes a highly hydrophilic protein with similarity to proteins from yeasts, Drosophila, and Caenorhabditis elegans. Consistent with a proposed role as a direct activator of FLC, VIP4 is expressed throughout the plant in a pattern similar to that of FLC. However, unlike FLC, VIP4 RNA expression is not down-regulated in vernalized plants, suggesting that VIP4 is probably not sufficient to activate FLC, and that VIP4 is probably not directly involved in a vernalization mechanism. Epistasis analysis suggests that VIP4 could act in a separate pathway from previously identified FLC regulators, including FRIGIDA and the autonomous flowering promotion pathway gene LUMINIDEPENDENS. Mutants lacking detectable VIP4 expression flower earlier than FLC null mutants, suggesting that VIP4 regulates flowering-time genes in addition to FLC. Floral morphology is also disrupted in vip4 mutants; thus, VIP4 has multiple roles in development.     

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.     

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.     

FLC基因表达在植物春化过程中的作用

在对以往有关不同开花途径研究简要总结的基础上综述了FLC基因在春化过程中的作用。近期以拟南芥不同生态型和突变体为模式的研究结果表明基因FLC可能是春化反应的关键基因。研究发现 ,FLC的表达水平与植株低温处理的时间呈数量关系 ,低温处理时间越长 ,FLC的表达越弱 ,去甲基化也可能对FLC起负调控的作用。同时FLC也存在于自主开花途径中 ,与其他基因共同作用以调节植株开花时间。而FLC的表达对开花起抑制作用。一系列研究表明 ,春化的低温作用可能在于相关基因的去甲基化 ,消除了FLC对开花的抑制作用 ,从而解除赤霉素合成途径的封锁最终导致植株在一定时期开花。     

FLC基因表达在植物春化过程中的作用

在对以往有关不同开花途径研究简要总结的基础上综述了FLC基因在春化过程中 的作用。近期以拟南芥不同生态型和突变体为模式的研究结果表明基因FLC可能是春化反应的关键基因。研究发现,FLC的表达水平与植株低温处理的时间呈数量关系,低温处理时间越长,FLC的表达越弱,去甲基化也可能对FLC起负调控的作用。同时FLC也存在于自主开花途径中,与其他基因共同作用以调节植株开花时间。而FLC的表达对开花起抑制作用。一系列研究表明,春化的低温作用可能在于相关基因的去甲基化,消除了FLC对开花的抑制作用,从而解除赤霉素合成途径的封锁最终导致植株在一定时期开花。