Polycomb Group蛋白复合体与植物春化作用

Polycomb Group(PcG)蛋白能形成Polycomb Repressive Complex 1(PRC1)和PRC2等复合体,通过一个保守且表观遗传的机制调节基因表达并控制动植物的发育。拟南芥中由VERNALIZATION2参与形成的PRC2复合体(VRN2-PRC2)在春化过程中能对主要开花抑制基因FLOWER LOCUS C(FLC)的染色质进行组蛋白甲基化修饰,形成H3K27me3(组蛋白H3第27位赖氨酸三甲基化)等转录抑制标记,从而抑制FLC转录,促进开花。虽然麦类作物的春化机理与拟南芥有较大差异,但最近的研究表明麦类作物春化过程也受PcG蛋白调控。文章对拟南芥PcG蛋白介导的春化调节机制进行综述,期望能对植物尤其是麦类作物的春化机理研究提供资料。     

Sexual and apomictic seed formation in Hieracium requires the plant polycomb-group gene FERTILIZATION INDEPENDENT ENDOSPERM

A Polycomb-Group (PcG) complex, FERTILIZATION INDEPENDENT SEED (FIS), represses endosperm development in Arabidopsis thaliana until fertilization occurs. The Hieracium genus contains apomictic species that form viable seeds asexually. To investigate FIS function during apomictic seed formation, FERTILIZATION INDEPENDENT ENDOSPERM (FIE), encoding a WD-repeat member of the FIS complex, was isolated and downregulated in sexual and apomictic Hieracium species. General downregulation led to defects in leaf and seed development, consistent with a role in developmental transitions and cell fate. PcG-like activity of Hieracium FIE was also supported by its interaction in vitro with the Arabidopsis CURLY LEAF PcG protein. By contrast, specific downregulation of FIE in developing seeds of sexual Hieracium did not result in autonomous endosperm proliferation but led to seed abortion after cross-pollination. Furthermore, in apomictic Hieracium, specific FIE downregulation inhibited autonomous embryo and endosperm initiation, and most autonomous seeds displayed defective embryo and endosperm growth. Therefore, FIE is required for both apomictic and fertilization-induced seed initiation in Hieracium. Since Hieracium FIE failed to interact with FIS class proteins in vitro, its partner proteins might differ from those in the FIS complex of Arabidopsis. These differences in protein interaction were attributed to structural modifications predicted from comparisons of Arabidopsis and Hieracium FIE molecular models.     

Antagonistic roles of SEPALLATA3, FT and FLC genes as targets of the polycomb group gene CURLY LEAF

In Arabidopsis, mutations in the Pc-G gene CURLY LEAF (CLF) give early flowering plants with curled leaves. This phenotype is caused by mis-expression of the floral homeotic gene AGAMOUS (AG) in leaves, so that ag mutations largely suppress the clf phenotype. Here, we identify three mutations that suppress clf despite maintaining high AG expression. We show that the suppressors correspond to mutations in FPA and FT, two genes promoting flowering, and in SEPALLATA3 (SEP3) which encodes a co-factor for AG protein. The suppression of the clf phenotype is correlated with low SEP3 expression in all case and reveals that SEP3 has a role in promoting flowering in addition to its role in controlling floral organ identity. Genetic analysis of clf ft mutants indicates that CLF promotes flowering by reducing expression of FLC, a repressor of flowering. We conclude that SEP3 is the key target mediating the clf phenotype, and that the antagonistic effects of CLF target genes masks a role for CLF in promoting flowering.     

基因组印迹与种子发育

胚乳介导营养物质从母体到胚的转运过程,是开花植物中发生印迹的重要部位。胚乳的发育异常会导致胚的败育。在拟南芥中已鉴定到三个FIS (fertilization-independent seed) 基因,能制止无需受精即形成种子的发育过程,即FIS1/ MEDEA、FIS2和FIS3/FIE。其中MEDEA基因是胚乳发育的主要调控基因,在胚乳中被印迹。FWA基因也在胚乳中被印迹。系统阐述了植物基因组印迹的机理以及MEA和FWA印迹机制的研究进展,并介绍了印迹发生的亲本冲突学说、印迹的方式及其它已报道的印迹基因。     

The Arabidopsis homeobox gene, ATHB16, regulates leaf development and the sensitivity to photoperiod in Arabidopsis

This report describes the characterisation of ATHB16, a novel Arabidopsis thaliana homeobox gene, which encodes a homeodomain-leucine zipper class I (HDZip I) protein. We demonstrate that ATHB16 functions as a growth regulator, potentially as a component in the light-sensing mechanism of the plant. Endogenous ATHB16 mRNA was detected in all organs of Arabidopsis, at highest abundance in rosette leaves. Reduced levels of ATHB16 expression in transgenic Arabidopsis plants caused an increase in leaf cell expansion and consequently an increased size of the leaves, whereas leaf shape was unaffected. Transgenic plants with increased ATHB16 mRNA levels developed leaves that were smaller than wild-type leaves. Therefore, we suggest ATHB16 to act as a negative regulator of leaf cell expansion. Furthermore, the flowering time response to photoperiod was increased in plants with reduced ATHB16 levels but reduced in plants with elevated ATHB16 levels, indicating that ATHB16 has an additional role as a suppressor of the flowering time sensitivity to photoperiod in wild-type Arabidopsis. As deduced from the response of transgenic plants with altered levels of ATHB16 expression in hypocotyl elongation assays, the gene may act to regulate plant development as a mediator of a blue light response.     

Polycomb-group mediated epigenetic mechanisms through plant evolution

Polycomb Group (PcG) proteins form an epigenetic "memory system", conserved in both plants and animals, controlling global gene expression during development via histone modifications. The role of PcG proteins in plants was primarily explored in Arabidopsis thaliana, where PcG regulation of developmental processes was demonstrated throughout the plant life cycle. Our knowledge about the PcG machinery in terrestrial plants other than Arabidopsis began to accumulate only in recent years. In this review we summarize recent emerging data on the evolution and diversification of PcG mechanisms in various phyla, from early-diverging plants, including members of the Chlorophyte algae, through bryophytes and flowering plants. We describe the compositions of the PcG gene families, their so-far studied expression profiles, and finally summarize commonalities vs. differences among PcG functions across the various species. This article is part of a Special Issue entitled: Epigenetic control of cellular and developmental processes in plants.     

Repression of FLOWERING LOCUS C and FLOWERING LOCUS T by the Arabidopsis Polycomb repressive complex 2 components

Polycomb group (PcG) proteins are evolutionarily conserved in animals and plants, and play critical roles in the regulation of developmental gene expression. Here we show that the Arabidopsis Polycomb repressive complex 2 (PRC2) subunits CURLY LEAF (CLF), EMBRYONIC FLOWER 2 (EMF2) and FERTILIZATION INDEPENDENT ENDOSPERM (FIE) repress the expression of FLOWERING LOCUS C (FLC), a central repressor of the floral transition in Arabidopsis and FLC relatives. In addition, CLF directly interacts with and mediates the deposition of repressive histone H3 lysine 27 trimethylation (H3K27me3) into FLC and FLC relatives, which suppresses active histone H3 lysine 4 trimethylation (H3K4me3) in these loci. Furthermore, we show that during vegetative development CLF and FIE strongly repress the expression of FLOWERING LOCUS T (FT), a key flowering-time integrator, and that CLF also directly interacts with and mediates the deposition of H3K27me3 into FT chromatin. Our results suggest that PRC2-like complexes containing CLF, EMF2 and FIE, directly interact with and deposit into FT, FLC and FLC relatives repressive trimethyl H3K27 leading to the suppression of active H3K4me3 in these loci, and thus repress the expression of these flowering genes. Given the central roles of FLC and FT in flowering-time regulation in Arabidopsis, these findings suggest that the CLF-containing PRC2-like complexes play a significant role in control of flowering in Arabidopsis.     

Epigenetic chromatin modifiers in barley: IV. The study of barley Polycomb group (PcG) genes during seed development and in response to external ABA

Background  

Epigenetic phenomena have been associated with the regulation of active and silent chromatin states achieved by modifications of chromatin structure through DNA methylation, and histone post-translational modifications. The latter is accomplished, in part, through the action of PcG (Polycomb group) protein complexes which methylate nucleosomal histone tails at specific sites, ultimately leading to chromatin compaction and gene silencing. Different PcG complex variants operating during different developmental stages have been described in plants. In particular, the so-called FIE/MEA/FIS2 complex governs the expression of genes important in embryo and endosperm development in Arabidopsis. In our effort to understand the epigenetic mechanisms regulating seed development in barley (Hordeum vulgare), an agronomically important monocot plant cultivated for its endosperm, we set out to characterize the genes encoding barley PcG proteins.     

Nucleostemin-like 1 is required for embryogenesis and leaf development in Arabidopsis

Arabidopsis NSN1 encodes a nucleolar GTP-binding protein and is required for flower development. Defective flowers were formed in heterozygous nsn1/+?plants. Homozygous nsn1 plants were dwarf and exhibited severe defects in reproduction. Arrests in embryo development in nsn1 could occur at any stage of embryogenesis. Cotyledon initiation and development during embryogenesis were distorted in nsn1 plants. At the seedling stage, cotyledons and leaves of nsn1 formed upward curls. The curled leaves developed meristem-like outgrowths or hyperplasia tissues in the adaxial epidermis. Long and enlarged pavement cells, characteristic of the abaxial epidermis of wild type plants, were found in the adaxial epidermis in nsn1 leaves, suggesting a disoriented leaf polarity in the mutant. The important role of NSN1 in embryo development and leaf differentiation was consistent with the high level expression of the NSN1 gene in the developing embryos and the primordia of cotyledons and leaves. The CLAVATA 3 (CLV3) gene, a stem cell marker in the Arabidopsis shoot apical meristem (SAM), was expressed in expanded regions surrounding the SAM of nsn1 plants, and induced ectopically in the meristem-like outgrowths in cotyledons and leaves. The nsn1 mutation up-regulated the expression levels of several genes implicated in the meristem identity and the abaxial cell fate, and repressed the expression of other genes related to the specification of cotyledon boundary and abaxial identity. These results demonstrate that NSN1 represents a novel GTPase required for embryogenesis, leaf development and leaf polarity establishment in Arabidopsis.     

Genetic networks regulated by ASYMMETRIC LEAVES1 (AS1) and AS2 in leaf development in Arabidopsis thaliana: KNOX genes control five morphological events

The asymmetric leaves 1 ( as1 ) and as2 mutants of Arabidopsis thaliana exhibit pleiotropic phenotypes. Expression of a number of genes, including three class-1 KNOTTED -like homeobox ( KNOX ) genes ( BP , KNAT2 and KNAT6 ) and ETTIN / ARF3 , is enhanced in these mutants. In the present study, we attempted to identify the phenotypic features of as1 and as2 mutants that were generated by ectopic expression of KNOX genes, using multiple loss-of-function mutations of KNOX genes as well as as1 and as2 . Our results revealed that the ectopic expression of class-1 KNOX genes resulted in reductions in the sizes of leaves, reductions in the size of sepals and petals, the formation of a less prominent midvein, the repression of adventitious root formation and late flowering. Our results also revealed that the reduction in leaf size and late flowering were caused by the repression, by KNOX genes, of a gibberellin (GA) pathway in as1 and as2 plants. The formation of a less prominent midvein and the repression of adventitious root formation were not, however, related to the GA pathway. The asymmetric formation of leaf lobes, the lower complexity of higher-ordered veins, and the elevated frequency of adventitious shoot formation on leaves of as1 and as2 plants were not rescued by multiple mutations in KNOX genes. These features must, therefore, be controlled by other genes in which expression is enhanced in the as1 and as2 mutants.     

In vitro morphogenesis of arrested embryos from lethal mutants of Arabidopsis thaliana

Summary Arrested embryos from lethal (emb) mutants of Arabidopsis thaliana were rescued on a nutrient medium designed to promote plant regeneration from immature wild-type cotyledons. The best response was observed with mutant embryos arrested at the heart to cotyledon stages of development. Embryos arrested at a globular stage produced callus but failed to turn green or form normal shoots in culture. Many of the mutant plants produced in culture were unusually pale with abnormal leaves, rosettes, and patterns of reproductive development. Other plants were phenotypically normal except for the presence of siliques containing 100% aborted seeds following self-pollination. These results demonstrate that genes with essential functions during plant embryo development differ in their pattern of expression at later stages of the life cycle. Most of the 15 genes examined in this study were essential for embryogenesis but were required again for subsequent stages of development. Only EMB24 appeared to be limited in function to embryo development. These differences in the response of mutant embryos in culture may facilitate the classification of embryonic lethals and the identification of genes with developmental rather than housekeeping functions.     

KNOX homeobox genes potentially have similar function in both diploid unicellular and multicellular meristems, but not in haploid meristems

Members of the class 1 knotted-like homeobox (KNOX) gene family are important regulators of shoot apical meristem development in angiosperms. To determine whether they function similarly in seedless plants, three KNOX genes (two class 1 genes and one class 2 gene) from the fern Ceratopteris richardii were characterized. Expression of both class 1 genes was detected in the shoot apical cell, leaf primordia, marginal part of the leaves, and vascular bundles by in situ hybridization, a pattern that closely resembles that of class 1 KNOX genes in angiosperms with compound leaves. The fern class 2 gene was expressed in all sporophyte tissues examined, which is characteristic of class 2 gene expression in angiosperms. All three CRKNOX genes were not detected in gametophyte tissues by RNA gel blot analysis. Arabidopsis plants overexpressing the fern class 1 genes resembled plants that overexpress seed plant class 1 KNOX genes in leaf morphology. Ectopic expression of the class 2 gene in Arabidopsis did not result in any unusual phenotypes. Taken together with phylogenetic analysis, our results suggest that (a) the class 1 and 2 KNOX genes diverged prior to the divergence of fern and seed plant lineages, (b) the class 1 KNOX genes function similarly in seed plant and fern sporophyte meristem development despite their differences in structure, (c) KNOX gene expression is not required for the development of the fern gametophyte, and (d) the sporophyte and gametophyte meristems of ferns are not regulated by the same developmental mechanisms at the molecular level.