Interaction of Polycomb-group proteins controlling flowering in Arabidopsis

In Arabidopsis, the EMBYRONIC FLOWER2 (EMF2), VERNALISATION2 (VRN2) and FERTILISATION INDEPENDENT ENDOSPERM2 (FIS2) genes encode related Polycomb-group (Pc-G) proteins. Their homologues in animals act together with other Pc-G proteins as part of a multimeric complex, Polycomb Repressive Complex 2 (PRC2), which functions as a histone methyltransferase. Despite similarities between the fis2 mutant phenotype and those of some other plant Pc-G members, it has remained unclear how the FIS2/EMF2/VRN2 class Pc-G genes interact with the others. We have identified a weak emf2 allele that reveals a novel phenotype with striking similarity to that of severe mutations in another Pc-G gene, CURLY LEAF (CLF), suggesting that the two genes may act in a common pathway. Consistent with this, we demonstrate that EMF2 and CLF interact genetically and that this reflects interaction of their protein products through two conserved motifs, the VEFS domain and the C5 domain. We show that the full function of CLF is masked by partial redundancy with a closely related gene, SWINGER (SWN), so that null clf mutants have a much less severe phenotype than emf2 mutants. Analysis in yeast further indicates a potential for the CLF and SWN proteins to interact with the other VEFS domain proteins VRN2 and FIS2. The functions of individual Pc-G members may therefore be broader than single mutant phenotypes reveal. We suggest that plants have Pc-G protein complexes similar to the Polycomb Repressive Complex2 (PRC2) of animals, but the duplication and subsequent diversification of components has given rise to different complexes with partially discrete functions.     

UBIQUITIN-SPECIFIC PROTEASE 26 is required for seed development and the repression of PHERES1 in Arabidopsis

The Arabidopsis mutant Atubp26 initiates autonomous endosperm at a frequency of approximately 1% in the absence of fertilization and develops arrested seeds at a frequency of approximately 65% when self-pollinated. These phenotypes are similar to those of the FERTILIZATION INDEPENDENT SEED (FIS) class mutants, mea, fis2, fie, and Atmsi1, which also show development of the central cell into endosperm in the absence of fertilization and arrest of the embryo following fertilization. Atubp26 results from a T-DNA insertion in the UBIQUITIN-SPECIFIC PROTEASE gene AtUBP26, which catalyzes deubiquitination of histone H2B and is required for heterochromatin silencing. The paternal copy of AtUBP26 is able to complement the loss of function of the maternal copy in postfertilization seed development. This contrasts to the fis class mutants where the paternal FIS copy does not rescue aborted seeds. As in the fis class mutants, the Polycomb group (PcG) complex target gene PHERES1 (PHE1) is expressed at higher levels in Atubp26 ovules than in wild type; there is a lower level of H3K27me3 at the PHE1 locus. The phenotypes suggest that AtUBP26 is required for normal seed development and the repression of PHE1.     

Embryo and endosperm development is disrupted in the female gametophytic capulet mutants of Arabidopsis

The female gametophyte of higher plants gives rise, by double fertilization, to the diploid embryo and triploid endosperm, which develop in concert to produce the mature seed. What roles gametophytic maternal factors play in this process is not clear. The female-gametophytic effects on embryo and endosperm development in the Arabidopsis mea, fis, and fie mutants appear to be due to gametic imprinting that can be suppressed by METHYL TRANSFERASE1 antisense (MET1 a/s) transgene expression or by mutation of the DECREASE IN DNA METHYLATION1 (DDM1) gene. Here we describe two novel gametophytic maternal-effect mutants, capulet1 (cap1) and capulet2 (cap2). In the cap1 mutant, both embryo and endosperm development are arrested at early stages. In the cap2 mutant, endosperm development is blocked at very early stages, whereas embryos can develop to the early heart stage. The cap mutant phenotypes were not rescued by wild-type pollen nor by pollen from tetraploid plants. Furthermore, removal of silencing barriers from the paternal genome by MET1 a/s transgene expression or by the ddm1 mutation also failed to restore seed development in the cap mutants. Neither cap1 nor cap2 displayed autonomous seed development, in contrast to mea, fis, and fie mutants. In addition, cap2 was epistatic to fis1 in both autonomous endosperm and sexual development. Finally, both cap1 and cap2 mutant endosperms, like wild-type endosperms, expressed the paternally inactive endosperm-specific FIS2 promoter GUS fusion transgene only when the transgene was introduced via the embryo sac, indicating that imprinting was not affected. Our results suggest that the CAP genes represent novel maternal functions supplied by the female gametophyte that are required for embryo and endosperm development.     

Endosperm cellularization defines an important developmental transition for embryo development

The endosperm is a terminal seed tissue that is destined to support embryo development. In most angiosperms, the endosperm develops initially as a syncytium to facilitate rapid seed growth. The transition from the syncytial to the cellularized state occurs at a defined time point during seed development. Manipulating the timing of endosperm cellularization through interploidy crosses negatively impacts on embryo growth, suggesting that endosperm cellularization is a critical step during seed development. In this study, we show that failure of endosperm cellularization in fertilization independent seed 2 (fis2) and endosperm defective 1 (ede1) Arabidopsis mutants correlates with impaired embryo development. Restoration of endosperm cellularization in fis2 seeds by reducing expression of the MADS-box gene AGAMOUS-LIKE 62 (AGL62) promotes embryo development, strongly supporting an essential role of endosperm cellularization for viable seed formation. Endosperm cellularization failure in fis2 seeds correlates with increased hexose levels, suggesting that arrest of embryo development is a consequence of failed nutrient translocation to the developing embryo. Finally, we demonstrate that AGL62 is a direct target gene of FIS Polycomb group repressive complex 2 (PRC2), establishing the molecular basis for FIS PRC2-mediated endosperm cellularization.     

Identification of new members of Fertilisation Independent Seed Polycomb Group pathway involved in the control of seed development in Arabidopsis thaliana

In higher plants, double fertilisation initiates seed development. One sperm cell fuses with the egg cell and gives rise to the embryo, the second sperm cell fuses with the central cell and gives rise to the endosperm. The endosperm develops as a syncytium with the gradual organisation of domains along an anteroposterior axis defined by the position of the embryo at the anterior pole and by the attachment to the placenta at the posterior pole. We report that ontogenesis of the posterior pole in Arabidopsis thaliana involves oriented migration of nuclei in the syncytium. We show that this migration is impaired in mutants of the three founding members of the FERTILIZATION INDEPENDENT SEED (FIS) class, MEDEA (MEA), FIS2 and FERTILIZATION INDEPENDENT ENDOSPERM (FIE). A screen based on a green fluorescent protein (GFP) reporter line allowed us to identify two new loci in the FIS pathway, medicis and borgia. We have cloned the MEDICIS gene and show that it encodes the Arabidopsis homologue of the yeast WD40 domain protein MULTICOPY SUPRESSOR OF IRA (MSI1). The mutations at the new fis loci cause the same cellular defects in endosperm development as other fis mutations, including parthenogenetic development, absence of cellularisation, ectopic development of posterior structures and overexpression of the GFP marker.     

Maternal control of seed development

Maternal control of higher plant seed development is likely to involve female sporophytic as well as female gametophytic genes. While numerous female sporophytic mutants control the production of the ovule and the embryo sac true maternal effect mutations affecting embryo and endosperm development are rare in plants. A new class of female gametophytic mutants has been isolated that controls autonomous development of endosperm. Molecular analyses of these genes, known as FIS class genes, suggest that they repress downstream seed development genes by chromatin remodelling. Expression of the FIS genes in turn is modulated by parent specific expression or genomic imprinting which in turn is controlled by DNA methylation. Thus maternal control of seed development is a complex developmental event influenced by both genetic and epigenetic processes.     

The Fertilization and the Development of Embryo and Endosperm in an Endangered Plant—Ranalisma rostratum Stapf (Alismataceae)

Ranalisma rostratum Stapf is a rare and endangered species． This paper deals with the fertilization and the development of embryo and endosperm in this plant．The embryogenesis is of Caryophyllad type and the development of endosperm belongs to Heobial type．Before fertilization,the two polar nuclei are located respectively at both ends of embryo sac. In most angiosperms with two polar nuclei,the polar nuclei may fuse eiher before fertilization to form a secondary nucleus or during fertilization called triple fusion． In Ranalisma rostratum Stapf, however, it is found that only in case when the micropylar polar nucleus is fertilized,it can move to the chalazal end and fuse with the chalazal polar nucleus．This phenomenon is very rare and the process must take more time to fulfil fertilization both polar nuclei． This feature of fusion of polar nuclei is therefore thought as a primitive character from the view of phylogeny．     

Mutations in FIE, a WD polycomb group gene, allow endosperm development without fertilization

A fundamental problem in biology is to understand how fertilization initiates reproductive development. Higher plant reproduction is unique because two fertilization events are required for sexual reproduction. First, a sperm must fuse with the egg to form an embryo. A second sperm must then fuse with the adjacent central cell nucleus that replicates to form an endosperm, which is the support tissue required for embryo and/or seedling development. Here, we report cloning of the Arabidopsis FERTILIZATION-INDEPENDENT ENDOSPERM (FIE) gene. The FIE protein is a homolog of the WD motif-containing Polycomb proteins from Drosophila and mammals. These proteins function as repressors of homeotic genes. A female gametophyte with a loss-of-function allele of fie undergoes replication of the central cell nucleus and initiates endosperm development without fertilization. These results suggest that the FIE Polycomb protein functions to suppress a critical aspect of early plant reproduction, namely, endosperm development, until fertilization occurs.     

Epigenetic mechanisms governing seed development in plants

Seed development in flowering plants is initiated by the fusion of two male gametes with two female gametes--the egg cell and the central cell--which leads to the formation of an embryo and an endosperm, respectively. Fertilization-independent seed formation is actively repressed by the FERTILIZATION-INDEPENDENT SEED (FIS) Polycomb group (PcG) proteins, an evolutionarily conserved class of proteins that ensures the stable transmission of developmental decisions. The FIS proteins act together in a complex and modify their target genes by applying repressive methylation on histone H3 lysine 27. In addition to its function before fertilization, the FIS complex restricts endosperm proliferation. This function is likely to be achieved by imprinting the maternal alleles of FIS target genes. However, imprinting in the endosperm is controlled not only by the FIS complex but also by DNA methylation, and the interconnections between these two processes are now being investigated.     

Epigenetic Resetting of a Gene Imprinted in Plant Embryos

Genomic imprinting resulting in the differential expression of maternal and paternal alleles in the fertilization products has evolved independently in placental mammals and flowering plants. In most cases, silenced alleles carry DNA methylation [1]. Whereas these methylation marks of imprinted genes are generally erased and reestablished in each generation in mammals [2], imprinting marks persist in endosperms [3], the sole tissue of reported imprinted gene expression in plants. Here we show that the maternally expressed in embryo 1 (mee1) gene of maize is imprinted in both the embryo and endosperm and that parent-of-origin-specific expression correlates with differential allelic methylation. This epigenetic asymmetry is maintained in the endosperm, whereas the embryonic maternal allele is demethylated on fertilization and remethylated later in embryogenesis. This report of imprinting in the plant embryo confirms that, as in mammals, epigenetic mechanisms operate to regulate allelic gene expression in both embryonic and extraembryonic structures. The embryonic methylation profile demonstrates that plants evolved a mechanism for resetting parent-specific imprinting marks, a necessary prerequisite for parent-of-origin-dependent gene expression in consecutive generations. The striking difference between the regulation of imprinting in the embryo and endosperm suggests that imprinting mechanisms might have evolved independently in both fertilization products of flowering plants.     

Polycomb group gene function in sexual and asexual seed development in angiosperms

In sexually reproducing angiosperms, double fertilization initiates seed development, giving rise to two fertilization products, the embryo and the endosperm. In the endosperm, a terminal nutritive tissue that supports embryo growth, certain genes are expressed differentially depending on their parental origin, and this genomic imbalance is required for proper seed formation. This parent-of-origin effect on gene expression, called genomic imprinting, is controlled epigenetically through histone modifications and DNA methylation. In the sexual model plant Arabidopsis, the Polycomb group (PcG) genes of the plant Fertilization Independent Seed (FIS)-class control genomic imprinting by specifically silencing maternal or paternal target alleles through histone modifications. Mutations in FIS genes can lead to a bypass in the requirement of fertilization for the initiation of endosperm development and seed abortion. In this review, we discuss the role of the FIS complex in establishing and maintaining genomic imprinting, focusing on recent advances in elucidating the expression and function of FIS-related genes in maize, rice, and Hieracium, and particularly including apomictic Hieracium species that do not require paternal contribution and thus form seeds asexually. Surprisingly, not all FIS-mediated functions described in Arabidopsis are conserved. However, the function of some PcG components are required for viable seed formation in seeds formed via sexual and asexual processes (apomixis) in Hieracium, suggesting a conservation of the seed viability function in some eudicots.     

千里光合子胚和胚乳形成及其早期发育的细胞学特征（英文）

千里光(Senecio scandens Buch.-Ham. ex D. Don)是传统中草药,抗菌功效显著。本研究从细胞学角度对千里光合子胚和胚乳的形成与发育进行观察研究。结果显示,结构和功能迥异的基细胞和顶细胞源自细胞质不均一分布的合子所致,推测合子的极性与胚囊的极性和生殖核分裂为"二态"精细胞有关;基细胞在合子胚胎"球型期"末期出现分化,早期胚胎的组织分化始于"三角期",可辨别的结构差异直到"鱼雷期"才出现。此外,胚乳形成遵循无细胞壁核化模型。本研究对千里光细胞分化、组织分化和结构差异各发育阶段特征的观察结果,不仅可为深入分析胚胎发育过程功能基因的时空表达提供依据,也为相关近缘物种的系统植物学研究提供参考资料。     

千里光合子胚和胚乳形成及其早期发育的细胞学特征(英文稿)

千里光（Senecio scandens Buch.-Ham. ex D. Don）是传统中草药, 抗菌功效显著。本研究从细胞学角度对千里光合子胚和胚乳的形成与发育进行观察研究。结果显示,结构和功能迥异的基细胞和顶细胞源自细胞质不均一分布的合子所致,推测合子的极性与胚囊的极性和生殖核分裂为“二态”精细胞有关;基细胞在合子胚胎“球型期”末期出现分化,早期胚胎的组织分化始于“三角期”,可辨别的结构差异直到“鱼雷期”才出现。此外,胚乳形成遵循无细胞壁核化模型。本研究对千里光细胞分化、组织分化和结构差异各发育阶段特征的观察结果,不仅可为深入分析胚胎发育过程功能基因的时空表达提供依据,也为相关近缘物种的系统植物学研究提供参考资料。     

Mutants in the imprinted PICKLE RELATED 2 gene suppress seed abortion of fertilization independent seed class mutants and paternal excess interploidy crosses in Arabidopsis

Endosperm cellularization is essential for embryo development and viable seed formation. Loss of function of the FERTILIZATION INDEPENDENT SEED (FIS) class Polycomb genes, which mediate trimethylation of histone H3 lysine27 (H3K27me3), as well as imbalanced contributions of parental genomes interrupt this process. The causes of the failure of cellularization are poorly understood. In this study we identified PICKLE RELATED 2 (PKR2) mutations which suppress seed abortion in fis1/mea by restoring endosperm cellularization. PKR2, a paternally expressed imprinted gene (PEG), encodes a CHD3 chromatin remodeler. PKR2 is specifically expressed in syncytial endosperm and its maternal copy is repressed by FIS1. Seed abortion in a paternal genome excess interploidy cross was also partly suppressed by pkr2. Simultaneous mutations in PKR2 and another PEG, ADMETOS (ADM), additively rescue the seed abortion in fis1 and in the interploidy cross, suggesting that PKR2 and ADM modulate endosperm cellularization independently and reproductive isolation between plants of different ploidy is established by imprinted genes. Genes upregulated in fis1 and downregulated in the presence of pkr2 are enriched in glycosyl‐hydrolyzing activity, while genes downregulated in fis1 and upregulated in the presence of pkr2 are enriched with microtubule motor activity, consistent with the cellularization patterns in fis1 and the suppressor line. The antagonistic functions of FIS1 and PKR2 in modulating endosperm development are similar to those of PICKLE (PKL) and CURLY LEAF (CLF), which antagonistically regulate root meristem activity. Our results provide further insights into the function of imprinted genes in endosperm development and reproductive isolation.     

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。