Morphological analysis of female gametophyte development in the bel1 stk shp1 shp2 mutant

Abstract

In Arabidopsis thaliana, cell fate in developing ovules is determined by the action of the homeodomain factor BELL1 (BEL1) and of the MADS-box factors SEEDSTICK (STK), SHATTERPROOF1 (SHP1) and SHP2. The analysis of the bel1 and the stk shp1 shp2 mutants revealed that the functional megaspore is formed, however, it does not proceed into megagametogenesis. In the bel1 stk shp1 shp2, quadruple mutant megasporogenesis does not take place. In this article we describe a detailed morphological analysis of the quadruple mutant, and we discuss the possibility that BELL1, STK, SHP1 and SHP2 not only control integument identity determination and development, but that they might also play a role during megasporogenesis.     

宁夏枸杞的大、小孢子发生和雌、雄配子体发育

在幼小花药横切面上,每个角隅处可见一层拱形孢原细胞,其经过平周分裂形成初生造孢细胞、次生造孢细胞,发育为小孢子母细胞。减数分裂过程中胞质分裂为同时型。四分体为四面体型。成熟花粉粒含二细胞,具三孔沟型萌发孔。花药绒毡层由二部分组成:药壁区的绒毡层由初生壁细胞所产生,药隔区的由药隔细胞直接转化成,为双重起源,呈二型性,属分泌型。雌蕊由二个心皮构成二室子房,中轴胎座。倒生胚珠具单珠被、薄珠心,珠被绒毡层。胚囊发育为蓼型。在胚囊细胞分化后,组成胚囊的四种细胞继续发育,表现出各自的形态学变化。     

粉叶小檗大孢子发生和雌配子体形成

采用石蜡切片方法对粉叶小檗（Berberis pruinosa Franch.)的大孢子发生和雌配子体形成过程进行了研究。主要结果如下：雌蕊1枚,子房单心皮,边缘胎座,2枚胚珠倒生,具双珠被,厚珠心,珠孔由内外两层珠被共同形成,呈“Z”字形;单孢原,位于珠心表皮下;直线形大孢子四分体,合点端的1个大孢子发育为功能大孢子,胚囊发育类型为蓼型;成熟胚囊中,2个极核在受精前融合为次生核;3个反足细胞不发达,较早退化;"品"字形卵器,其中助细胞发达且具丝状器。     

马哈利樱桃大孢子发生和雌配子体的发育

采用石蜡切片法对马哈利樱桃大孢子发生和雌配子体发育过程进行观察研究。结果表明:(1)马哈利樱桃雌配子体发育早期,在单室子房内可以看到2个倒生胚珠,但在后期其中一个退化,另一个发育为种子;其胚珠具双珠被,为厚珠心。(2)大孢子母细胞减数分裂形成直线型四分体,功能大孢子位于合点端;胚囊发育为蓼型,成熟胚囊为七细胞八核。(3)根据不同时间花的外部形态特征与内部解剖学对比的观察结果,在陕西关中地区,三月下旬是马哈利樱桃雌性生殖细胞分化和发育的重要时期,果园在此期间应加强肥水管理。     

Altering sexual development inArabidopsis

The reproductive system determines the way in which gametes develop and interact to form a new organism. Therefore, it exerts the primary level of control of genotypic frequencies in plant populations, and plays a fundamental role in plant breeding. A basic understanding of plant reproductive development will completely transform current breeding strategies used for seed production. Apomixis is an asexual form of reproduction in which embryogenesis occurs in a cell lineage lacking both meiosis and fertilization, and that culminates in the formation of viable progeny genetically identical to the mother plant. The transfer of apomixis into sexual crops will allow the production of self-perpetuating improved hybrids, and the fixation of any desired heterozygous genotype. The initiation of apomictic development invariably takes place at early stages of ovule ontogeny, before the establishment of the megagametophytic phase. The developmental versatility associated with megagametophyte formation suggests that the genetic and molecular regulation of apomixis is intimately related to the regulation of sexuality. Differences between the initiation of sexual and apomictic development may be determined by regulatory genes that act during megasporogenesis, and that control events leading to the formation of unreduced female gametophytes. To test this hypothesis, we are isolating and characterizing genes that act during megasporogenesis inArabidopsis thaliana and investigating their potential role in the induction of apomixis. We are using a recently established transposon-based enhancer detection and gene trap insertional mutagenesis system that allows the identification of genes based on their expression patterns. An initial screen of transposants has yielded over 20 lines conferring restricted GUS expression during early ovule development. We have obtained the sequence of genomic fragments flanking the transposon insertion. Several have homology to genes playing important roles in plant and animal development. They include cell cycle regulators, enzymes involved in callose hydrolysis, leucine-rich repeat protein kinase receptors, and expressed sequence tags (ESTs) of unknown function. Independently, a genetic screen allows the identification of female sterile mutants defective in megasporogenesis. Results from these experiments will improve our basic understanding of reproductive development in plants, and will set the basis for a sustained effort in plant germ line biotechnology, a first step toward a flexible transfer of apomixis into a large variety of sexual crops.     

Development of megagametophyte, embryo, and seed in Senna corymbosa (Lam.) H.S. Irwin & Barneby (Leguminosae – Caesalpinioideae)

The main aspects of seed ontogeny in Senna corymbosa were studied by standard anatomical microtechniques for light microscope observations. The results revealed an ana-campylotropous, bitegmic, and crassinucelate mature ovule. A single archesporocyte developed by an archesporial cell enlargement from the subhypodermal multicellular archesporium. Meiosis originated linear or T-shaped megasporic tetrads. The functional megaspore was the chalazal one. Megagametophytic development conformed to the Polygonum type. Fertilization was porogamic. Endosperm development was free nuclear and conformed to a chalazal haustorium. Cellular endosperm was initiated from the micropylar end during the globular embryo stage. Embryogeny derived from a linear proembryonal tetrad. The mature embryo showed an oblique axis. The testa derived from the outer ovular integument. Nucellar and endosperm remnants, and the micropylar region of the inner ovular integument, persisted at embryo maturity. The absence of a pleurogram would be adaptative to wetland habitats. The taxonomic use of the mature embryo axis in the Cassieae and the phylogenetic employment of megasporic arrangements in Leguminosae needs some reinterpretation.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 169–179.     

'土佐文旦'胚囊发育及其子代体细胞染色体的观察研究

选取母本‘土佐文旦’不同时期的花蕾及文旦杂交F1子代3~5 mm长的幼叶,采用石蜡制片法及柑橘染色体制片技术,观察不同时期母本子房结构及杂交F1子代体细胞染色体数目。结果显示:(1)追踪到母本大孢子的发生及不同时期胚囊的发育特征,获得了杂交F1子代清晰可靠、分散良好的体细胞染色体图像,母本石蜡切片观察结果为进一步确定2n雌配子的最佳诱导期提供依据;(2)杂交F1子代体细胞染色体观察结果表明,杂交F1子代中既有正常的二倍体也有三倍体,为进一步原位杂交技术及确认2n配子类型和可能的遗传效应奠定了基础。     

Embryological Studies on Narcissus tazetta var. chinensis

Chinese narcissus (Narcissus tazetta var.chinensis Roem) blooms but has no seeds.Embryological studies on the species were conducted to discover the causes of its sterility.Its anther wall is composed of four layers of cells,and its tapetum is of the secretory type.The cytokinesis of microspore mother cells is of the successive type,and the tetrad is tetrahedral.During meiosis of microspore mother cells,some chromosomes lagged,and several micronuclei were found in tetrads.Only 27.7% of the pollen grains contained full cytoplasm,and 1.3% of them germinated in culture medium.No pollen grain,however,could germinate on the stigma.The ovary is trilocular with axile placenta,and the ovules are bitegmic,tenuinucellate,and anatropous.Its embryo sac is of the polygonum type.Most embryo sacs degenerated,and only about 4.5% of the ovules contained a normal embryo sac with an egg cell,two synergids,three antipodal,and a central cell containing two polar nuclei.One reason for the sterility of Chinese narcissus is the abnormality of microsporogenesis and megasporogenesis,in which only a few functional pollen grains and embryo sacs are produced.The other reason is that the pollen grains cannot germinate on the stigma.     

巨龙竹生殖器官形态结构及雌、雄配子体的发育

通过石蜡切片的方法对巨龙竹生殖器官结构、大小孢子的发生和雌、雄配子体的发育过程进行了观察研究。 巨龙竹为一心皮组成的单室单子房,子房内具有一个胚珠,倒生、双珠被、厚珠心。大孢子母细胞减数分裂形成线形排列的4个大孢子,合点端大孢子具功能。胚囊的发育为蓼型,具多个反足细胞。巨龙竹的花药壁由4层结构组成,包括表皮、药室内壁、中层、绒毡层;花药壁发育为单子叶型,绒毡层为腺质型。小孢子母细胞减数分裂中的胞质分裂为连续型,四分孢子为四面体型;成熟花粉粒为2细胞型,具1个萌发孔。小穗发育雌雄异熟,雌蕊的发育早于雄蕊的发育。