被子植物核型胚乳的细胞化

文章介绍被子植物胚乳多核体的产生及其意义、初始垂周壁和初始平周壁的产生以及微管在初始垂周壁和初始平周壁产生中的作用、胚乳发育及胚乳核细胞化的分子机制以及胚乳细胞壁的物质来源和组成的研究进展。     

花生胚乳细胞化的超微结构观察

花生（ＡｒａｃｈｉｓｈｙｐｏｇｅａｅＬ．）心形胚期的胚乳游离核多瓣裂,或具长尾状结构。胚乳细胞质内有大量线粒体、质体、高尔基体、小泡及少量内质网。中央细胞壁有壁内突。球胚及心形胚期常见胚乳瘤。心形胚晚期,胚乳开始细胞化,胚乳细胞壁形成有３种方式,分别存在于不同的胚珠中：（１）从胚囊壁产生自由生长壁形成初始垂周壁,具有明显的电子密度深的中层,其生长主要靠末端的高尔基体小泡及内质网囊泡的融合。两相邻的自由生长壁末端或其分枝末端相连形成胚乳细胞。（２）核有丝分裂后产生细胞板,细胞板向外扩展并可分枝。间期的非姊妹核间也观察到形成了细胞板。小泡与微管参与细胞板的扩展,高尔基体和内质网是小泡的主要来源。细胞板的扩展末端相互连接,形成胚乳细胞的前身。小泡继续加入细胞板的组成,以后形成胚乳细胞壁。（３）胚乳细胞质中,出现一些比较大的不规则形的片段性泡状结构,它们可能来源于高尔基体小泡,这些片段性泡状结构随机相连形成细胞壁,未见微管参与。胚乳细胞外切向壁及经向壁上有壁内突。     

何首乌胚和胚乳的发育

何首乌为直生胚珠,双珠被。胚发育属于柳叶菜型。心形胚柄最为发达,鱼雷形胚期胚柄奶化。早期胚胎发育营养的主要来源可能是合了中积累的淀粉和胚柄吸收来的营养。成熟胚中积累了大量的蛋白质和淀粉粒。胚乳发育属核型。从球形胚期起,胚乳细胞化过程由珠孔端向合点端逐渐推进。初始垂周壁源于姊妹核间的细胞反或非姊妹核间由次生成膜生的细胞板。初始平周壁源于有丝分裂所产生的细胞板。心形胚期,除合点端保持游离核胚乳吸器外,     

荞麦胚和胚乳的发育及贮藏营养物质的积累

荞麦原胚期,胚乳为游离核期。球形胚晚期,胚乳开始细胞化。心形胚期,胚囊中部形成一层“开放细胞”。鱼雷形胚期,胚囊中部有５－７层胚乳细胞。子叶弯曲胚期,胚乳全部形成胚乳细胞,具传递细胞特征的合点胚乳吸器形成。胚乳细胞的初始垂周壁来自于自由生长壁和胞质分裂形成的细胞板;初始平周壁由自由生长垂周壁分友相接形成,及有丝分裂的细胞板形成。开花后９ｄ,胚乳细胞积累淀粉,比胚细胞积累早６ｄ。开花后１５ｄ,胚乳最     

白刺胚乳早期发育的超微结构研究

白刺(Nitraria sibirica)胚乳发育经历游离核阶段、细胞化阶段和被吸收解体阶段。游离核胚乳沿胚囊壁均匀排列为一层,胞质浓厚,其中有丰富的质体、线粒体、高尔基体、内质网和各种小泡等细胞器。珠孔区域的胚囊壁具发达的分枝状壁内突,而周缘区域的胚囊壁具间隔的钉状内突,内突周围的细胞质中具多数线粒体和小泡。胚乳细胞化时,初始垂周壁源于核有丝分裂产生的细胞板。在细胞板两端开始壁的游离生长,一端与胚囊壁相连接,另一端向心自由延伸。壁的游离生长依赖于小泡的融合。早期胚乳细胞具大液泡,具核或无核,细胞质中有大量的线粒体,质体缺乏,其壁仍由多层膜结构组成。     

黄花油点草胚胎发育研究

利用石蜡切片技术对百合科植物黄花油点草[Tricyrtis maculata(D.Don)Machride]双受精、胚及胚乳发育进行了研究,以明确其胚胎发育的特征,为百合科植物的系统研究提供生殖生物学资料。结果表明:(1)黄花油点草为珠孔受精;进入胚囊的2枚精子分别与卵细胞和中央细胞进行正常的双受精,其受精作用属有丝分裂前型。(2)受精后的初生胚乳核立即分裂,其发育方式为核型胚乳;早期的游离胚乳核沿胚囊的边缘分布,胚囊中央部位主要为胚乳细胞质,随着游离胚乳核数量的增加,胚乳核慢慢充满整个胚囊;当发育至球形胚早期阶段,在各胚乳核周围产生胚乳细胞壁,形成完整的胚乳细胞。(3)合子有较长的休眠时间,胚的发育方式为茄型;合子第一次有丝分裂为横裂,分裂后形成基细胞和顶细胞;基细胞经过3次横裂,形成一列胚柄细胞;顶细胞经过分裂形成胚体,依次形成球形胚、棒状胚和盾形胚。(4)种子成熟时胚无器官分化;成熟种子由种皮、胚和胚乳三部分组成。     

粗茎鳞毛蕨原叶体细胞有丝分裂过程中微管列阵的变化

应用Steedman‘s wax切片法,间接免疫荧光标记技术和激光共聚焦扫描显微镜技术研究了粗茎鳞毛蕨（Dryopteris crassirhizoma Nakai)原叶体大液泡化细胞和分生组织细胞有丝分裂过程中微管列阵的变化。结果显示：应用高浓度的多聚甲醛（8%）可以很好地保持大液泡化细胞的结构和微管的抗原性。结果也显示Steedman‘s wax切片法和间接免疫荧光标记技术的优点;（1）避免在微管标记过程中酶解细胞壁;（2）在乙醇脱水过程中样品中叶绿素的自发荧光被减到最小;（3）能够详细观察到有丝分裂过程中微管骨架的变化。因此,这种方法可以被广泛用来调查简单植物体和复杂植物体中细胞的有丝分裂过程以及发育过程中微管骨架的变化。     

刺五加胚和胚乳发育的研究

刺五加Eleutherococcus senticosus(Rupr .et Maxim．)Maxim的胚胎发生类型为茄型。其卵细胞受精后,合子经历15天左右的休眠期才进行第一次分裂。合子分裂通常发生在胚乳细胞化之后,经棒形胚、球形胚、至果实成熟时发育到心形胚。棒形胚后期至心形胚初期,胚柄最为发达。刺五加的胚乳发育类型为核型。其初生胚乳核的休眠期为1天左右。当胚乳游离核数目增加到200至300时,胚乳以自由生长细胞壁的方式细胞化,胚乳细胞以典型的有丝分裂方式进一步增殖,增加细胞数目。球形胚时期,胚乳细胞内开始贮藏营养物质。少数种子的胚乳里存在巨大细胞核的异型胚乳细胞。在胚乳游离核为32至64个时,分化出珠被绒毡层;球形胚时期,珠被绒毡层解体。珠被绒毡层解体后,胚乳表层细胞分化为分泌层。球形胚至心形胚阶段,约有5％的种子里,胚与胚乳组织发生弥散样降解。成熟果实中,含有大量的瘪粒种子和虫咬种子;饱满种子率为40％左右。饱满种子中,胚乳组织占据种子体积的绝大部分,胚所占比率很小。讨论了不同发育时期胚和胚乳的营养供应。     

薏苡胚乳细胞化的超微结构观察

采用透射电镜对薏苡早期的胚乳细胞化进行了研究,在胚乳游离核时期,胚乳游离核及细胞质绕中央细胞分布,游离核间没有发现胚囊壁内突、成膜体等结构。胚乳细胞化过程中初始垂周壁形成过程如下：（１）胚乳细胞质中出现液泡,使细胞质和核向中央液泡推进：（２）一对相邻细胞核间液泡成对存在,且呈垂周分布,而且两液泡间的细胞质很狭窄;（３）在这狭窄的细胞质中出现成行排列的小泡;（４）小泡融合形成细胞板,细胞板悬于两液泡     

短柄五加胚和胚乳发育的研究

短柄五加胚发育属茄型,棒形胚后期胚柄最为发达,球形胚胚柄开始退化,心形胚期胚柄解体。胚乳发育属核型,当胚乳游离核为２００￣３００个时,以自由生长细胞壁的方式进行胚乳细胞化。胚乳细胞进一步增殖以有丝分裂方式进行,球形胚时,胚乳细胞最外层细胞特化为分泌层,胚乳细胞贮藏蛋白质和脂类物质。在胚乳游离核为３２￣６４个时,单珠被的内表皮层分化为珠被绒毡层。合子分裂后,珠被绒毡层发育最为充分;棒形胚后期,珠被绒     

Studies on the Development of the Embryo and Endosperm of Nitraria sibirica Pall

The pollen tube enters the embryo sac through the crassinucellus at the micropylar end, and brings about the porogamy. The embryogeny corresponds to the Solanad type. The defference of the suspensor structure is notable by comparing it with the other genera of Zygophyllaceae that have been studied. The endosperm is of the Nuclear type. Mitosis is the main form of the free endosperm nuclei proliferation. No cell plates develop in the early free nuclear division, however, they appear in late development, without developing into the cell wall and disappear ultimately. At the late globular embryo stage, cell formation in endosperm starts first from the micropylar end. The first anticlinal walls develop from the cell plate that is initiated from tile phragmoplast as a result of normal cytokinesis. Follwing this a wall begins to grow from the base of the cell plates, the outer growing margin soon fuses with the wall of the central cell, and the inner growing margin continues to grow towards the central vacuole. The growing walls branch and eventually fuse on the side nearest the central vacuole. Thus, the first periclinal walls are initiated, and a complete endosperm cell is formed. Along with the development of embryo, cell is gradually formed in the endosperm from the micropylar end towards the chalazal end, but the chalazal endosperm is still coenocytic until the endosperm disintegrate completely. The mature seed has no endosperm.     

Endosperm Development in Barley: Microtubule Involvement in the Morphogenetic Pathway

An immunofluorescence study of sectioned barley endosperm imaged by confocal laser scanning microscopy provided three-dimensional data on the relationship of microtubules to the cytoplasm, nuclei, and cell walls during development from 4 to 21 days after pollination (DAP). Microtubules play an important role throughout endosperm ontogeny. The syncytium is organized into units of nuclear-cytoplasmic domains by nuclear-based radial microtubule systems that appear to control the pattern of the first anticlinal walls at 5 to 6 DAP. After 7 DAP, phragmoplasts of two origins (interzonal and cytoplasmic) guide wall formation. Large compartments formed by the "free growing" walls in association with cytoplasmic phragmoplasts formed adventitiously at interfaces of opposing microtubule systems are subsequently subdivided by interzonal phragmoplast/cell plates to give rise to the starchy endosperm. During development of the aleurone layer from 8 to 21 DAP, the microtubule cycle is typical of plant histogenesis; cortical microtubules are hooplike, and preprophase bands of microtubules predict the division plane.     

Polarization predicts the pattern of cellularization in cereal endosperm

Summary The endosperm of cereal grains develops as a multinucleate mass of wall-less cytoplasm (syncytium) that lines the periphery of the central cell before becoming cellular. The pattern of initial wall formation is precisely oriented and is followed by a round of precisely oriented formative cell division that gives rise to initials for the two tissues of endosperm. The initial anticlinal walls form at boundaries of nuclear-cytoplasmic domains (NCDs) defined by radial microtubules emanating from nuclei in the syncytium. Polarized growth of the NCDs in axes perpendicular to the embryo sac wall and centripetal elongation of the anticlinal walls results in a single layer of open ended alveoli overtopped by the remaining syncytial cytoplasm. This arboreal stage, so named because the elongate nucleate columns of cytoplasm resemble an orchard of trees, predicts the division polarity of the imminent formative division. Mitosis occurs as a wave which, like polarization, moves in both directions from ventral to dorsal. Spindles are oriented parallel to the long axis of the alveoli and cell plates give rise to periclinal walls. The outer daughter nuclei (aleurone initials) are thus completely enclosed by walls and the inner nuclei (starchy endosperm initials) are in alveoli adjacent to the central vacuole.     

FTIR imaging of wheat endosperm cell walls in situ reveals compositional and architectural heterogeneity related to grain hardness.

Endosperm cell walls of cultivars of wheat (Triticum aestivum L.) selected for their endosperm texture (two soft and two hard) were analysed in situ by Fourier transform infrared (FTIR) microspectroscopy. FTIR imaging coupled with statistical analysis was used to map the compositional and structural heterogeneity within transverse sections from which cell contents had been removed by sonication. In the majority of grains analysed, two distinct populations of endosperm cells could be identified by spectral features that were related to cell morphology and age, regardless of cultivar. The main cell-wall component responsible for these differences was the polysaccharide arabinoxylan. In a few samples, this heterogeneity was absent, for reasons that are not understood, but this was not correlated to endosperm texture or growth conditions. Within the same population of endosperm cells, cell walls of hard endosperm could be distinguished from those of soft endosperm by their spectral features. Compared to hard cultivars, the peripheral endosperm of soft cultivars was characterised by a higher amount of polymer, whose spectral feature was similar to water-extractable arabinoxylan. In contrast, no specific compound has been identified in the central endosperm: structural differences within the polysaccharides probably contribute to the distinction between hard and soft cultivars. In developing grain, a clear difference in the composition of the endosperm cell walls of hard and soft wheat cultivars was observed as early as 15 days after anthesis.     

The Development of Green Onion (Allium Fistulosum L. )Embryo and Endosperm

Embryo development of Zhangqiu green onion conforms to the Asterad type and goes through the following stages: proembryo, globular, ellipsoidal, laterally concave, stick-shaped, and curved and mature. The persistent synergid is present until the late globular stage of embryogenesis. Endosperm development of Zhangqiu green onion follows the nuclear pattern. Endosperm cell formation begins at both the micropylar end and the chalazai end of the embryo sac when the embryo is in the late globular stage. Due to the anticlinal wall formation, a layer of free nuclei becomes a layer of “open cells” which lack the inner periclinat wall. The open cells undergo cell division periclinally, and a layer of complete cells is cut off outside and a new layer of open cells inside. The subsequent cell divisions give rise to the endosperm cells centripetally until those from the opposite of the embryo sac meet. The first anticlinal walls arise from the cell plates without phragmoplasts between the free nuclei in interphase. The first periclinal walls are formed by normal cytokinesis. When a few layers of endosperm cells are formed at the micropylar end and the chalazal end of the embryo sac, free cells are present in the central vacuole.     

Change in wall composition of transfer and aleurone cells during wheat grain development

In addition to the starchy endosperm, a specialized tissue accumulating storage material, the endosperm of wheat grain, comprises the aleurone layer and the transfer cells next to the crease. The transfer cells, located at the ventral region of the grain, are involved in nutrient transfer from the maternal tissues to the developing endosperm. Immunolabeling techniques, Raman spectroscopy, and synchrotron infrared micro-spectroscopy were used to study the chemistry of the transfer cell walls during wheat grain development. The kinetic depositions of the main cell wall polysaccharides of wheat grain endosperm, arabinoxylan, and (1–3)(1–4)-β-glucan in transfer cell walls were different from kinetics previously observed in the aleurone cell walls. While (1–3)(1–4)-β-glucan appeared first in the aleurone cell walls at 90°D, arabinoxylan predominated in the transfer cell walls from 90 to 445°D. Both aleurone and transfer cell walls were enriched in (1–3)(1–4)-β-glucan at the mature stage of wheat grain development. Arabinoxylan was more substituted in the transfer cell walls than in the aleurone walls. However, arabinoxylan was more feruloylated in the aleurone than in the transfer cell walls, whatever the stage of grain development. In the transfer cells, the ferulic acid was less abundant in the outer periclinal walls while para-coumarate was absent. Possible implications of such differences are discussed.     

Microtubular configurations during the cellularization of coenocytic endosperm in Ranunculus sceleratus L.

Summary Endosperm cellularization in Ranunculus sceleratus was studied in terms of the initiation of cell-wall formation in the coenocytic endosperm. The first endosperm cell walls were in an anticlinal position relative to the cell wall of the embryo sac and originated from the cell plates and not from wall ingrowths from the embryo-sac wall itself. Alveolar endosperm was formed 3 days after pollination. Microtubules were associated with the freely growing wall ends of the anticlinal walls and were observed in various orientations that generally ranged from angles of 45 ° to 90 ° to the plane of the wall. They were absent in the regions where vesicles had already fused. These microtubules may function in maintaining the growth and the direction of growth of the anticlinal wall until cellularization is completed. At the site where three neighbouring alveoli share their freely growing wall ends, remarkable configurations of microtubules were observed: in each alveolus, microtubules ran predominantly parallel to the bisector of the angle formed by the common walls. These microtubules may form a physically stable framework and maintain the direction of growth of the wall edges. It is concluded that the growing edge of the anticlinal endosperm wall and its associated microtubules are a special continuum of the original phragmoplast that gave rise to the anticlinal wall.     

The emergence of embryos from hard seeds is related to the structure of the cell walls of the micropylar endosperm, and not to endo-beta-mannanase activity

BACKGROUND AND AIMS: Seeds of carob, Chinese senna, date and fenugreek are hard due to thickened endosperm cell walls containing mannan polymers. How the radicle is able penetrate these thickened walls to complete seed germination is not clearly understood. The objective of this study was to determine if radicle emergence is related to the production of endo-beta-mannanase to weaken the mannan-rich cell walls of the surrounding endosperm region, and/or if the endosperm structure itself is such that it is weaker in the region through which the radicle must penetrate. METHODS: Activity of endo-beta-mannanase in the endosperm and embryo was measured using a gel assay during and following germination, and the structure of the endosperm in juxtaposition to the radicle, and surrounding the cotyledons was determined using fixation, sectioning and light microscopy. KEY RESULTS: The activity of endo-beta-mannanase, the major enzyme responsible for galactomannan cell wall weakening increased in activity only after emergence of the radicle from the seed. Thickened cell walls were present in the lateral endosperm in the hard-seeded species studied, but there was little to no thickening in the micropylar endosperm except in date seeds. In this species, a ring of thin cells was visible in the micropylar endosperm and surrounding an operculum which was pushed open by the expanding radicle to complete germination. CONCLUSIONS: The micropylar endosperm presents a lower physical constraint to the completion of germination than the lateral endosperm, and hence its structure is predisposed to permit radicle protrusion.     

Syncytial-type cell plates: a novel kind of cell plate involved in endosperm cellularization of Arabidopsis

Cell wall formation in the syncytial endosperm of Arabidopsis was studied by using high-pressure-frozen/freeze-substituted developing seeds and immunocytochemical techniques. The endosperm cellularization process begins at the late globular embryo stage with the synchronous organization of small clusters of oppositely oriented microtubules ( approximately 10 microtubules in each set) into phragmoplast-like structures termed mini-phragmoplasts between both sister and nonsister nuclei. These mini-phragmoplasts produce a novel kind of cell plate, the syncytial-type cell plate, from Golgi-derived vesicles approximately 63 nm in diameter, which fuse by way of hourglass-shaped intermediates into wide ( approximately 45 nm in diameter) tubules. These wide tubules quickly become coated and surrounded by a ribosome-excluding matrix; as they grow, they branch and fuse with each other to form wide tubular networks. The mini-phragmoplasts formed between a given pair of nuclei produce aligned tubular networks that grow centrifugally until they merge into a coherent wide tubular network with the mini-phragmoplasts positioned along the network margins. The individual wide tubular networks expand laterally until they meet and eventually fuse with each other at the sites of the future cell corners. Transformation of the wide tubular networks into noncoated, thin ( approximately 27 nm in diameter) tubular networks begins at multiple sites and coincides with the appearance of clathrin-coated budding structures. After fusion with the syncytial cell wall, the thin tubular networks are converted into fenestrated sheets and cell walls. Immunolabeling experiments show that the cell plates and cell walls of the endosperm differ from those of the embryo and maternal tissue in two features: their xyloglucans lack terminal fucose residues on the side chain, and callose persists in the cell walls after the cell plates fuse with the parental plasma membrane. The lack of terminal fucose residues on xyloglucans suggests that these cell wall matrix molecules serve both structural and storage functions.