Development of the endosperm in rice (Oryza sativa L.): Cellularization

The syncytial endosperm of rice undergoes cellularization according to a regular morphogenetic plan. At 3 days after pollination (dap) mitosis in the peripheral synctium ceases. Radial systems of microtubules emanating from interphase nuclei define nuclear-cytoplasmic domains (NCDs) which develop axes perpendicular, to the embryo sac wall. Free-growing anticlinal walls between adjacent NCDs compart-mentalize the cytoplasm into open-ended alveoli which are overtopped by syncytial cytoplasm adjacent to the central vacuole. At 4 dap, mitosis resumes as a wave originating adjacent to the vascular bundle. The spindles are oriented parallel to the alveolar walls and cell plates formed in association with interzonal phragmoplasts result in periclinal walls that cut off a peripheral layer of cells and an inner layer of alveoli displaced toward the center. Polarized growth of the newly formed alveoli and elongation of the anticlinal walls occurs during interphase. The next wave of cell division in the alveoli proceeds as the first and a second cylinder of cells is cut off inside the peripheral layer. The periods of polarized growth/anticlinal wall elongation alternating with periclinal cell division are repeated 3–4 times until the grain is filled by 5 dap.     

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。     

Early Endosperm Development in Ovules of Soybean, Glycine max (L) Merr. (Fabaceae)

Endosperm development was studied in normally setting flowersand pods of soybean from anthesis to a pod length of 10–20mm. The free-nuclear stage following double fertilization istypified by loss of starch and increasing vacuolation. The cytoplasmprovides evidence of extensive metabolic activity. Wall ingrowths,already present at the micropylar end of the embryo sac wallprior to fertilization, develop along the lateral wall of thecentral cell as well as at the chalazal endosperm haustorium.Endosperm cellularization begins when the embryo has developeda distinct globular embryo proper and suspensor. Cellularizationstarts at the micropylar end of the embryo sac as a series ofantidinal walls projecting into the endosperm cytoplasm fromthe wall of the central cell. The free, growing ends of thesewalls are associated with vesicles, microtubules, and endoplasrnicreticulum. Pendinal walls that complete the compartmentalizalionof portions of the endosperm cytoplasm are initiated as cellplates formed during continued mitosis of the endosperm nuclei.Endosperm cell walls are traversed by plasmodesmata. This studywill provide a basis for comparison with endosperin from soybeanflowers programmed to abscise. Glycine max, soybean, endosperm, ovules     

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。     

Observation on the Characteristic Features of Cell Wall Formation in Free-Nuclear Endosperm During the Early Stages of Cellularization in Triticum aestivum

Triticum aestivum cv. “Yang Mai # 1” was used to study the celI wall formation in free-nuclear endosperm with electronmicroscopy and fluorescence microscopy. During the initiation of cellularization the peg-like wall ingrowths developed, and the freely growing walls gradually cleaved the cytoplasm into small compartments in both regions (ventral and dorsal) of the 'endosperm sac, but in ventral region the anastomose of freely growing wall often occured. The striking resemblance and close connection betwteen nucellar debris and freely growing walls showed the possibility that the disintegrated nucellus played an important role in supporting the wall formation of endosperm during the early stages of cellularization.     

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     

Development of endosperm in Arabidopsis thaliana

 The process of endosperm development in Arabidopsis was studied using immunohistochemistry of tubulin/microtubules coupled with light and confocal laser scanning microscopy. Arabidopsis undergoes the nuclear type of development in which the primary endosperm nucleus resulting from double fertilization divides repeatedly without cytokinesis resulting in a syncytium lining the central cell. Development occurs as waves originating in the micropylar chamber and moving through the central chamber toward the chalazal tip. Prior to cellularization, the syncytium is organized into nuclear cytoplasmic domains (NCDs) defined by nuclear-based radial systems of microtubules. The NCDs become polarized in axes perpendicular to the central cell wall, and anticlinal walls deposited among adjacent NCDs compartmentalize the syncytium into open-ended alveoli overtopped by a crown of syncytial cytoplasm. Continued centripetal growth of the anticlinal walls is guided by adventitious phragmoplasts that form at interfaces of microtubules emanating from adjacent interphase nuclei. Polarity of the elongating alveoli is reflected in a subsequent wave of periclinal divisions that cuts off a peripheral layer of cells and displaces the alveoli centripetally into the central vacuole. This pattern of development via alveolation appears to be highly conserved; it is characteristic of nuclear endosperm development in angiosperms and is similar to ancient patterns of gametophyte development in gymnosperms. Received: 21 September 1998 / Revision accepted: 17 November 1998     

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.     

Initital cellularization and differentiation of the aleurone cells in the ventral region of the developing wheat grain

Early cellularization of the free-nuclear endosperm and subsequent differentation of the aleurone cells in the ventral region of the developing wheatgrain (Triticumaestivum L. cv. Heron) were examined using both light and electron microscopy. In ovules harvested 1 d after anthesis, irregular wall ingroths typical of transfer cells protrude into the multinucleate cytoplasm. Initital cellularization occurs by a process of free wall formation in much the same fashion as in the dorsal region of the grain. In places, sheets of endoplasmic reticulum and dictyosomes appear to be closely associated with the growing wall. Like the wall ingrowths noted earlier, the freely growing walls are intensely fluorescent after staining with aniline blue. Initiatal cellularization is complete 2–3 days after anthesis. Unlike the first-formed cells in the dorsal region of the developing grain, those in the ventral region are not meristematic. These amitotic cells become the groove aleurone cells which at an early stage of development are set apart from the rest of the endosperm by their irregularly thickened walls and dense cytoplasm. Autofluorescence is first apparent in the walls of those cells next to the degenerating nucellus. In contrast to the aleurone cells in the dorsal region of the grain, at maturity only the inner wall layer of each of the groove aleurone cells remains autofluorescent. The aleurone grains are highly variable in appearance and contain no Type II inclusions.     

Cell wall (1 → 3)- and (1 → 3, 1 → 4)-β-glucans during early grain development in rice (Oryza sativa L.)

Immunogold labeling was used to study the distribution of (1 → 3)-β-glucans and (1 → 3, 1 → 4)-β-glucans in the rice grain during cellularization of the endosperm. At approximately 3–5 d after pollination the syncytial endosperm is converted into a cellular tissue by three developmentally distinct types of wall. The initial free-growing anticlinal walls, which compartmentalize the syncytium into open-ended alveoli, are formed in the absence of mitosis and phragmoplasts. This stage is followed by unidirectional (centripetal) growth of the anticlinal walls mediated by adventitious phragmoplasts that form between adjacent interphase nuclei. Finally, the periclinal walls that divide the alveoli are formed in association with centripetally expanding interzonal phragmoplasts following karyokinesis. The second and third types of wall are formed alternately until the endosperm is cellular throughout. All three types of wall that cellularize the endosperm contain (1 → 3)-β-glucans but not (1 → 3, 1 → 4)-β-glucans, whereas cell walls in the surrounding maternal tissues contain considerable amounts of (1 → 3, 1 → 4)-β-glucans with (1 → 3)-β-glucans present only around plasmodesmata. The callosic endosperm walls remain thin and cell plate-like throughout the cellularization process, appearing to exhibit a prolonged juvenile state. Received: 7 January 1997 / Accepted: 11 February 1997     

胚乳细胞壁的建成及其与胞质微管的关系

综述了被子植物核型胚乳发育的不同阶段(合胞体时期、初始垂周壁形成期、初始平周壁形成期和胚乳细胞分裂增生期)胚乳细胞壁建成的各种观点,以及在各个阶段中微管与胚乳细胞壁建成之间关系的最新研究进展。并分别比较了胚乳细胞化过程不同阶段胚乳游离核或胚乳细胞有丝分裂与分生组织细胞有丝分裂微管周期的差异。     

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.     

The cytoskeleton and spatial control of cytokinesis in the plant life cycle

Summary One of the intriguing aspects of development in plants is the precise control of division plane and subsequent placement of walls resulting in the specific architecture of tissues and organs. The placement of walls can be directed by either of two microtubule cycles. The better known microtubule cycle is associated with control of the future division plane in meristematic growth where new cells become part of tissues. The future daughter domains are determined before the nucleus enters prophase and the future site of cytokinesis is marked by a preprophase band (PPB) of cortical microtubules. The spindle axis is then organized in accordance with the PPB and, following chromosome movement, a phragmoplast is initiated in the interzone and expands to join with parental walls at the site previously occupied by the PPB. The alternative microtubule cycle lacks both the hooplike cortical microtubules of interphase and the PPB. Wall placement is determined by a radial microtubule system that defines a domain of cytoplasm either containing a nucleus or destined to contain a nucleus (the nuclear cytoplasmic domain) and controls wall placement at its perimeter. This more flexible system allows for cytoplasmic polarization and migration of nuclei in coenocytes prior to cellularization. The uncoupling of cytokinesis from karyokinesis is a regular feature of the reproductive phase in plants and results in specific, often unusual, patterns of cells which reflect the position of nuclei at the time of cellularization (e.g., the arrangement of spores in a tetrad, cells of the male and female gametophytes of angiosperms, and the distinctive cellularization of endosperm). Thus, both microtubule cycles are required for completion of plant life cycles from bryophytes to angiosperms. In angiosperm seed development, the two methods of determining the boundaries of domains where walls will be deposited are operative side by side. Whereas the PPB cycle drives embryo development, the radial-microtubule-system cycle drives the common nuclear type of endosperm development from the syncytial stage through cellularization. However, a switch to the PPB cycle can occur in endosperm, as it does in barley, when peripheral cells divide to produce a multilayered aleurone. The triggers for the switch between microtubule cycles, which are currently unknown, are key to understanding plant development.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

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.     

Differential Growth in Periclinal and Anticlinal Walls during Lobe Formation in Arabidopsis Cotyledon Pavement Cells

Lobe development in the epidermal pavement cells of Arabidopsis thaliana cotyledons and leaves is thought to take place via tip-like growth on the concave side of lobes driven by localized concentrations of actin filaments and associated proteins, with a predicted role for cortical microtubules in establishing the direction of restricted growth at the convex side. We used homologous landmarks fixed to the outer walls of pavement cells and thin-plate spline analysis to demonstrate that lobes form by differential growth of both the anticlinal and periclinal walls. Most lobes formed within the first 24 h of the cotyledons unfurling, during the period of rapid cell expansion. Cortical microtubules adjacent to the periclinal wall were persistently enriched at the convex side of lobes during development where growth was anisotropic and were less concentrated or absent at the concave side where growth was promoted. Alternating microtubule-enriched and microtubule-free zones at the periclinal wall in neighboring cells predicted sites of new lobes. There was no particular arrangement of cortical actin filaments that could predict where lobes would form. However, drug studies demonstrate that both filamentous actin and microtubules are required for lobe formation.     

Cytoskeletal organization of the micropylar endosperm in Coronopus didymus L. (Brassicaceae)

Summary. The micropylar chamber of the mustard Coronopus didymus is a developmental domain distinct from the contiguous central chamber and the more extreme chalazal chamber. Early in syncytial development the micropylar endosperm surrounding the embryo becomes populated with unusual fusiform to multilobed nuclei. These nuclei are sheathed by unique parallel arrays of microtubules that focus at tips of the nuclei and flare to connect with a reticulate network in the common cytoplasm. F-actin does not closely invest the nuclei but instead forms an extensive but separate cytoplasmic reticulum. When the embryo is in the early heart stage, the cytoskeleton of the endosperm undergoes a remarkable transition in preparation for cellularization. Microtubules become reorganized into radial arrays emanating from the nuclei, which themselves become spherical. Radial microtubule systems (RMSs), which replace both the parallel microtubules and the cytoplasmic reticulum, organize the common cytoplasm into evenly spaced nuclear cytoplasmic domains (NCDs). F-actin gradually becomes coaligned with the RMSs. Phragmoplasts are initiated adventitiously at the interfaces of opposing RMSs in the absence of mitosis. Cell plate deposition, which is initiated at multiple sites, results in a network of walls formed more or less simultaneously around the densely packed NCDs. The walls, which are rich in 1–3-β-glucans, join with one another and with the existing walls of both the central cell and embryo to complete cellularization in the micropylar chamber. In the adjacent central chamber where the syncytium is restricted to a thin peripheral layer by the large central vacuole, basic organization of the syncytium into NCDs is followed by alternating cycles of alveolation and periclinal cell division resulting in cellularization. Received July 19, 2001 Accepted October 16, 2001     

Pattern and process of wall formation in developing endosperm

Endosperm is emerging as a system for investigating the genetic control of wall placement and deposition in plant development. Development of endosperm progresses in distinct stages from a wall-less syncytial stage to a cellular stage that is entirely typical of plant meristems where the division plane is predicted by a preprophase band of microtubules (PPB) and cytokinesis is completed by formation of a cell plate in association with a phragmoplast. Four developmentally different types of walls, each associated with a different microtubule system, are sequentially produced: (1) free growing walls deposited in the absence of mitosis and phragmoplasts; (2) walls guided by cytoplasmic phragmoplasts formed adventitiously in the absence of mitosis; (3) walls formed by interzonal phragmoplasts in a cell cycle that lacks PPBs; and (4) wall deposition driven by interzonal phragmoplasts in a cycle that includes PPBs. We are using methods of differential screening to isolate cDNA clones corresponding in temporal and spatial pattern to the types of wall development, and are studying mutants for clues to the genetic controls of wall development.     

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.     

Structure and function of the microtubular cytoskeleton during endosperm development in wheat: an immunofluorescence study

Summary The three-dimensional structure of the microtubular cytoskeleton of developing wheat endosperm was investigated immunocytochemically. Semi-thin sections were prepared from polyethylene glycol embedded ovaries. At the free-nuclear stage the endosperm cytoplasm with regularly distributed nuclei surrounded a large central vacuole and exhibited an extensive network of fluorescent labelled microtubular assemblies radiating from each nucleus. As was found in other coenocytes, this particular and nuclear-dependent cytoskeletal configuration functions in the arrangement of nuclei and in the stabilization of the nuclear positions. At the beginning of cellularization of the endosperm the formation of vacuoles altered the radiating networks. It is likely that the radiating microtubular arrays function in the formation of phragmoplasts, independent of nuclear divisions. The formation of anticlinal cell walls, giving rise to openended cell cylinders, coincides with the occurrence of phragmoplast microtubular arrays which were demonstrated during the period of cell wall elongation. The microtubular system radiating from the nuclei in these cell cylinders anchored the nuclei in stage- and locus-specific positions. During the development of aleurone and inner endosperm cells, cell morphogenesis was related to earlier demonstrated types of microtubular configurations in the cortical cytoplasm. This suggests that a general mechanism is involved.Abbreviations A alveolus - AL aleurone layer - CE central endosperm - CV central vacuole - DAP days after pollination - END endosperm - FITC fluorescein isothiocyanate - GAR-FITC goat anti-rabbit antibodies conjugated with FITC - I integument - IE PC inner epidermis pericarp - II inner integument - N nucleus - NC nucellus cells - NE nucellar epidermis - NUC nucellus - OI outer integument - PBS phosphate buffered saline - PC pericarp - PEG polyethylene glycol - V vacuole     

Microtubules and morphogenesis in ordinary epidermal cells ofVigna sinensis leaves

Summary Undifferentiated ordinary epidermal cells (ECs) ofVigna sinensis leaves possess straight anticlinal walls and cortical microtubules (Mts) scattered along them. At an early stage of EC differentiation cortical Mts adjacent to the above walls form bundles normal to the leaf plane, loosely interconnected through the cortical cytoplasm of the internal periclinal wall. At the upper ends of the Mt bundles, Mts fan out towards the external periclinal wall and form radial arrays. Mt bundles and radial arrays exhibit strict alternate disposition between neighbouring ECs. An identical reticulum of cellulose microfibril (CM) bundles is deposited outside the Mt bundles. Local wall pads rise at the junctions of anticlinal walls with the external periclinal one, where the CM bundles terminate. They display radial CMs fanning towards the external periclinal wall. The CM bundles and radial CM systems prevent local cell bulging, but allow it in the intervening wall areas. In particular, the radial CM systems dictate the pattern of EC waviness by favouring local tangential expansion of external periclinal wall. As a result, ECs obtain an undulate appearance. Constrictions in one EC correspond with protrusions of adjacent ECs. ECs affected by colchicine entirely lose their Mts and do not develop wavy walls, an observation substantiating the role of cortical Mts in EC morphogenesis.Abbreviations CM cellulose microfibril - DTT dithiothreitol - EC epidermal cell - MSB microtubule stabilizing buffer - Mt microtubule - PBS phosphate buffered saline - PMSF phenylmethylsulfonyl fluoride