Integument cell gelatinisation—the fate of the integumentary cells in <Emphasis Type="Italic">Hieracium</Emphasis> and <Emphasis Type="Italic">Pilosella</Emphasis> (Asteraceae)

Members of the genera Hieracium and Pilosella are model plants that are used to study the mechanisms of apomixis. In order to have a proper understanding of apomixis, knowledge about the relationship between the maternal tissue and the gametophyte is needed. In the genus Pilosella, previous authors have described the specific process of the “liquefaction” of the integument cells that surround the embryo sac. However, these observations were based on data only at the light microscopy level. The main aim of our paper was to investigate the changes in the integument cells at the ultrastructural level in Pilosella officinarum and Hieracium alpinum. We found that the integument peri-endothelial zone in both species consisted of mucilage cells. The mucilage was deposited as a thick layer between the plasma membrane and the cell wall. The mucilage pushed the protoplast to the centre of the cell, and cytoplasmic bridges connected the protoplast to the plasmodesmata through the mucilage layers. Moreover, an elongation of the plasmodesmata was observed in the mucilage cells. The protoplasts had an irregular shape and were finally degenerated. After the cell wall breakdown of the mucilage cells, lysigenous cavities that were filled with mucilage were formed.     

葫芦藓精子发生过程中胞间连丝与胞质桥的超微结构

从超微结构水平上对葫芦藓(Funaria hygrometrica Hedw.)精子发生过程中胞间连接系统的结构及其变化动态进行了研究.结果表明,同一区中的相邻生精细胞由大量胞质桥相连,而不同区的细胞之间则不存在胞质桥.胞间连丝存在于套细胞之间以及套细胞与生精细胞之间,但它在生精细胞间不存在.在精子器发生的后期,当精子细胞壁开始降解时,同一个精子器中所有的精子细胞似乎都由扩大的胞质桥相互连接.胞质桥一直保持到精子分化的后期,最终精子细胞同步分化成精子.胞间连丝与胞质桥具有不同的内部结、分布以及生物发生机制,这表明它们在精子器的发育过程中可能扮演着不同的角色.     

葫芦藓精子发生过程中胞间连丝与胞质桥的超微结构

从超微结构水平上对葫芦藓(Funaria hygrometrica Hedw.)精子发生过程中胞间连接系统的结构及其变化动态进行了研究。结果表明,同一区中的相邻生精细胞由大量胞质桥相连,而不同区的细胞之间则不存在胞质桥。胞间连丝存在于套细胞之间以及套细胞与生精细胞之间, 但它在生精细胞间不存在。在精子器发生的后期,当精子细胞壁开始降解时,同一个精子器中所有的精子细胞似乎都由扩大的胞质桥相互连接。胞质桥一直保持到精子分化的后期,最终精子细胞同步分化成精子。胞间连丝与胞质桥具有不同的内部结、分布以及生物发生机制,这表明它们在精子器的发育过程中可能扮演着不同的角色。     

Mucilage Cells, Calcium Oxalate Crystals and Soluble Calcium in Opuntia ficus-indica

The subcellular location of soluble calcium in parenchymatousand mucilage cells of Opuntia ficus-indica was determined histochemically.Soluble calcium was observed in crystal chambers containingcalcium oxalate on the membrane of the vesicles. Calcium wasalso present in the plasmalemma, in plasmodesmata, in cell walls,in mitochondria and in the vacuoles. Especially marked was thepresence of soluble calcium in vesicles free or fused with theplasmalemma. Little free calcium was observed in other cellcompartments. In the calcium economy of tissues the location of soluble calciumand the transport of calcium to and from mucilage cells to parenchymatouscells and calcium oxalate idioblasts will play a role. Chelationof calcium by mucilage or oxalate, which depends on pH, ionicstrength, etc., will be important in this respect. Opuntia ficus-indica, calcium oxalate, mucilage cells, transport of calcium     

The positional differentiation of ethylene-responsive cells in rachis abscission zones in leaves of Sambucus nigra and their growth and ultrastructural changes at senescence and separation

Summary In leaves of S. nigra, fragmentation of the rachis follows the autumnal abscission of leaflets and the high levels of ethylene produced by the senescing blades. Fragmentation is accompanied by cell growth and ultrastructural changes in a zone of cells precisely differentiated at the separation zone. Studies with explants from the rachis show that those that contain an abscission zone increase in freshweight by as much as 50% before and during cell separation. Cell growth changes are induced by ethylene but not by auxin, and are restricted to explants that contain the separation zone cells. In ethylene, enlarging cells of the zone show cytoplasmic activation indicated by dilated dictyosomes, enhanced production of Golgi vesicles, elongated profiles of rough endoplasmic reticulum, a crenellated plasmalemma, and the apparent discharge and accumulation of cytoplasmic vesicles within the desmotubules of the branched plasmodesmata. Degradation of the middle lamella and cell wall matrix could be associated with the release of hydrolytic enzymes on the disruption of the vesicles. Although ultrastructural changes of a similar but limited nature occur in all cells of the rachis in response to ethylene, only those that are morphologically delimited as zone cells exhibit the growth and separation that leads to rachis fragmentation. It is proposed that abscission can occur only at the sites of the positional differentiation of these special ethylene-responsive target cells.Abbreviation IAA indole-acetic acid (auxin)     

The formation of cytoplasmic channels between tapetal cells inZea mays

Summary In this report we show that large cytoplasmic channels form between the tapetal cells ofZea mays (maize) during the period of tapetal cell differentiation. Tapetal cells are connected by plasmodesmata through their cellulosic cell walls prior to the first meiotic division of the meiocytes. As the tapetal cellulose wall is degraded at the onset of meiosis, both plasmodesmata and cytoplasmic channels measuring 50–200 nm are detectable between tapetal cells. By the time the meiotic tetrad is formed, the cytoplasmic channels are well-established and vary in size from 100–400 nm. The channels, with an average diameter of 200–300 nm, persist after the microspores are released from the callose wall and throughout the period of exine development in microsporogenesis. The channels could potentially allow for free exchange of cytoplasm and organelles. As the tapetal cells begin to pull apart and become vacuolate prior to microspore mitosis, the connecting channels are no longer detectable.     

Plasmodesmal widening accompanies the short-term increase in symplasmic phloem unloading in pea root tips under osmotic stress

Summary Root tips ofPisum sativum seedlings were exposed to 350 mM mannitol, which was shown to effect a transient but dramatic increase in phloem unloading, and investigated by electron microscopy. After chemical fixation and embedding, extremely thin sections of the root extension zone were examined. Outer, inner, and desmotubule diameters of 830 primary plasmodesmata in transverse walls of cortical cells were measured. Statistical analysis indicated that the majority of plasmodesmata had no neck constriction during osmoregulation. Compared to controls, a highly significant increase in mean plasmodesmata diameter was found, but the desmotubule diameter remained unchanged. Both loss of neck constriction and widening of the cytoplasmic sleeve indicate an increase in effective passage area of plasmodesmata. Spokes between plasma membrane and desmotubule were preserved. Continued exposure of the root tips to mannitol led to a return to control values for plasmodesmal diameters. In contrast to these responses, plasmolysis of cortical cells by 1,000 mM sucrose, diminishing phloem unloading, was accompanied by a reduction in those plasmodesmata classified as open. This is the first report showing a correlation between the ultrastructure of plasmodesmata and the rate of symplasmic transport. The role of the different plasmodesmal components in controlling the passage area of symplasmic transport is discussed.     

Studies on the Development of Glandular Hairs in Nymphoides peltatum and the Ultrastructure During Their Secretory P

Each glandular hair of Nyrnphoides peltaturn (Gmel.) O. Kuntz consisted of only one row of cylindar cells with secretory function. The hairs originated from the protoderm cells on the adaxial surface of the second leaf primordium from the shoot apex. Cells of the glandular hairs prossessed dense cytoplast during the secretory period, but the vacuoles were very small. There were not only abundant mitochondria, Golgi bodies and endoplasmic reticulum in the glandular hair cells, but also many plasmodesmata. The authors' research indicated that the mucilage was carried to the edge of the cells by the membranous multilamellar bodies and the vesicles from both Golgi bodies and endoplasmic reticulum. The mucilage was secreted extracellularly by either exocytosis or ecrine secretion. The side walls of the glandular hairs swelled because of mucilage mass accumulation in the walls. The mucilage, being tested to be composed of polysaccharides and a trace of protein, played an important role in protecting the development of the vegetative buds of N. peltatum.     

Studies on graft unions

Summary De novo formation of cytoplasmic cell connections are studied at the graft interface of 5 day old in vitro heterografts ofVicia faba onHelianthus annuus. Continuous and half plasmodesmata, both branched and unbranched, are described at various stages of development in non-division walls between unlike and like dedifferentiated callus cells. In apical portions of protruding callus cells and in the contact zone between opposing cells extremely thin wall parts with a striking ER/plasmalemma contact are observed. During subsequent thickening of the modified wall parts cytoplasmic strands enclosing constricted ER cisternae are entrapped within the newly deposited wall material. These cytoplasmic strands represent half plasmodesmata which—in case of fusion with corresponding structures of adjoining cells across the loosened wall matrix — form continuous cell connections. Golgi vesicles secreting wall material are involved in the process of forming half and continuous plasmodesmata, thus following the same mechanism of plasmodesmata development as described for isolated protoplasts in cell cultures. The findings suggest the existence of a unifying mechanism of secondary formation of plasmodesmata showing far-reaching similarities with the establishment of primary cell connections.     

Morphometric analysis of epidermal differentiation in primary roots of Zea mays

Epidermal differentiation in primary roots of Zea mays was divided into six cell types based on cellular shape and cytoplasmic appearance. These six cell types are: 1) apical protoderm, located at the tip of the root pole and characterized by periclinally flattened cells; 2) cuboidal protoderm, located approximately 230 microns from the root pole and characterized by cuboidal cells; 3) tabular epidermis, located approximately 450 microns from the root pole and characterized by anticlinally flattened cells; 4) cuboidal epidermis, located approximately 900 microns from the root pole and characterized by cuboidal cells having numerous small vacuoles; 5) vacuolate cuboidal epidermis, located approximately 1,500 microns from the root pole and characterized by cuboidal cells containing several large vacuoles; and 6) columnar epidermis, located approximately 2,200 microns from the root pole (i.e., at the beginning of the zone of elongation) and characterized by elongated cells. We also used stereology to quantify the cellular changes associated with epidermal differentiation. The quiescent center and the apical protoderm have significantly different ultrastructures. The relative volume of dictyosomes increases dramatically during the early stages of epidermal differentiation. This increase correlates inversely with the amount of coverage provided by the root cap and mucilage.     

Tobacco mosaic virus movement protein-mediated protein transport between trichome cells.

Tobacco mosaic virus movement protein (TMV MP) is required to mediate viral spread between plant cells via plasmodesmata. Plasmodesmata are cytoplasmic bridges that connect individual plant cells and ordinarily limit molecular diffusion to small molecules and metabolites with a molecular mass up to 1 kD. Here, we characterize functional properties of Nicotiana clevelandii trichome plasmodesmata and analyze their interaction with TMV MP. Trichomes constitute a linear cellular system and provide a predictable pathway of movement. Their plasmodesmata are functionally distinct from plasmodesmata in other plant cel types; they allow cell-to-cell diffusion of dextrans with a molecular mass up to 7 kD, and TMV MP does not increase this size exclusion limit for dextrans. In contrast, the 30-kD TMV MP itself moves between trichome cells and specifically mediates the translocation of a 90-kD beta-glucuronidase (GUS) reporter protein as a GUS::TMV MP fusion. Neither GUS by itself nor GUS in the presence of TMV MP moves between cells. These data imply that a plasmodesmal transport signal resides within TMV MP and is essential for movement. This signal confers selectivity to the translocated protein and cannot function in trans to support movement of other molecules.     

CERCIDIUM FLORIDUM SEED COAT,LIGHT AND ELECTRON MICROSCOPIC STUDY

Scott , Flora Murray , B. G. Bystrom , and E. Bowler . (U. California, Los Angeles.) Cercidium floridum seed coat, light and electron microscopic study. Amer. Jour. Bot. 49(8): 821–833. Illus. 1962.—The structure of the seed coat of the desert tree Cercidium is typical of the family Leguminosae, but the inner integument is mucilaginous. The characteristic palisade cells of the epidermis and the many-armed cells of the mucilaginous zone are discussed in detail. The minute wax rodlets on the surface of the young seed fuse later into a film of wax. The epidermal palisade cells are polygonal in transection and the walls are strengthened by cellulose flanges. The palisade cells are enclosed in a thin suberin-like sheath which connects with the film which lines all air-filled intercellular spaces in the outer integument. Plasmodesmata extend from the protoplast through the subcuticular zone to the cuticle and also interconnect with adjacent cells through numerous pits in the interflange areas of the main wall. The cells of the inner integument are many-armed. Intercellular spaces at all stages of growth observed are occluded with mucilage; nevertheless, they are lined with a suberin-like film similar in reaction to that of the air-filled intercellular spaces of the outer integument. The distribution of wall materials and ergastic substances was determined by microchemical tests. For electron microscopic study, the epidermis and the mucilaginous zone were fragmented ultrasonically, after chemical treatment when necessary. Cuticular fragments, frequently polygonal in outline, are dense to the electron beam at photographic illumination. When exposed to the electron beam, full-strength, volatile components are driven off and condense on other wall fragments or on substrate. Unique structures occur in the subcuticular zone, termed here, collar, cone and paddle structure. The basic material of the cell wall as a whole is cellulose impregnated with matrix materials, mainly non-cellulosic polysaccharides. In addition, lipid-like substances are conspicuous particularly in the flanges of the palisade cells. Under the electron beam, full-strength, these substances volatilize and leave the flanges with a blistered outline except in the region of the light line. The volatile substances may be absent from this zone or the microfibrillar structure may prevent distortion. The microfibrillar pattern throughout the length of the flanges is dominantly parallel, but a change to reticulate arrangement occurs in the light-line region. Numerous pits mark the paths of the plasmodesmata in the subcuticular zone and also through flange and interflange areas of the wall. The dominant microfibrillar pattern in the mucilage cells is reticulate in young and old cells, except for helical orientation in extending arms. Pits occur on arm ends but are infrequent elsewhere. The loose microfibrillar structure presumably allows continuous secretion of mucilage.     

Cortical microtubules mark the mucilage secretion domain of the plasma membrane in Arabidopsis seed coat cells

During their differentiation Arabidopsis thaliana seed coat cells undergo a brief but intense period of secretory activity that leads to dramatic morphological changes. Pectic mucilage is secreted to one domain of the plasma membrane and accumulates under the primary cell wall in a ring-shaped moat around an anticlinal cytoplasmic column. Using cryofixation/transmission electron microscopy and immunofluorescence, the cytoskeletal architecture of seed coat cells was explored, with emphasis on its organization, function and the large amount of pectin secretion at 7 days post-anthesis. The specific domain of the plasma membrane where mucilage secretion is targeted was lined by abundant cortical microtubules while the rest of the cortical cytoplasm contained few microtubules. Actin microfilaments, in contrast, were evenly distributed around the cell. Disruption of the microtubules in the temperature-sensitive mor1-1 mutant affected the eventual release of mucilage from mature seeds but did not appear to alter the targeted secretion of vesicles to the mucilage pocket, the shape of seed coat cells or their secondary cell wall deposition. The concentration of cortical microtubules at the site of high vesicle secretion in the seed coat may utilize the same mechanisms required for the formation of preprophase bands or the bands of microtubules associated with spiral secondary cell wall thickening during protoxylem development.     

Cytoplasmic acidification with butyric acid does not alter the ionic conductivity of plasmodesmata

Summary The effect of lowering cytoplasmic pH on the ionic conductivity of higher-plant plasmodesmata was investigated with corn (Zea mays L. cv. Black Mexican Sweet) suspension culture cells. Exposure to butyric acid decreased the cytoplasmic pH by 0.8 units. Intercellular communication was monitored by electrophysiological techniques that allowed the measurement of membrane resistances of sister cells and the electrical resistance of the plasmodesmata connecting them. The decrease in cytoplasmic pH did not affect the resistance of plasmodesmata, despite the fact that the butyric acid treatment more than doubled the concentration of cytoplasmic calcium. This is discussed in light of previous findings that increases in cytoplasmic calcium increase the electrical resistance of plasmodesmata.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Differentiation of mucilage secretory cells of the Arabidopsis seed coat

In some plant species, including Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type with a unique morphology and containing large quantities of polysaccharide mucilage (pectin). Such seed coat mucilage cells are necessary for neither viability nor germination under normal laboratory conditions. Thus, the Arabidopsis seed coat offers a unique system with which to use genetics to identify genes controlling cell morphogenesis and complex polysaccharide biosynthesis and secretion. As a first step in the application of this system, we have used microscopy to investigate the structure and differentiation of Arabidopsis seed coat mucilage cells, including cell morphogenesis and the synthesis, secretion, and extrusion of mucilage. During seed coat development in Arabidopsis, the epidermal cells of the outer ovule integument grow and differentiate into cells that produce large quantities of mucilage between the primary cell wall and plasma membrane. Concurrent with mucilage production, the cytoplasm is shaped into a column in the center of the cell. Following mucilage secretion the cytoplasmic column is surrounded by a secondary cell wall to form a structure known as the columella. Thus, differentiation of the seed coat mucilage cells involves a highly regulated series of events including growth, morphogenesis, mucilage biosynthesis and secretion, and secondary cell wall synthesis.     

Plasmodesmata: the intercellular organelles of green plants

Cytokinesis in higher plants results in the incomplete separation of daughter cells, due to the formation of special plasma-membrane-lined cytoplasmic bridges, called plasmodesmata. Within the green algae, these structures coordinate biochemical and physiological processes by facilitating the cell-to-cell diffusion of simple metabolites and ions. Until recently, most plant biologists thought that plasmodesmata perform a similar function in higher plants. However, it is now known that the more structurally advanced plasmodesmata of higher plants can also traffic macromolecules, including proteins and nucleic acids. These findings give new insights into how green plants evolved the ability to orchestrate their developmental and physiological processes in a supracellular rather than a multicellular manner.     

Inhibitors of myosin,but not actin,alter transport through <Emphasis Type="Italic">Tradescantia</Emphasis> plasmodesmata

Actin and myosin are components of plasmodesmata, the cytoplasmic channels between plant cells, but their role in regulating these channels is unclear. Here, we investigated the role of myosin in regulating plasmodesmata in a well-studied, simple system comprising single filaments of cells which form stamen hairs in Tradescantia virginiana flowers. Effects of myosin inhibitors were assessed by analysing cell-to-cell movement of fluorescent tracers microinjected into treated cells. Incubation in the myosin inhibitor, 2,3-butanedione monoxime (BDM) or injection of anti-myosin antibodies increased cell–cell transport of fluorescent dextrans, while treatment with the myosin inhibitor N-ethylmaleimide (NEM) decreased cell–cell transport. Pretreatment with the callose synthesis inhibitor, deoxy-d-glucose (DDG), enhanced transport induced by BDM treatment or injection of myosin antibodies but did not relieve NEM-induced reduction in transport. In contrast to the myosin inhibitors, cell-to-cell transport was unaffected by treatment with the actin polymerisation inhibitor, latrunculin B, after controlling for callose synthesis with DDG. Transport was increased following azide treatment, and reduced after injection of ATP, as in previous studies. We propose that myosin detachment from actin, induced by BDM, opens T. virginiana plasmodesmata whereas the firm attachment of myosin to actin, promoted by NEM, closes them.     

ULTRASTRUCTURE OF CARPOSPOROGENESIS IN THE PARASITIC RED ALGA FAUCHEOCOLAX ATTENUATA SETCH. (RHODYMENIALES,RHODYMENIACEAE)

Carpospore differentiation in Faucheocolax attenuata Setch. can be separated into three developmental stages. Immediately after cleaving from the multinucleate gonimoblast cell, young carpospores are embedded within confluent mucilage produced by gonimoblast cells. These carpospores contain a large nucleus, few starch grains, concentric lamellae, as well as proplastids with a peripheral thylakoid and occasionally some internal (photosynthetic) thylakoids. Proplastids also contain concentric lamellar bodies. Mucilage with a reticulate fibrous substructure is formed within cytoplasmic concentric membranes, thus giving rise to mucilage sacs. Subsequently, these mucilage sacs release their contents, forming an initial reticulate deposition of carpospore wall material. Dictyosome vesicles with large, single dark-staining granules also contribute to wall formation and may create a separating layer between the mucilage and carpospore wall. During the latter stages of young carpospores, starch is polymerized in the perinuclear cytoplasmic area and is in close contact with endoplasmic reticulum. Intermediate-aged carpospores continue their starch polymerization. Dictyosomes deposit more wall material, in addition to forming fibrous vacuoles. Proplastids form thylakoids from concentric lamellar bodies. Mature carpospores are surrounded by a two-layered carpospore wall. Cytoplasmic constituents include large floridean starch granules, peripheral fibrous vacuoles, mature chloroplasts and curved dictyosomes that produce cored vesicles which in turn are transformed into adhesive vesicles. Pit connections remain intact between carpospores but begin to degenerate. This degeneration appears to be mediated by microtubules.