Microspore ofSecale cereale as a transfer cell type

Summary In the mature microspore ofSecale cereale, a set of wall ingrowths deposited as the first (outer) intine layer between exine and the microspore plasma membrane, are revealed by electron microscopy. The wall ingrowths form a girdle in the vicinity of the apertural region at the external pole of microspore which is in contact with the tapetum, so the microspore can be considered as a transfer cell which is polarized. After microspore division the second (inner) intine layer is deposited by the vegetative cell and forms a labyrinth of branched wall ingrowths. As a result, the periphery of a vegetative cell is also irregular and appears as very thin plasmatubules or evaginations delimited by plasma membrane and penetrating the pollen wall.The possible functions of the microspore as a transfer cell and the wall-membrane system of the vegetative cell are discussed.     

DEVELOPMENT AND GERMINATION OF THE AZOTOBACTER CYST

The fine structure of Azotobacter vinelandii has been studied by means of electron microscopy of ultrathin sections made of the encysting and germinating cells. The organisms were fixed with KMnO₄ and embedded in epoxy resin. On an encystment medium the rod-shaped bacteria begin to assume an almost spherical form and then bark-like exine appears in 1½ to 2 days. The exine thickens and an electron permeable intine forms between it and the shrinking cell body. In 5 days the intine makes up more than half of the cyst volume and begins to show a definite two-layered structure. Meanwhile the peripheral bodies, which may be extensions of the cell membrane of the vegetative cell, disappear as the encystment progresses. The cell wall and membrane of the vegetative cell remain demonstrable as the confining structure of the shrinking central body of the mature cyst. In this central body lipoidal globules appear together with aggregations of nuclear material. Cyst germination begins with an increase in the size of the central body at the expense of the intine. The nuclear aggregations become more diffuse and the lipoidal globules disappear. The exine may be pushed outward and the bark-like fragments separate as the emerging vegetative cell develops. Invagination of the cell wall and membrane may occur at this stage leading to cell division. Empty exines remain as horseshoe-shaped structures.     

Dynamic of polysaccharide and lipid in the developing microsporangiums of Chinese fir (Cunninghamia lanceolata)

Microsporongium development in Chinese fir (Cunninghamia lanceolata) was investigated using cytochemical methods with a special attention to the fluctuations (in amount and distribution) of polysaccharide and lipid reserves along the development of the microsporangium. Semi-thin sections of microsporangia at different developmental stages were stained with periodic acid–Schiff (PAS) reagent and Sudan Black B to detect insoluble polysaccharides and neutral lipids, respectively. In young microsporangia, microspore mother cells began to accumulate starch grains and lipids, which disappeared during microspore development. Following microspore division, the starch grains present in bicellular pollen disappeared and abundant lipid deposits were accumulated. In mature pollen, only abundant lipids accumulated as storage material. The pollen wall of C. lanceolata is predominantly composed of polysaccharidic intine, and the sporopollenin-containing exine is weakly developed and only forms a thin layer covering the intine.     

Pectin secretion and distribution in the anther during pollen development in Lilium

Using the monoclonal antibodies JIM 5 and 7, pectin was immunolocalized and quantitatively assayed in three anther compartments of Lilium hybrida during pollen development. Pectin levels in both the anther wall and the loculus increased following meiosis, were maximal during the early microspore stages and declined during the remainder of pollen ontogenesis. In the microspores/pollen grains, pectin was detectable at low levels during the microspore stages but accumulated significantly during pollen maturation. During early microspore vacuolation, esterified pectin epitopes were detected both in the tapetum cytoplasm and vacuoles. In the anther loculus, the same epitopes were located simultaneously in undulations of the plasma membrane and in the locular fluid. At the end of microspore vacuolation, esterified pectin epitopes were present within the lipids of the pollenkitt, and released in the loculus at pollen mitosis. Unesterified pectin epitopes were hardly detectable in the cytoplasm of the young microspore but were as abundant in the primexine matrix as in the loculus. During pollen maturation, both unesterified and esterified pectin labelling accumulated in the cytoplasm of the vegetative cell, concurrently with starch degradation. In the mature pollen grain, unesterified pectin epitopes were located in the proximal intine whereas esterified pectin epitopes were deposited in the distal intine. These data suggest that during early microspore development, the tapetum secretes pectin, which is transferred to the primexine matrix via the locular fluid. Further, pectin is demonstrated to constitute a significant component of the pollen carbohydrate reserves in the mature grain of Lilium. Received: 3 July 2000 / Accepted: 19 October 2000     

THE POLLEN WALL OF CANNA AND ITS SIMILARITY TO THE GERMINAL APERTURES OF OTHER POLLEN

The pollen wall of Canna generalis Bailey is exceptionally thick, but only a minor part of it contains detectable amounts of sporopollenin. The sporopollenin is in isolated spinules at the exine surface and in the intine near the plasma membrane. There is no sporopollenin in the > 10 μ thick channeled region between spinules and intine. We suggest that the entire pollen wall of C. generalis is similar to the thick intine and thin exine typical for germinal apertures in many pollen grain types. Considered functionally, the Canna pollen wall may offer an infinite number of sites for pollen tube initiation and would differ significantly from grains that are inaperturate in the sense of an exine lacking definite germinal apertures.     

洋葱花粉发育过程中ATP酶的分布特征(简报)

在真核细胞中,除了线粒体和叶绿体ATPase的功能是合成ATP外,其余部位ATPase是水解ATP以获取生物能量的代谢酶,在生物体细胞内广泛存在。探索ATPase在细胞中的分布状态是研究细胞生理状态的一种重要手段。ATPase在细胞中的多少可反映出细胞当时的生活状态,这一特征已被初步用于探索小麦和水稻雄性不育的细胞生物学研究中,希望通过比较可育花药和不育花药中ATPase的分布差异寻找雄性不育的机理,发现     

Ultrastructural evidence of endoplasmic reticulum changes during the maturation of the olive pollen grain (Olea europaea L., Oleaceae)

Characteristic features of rough endoplasmic reticulum (rER) distribution and proliferation were noted during olive pollen (Olea europaea L.) development, suggesting the physiological significance of this organelle. Initially scarce in the young microspore, ER increases as cytoplasmic vacuoles form. At the vacuolated microspore stage the cytoplasm contains numberous polysomes and elongated rER cisternae arranged preferentially in stacks, with an average intracisternal width of 0.07 µm. Stacks persist in the bicellular pollen grain but consist of fewer, shorter, dilated cisternae (mean intracisternal width 0.1 µm) containing a considerable electron-dense matrix. Cisternae in the mature grain are fragmented, leaving behind an ER of swollen pockets. Pockets of ER containing a material of greater electron density are evenly deposited along the plasmalemma, in close relation with it. A dense material is seen in the tubules of the apertural region, which was lacking in earlier stages. Our results show that ER may be involved in protein transport to the intine.     

Sporoderm in Populus and Salix

Four phenomena were observed in a study of Populus tremula and P. tremula f. gigas microspores from before microspore mitosis through mature pollen which may have general significance in the ontogeny of pollen grains: 1) The exine and orbicules (Ubisch bodies) were covered by membranes. 2) The exine and the tapetal surfaces where orbicules form were covered by a polysaccharide (PAS positive) coat until after microspore mitosis; subsequently the tapetum became plasmodial. 3) Material having the staining characteristics of the nexine 2 (endexine in the sense of Fægri) accumulated on membranes in microspores in the space between the exine and the plasma membrane. That material was almost completely gone from the wall in mature pollen. The membranes on which material had accumulated migrated through the exine. Following passage through the exine these membranes were seen as empty fusiform vesicles in micrographs of anthers prepared by commonly used methods. 4) At about microspore mitosis when the cellulosic intine begins to form, microtubules about 240 A in diameter occurred near the plasma membrane and generally parallel with it. Positive acid phosphatase reactions in tapetal cells together with the morphology of orbicules and other tapetal organelles suggest that the wall of orbicules, which is like the pollen exine, may form as a residual product of a lysosome system.

Sections of mature Salix humilis pollen were compared with Populus.     

Development of heterocolpate pollen in Myosotis scorpioides L.(Cynoglosseae,Boraginaceae)

Structure of mature pollen of Myosotis scorpioides and sporoderm development were examined using light, scanning and transmission electron microscopy. Pollen grains of M. scorpioides are heterocolpate with three colpori alternating with three pseudocolpi (pseudoapertures). Functional pseudoapertures take part in harmomegathy but not in pollen germination. Formation of colpori and pseudocolpi starts simultaneously. Structural difference between the aperture and pseudoaperture becomes clearly noticeable at the early free microspore stage when the endexine is initiated: at the colpori (at the area of ora) the endexine is composed of only a thin loose layer, at the pseudocolpi, the endexine is homogeneous and thick. Later, on the late free microspore stage, the difference appears in the structure of the intine: at the area of ora, the intine is three times thicker comparing with the pseudoapertures and mesocolpia. Besides zonal pseudoapertures, the triangular poroid areas are present at both poles of M. scorpioides pollen. Their structure and developmental pathway are similar to the zonal pseudoapertures. Pseudocolpi and polar pseudoapertures should be considered as the structures originated de novo specialised for harmomegathy.     

鹅掌楸属植物花粉萌发前后壁的超微结构

观察描述了在电镜下中国鹅掌楸（Ｌｉｒｉｏｄｅｎｄｒｏｎｃｈｉｎｅｎｓｅ）和北美鹅掌楸（Ｌ．ｔｕｌｉｐｉｆｅｒａ）２种植物花粉壁的超微结构及其水合后的变化。（１）成熟花粉壁由６层组成,即外壁３层──外层,中层１和中层２,内壁３层──内壁１,内壁２和内壁３。（２）花粉水合时,在内壁３与质膜之间由Ｐ一粒子（多糖－粒子）和被膜小泡参与形成新层。（３）花粉萌发时,由内壁３的一部分和新层突出萌发孔共同形成花粉管壁。（４）新层于花粉管形成早期分成２层──外染色深的果胶层和内电子透明的胼胝质层。     

CHANGES IN PLASTID AND MITOCHONDRION CONTENT DURING MATURATION OF GENERATIVE CELLS OF SOLANUM (SOLANACEAE)

A study was made of the number of plastids and mitochondria present in generative cells of Solanum immediately after microspore mitosis, and the fate of these organelles during development of the pollen was determined. Changes were followed via electron microscopy of anthers of S. chacoense and S. tuberosum Group Phureja × S. chacoense. In earliest stages the generative cells were oval and had one surface along the intine and other surfaces in contact with the vegetative cell. As the pollen matured the generative cells elongated, became spindle-shaped, and were completely engulfed in the vegetative cells. At the earliest stages studied, both mitochondria and plastids were present in the generative cell. Plastids of the generative cell were, in contrast to those of the vegetative cells, fewer, smaller, and lacking in starch. Through the maturation stages the content of these organelles in the vegetative cells remained unchanged. While the generative cells retained mitochondria until anthesis, their plastids disappeared completely during maturation. This selective loss during generative cell maturation could lead to transmission of those characteristics encoded in plastid DNA through the pistillate parent only. The mechanism could explain earlier genetic evidence that plastid characters of Solanum were transmitted uniparentally.     

The formation of the generative cell in Polystachia pubescens (Orchidaceae)

Summary The formation and nature of the generative cell wall and the detachment mode of the generative cell from the intine in Polystachia pubescens were observed by LM and TEM. Vesicles evenly positioned within the phragmoplast fuse to form a cell plate that divides the microspore into the generative and vegetative cell. This cell plate consists of callose. Before the generative cell leaves the intine, however, the callose is completely resorbed and is not replaced by any other substance. The generative cell becomes detached from the intine by moving towards the centre of the pollen grain. A constriction formed thereby gives the generative cell a bulb-like appearance and leads ultimately to the generative cell being pinched off. Plasma-filled vesicles originating from the generative cell remain between the intine and the plasma membrane of the vegetative cell.     

Pollen wall development in Vigna vexillata I. Characterization of wall layers

Light and electron microscope observations characterized the layers that comprise Vigna vexillata L. pollen walls, and identified the timing of their development. Exine sculpturings form an unusually coarse ektexinous reticulum. The structure of the ektexine is granular; this differs from the columellate/tectate type of structure typical of most angiosperm pollen. The ektexine overlies a homogeneous-to-lamellar, electron-dense endexine, which in turn surrounds a thick, microfibrillar intine. Pollen grains are triporate and operculate, with Zwischenkörper and thickened intine underlying the apertures. The ektexine forms during the tetrad period of microspore development, the endexine and Zwischenkörper during the free microspore stage, and the intine during the bicelled (pollen) stage. Coarsely reticulate exine sculpturings and the granular structure of the patterned exine wall of the pollen grains are features that make this species suitable for detailed studies of pollen wall pattern formation.     

Three-dimensional aspects of EXINE INITIATION AND DEVELOPMENT IN LILIUM LONGIFLORUM (Liliaceae)

Pollen development in Lilium longiflorum was reinvestigated with high resolution scanning electron microscopy, with special attention to three-dimensional conformation in the exine pattern formation. At the early tetrad stage, the invaginated plasma membrane takes the form of a reticulate pattern that corresponds to the mature exine tectum. Protectum is the first exine layer to be deposited on the reticulate-patterned plasma membrane. The initial protectum consists of aggregated fibrous threads and granules. Subsequently, probacules are formed under the protectum on the plasma membrane. At the free microspore stage the developing exine becomes further enlarged and the protectum develops into mature verrucate muri. The present three-dimensional investigation conflicts with the previous studies on exine development in Lilium.     

SPORE WALL FORMATION AND CHLOROPLAST DEVELOPMENT DURING SPOROGENESIS IN THE MOSS FISSIDENS LIMBATUS

The sequence of wall formation in spores of Fissidens limbatus Sullivant is as follows: The exine is formed around the protoplasts after the sporocyte has undergone meiosis. The fully enlarged spores then become coated by the perine; this is followed by intine formation. The source of the intine and exine appears to be from within the spore, but the perine is of an apparent exogenous origin. Ornamentation of the spore is due solely to deposition of the perine. Each spore originally has a single plastid. Plastids increase in number by fission, resulting in mature spores with numerous plastids with well differentiated lamellae.     

Sporoderm ultrastructure and development in Aristolochia manshuriensis Komarov (Aristolochiaceae)

Pollen ontogeny and sporoderm development in Aristolochia manshuriensis were studied for elaboration of the inaperturete pollen ontogeny in Aristolochia. Despite the formation of apertures in the tetrad period, the sporoderm in A. manshuriensis becomes inaperturate at the end of the free microspore period. A similar immature exine is also detected in A. macrophylla. Variants of aperture formation in the tetrad period in A. manshuriensis or formation of a polar aperture in the free microspore period in A. clematitis are associated with types of microsporogenesis. The ectexine and endexine in A. manshuriensis are formed over a longer time and reached much greater thickness than those in A. clematitis. The endexine and intine in A. manshuriensis do not reach a mature state, similar to A. clematitis. The exine of A. manshuriensis cracks, releasing a pollen tube enveloped by the intine. This fact does not hinder the functioning of the male gametophyte of A. manshuriensis.     

麻疯树花药发育过程中Ca2+的分布特征

采用超薄切片技术,在透射电镜下观察麻疯树(Jatropha curcasL.)花药发育过程中Ca2 的分布特征。在孢原细胞时期的花药中几乎看不到Ca2 沉淀,但花药维管束周围的细胞中有较多的Ca2 沉淀;到小孢子母细胞时期,细胞质中Ca2 沉淀依然较少,绒毡层壁上Ca2 沉淀明显增多;四分体形成时,小孢子细胞质和绒毡层细胞质中出现了较多的Ca2 沉淀;在小孢子发育早期,细胞质中Ca2 沉淀增加不明显,花粉壁部位累积有很多的Ca2 沉淀,绒毡层中Ca2 沉淀数量达到最多;到小孢子发育晚期,小孢子大液泡的液泡膜上有大量的Ca2 沉淀,绒毡层中Ca2 沉淀明显减少;随着二胞花粉中的大液泡消失,细胞质中积累淀粉粒以后,花粉中看到的Ca2 沉淀极少,同时,在花药维管束周围的薄壁细胞中,又出现了较多的Ca2 沉淀,表明花粉对Ca2 的需求可能降低。麻疯树花药发育过程中钙的动态分布特征暗示着钙参与了调控花粉发育过程,Ca2 的运输途径是由药隔薄壁组织运输到绒毡层,再进一步转移到小孢子表面和细胞质中,整个花药发育过程中,Ca2 沉淀表现为少—增加—减少的变化趋势。     

SPORE WALL DEVELOPMENT IN ANDREAEA (MUSCI: ANDREAEOPSIDA)

The spore wall of Andreaea rothii (Andreaeopsida) is unique among mosses studied by transmission electron microscopy. The exine of other mosses is typically initiated on trilaminar structures of near unit membrane dimensions just outside the plasma membrane. The exine of Andreaea is initiated in the absence of such structures as discrete globules within the coarsely fibrillar network of the sporocyte wall. The sequence of wall layer development, nevertheless, is essentially like that of other mosses. The intine is deposited within the exine and the perine accumulates on the surface of the exine during the latter stages of spore maturation. The mature spore is weakly trilete and inaperturate. The wall consists of three layers, the inner intine, the spongy exine consisting of loosely compacted irregular globules of sporopollenin, and an outer layer of perine. The perine differs ultrastructurally from the exine only in its greater degree of electron opacity. This ultrastructural evidence of departure from the fundamental pattern of exine development in mosses supports the taxonomic isolation of Andreaea from mosses of the Sphagnopsida and Bryopsida.     

Ultrastructure of microsporogenesis and microgametogenesis in Campsis radicans (L.) Seem. (Bignoniaceae)

In the present study, microsporogenesis, microgametogenesis and pollen wall ontogeny in Campsis radicans (L.) Seem. were studied from sporogenous cell stage to mature pollen using transmission electron microscopy. To observe the ultrastructural changes that occur in sporogenous cells, microspores and pollen through progressive developmental stages, anthers at different stages of development were fixed and embedded in Araldite. Microspore and pollen development in C. radicans follows the basic scheme in angiosperms. Microsporocytes secrete callose wall before meiotic division. Meiocytes undergo meiosis and simultaneous cytokinesis which result in the formation of tetrads mostly with a tetrahedral arrangement. After the development of free and vacuolated microspores, respectively, first mitotic division occurs and two-celled pollen grain is produced. Pollen grains are shed from the anther at two-celled stage. Pollen wall formation in C. radicans starts at tetrad stage by the formation of exine template called primexine. By the accumulation of electron dense material, produced by microspore, in the special places of the primexine, first of all protectum then columellae of exine elements are formed on the reticulate-patterned plasma membrane. After free microspore stage, exine development is completed by the addition of sporopollenin from tapetum. Formation of intine layer of pollen wall starts at the late vacuolated stage of pollen development and continue through the bicellular pollen stage.     

DEVELOPMENT OF STRIATE EXINE IN IPOMOPSIS RUBURA (POLEMONIACEAE)

Exine formation in Ipomopsis rubura (L.) Wherry (Polemoniaceae) was traced with transmission and scanning electron microscopy. At the early tetrad stage, plasma membranes of microspores invaginate and form a punctate pattern within the callose wall. Protectum is then deposited, taking a punctate pattern corresponding to the plasma membrane. After dissolution of the callose wall, the punctate pattern develops into a striate sculpture by partial thickening of tectum. The mature tectum is thus composed of two layers, the inner punctate and outer striate layers. With the plasma membrane determining the initial tectum pattern within the tetrad, additional sculptural elements are later formed above this pattern during the free microspore stage.