Vegetative Cell Cytoplasm Research Articles

Generative reproduction of Antarctic grasses, the native species Deschampsia antarctica Desv. and the alien species Poa annua L.

AbstractThe embryology of two species, Deschampsia antarctica, a native species, and Poa annua, an alien species in the Antarctic we studied. Flowering buds of plants growing in their natural habitats on King George Island and generative tissues of both plant species grown in a greenhouse were analyzed. Adaptations to autogamy and anemogamy were observed in the flower anatomy of both species. The microsporangia of the evaluated grasses produce a small number of three−celled pollen grains. Numerous pollen grains do not leave the microsporangium and germinate in the thecae. Deschampsia antarctica and P. annua plants harvested in Antarctica developed a particularly small number of microspores in pollen chambers. In D. antarctica, male gametophytes were produced at a faster rate: generative cells in pollen did not become detached from the wall of the pollen grain, they were not embedded in the cytoplasm of vegetative cells, and they divided into two sperm cells situated close to the wall. The monosporous Polygonum type of embryo sac development was observed in the studied species. The egg apparatus had typical polarization, and the filiform apparatus did not develop in synergids. Large antipodals with polyploidal nuclei were formed in the embryo sacs of D. antarctica and P. annua. Poa annua was characterized by numerous antipodal cells which formed antipodal tissue in the chalazal region of the embryo sac. Three distinct antipodals with atypical, lateral position in the vicinity of the egg apparatus were observed in D. antarctica. The diaspores of the investigated grass species were characterized by small size, low weight and species-specific primary and secondary sculpture of the testa and caryopsis coat.

Ultrastructural study of maturing pollen in Arabidopsis thaliana (L.) Heynh. (Brassicaceae)

Ultrastructural changes in <em>Arabidopsis thaliana</em> pollen, between late microspore stage and mature pollen stage were described. When the generative cell was peeled off from the intine, it was of spherical shape and had all usual organelles with the exception of plastids. The cytoplasm transformation of the vegetative cell included an increase in the number of mitochondria and changes in the accumulation of starch and lipid bodies. The starch plastids were observed at the bicellular and early tricellular pollen stages and next starch was utilized during the maturation procces. The lipid bodies of the vegetative cell form a very regular sheath around the generative cell and then, around the sperm cells. Before anthesis the lipid bodies were dispersed within the whole vegetative cell cytoplasm.

CHARACTERIZATION OF A MARINE CYANOBACTERIUM THAT BORES INTO CARBONATES AND THE REDESCRIPTION OF THE GENUS MASTIGOCOLEUS(1).

A marine, filamentous, endolithic cyanobacterium, strain BC008, was obtained in pure culture and characterized using a polyphasic approach. BC008 could bore into calcium carbonate minerals (calcite, aragonite) and, weakly, into strontium carbonate (strontianite), but not into other carbonates, phosphates, sulfates, silicates, or oxides, including those of calcium. We describe procedures for its continued cultivation in an actively boring state. BC008 was developmentally complex: it displayed lateral, terminal, and intercalary heterocysts; true branching; trichome tapering; and motile hormogonia. It also displayed considerable morphological plasticity between boring and nonboring modes. Boring brought about a halving of trichome diameter, a marked decrease in the ratio of heterocysts to vegetative cells, and a significant preference for lateral versus terminal heterocyst development. The cytoplasm of vegetative cells was filled with 20 nm thick, nanocompartment-like structures of polyhedral appearance and of unknown function. BC008 was capable of complementary chromatic adaptation but did not produce sheath pigments. When boring, it conformed well morphologically to Lagerheim's (1886) description of Mastigocoleus testarum, one of the most common and pervasive bioerosive agents of marine carbonates. We propose strain BC008 as type strain for the species. Multigene (16S rRNA, nif H, rbcL) phylogenies confirm that Mastigocoleus is a distinct, deeply branching genus of cyanobacteria that shares affinities and critical traits with two major taxonomic groups in the heterocystous clade (Nostocales and Stigonematales). We provide a revision of the genus and species descriptions based on our strain and findings.

A RelA‐dependent two‐tiered regulated proteolysis cascade controls synthesis of a contact‐dependent intercellular signal in Myxococcus xanthus

Proteolytic cleavage of precursor proteins to generate intercellular signals is a common mechanism in all cells. In Myxococcus xanthus the contact-dependent intercellular C-signal is a 17 kDa protein (p17) generated by proteolytic cleavage of the 25 kDa csgA protein (p25), and is essential for starvation-induced fruiting body formation. p25 accumulates in the outer membrane and PopC, the protease that cleaves p25, in the cytoplasm of vegetative cells. PopC is secreted in response to starvation, therefore, restricting p25 cleavage to starving cells. We focused on identifying proteins critical for PopC secretion in response to starvation. PopC secretion depends on the (p)ppGpp synthase RelA and the stringent response, and is regulated post-translationally. PopD, which is encoded in an operon with PopC, forms a soluble complex with PopC and inhibits PopC secretion whereas the integral membrane AAA+ protease FtsH(D) is required for PopC secretion. Biochemical and genetic evidence suggest that in response to starvation, RelA is activated and induces the degradation of PopD thereby releasing pre-formed PopC for secretion and that FtsH(D) is important for PopD degradation. Hence, regulated PopC secretion depends on regulated proteolysis. Accordingly, p17 synthesis depends on a proteolytic cascade including FtsH(D) -dependent degradation of PopD and PopC-dependent cleavage of p25.

Effects ofMentha longifoliaL. Essential Oil and Nisin Alone and in Combination onBacillus cereusandBacillus subtilisin a Food Model and Bacterial Ultrastructural Changes

In the face of emerging new pathogens and ever-growing health-conscious customers, food preservation technology remains on the top agenda of food industry. This study was aimed at determining the effects of the essential oil of Mentha longifolia L., alone and in combination with nisin, on Bacillus cereus and Bacillus subtilis at 8°C and 25°C in a food model (commercial barley soup) during 15 days. The essential oil alone at 8°C inhibited bacterial growth significantly compared with the control (p < 0.05). However, at 25°C, none of the concentrations of the essential oil alone showed inhibitory effect on bacterial growth. At 8°C, the combination effect of the essential oil and nisin on bacteria was noted at 0.25 μg mL(-1) for nisin and 0.05 μL mL(-1) for the essential oil (p < 0.05). The combination of nisin and the essential oil demonstrated significant inhibitory effects on the vegetative forms of bacteria at 25°C, although it was comparable to that of nisin alone at the same concentrations. Electron microscopy studies revealed a great deal of damage to B. cereus treated with a combination of nisin and the essential oil. However, the combination of nisin with the essential oil led to a complete destruction of cell wall and cytoplasm of vegetative cells of B. subtilis.

Characterization of bacterial-type phosphoenolpyruvate carboxylase expressed in male gametophyte of higher plants

BackgroundPhosphoenolpyruvate carboxylase (PEPC) is a critical enzyme catalyzing the β-carboxylation of phosphoenolpyruvate (PEP) to oxaloacetate, a tricarboxylic acid (TCA) cycle intermediate. PEPC typically exists as a Class-1 PEPC homotetramer composed of plant-type PEPC (PTPC) polypeptides, and two of the subunits were reported to be monoubiquitinated in germinating castor oil seeds. By the large-scale purification of ubiquitin (Ub)-related proteins from lily anther, two types of PEPCs, bacterial-type PEPC (BTPC) and plant-type PEPC (PTPC), were identified in our study as candidate Ub-related proteins. Until now, there has been no information about the properties of the PEPCs expressed in male reproductive tissues of higher plants.ResultsExpression analyses showed that lily BTPC (LlBTPC) and Arabidopsis BTPC (AtBTPC) were significantly expressed in pollen. The fusion protein AtBTPC-Venus localized in the cytoplasm of the vegetative cell (VC). Both LlBTPC and AtBTPC expression initiated after the last mitosis before pollen germination. Lily PTPC (LlPTPC) and monoubiquitinated LlPTPC (Ub-LlPTPC) remained at constant levels during pollen development. In late bicellular pollen of lily, LlBTPC forms a hetero-octameric Class-2 PEPC complex with LlPTPC to express PEPC activity.ConclusionOur results suggest that an LlBTPC:Ub-LlPTPC:LlPTPC complex is formed in the VC cytoplasm during late pollen development. Both LlBTPC and AtBTPC expression patterns are similar to the patterns of the appearance of storage organelles during pollen development in lily and Arabidopsis, respectively. Therefore, BTPC is thought to accelerate the metabolic flow for the synthesis of storage substances during pollen maturation. Our study provides the first characterization of BTPC in pollen, the male gametophyte of higher plants.

Interaction of lipid bodies with other cell organelles in the maturing pollen of Magnolia × soulangeana (Magnoliaceae)

The pollen grain maturation in Magnolia x soulangeana was studied ultrastructurally and cytochemically using both the light and transmission electron microscope. Emphasis was given on the storage lipid bodies of the vegetative cell (VC) and their interaction with other cell organelles. Stereological analysis of electron micrographs was performed to evaluate the variation in volume density (V(V)), surface density, and surface-to-volume ratio (S/V) of various cell organelles during pollen maturation. The size and numerical density of the lipid bodies, and their frequency of association with other cell organelles, were also determined. It was noted that during pollen ontogeny and maturation, the lipid bodies changed their pattern of distribution in the VC cytoplasm, which may be a good marker for the succeeding stages of pollen development. Also, the size, osmiophily, and V(V) of the lipid bodies were progressively reduced during pollen maturation whereas the S/V was significantly increased. This seems to indicate that the lipid bodies are mobilized in part during this period of pollen maturation. In particular, the intermediate and mature pollen showed a high percentage of lipid bodies establishing a physical contact with either glyoxysomes, either protein storage vacuoles, or small vesicles presumably originated from dictyosomes. This physical contact was found in both the chemically fixed and rapid freeze-fixed pollen indicating that it is neither artifactual nor casual. On the basis of this intimate association with other cell organelles and the morphometric analysis performed, we suggest that the mobilization of lipid bodies is likely mediated not only by glyoxysomes but also by other catabolic pathways involving the interaction of lipid bodies with either protein storage vacuoles or small Golgi vesicles.

Regulated Secretion of a Protease Activates Intercellular Signaling during Fruiting Body Formation in M. xanthus

In response to starvation Myxococcus xanthus initiates a developmental program that culminates in fruiting body formation. There are two morphogenetic events in this program, aggregation and sporulation, which are temporally and spatially coordinated by the contact-dependent intercellular C-signal protein (p17). p17 is generated by proteolytic cleavage of the p25 precursor protein, which accumulates in the outer membrane of vegetative and starving cells. However, p17 generation is restricted to starving cells. Here we identify the subtilisin-like protease PopC that is directly responsible for cleavage of p25. PopC accumulates in the cytoplasm of vegetative cells but is selectively secreted during starvation coinciding with the generation of p17. Consequently, p25 and PopC only encounter each other in starving cells. Thus, restriction of p25 cleavage to starving cells occurs by a compartmentalization mechanism that depends on selective secretion of PopC during starvation. Our results provide evidence for regulated proteolysis via regulated secretion.

Ultrastructure and cytochemistry of Eucalyptus globulus (Myrtaceae) pollen grain

The morphology, ultrastructure and cytochemistry of Eucalyptus globulus mature pollen were investigated using light (LM), scanning electron (SEM) and transmission electron microscopy (TEM). The pollen morphology is typically myrtaceous with a suite of characters that allows its distinction from the pollen of other Eucalyptus species. The exine consists of a thick endexine and a massive ectexine with hardly distinguishable columellae. The endexine is 2‐layered towards the apertural regions where the inner and spongy‐granulate layer forms a continuous colpus membrane and a thinner pore membrane. Under the pores the intine is 3‐layered forming complex onci. The spindle‐shaped generative cell (GC) is deeply undulated and located in a cup‐shaped depression of the vegetative cell (VC) nucleus. In the dense VC cytoplasm the main storage reserves are lipid bodies and insoluble carbohydrates in the cytosol, although proteins are also present. The most characteristic feature of the VC cytoplasm is the extremely well‐developed rough endoplasmic reticulum (RER), which forms extensive stacks filling large areas of the central cytoplasm. Single RER cisternae are also scattered throughout the cytoplasm, most of them establishing an intimate association with lipid bodies, storage vacuoles, the VC plasmalemma and a few proplastids. The physiological significance of the RER stacks and of the RER cisternae association with other cell components, as well as the structure and function of an endomembrane compartment, found only in the freeze‐fixed pollen, are discussed.

Sperm cells of Zea mays have a complex complement of mRNAs

Although double fertilization in angiosperm was discovered in 1898, we still know nothing about the proteins that mediate gamete recognition and fusion in plants. Because sperm are small and embedded within the large vegetative cell of the pollen grain, mRNAs from sperm are poorly represented in EST databases. We optimized fluorescence-activated cell sorting (FACS) in order to isolate Zea mays sperm free of contaminating vegetative cell cytoplasm, and constructed a cDNA library. Sequencing of over 1100 cDNAs from the unamplified library revealed that sperm have a diverse complement of mRNAs. Most transcripts were singletons; the most abundant was sequenced only 17 times. About 8% of the sequences are predicted to encode secreted or plasma membrane-localized proteins and are therefore candidates that might mediate gamete interactions. About 8% of the sequences correspond to retroposons. Plant sperm have condensed chromatin and are thought to be transcriptionally inactive. We used RT-PCR and in situ hybridization to determine when selected sperm mRNAs were transcribed. Sperm transcripts encoding proteins involved in general cell functions were present throughout pollen development and were more abundant in tricellular pollen than in sperm cells, suggesting that these transcripts were also present in the larger vegetative cell. However, several transcripts, which encode proteins that are most similar to hypothetical Arabidopsis proteins, appeared to be present exclusively in the sperm cells inside mature pollen, but were already present in unicellular microspores. This suggests that certain transcripts might be transcribed early during pollen development and later partitioned into the sperm cells.

Ultrastructural characterization of male sterile33 (ms33) mutant in Arabidopsis affected in pollen desiccation and maturation

The MS33 gene in Arabidopsis is required for stamen filament growth and for pollen maturation. The objective of this study was to characterize the effects of ms33 mutation on pollen development at the ultrastructural level. There were no differences between the wild type and ms33 mutant pollen development before the first mitotic division of microspores. At the bicellular pollen stage, the first signs of abnormalities were observed in the ms33 tapetum, which started to degenerate early and released osmiophilic material in the anther locule. In ms33 pollen, the endintine was thicker, and exintine thinner, than in the wild type, and the mutant pollen had large vacuoles. Later in development, the mutant pollen underwent second mitosis and produced two normal-looking sperm cells; however, the intine was precociously formed, and there were abnormalities in tryphine deposition on the pollen wall, in the size of vacuoles, and in the formation of lipid bodies in the vegetative cell cytoplasm. Based on these observations it is suggested that mutation in the MS33 gene interferes with intine formation and tryphine deposition, both of which negatively affect pollen desiccation resulting in large, highly vacuolate pollen that are nonviable.Key words: Arabidopsis, male sterility, mutant, pollen, tapetum, ultrastructure.

Hermodactylus tuberosus L. (Iridaceae) pollen organisation before and after anther dehiscence

ABSTRACT The morphology, cytology and viability of Hermodactylus tuberosus L. (Iridaceae) pollen were examined from the first mitosis until maturation and after anther opening. During maturation, the pollen coat becomes modified, and the vegetative cell cytoplasm accumulates several types of reserve substances. In the vegetative cell cytoplasm, starch is quickly utilised whereas lipid inclusions of different dimensions, shape and composition occur during pollen maturation. Pollen from opened anthers have a thin pollen coat; the cytoplasm has mostly lipid reserves, and many small vesicles and vacuoles. It is similar in size or larger than pollen located inside the anther, and its viability does not decrease until one day after anther dehiscence. Large osmiophilic bodies, different from those of the vegetative cell cytoplasm, are present in the generative cell cytoplasm starting from the first stage of pollen development. The poorly developed pollen coat in pollen from opened anthers suggests that it plays a minor role in attracting insects for pollination. The size and structural and ultrastructural features of mature pollen indicate that it does not undergo dehydration and possesses sufficient vigour for immediate germination.

The spermatogenous body cell of the coniferPicea abies (Norway spruce) contains actin microfilaments

In conifer pollen, the generative cell divides into a sterile stalk cell and a body cell, which subsequently divides to produce two sperm. InPicea abies (Norway spruce, Pinaceae) this spermatogenous body cell contains actin microfilaments. Microfilament bundles follow the spherical contour of the body cell within the cell cortex, and also traverse the cytoplasm and enmesh amyloplasts and other organelles. In addition, microfilaments are associated with the surface of the body cell nucleus. The sterile stalk cell also contains microfilament bundles in the cytoplasm, around organelles, and along the nuclear surface. Within the pollen grain, microfilament bundles traverse the vegetative-cell cytoplasm and are enriched in a webbed cage which surrounds the body cell. Microfilaments were identified with rhodamine-phalloidin and with indirect immunofluo-rescence using a monoclonal antibody to actin. The majority of evidence in literature suggests that the spermatogenous generative cell in angiosperms does not contain actin microfilaments, so the presence of microfilaments within the spermatogenous body cell inP. abies appears to be a fundamental difference in sexual reproduction between conifers and angiosperms.

Intracellular motility and the evolution of the actin cytoskeleton during development of the male gametophyte of wheat (Triticum aestivumL.)

The uniaperturate pollen of wheat is dispersed in a partially hydrated condition. Amyloplasts are concentrated in the apertural hemisphere where they surround the two sperms, while vigorously moving polysaccharide–containing wall precursor bodies (P–particles) together with the vegetative nucleus occupy the other. This disposition is the product of a post–meiotic developmental sequence apparently peculiar to the grasses. During vacuolation of the spore after release from the tetrad, the nucleus is displaced to the pole of the cell opposite the site of the germination aperture, already defined in the tetrad. Following pollen mitosis, the vegetative nucleus migrates along the wall of the vegetative cell towards the aperture, leaving the generative cell at the opposite pole isolated by a callose wall. As the vacuole is resorbed, the generative cell rounds up, loses its wall and follows the vegetative nucleus, passing along the wall of the vegetative cell towards the aperture where it eventually divides to produce the two sperms. Throughout this period of nucleus and cell manoeuvrings, minor inclusions of the vegetative cell cytoplasm, including mitochondria, lipid globuli and developing amyloplasts, move randomly. Coordinated vectorial movement begins after the main period of starch accumulation, when the amyloplasts migrate individually into the apertural hemisphere of the grain, a final redistribution betokening the attainment of germinability. In the present paper we correlate aspects of the evolution of the actin cytoskeleton with these events in the developing grain, and relate the observations to published evidence from another monocotyledonous species concerning the timing of the expression of actin genes during male gametophyte development, as revealed in the synthesis of actin mRNA.

Distribution and characterization of mosquitocidal toxin genes in some strains of Bacillus sphaericus.

The binary toxin of Bacillus sphaericus strains forms a crystal in sporulating cells, while the mosquitocidal toxin is located in the cytoplasm of vegetative cells. The distribution of binary toxin (btx) and mosquitocidal toxin (mtx) genes in 53 strains of B. sphaericus was determined by hybridization of specific gene probes to chromosomal DNA in Southern blots. btx genes were found in all strains of serotype 5a5b examined and in some strains of serotypes 1a, 3, 6, 25, and 48, while mtx genes were detected in strains of serotypes 1a, 2a2b, 5a5b, 6, 9a9c, 25, and 48. Serotype 26a26b strains lacked both toxin genes, as did some strains of serotypes 2a2b, 3, 6, and 48. Partial DNA sequences of btx genes from five strains, together with published sequences, revealed four types of toxin among mosquitocidal B. sphaericus strains. most of the 42-kDa toxin gene of btx was identical in strains from serotypes 1a, 3, 6, and 48, and the gene is here classified as a type 1 btx gene. A serotype 3 strain isolated in Singapore possessed a unique 42-kDa toxin gene, here designated type 4; while the btx genes from strains of serotypes 5a5b and 25 are referred to as types 2 and 3, respectively.

Diphtheria toxin-mediated cell ablation in developing pollen: Vegetative cell ablation blocks generative cell migration

The technique of genetic cell ablation involves the targeted expression of a cell autonomous cytotoxic protein under the control of cell-specific regulatory sequences. This technique allows the investigation of cell-cell interactions by inducing selective death in a precisely controlled and cell autonomous manner. Here, targeted vegetative cell-specific ablation was used to examine the role of the vegetative cell (VC) in controlling generative cell (GC) behaviour and differentiation during pollen development. The tomatolat 52 late-pollen promoter, which has been shown to be activated specifically in the nascent VC immediately following pollen mitosis I (PMI), was used to direct expression of the cytotoxic diphtheria toxin A chain (DTA) in both transient expression assays using microprojectile bombardment and in transgenic tobacco plants. Transient expression of DTA linked to thelat 52 promoter (lot 52-DTA) in pollen dramatically reduced the expression of a co-transfected reporter gene fusion, demonstrating the cytotoxicity of DTA to pollen. Genetic and phenotypic analysis oflat 52-DTA transformants demonstrated that DTA expression led to a pollen-lethal phenotype, recognisable as small acytoplasmic pollen grains at anthesis, which affected 50% of the pollen population in single locus transformants. Detailed cytological analysis using confocal laser scanning microscopy and vital staining using fluorescein diacetate (FDA), showed that the first sign of cell ablation during pollen development was a loss of vital staining of the VC immediately following PMI. In contrast, the GC retained viability for up to several days following VC ablation, but progressively lost viability in the absence of a functional VC. Of particular interest was the observation that in the absence of VC function the generative cell (GC) failed to undergo normal migration away from the pollen grain wall into the VC cytoplasm. These results directly demonstrate the dependence of the GC on VC cell functions and highlight the importance of VC-GC interactions in controlling GC migration.

An intermediate-voltage electron microscopic study of freeze-substituted generative cell in pear (Pyrus communis L.): features with relevance to cell-cell communication between the two cells of a germinating pollen

The ultrastructure of the generative cell (GC) wall complex in germinating pear (Pyrus communis L.) pollen was studied with the aim of identifying features that may shed light on the mechanism of uptake of substances by the GC from its host, the vegetative cell (VC). The techniques of rapid freeze-fixation and freeze-substitution, serial sectioning, and conventional and intermediate-voltage transmission electron microscopy were employed. The wall complex consisted of two plasma membranes (PMs), one derived from the GC and the other from the VC. A nonfibrillar wall material occurred in the space between the two PMs. Plasmodesmata could not be identified in this wall complex. However, in localized areas the wall complex formed processes that protruded into the VC cytoplasm. In other areas, the wall complex showed certain cup-shaped invaginations. Certain double membrane bound multivesicular bodies occurred in the GC cytoplasm; their morphological characteristics indicated that they may have been derived from the GC wall complex. The data indicate that in pear the GC surface is amplified by wall processes, presumably to perform a role analogous to transfer cells.

Electron microscopy of infection of nematodes by Dactylaria haptotyla

Infection of nematodes byDactylaria haptotyla, a nematode-trapping hyphomycete, was studied by electron microscopy. The cytoplasm of the adhesive knob in the fungus contained a number of electron-dense, membrane-bound vesicles, 0.2–0.5 µm in diam. The vesicles were rarely seen in the stalk cell or vegetative cell cytoplasm. When the adhesive knob came into contact with the nematode's cuticle, it secreted an adhesive which was seen in ultrathin sections between the knob and the cuticle as an amorphous mass. At the same time, electron-dense vesicles in the cytoplasm were reduced in number and many small vacuoles developed. Soon after capture of a nematode, the cell wall of the adhesive knob became obscure at the prospective site of penetration, where a vesicle, 0.7 µm in diam, was found in serial thin sections of the knob's cytoplasm. At the site facing the vesicle, the peripheral part of the nematode's cell exhibited a high electron density. The vesicle, which appeared to be derived from smaller electron-dense vesicles coalesced with each other, released its enzymic contents toward the captured nematodes before penetration by the fungus.

Intimate Association between the Generative Cell and Endoplasmic Reticulum during the In vitro Lag Period of Magnolia x Soulangeana Pollen Germination

During the in vitro lag period of Magnolia x soulangeana pollen germination, a conspicuous intimate association is established between the inner plasma membrane of the vegetative cell (VC) and the cisternae of the rough endoplasmic reticulum (RER) existing in the VC cytoplasm around the generative cell (GC). The cisternae run parallel and are tightly appressed to the inner plasma membrane of the VC without space between them. The same single cisterna often parallels the convoluted outline of the GC surface over an extensive length, almost completely covering it with RER. Possibly, the attachment of RER cisternae to the inner plasma membrane of the VC, apart from providing local synthesis, may have a role in (1) canalizing products from elsewhere in the VC cytoplasm toward the GC or (2) shielding the GC from events occurring in the VC cytoplasm. During this period the GC and the vegetative cell nucleus (VN) are always closely associated with each other, forming the male germ unit. Examination of thin sections indicates that they are probably interlocked with one another, the distance between the inner plasma membrane of the VC and the nuclear envelope of the VN being 50-70 nm in some regions. Although components that could link both the structures of the GC and VN are not detected, it is noteworthy that cisternae covering the inner plasma membrane of the VC do not normally exist in the regions where the GC is more closely associated with the VN. The possibility that this lack of cisternae is connected with the relationship between the GC and VN is discussed.

An ultrastructural study of pollen development in tomato (Lycopersicon esculentum). II. Pollen maturation

The maturation of tomato pollen grains encompassed several ultrastructural changes. The generative cell separated from the intine and was free in the cytoplasm of the vegetative cell. This process coincided with the appearance of starch in plastids and the division of elongated mitochondria. This stage was followed by a second phase of vacuolation in the vegetative cell cytoplasm. Starch was still abundant at this stage, as were mitochondria, endoplasmic reticulum (ER), and ribosomes. Lipid droplets were the prominent feature of mature pollen grains. Each droplet was surrounded by rough ER (RER), suggesting the role of RER in lipid accumulation and mobilization. Long stretches of ER were present at early stages of maturation, and stacks of up to 50 strands of RER were abundant in mature pollen. The plastids in mature pollen were devoid of starch and had few internal membranes. Mitochondria were abundant and spherical with parallel cristae. In many cases, the cytoplasm at the periphery of the mature pollen grain was dense, forming a distinct zone, and contained only ER. The generative cell cytoplasm had mitochondria, ER, and actin-like filaments but no plastids. The pollen wall at maturity had a lamellated foot layer, a lightly sculptured tectum, and broad intine. The intine was layered in the region of the pollen aperture. Key words: Lycopersicon esculentum, pollen grains, tomato, ultrastructure.