Dictyosome Vesicles Research Articles

Genome-Wide Association Study Reveals Key Genes for Differential Lead Accumulation and Tolerance in Natural Arabidopsis thaliana Accessions.

Soil contamination by lead (Pb) has become one of the major ecological threats to the environment. Understanding the mechanisms of Pb transport and deposition in plants is of great importance to achieve a global Pb reduction. We exposed a collection of 360 Arabidopsis thaliana natural accessions to a Pb-polluted soil. Germination rates, growth, and leaf Pb concentrations showed extensive variation among accessions. These phenotypic data were subjected to genome wide association studies (GWAs) and we found a significant association on chromosome 1 for low leaf Pb accumulation. Genes associated with significant SNP markers were evaluated and we selected EXTENSIN18 (EXT18) and TLC (TRAM-LAG1-CLN8) as candidates for having a role in Pb homeostasis. Six Pb-tolerant accessions, three of them exhibiting low leaf Pb content, and three of them with high leaf Pb content; two Pb-sensitive accessions; two knockout T-DNA lines of GWAs candidate genes (ext18, tlc); and Col-0 were screened under control and high-Pb conditions. The relative expression of EXT18, TLC, and other genes described for being involved in Pb tolerance was also evaluated. Analysis of Darwinian fitness, root and leaf ionome, and TEM images revealed that Pb-tolerant accessions employ two opposing strategies: (1) low translocation of Pb and its accumulation into root cell walls and vacuoles, or (2) high translocation of Pb and its efflux to inactive organelles or intracellular spaces. Plants using the first strategy exhibited higher expression of EXT18 and HMA3, thicker root cell walls and Pb vacuolar sequestration, suggesting that these genes may contribute to the deposition of Pb in the roots. On the other hand, plants translocating high amounts of Pb showed upregulation of TLC and ABC transporters, indicating that these plants were able to properly efflux Pb in the aerial tissues. We conclude that EXT18 and TLC upregulation enhances Pb tolerance promoting its sequestration: EXT18 favors the thickening of the cell walls improving Pb accumulation in roots and decreasing its toxicity, while TLC facilitates the formation of dictyosome vesicles and the Pb encapsulation in leaves. These findings are relevant for the design of phytoremediation strategies and environment restoration.

Diaphragm development in cytokinetic vegetative cells of brown algae

Abstract We studied development of the cytokinetic diaphragm (i.e., the membranous septum formed during cytokinesis) in vegetative cells of the brown algae Sphacelaria rigidula, Halopteris congesta and Dictyota dichotoma after cryofixation-freeze-substitution. Cytokinesis began by the gathering of organelles and large vesicles between daughter nuclei following telophase. Subsequently, a thin cytoplasmic strand was formed along this plane, where endoplasmic reticulum (ER), dictyosome vesicles and particular membranous elements, the flat cisternae, were accumulated. Their fusion formed a patchy diaphragm with irregular gaps. Fine tubular channels perforated the diaphragm during all stages of its formation. Following diaphragm completion, cell wall material was deposited in it. The new walls had ER-free plasmodesmata. In D. dichotoma, diaphragm development did not follow a definite pattern, i.e., centripetal or centrifugal, a phenomenon also confirmed in H. congesta. In contrast, in apical cells of S. rigidula the diaphragm started developing from the periphery, growing to some extent centripetally. In these cells, local cell wall deposition was greatest at the division site. In apical cells in which cytokinesis was experimentally inhibited a ring of wall material was usually deposited at the cytokinetic plane.

Cytogenetical and ultrastructural effects of copper on root meristem cells of Allium sativum L.

Different copper concentrations, as well as different exposure times, were applied to investigate both cytogenetical and ultrastructural alterations in garlic (Allium sativum L.) meristem cells. Results showed that the mitotic index decreased progressively when either copper concentration or exposure time increased. C-mitosis, anaphase bridges, chromosome stickiness and broken nuclei were observed in the copper treated root tip cells. Some particulates containing the argyrophilic NOR-associated proteins were distributed in the nucleus of the root-tip cells and the amount of this particulate material progressively increased with increasing exposure time. Finally, the nucleolar material was extruded from the nucleus into the cytoplasm. Also, increased dictyosome vesicles in number, formation of cytoplasmic vesicles containing electron dense granules, altered mitochondrial shape, disruption of nuclear membranes, condensation of chromatin material, disintegration of organelles were observed. The mechanisms of detoxification and tolerance of copper are briefly discussed.

Floral nectar production and nectary anatomy and ultrastructure of Echinacea purpurea (Asteraceae).

In spite of the impressive species diversity in the Asteraceae and their widespread appeal to many generalist pollinators, floral-nectary ultrastructure in the family has rarely been investigated. To redress this, a study using Echinacea purpurea, a plant of horticultural and nutraceutical value, was undertaken. Nectar secretion of disc florets was compared with floral nectary ultrastructure taking into account nectar's potential impact upon the reproductive success of this outcrossing species. Micropipette collections of nectar in conjunction with refractometry were used to determine the volume and nectar-sugar quantities of disc florets throughout their phenology, from commencement of its production to cessation of secretion. Light, scanning-electron and transmission-electron microscopy were utilized to examine morphology, anatomy and ultrastructure of nectaries of the disc florets. Florets were protandrous with nectar being secreted from anthesis until the third day of the pistillate phase. Nectar production per floret peaked on the first day of stigma receptivity, making the two innermost whorls of open florets most attractive to foraging visitors. Modified stomata were situated along the apical rim of the collar-like nectary, which surrounds the style base and sits on top of the inferior ovary. The floral nectary was supplied by phloem only, and both sieve elements and companion cells were found adjacent to the epidermis; the latter participated in the origin of some of the precursor cells that yielded these specialized cells of phloem. Companion cells possessed wall ingrowths (transfer cells). Lobed nuclei were a key feature of secretory parenchyma cells. The abundance of mitochondria suggests an eccrine mechanism of secretion, although dictyosomal vesicles may contribute to a granulocrine process. Phloem sap evidently is the main contributor of nectar carbohydrates. From the sieve elements and companion cells, an apoplastic route via intercellular spaces and cell walls, leading to the pores of modified stomata, is available. A symplastic pathway, via plasmodesmata connecting sieve elements to companion, parenchyma and epidermal cells, is also feasible. Uncollected nectar was reabsorbed, and the direct innervation of the nectary by sieve tubes potentially serves a second important route for nectar-sugar reclamation. Microchannels in the outer cuticle may facilitate both secretion and reabsorption.

Ultrastructure of tetrasporogenesis in the red alga Osmundea spectabilis var. spectabilis (Rhodomelaceae: Ceramiales: Rhodophyta)

Tetrasporogenesis in Osmundeaspectabilis (Postels and Ruprecht) K.W. Nam var. spectabilis proceeds through three developmental stages. A uninucleate tetraspore mother cell with synaptonemal complexes depicted during early prophase of meiosis I comprises the youngest stage. Subsequently, the nuclear envelope obtains a highly irregular profile and is surrounded by perinuclear endoplasmic reticulum (ER) and numerous mitochondria. Proplastids with peripheral thylakoid and numerous straight-profiled dictyosomes are present. Dictyosome vesicles discharge their contents, initiating tetraspore wall formation. A four-nucleate tetraspore mother cell is formed prior to tetrahedral cleavage. During the second stage of tetraspore formation the nuclei are very irregular in outline and move toward the central region where they fuse. The formation of fibrous vacuoles originates from the fibrous vacuole associated organelles (FVAOs). Thylakoid formation from concentric lamellar bodies (CLBs) seems to occur in plastids. Extremely hypertrophied dictyosomes with two tightly compressed and densely laminated mid-regions produce vesicles which contribute to enlargement of the fibrous vacuoles. During the second stage of tetraspore development striated vesicles are formed and starch granule formation increases. Mature tetraspores are characterized by numerous cored vesicles and abundant starch granules. Plastids have a fully developed system of internal thylakoids, and cleavage is usually complete at this stage. Mature tetraspores are surrounded by a bi-layered wall and the tetrasporangial wall. Unbranched, tubular structures connecting the plasmalemma with the peripheral sheets of ER are possibly involved in wall deformation. A distinct blebbing nuclear envelope appears to sequester vesicles into the cytoplasm.

Ultrastructure of spermatiogenesis in the red alga Osmundea spectabilis var. spectabilis (Rhodomelaceae, Ceramiales, Rhodophyta)

Spermatiogenesis in Osmundea spectabilis var. spectabilis (= Laurencia spectabilis var. spectabilis) begins with the differentiation of the epidermal cells within the conceptacle. These cells divide to form filaments that develop toward the centre of the pit. Spermatangial mother cells originate from the cells of the filaments and produce several spermatangia. All young spermatia have some chloroplasts and are surrounded by a spermatangial wall. As the spermatia mature, rough endoplasmic reticulum aggregates and the formation of spermatial vesicles containing granular material commences. Subsequently, spermatial vesicles enlarge by contribution of dictyosome vesicles containing fibrillar material. Spermatia develop a two-layered wall and can be released at any stage of differentiation. Finally, the spermatial vesicles discharge their contents, forming an extracellular, confluent, mucilaginous matrix that surrounds the released mature spermatia. Bacteria cannot enter the spermatia since they are well protected by this matrix. Mature released spermatia are spherical and devoid of a wall, and possess mitochondria, vesicles with electron-dense cores, some concentric membrane bodies and an intact nucleus, but they usually lack chloroplasts.

Callose deposition and breakdown, followed by phytomelan synthesis in the seed coat ofGasteria verrucosa (Mill.) H. Duval

In the seed coat ofGasteria verrucosa the deposition of phytomelan takes place during seed development in three stages. Phytomelan is a black cell wall material which is chemically very inert. First the radial walls and part of the transverse cell wall of the outer epidermis of the outer integument become thickened by exocytosis of dictyosome vesicles. Callose is deposited at the tangential plasma membrane against those walls. After the callose deposition about two thirds of the original cell volume is filled with callose. During the second stage the callose is broken down, probably into glucose monomers or small polymers. At the same time cellulose is deposited at the outer tangential plasma membrane, forming a wall between the dissolving callose and the plasma membrane. In the third phase small granules appear in the solution of dissolved callose. which grow out and finally fuse to form a block of phytomelan, consisting of spherical 15-nm units. Remarkable is the function of the callose: it determines the size of the phytomelan block, and it probably functions as carbohydrate source for the phytomelan synthesis and/or for the cellulose inner layer. In this study transmission electron microscopy and cryo scanning electron microscopy are used to study the three developmental stages of the formation of the phytomelan layer.

Regions of lead uptake inLemna minor plants and localization of this metal within selected parts of the root

Investigations were carried out to determine the sites of lead uptake within the frond and the root ofLemna minor. With the sodium rhodizonate four regions favoured in lead uptake were distinguished: the frond region between the base and the node, the basal part of the root, and the regions at the proximal and distal ends of the root cap. For analysis in electron microscope only the root regions were chosen. The highest rate of lead uptake was found in the basal part of the root. Lead was present in the apoplast of this region after 5 min of exposure and was observed in the stelar cells after 30 min of incubation. Lead deposits were detected mostly in the cell walls adjacent to the plasma membrane and in the lumen of several endomembrane compartments - the endoplasmic reticulum (ER), dictyosomal vesicles, nuclear envelope and the vacuoles. Lead induced changes of cell ultrastructure; an increase in the number of membraneous structures, swelling of ER cisternae and distortion of the dictyosomal cisternae were observed after 2 to 6 h of exposure.

Freeze-fracture studies in the brown algaAsteronema rhodochortonoides

The gross structure of the cell wall and the organization of the plasmalemma of the filamentous brown algaAsteronema rhodochortonoides were examined in replicas of freeze-fractured cells. The protoplasmic fracture face (PF) of the plasmalemma, apart from the single particles, exhibits two particular particle complexes, i.e., single linear arrays of closely packed particles, and well defined particle pentads. The former display a consistent relationship with the ends of microfibril imprints and therefore are considered as terminal complexes (TCs). They seem to be composed of subunits, each one consisting of two particles. The average diameter of the particles is 7 nm. The number of the subunits forming the TCs varies between 2 and 40. Short TCs, consisting of 3–5 subunits were also found on the PF of dictyosome vesicles, a fact suggesting the involvement of the Golgi apparatus in exocytosis of preformed TC portions. The occurrence, distribution and size of the TCs appear to be related to the developmental stage of the cell. A large number of TCs occur in actively growing cells, while a few or no TCs are found in differentiated cells. The pentads are rectangular structures consisting of five particles, four in the corners and one in the centre. Their dimensions are very constant, but their occurrence and distribution varies. They occur in young developing cells where TCs are few or absent, but were also observed in areas showing many TCs. In differentiated cells no pentads were found. Pentad-like structures were rarely observed on the PF of dictyosome vesicles or cisternae. The observations support the hypothesis that pentads are involved in the synthesis of matrix polysaccharides, which are the major components of brown algal cell wall and their synthesis begins before that of cellulose.

Pollination Drop and Microdrop Secretions in Cedrus

In Cedrus pollen adheres by microdrops to the apical portion of the integument. The microdrops originate from dictyosome vesicles in cells of the integument apex. Electron-dense substances are present in these dictyosome vesicles and accumulate between the surface cell walls and plasma membrane. The substances traverse the walls and form microdrops. The cuticle layer on the surface cell walls is less developed and does not serve as an obstacle to the passage. During pollination drop secretion, nucellar cells show characteristics such as undulation of the plasma membrane, numerous dictyosomes, and a prominent amount of endoplasmic reticulum. The cuticle layer is absent from the nucellar apex. After the secretion, cell walls of the nucellus and chalaza become transparent. We describe ultrastructural changes associated with the winter dormancy of the ovule.

The Transmitting Tissue in Brugmansia suaveolens L.: Ultrastructure of the Stylar Transmitting Tissue

Abstract The stylar transmitting tissue of the angel's trumpet, Brugmansia (Datura) suaveolens, was studied at two developmental stages: about 6 d before anthesis and after anthesis. Histochemical localization of polysaccharides was carried out with PATAg and immunohistochemistry with gold-conjugated antibodies recognizing pectins. Before anthesis the transmitting tissue forms a central core of polyhedral meristematic, still dividing, cells with narrow intercellular spaces. Epitopes for unesterified pectins are present in the walls and the spaces between the cells, while methylesterified pectins are confined to the middle lamella and intercellular spaces. PATAg positive material and the antibody against unesterified pectin was found in plasmalemma invaginations and multivesicular bodies. Dictyosome cisternae and vesicles contained epitopes for both kinds of pectins. Plastids are poorly differentiated and lack starch. Nutrients are stored as lipid bodies, which are digested by small vacuoles. After anthesis the transmitting tract cells form cylindrical files separated by voluminous spaces filled with a mucous secretion reacting with PATAg and with the antibody against unesterified pectins. Dictyosome vesicles contain epitopes for the same kind of pectins. The cells are vacuolized and have leucoplasts. This study shows pectin synthesis by different parts of the endomembrane system and changes in pectin esterification during stylar development.

How lead can easily enter the food chain — a study of plant roots

Using the root test, the relationship between the amount of lead in plant tissues and the level of the root growth inhibition was examined for twelve food plant species. 345 mg to 8152 mg Pb·kg −1 dry wt. inhibited root growth only by 5 to 36% as compared to the control. At least 96.6% of lead was bound in cells of root tips. Most of the lead was accumulated in cell walls, vacuoles and sometimes in dictyosomal vesicles. Lead accumulating in these cell compartments is separated from cell cytoplasm and therefore is no longer toxic for root cells. The results show that amounts of lead much bigger than those observed in the environment can now easily enter the food chain via plants. High tolerance to lead in plant roots is quite unfavourable for other members in the food chain, including man.

Subcellular alterations in Phytophthora infestans infecting tomato leaves treated with fosetyl-Al

The quantitative changes in organelles of Phytophthora infestans in tomato leaves that had previously been treated with the systemic fungicide fosetyl-Al (2000 μg · ml −1) were measured by electron microscopy 15 and 18 hr after inoculation. The fungal nuclei and ribosomes did not show alterations in fine structure and numbers, although the nuclear form, and shape of some haustoria, had changed by 18 hr after inoculation. Some components of the endomembrane system such as vacuoles and dictyosome vesicles were apparent in thin sections 15 hr after inoculation. Invagination of the plasmalemma increased. The width of dictyosomes and the amount of endoplasmic reticulum increased by 18 hr. Mitochondrial size differed from that of the control after 15 hr. In addition, vacuoles were present in the subapical region of hyphal tips in fosetyl-Al-treated tissues. Changes in these fungal structures and their possible relation to the mode of action of the fungicide are discussed.

Embryology of Brassica campestris: the entrance and discharge of the pollen tube in the synergid and the formation of the zygote

The postanthesis synergids and zygote of Brassica campestris cv. Candle were examined using techniques of light, fluorescence, and electron microscopy. The pollen tube enters the degenerate synergid by way of the filiform apparatus. A degeneration of one of the two synergids occurs after anthesis and is independent of pollination. The first sign of synergid degeneration is a more intense staining of one of the synergids, followed by a loss of organelle membrane integrity. There is a disappearance of the plasma membrane and dictyosome cisternae; however, profiles of degenerate synergid mitochondria, plastids, and dilated endoplasmic reticulum remain along with dictyosome vesicles that contain periodic acid – thiocarbo-hydrazide – silver proteinate positive substances. The zygote, shortly after fertilization, is reduced in size and lacks the large micropylar vacuole characteristic of the mature unfertilized egg cell. Plastids and mitochondria are concentrated around the centrally located nucleus of the zygote, and dictyosomes, active in vesicle production, are located in the lateral and chalazal regions of the cell, adjacent to the cell wall. The lateral cell walls are periodic acid – Schiff's and Calcofluor positive, while the ampulliform chalazal tip of the cell is weakly periodic acid – Schiff s positive and Calcofluor negative. Microtubules, with the long axis perpendicular to the long axis of the zygote, are abundant in the ampulliform chalazal tip of the cell. Following fertilization the central cell becomes highly vacuolate. There is continuity between the zygote – central cell plasma membrane, the central cell vacuole tonoplast, and membranes of the central cell endoplasmic reticulum. Central cell wall projections, of the transfer cell type, are located in the lateral regions of the megagametophyte adjacent to the developing zygote cell and are positioned adjacent to the region of inner and outer integument starch. Key words: Brassica, ultrastructure, synergid, megagametophyte, pollen tube, zygote.

Studies of pollen maturation in cotton: the storage reserve accumulation phase

This study follows the maturation of the pollen grain of cotton (Gossypium hirsutum L.), particularly the development of the vegetative cytoplasm and the various storage products formed. CTEM, HVEM, stereoscopy, and cyto-histochemistry were used to examine the events occurring during the 9 days before anthesis. Starch began to accumulate in plastids at anthesis minus 9 days and reached a peak concentration shortly before anthesis; lipid deposition followed a similar pattern, but started at 6 days before anthesis. Lipid bodies were always seen closely oppressed to the endoplasmic reticulum (ER). Dictyosomes appear active during the entire 9 days; first producing vesicles involved in the formation of the intine and, later, producing vesicles stored in the pollen grain. The dictyosome vesicles appear to contain polysaccharides and concentrate in layers around the lipid bodies. Ribosomes increase in number from 6 days before anthesis and are particularly numerous in the mature pollen. From anthesis minus 6 days until anthesis, the ER cisternae become increasingly inflated and, in the hours immediately before pollen release, form pockets filled with lipid bodies and dictysosome vesicles. The mature pollen has a core region filled with ER pockets and a peripheral cytoplasm in which such pockets are generally lacking.

Effects of oxadixyl treatment on the ultrastructure of Phytophthora infestans infecting tomato leaves

AbstractOxadixyl is a systemic fungicide controlling diseases caused by Oomycetes. In this ultrastructural study of the mode of action, it was found that more than 40% of 60 observed haustoria of Phytophthora infestans were less than I am in diameter in tomato leaf inner spongy mesophyll tissues previously treated with the fungicide (8 μg ml‐1) and were on average smaller than in untreated tissues. Ultrastructural observations on treated tissues also revealed that dictyosome vesicles were fewer near dictyosomes in the region of the intercellular hyphae which subtended the haustorium and in haustoria, during haustorium development. Fungal nucleoli degenerated and. in contrast to previous observations of the fungitoxic effects, numbers of ribosomes decreased in the cytoplasm, indicating that synthesis of ribosomes was probably inhibited. In addition, fragmentation of the endoplasmic reticulum and increases in the number and size of vacuoles and lomasomes were also observed in treated tissues. These results were considered to be fungistatic effects of the fungicide on the fungus. In some infected plant cells, electron‐dense deposits appeared in the cytosol in the vicinity of the extrahaustorial membrane. Compared with the nature of the related compound, metalaxyl, there appear to be differences between their effects in some aspects, e.g. haustorium development and haustorial encasement formation.

Development of oil and mucilage cells inCinnamomum burmanni. An ultrastructural study

SUMMARY The development of oil and mucilage cells in the shoot apex and young leaves in Cinnamomum burmanni is described. Three arbitrary developmental stages are distinguished. At stage 1, the cells differ from the surrounding cells by the absence of osmiophilic deposits in the vacuoles and the presence of distinct small plastids with reduced thylakoids. At stage 2, a suberized layer is deposited in the cells. At stage 3, the oil or mucilage secretion has started in the cells. For both oil and mucilage cells, three intergrading stages (3a, 3b, 3c) are recognized. In oil cells, the subdivision is based on the thickness of the inner-wall layer, the composition of the cytoplasm and the extent of the oil cavity. The thickness of the inner-wall layer increases with development. The oil cavity, enclosed by the plasmalemma and attached to the wall by a cupule, enlarges. Oil formation presumably takes place within the plastids and apparently results in the disintegration of the plastids. In mucilage cells, the substages are distinguished based on the amount of mucilage deposited between the suberized layer and the plasmalemma, and the composition of the cytoplasm which contains an increasing amount of dictyosome vesicles. For the first time, cupule-like wall structures are reported in mucilage cells. Therefore, the number of similarities in both oil and mucilage cells increases to four: the presence of a suberized layer, an extraplasmatic accumulation of the secretory product, and the presence of a cupule and characteristic plasmodesmata. These resemblances strongly support their hypothesized homology.

The transmitting tract in Trimezia fosteriana (Iridaceae).

Observations of the transmitting tract cells in Trimezia fosteriana were made from the pre‐secretory stage until anthesis. Secretory products appear about 14 days before anthesis in all parts of the pistil. Simultaneously starch disappears from the plastids and the dictyosomes are surrounded by more and larger vesicles than before. In the beginning of the secretory stage multivesicular bodies may be in contact with ER profiles and an ER origin is therefore presumed. Later during the secretory stage the multivesicular bodies are larger and more abundant. Their envelope is often partly fused with the plasma membrane and vesicle swarms are common on the outside of it. Close to flower opening many organelles change in appearence. RER becomes more abundant and the mitochondrial matrix highly electron dense and the cristae expanded. Starch grains reappear and large ones are common in the stigma and ovary plastids. The dictyosomes are numerous and surrounded by the largest vesicles observed during the secretory stage. Osmiophilic granules are common in the dictyo‐some vesicles and under the cell walls abutting the transmitting tract. In the ovary they are present one week before anthesis. In the stigma and style corresponding granules of high electron density appears at the day of flower opening. The extracellular secretory product contains fibrillar and granular substances consisting of carbohydrates, including pectic substances, and proteins.

The development of the central cell of Brassica campestris prior to fertilization

The development of the central cell of Brassica campestris cv. Candle (canola-rapeseed) was examined using techniques of light and electron microscopy and cytochemistry. The mature central cell is devoid of the large central vacuole characteristic of the early cellular stage of megagametophyte development. Prior to anthesis, cell wall projections, of the transfer cell type, develop on the lateral wall of the central cell. These central cell wall projections extend from the midregion of the megagametophyte to the egg apparatus and are immediately adjacent to the starch-containing region of the inner and outer integuments. The cell wall projections are periodic acid – thiocarbohydrazide – silver proteinate positive as are the contents of dictyosome vesicles that appear to contribute to their formation. Mitochondria are associated with the wall projections as is a network of central cell endoplasmic reticulum that extends from the wall projections to the egg apparatus. Microtubules are associated with the migrating chalazal polar nucleus. The two polar nuclei partially fuse prior to double fertilization, united by nuclear bridges and endoplasmic reticulum interconnections. Proplastids are a characteristic feature of the immature cellular megagametophyte. By anthesis, the proplastids of the mature central cell develop into chloroplasts with stacked thylakoids and starch deposits. Microbodies are frequently found associated with lipid bodies, and polysomes with the endoplasmic reticulum of the mature central cell. Key words: Brassica, central cell, megagametophyte, ovule, transfer cell.

Effects of anoxia on pollen tube growth and tube wall formation of Impatiens glandulifera

When pollen of Impatiens glandulifera was cultured in aerated liquid medium for 1 h, 70% of the pollen grains germinated; these attained an average tube length of 1 mm. Subsequently, these aerobic growth conditions were changed to anaerobic by substituting a nitrogen inlet for the air inlet. As a result, the pollen tubes stopped elongating and burst. The ultrastructural changes which occurred upon inducing anoxia were studied with samples taken at 0 s, 45 s, and 4 min after changing the gas. Anoxia caused rapid and considerable changes in the ultrastructure of the dictyosome vesicles involved in cell wall formation. There was an increase in the osmiophyly of the vesicle content, and the presence of fibrillar material became apparent. Simultaneously, the fusion behavior of the dictyosome vesicles changed. Instead of the normal fusion of the dictyosome vesicles with the plasma membrane, there was a premature fusion of the vesicles with each other inside the cytoplasm that resulted in the formation of aggregates. Furthermore, the cell wall precursors that were excreted were not incorporated in their usual configuration into the growing cell wall. Instead of a smooth inner cell wall surface, irregular thickenings were formed.