Accumulation and post-translational modifications of plant tubulins

The microtubular cytoskeleton of plant cells provides support for several functions (including the anchoring of proteins, assembly of the mitotic spindle, cytoplasmic streaming and construction of cell walls). Both α- and β-tubulins are encoded through multigene families that are differentially expressed in different organs and tissues. To increase the variability of expression, both protein subunits are subjected to post-translational modifications, which could contribute to the assembly of specific microtubule structures. This review aims to highlight the role of specific post-translational modifications of tubulin in plant cells. We initially describe the expression and accumulation of α- and β-tubulin isoforms in different plants and at different stages of plant development. Second, we discuss the different types of post-translational modifications that, by adding or removing specific functional groups, increase the isoform heterogeneity and functional variability of tubulin. Modifications are proposed to form a 'code' that can be read by proteins interacting with microtubules. Therefore, the subpopulations of microtubules may bind to different associated proteins (motor and non-motor), thus creating the physical support for various microtubule functions.

Identification and characterization of a novel microtubule-based motor associated with membranous organelles in tobacco pollen tubes

Pollen tube growth depends on the differential distribution of organelles and vesicles along the tube. The role of microtubules in organelle movement is uncertain, mainly because information at the molecular level is limited. In an effort to understand the molecular basis of microtubule-based movement, we isolated from tobacco pollen tubes polypeptides that cosediment with microtubules in an ATP-dependent manner. Major polypeptides released from microtubules by ATP (ATP-MAPs) had molecular masses of 90, 80, and 41 kD. Several findings indicate that the 90-kD ATP-MAP is a kinesin-related motor: binding of the polypeptide to microtubules was enhanced by the nonhydrolyzable ATP analog AMP-PNP; the 90-kD polypeptide reacted specifically with a peptide antibody directed against a highly conserved region in the motor domain of the kinesin superfamily; purified 90-kD ATP-MAP induced microtubules to glide in motility assays in vitro; and the 90-kD ATP-MAP cofractionated with microtubule-activated ATPase activity. Immunolocalization studies indicated that the 90-kD ATP-MAP binds to organelles associated with microtubules in the cortical region of the pollen tube. These findings suggest that the 90-kD ATP-MAP is a kinesin-related microtubule motor that moves organelles in the cortex of growing pollen tubes.

Localization of pectins in the pollen tube wall of Ornithogalum virens L. Does the pattern of pectin distribution depend on the growth rate of the pollen tube?

Monoclonal antibodies that recognize pectins were used for the localization of esterified (JIM7) and acidic, unesterified (JIM5) forms of pectin in pollen tube walls of Ornithogalum virens L. (x = n = 3). The results indicated that the distribution of the two forms of pectin in the pollen tube wall depended on the medium (liquid or solid) used for pollen germination. In pollen tubes grown in the liquid medium, the localization of JIM7 was limited to the very tip of the pollen tube, whereas the localization of JIM5 indicated a uniform distribution of unesterified pectins in the very tip of the tube and along the subapical parts of the tube wall. In tubes germinated on the medium stabilized with agar (1-2%) the localization of JIM7 and JIM5 indicated the presence of both forms of pectin in the tube tip and along the whole length of the pollen tube wall in a ring-like pattern. Thus, the localization of esterified pectins in the sub-apical part of the pollen tube wall, below the apex of the tube, is described for the first time. Measurements of the growth rates of pollen tubes growing on the two types of medium indicated that oscillations in tube growth rate occur but these do not coincide with the pattern of pectin distribution in the tube wall. Our results complement the previous data obtained for the localization of JIM5 and JIM7 in pollen tube walls of other plant species. (Y.-Q. Li et al. 1994, Sex Plant Reprod 7: 145-150) and provide new insight into an understanding of the construction of the pollen tube wall and the physiology of pollen grain germination.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

Comparison of flagellated and nonflagellated sperm in plants

Differences among flagellated and nonflagellated sperm in land plants are striking, but close examination reveals similarities in pattern of cytoskeleton and in nuclear structure. The microtubular cytoskeleton of flowering plant sperm consists of microtubule bundles arranged obliquely around the nucleus, terminating in cellular extensions. Microtubules are linked into bundles that branch and rejoin along the axis of the sperm cell, forming a cytoskeleton that determines cell shape but does not actively participate in cell movement. Generative cells and sperm share a pattern of microtubules not found in somatic cells. This pattern is initiated in the generative cell, one division before sperm formation, a situation parallel to spermatogenous cell development in vascular plants with flagellated sperm. Chromatin in flagellated and nonflagellated sperm is condensed by specialized histones.

Confocal imaging and immunogold electron microscopy of changes in distribution of myosin during pollen hydration, germination and pollen tube growth in Nicotiana tabacum L

Using anti-myosin antibodies, standard immunocytochemical techniques in conjunction with confocal scanning laser microscopy and colloidal gold immunoelectron microscopy we compare changes in the distribution patterns of myosin during the early stages of pollen hydration, germination, tube growth, and myosin associated with isolated vegetative nucleus and the generative cell in Nicotiana tabacum L. Furthermore, on the Western blots of pollen tube proteins, the antimyosin antibodies crossreact only with one polypeptide of approximately 174 kDa. Confocal immunofluorescence microscopy reveals that in hydrated pollen, myosin is discretely associated with the cytoplasmic organelles and numerous punctate structures present in the center of the pollen. Within 30 min following transfer of pollen into the germination medium, that is, with the onset of germination, the centrally located punctate structures are displaced, and we find accumulation of myosin-associated organelles towards one of the germinal apertures from which the pollen tube would emerge. Subsequently, after 45 min of germination with the emergence of germination structure, few punctate structures are detected in the vegetative cytoplasm while intense immunostain is detected just below the plasma membrane of the emerging pollen tube tip. In the older parts of both short and long pollen tubes after 90 to 120 min of pollen germination, few fluorescent structures were found in the pollen tubes, however, numerous punctate fluorescent spots were concentrated in the tip region over a distance of 2 to 3 microns below the plasma membrane of the tube tip. This is further substantiated by colloidal gold immunoelectron microscopy wherein clusters of gold particles are associated with vesicle-like structures in the tip region of the pollen tubes.(ABSTRACT TRUNCATED AT 250 WORDS)

High molecular weight polypeptides related to dynein heavy chains in Nicotiana tabacum pollen tubes

Nicotiana tabacum pollen tubes contain two high molecular weight polypeptides (about 400 kDa), which are specifically expressed during pollen germination and pollen tube growth in BK medium. The high molecular weight doublet resembles the dynein heavy chains in some biochemical properties. Sedimentation profiles of pollen tube extracts show that the high molecular weight bands have sedimentation coefficients of 22 S and 12 S, respectively. ATPase assay of sedimentation fractions shows an activity ten times higher when stimulated by the presence of bovine brain microtubules in fractions containing the 22 S high molecular weight polypeptide. Both these high molecular weight polypeptides can bind microtubules in an ATP-dependent fashion. A mouse antiserum to a synthetic peptide reproducing the sequence of the most conserved ATP-binding site among dynein heavy chains recognized the two high molecular weight polypeptides. Therefore these polypeptides have sequences immunologically related to the ATP binding sites of dynein heavy chains.

Characterisation of isolated egg cells, in vitro fusion products and zygotes of Zea mays L. using the technique of image analysis and confocal laser scanning microscopy

Changes in membrane Ca2+, calcium receptor protein calmodulin, endoplasmic reticulum (ER), mitochondria and cellulose in unfixed, living, isolated egg cells and fusion products of pairs of one egg and one sperm cell of Zea mays L. have been investigated using chlorotetracycline, fluphenazine, immunocytochemical techniques, 3,3'-dihexyloxa-carbocyanine iodide (DiOC6(3)) and calcofluor white in conjunction with computer-controlled video image analysis. In addition, confocal laser scanning microscopy has been used in conjunction with ethidium bromide to detect the nature and location of the sperm cell nuclear chromatin before and after karyogamy. Digitised video images of chlorotetracycline (CTC) fluorescence reveal that egg cells contain high levels of membrane Ca2+ in organelles present around the nucleus while the cytosolic signal is relatively low. Intense CTC fluorescence is invariably present just below the plasma membrane of egg cells and a certain degree of regionalised distribution of Ca2+ in cytoplasm is also discernible. Similarly, the fluphenazine (FPZ)-detectable calmodulin (CaM) and that localised immunocytochemically using monoclonal anti-CaM antibodies reveal high levels of CaM in the vicinity of the nucleus in egg cells. Only a few ER profiles and mitochondria could be visualised in the egg cell and no calcofluor fluorescence could be detected. Following in vitro fertilisation of single isolated eggs substantial changes in the Ca2+ levels occur which include an increase in the membrane Ca2+ of the fusion product, particularly in the cytosol and around the nucleus. Unlike in the eggs the fine CTC fluorescence signal below the plasma membrane is not detectable in the fusion products.(ABSTRACT TRUNCATED AT 250 WORDS)

Pollen tube growth is coupled to the extracellular calcium ion flux and the intracellular calcium gradient: effect of BAPTA-type buffers and hypertonic media

Lily pollen tubes possess a steep, tip-focused intracellular Ca2+ gradient and a tip-directed extracellular Ca2+ influx. Ratiometric ion imaging revealed that the gradient extends from above 3.0 microM at the apex to approximately 0.2 microM within 20 microns from the tip, while application of the Ca(2+)-specific vibrating electrode indicated that the extracellular influx measured between 1.4 and 14 pmol cm-2 sec-1. We examined the relationship between these phenomena and their role in tube growth by using different 1,2-bis(o-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)-type buffers and hypertonic media. Injection of active BAPTA-type buffers or application of elevated levels of sucrose reversibly inhibited growth, destroyed tip zonation of organelles, and modified normal patterns of cytoplasmic streaming. Simultaneously, these treatments dissipated both the intracellular tip-focused gradient and the extracellular Ca2+ flux. Of the BAPTA-type buffers, 5,5'-dibromo-BAPTA (dissociation constant [Kd] is 1.5 microM) and 4,4'-difluoro-BAPTA (Kd of 1.7 microM) exhibited greater activity than those buffers with either a higher affinity (5,5'-dimethyl-BAPTA, Kd of 0.15 microM; BAPTA, Kd of 0.21 microM; 5,5'-difluoro-BAPTA, Kd of 0.25 microM) or lower affinity (5-methyl, 5'-nitro-BAPTA, Kd of 22 microM) for Ca2+. Our findings provide evidence that growing pollen tubes have open Ca2+ channels in their tip and that these channels become inactivated in nongrowing tubes. The studies with elevated sucrose support the view that stretching of the apical plasma membrane contributes to the maintenance of the Ca2+ signal.

Pollen tube growth is coupled to the extracellular calcium ion flux and the intracellular calcium gradient: effect of BAPTA-type buffers and hypertonic media

Lily pollen tubes possess a steep, tip-focused intracellular Ca2+ gradient and a tip-directed extracellular Ca2+ influx. Ratiometric ion imaging revealed that the gradient extends from above 3.0 microM at the apex to approximately 0.2 microM within 20 microns from the tip, while application of the Ca(2+)-specific vibrating electrode indicated that the extracellular influx measured between 1.4 and 14 pmol cm-2 sec-1. We examined the relationship between these phenomena and their role in tube growth by using different 1,2-bis(o-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)-type buffers and hypertonic media. Injection of active BAPTA-type buffers or application of elevated levels of sucrose reversibly inhibited growth, destroyed tip zonation of organelles, and modified normal patterns of cytoplasmic streaming. Simultaneously, these treatments dissipated both the intracellular tip-focused gradient and the extracellular Ca2+ flux. Of the BAPTA-type buffers, 5,5'-dibromo-BAPTA (dissociation constant [Kd] is 1.5 microM) and 4,4'-difluoro-BAPTA (Kd of 1.7 microM) exhibited greater activity than those buffers with either a higher affinity (5,5'-dimethyl-BAPTA, Kd of 0.15 microM; BAPTA, Kd of 0.21 microM; 5,5'-difluoro-BAPTA, Kd of 0.25 microM) or lower affinity (5-methyl, 5'-nitro-BAPTA, Kd of 22 microM) for Ca2+. Our findings provide evidence that growing pollen tubes have open Ca2+ channels in their tip and that these channels become inactivated in nongrowing tubes. The studies with elevated sucrose support the view that stretching of the apical plasma membrane contributes to the maintenance of the Ca2+ signal.

Confocal image analysis of spatial variations in immunocytochemically identified calmodulin during pollen hydration, germination and pollen tube tip growth in Nicotiana tabacum L

Using monoclonal anti-calmodulin antibodies in conjunction with confocal scanning laser microscopy we have analysed the spatial variations in the distribution pattern of calmodulin (CaM) during the sequential events of pollen hydration, germination and tube growth in Nicotiana tabacum. These immunocytochemical observations have been complemented by immunochemical studies wherein the anti-calmodulin antibody raised against pea CaM recognises a polypeptide of c. 18 kDa in the pollen extracts. Digitisation of confocally acquired optical sections of immunofluorescence images reveals that in hydrated pollen a high level of CaM is consistently present in the region of the germinal apertures. Subsequently, with the onset of germination a high CaM concentration was found associated with the plasma membrane of the germination bubble and in the cytoplasm in its vicinity, while in the vegetative cytoplasm a weak diffuse and intense punctate signal was registered. CaM immunostain was also detected in association with the plasma membrane of the tube tips in both short and long pollen tubes. Furthermore, the cytosol of the tubes invariably manifested an apically focused CaM gradient. We were, however, unable to detect any vacuolar association of CaM in the older regions of the pollen tubes. Although punctate immunostain was obvious across the pollen tube numerous punctate structures were invariably present in the extreme tip. The possible implications of these findings in development of cell polarity, polarised growth, maintenance of calcium homeostasis and CaM interactions with other mechanochemical motor proteins in effecting propulsion of organelles during pollen hydration, germination and pollen tube growth are discussed.

Kinesin-related polypeptide is associated with vesicles from Corylus avellana pollen

A 100-kDa polypeptide with microtubule-interacting properties was identified in a Golgi vesicle-enriched fraction from Corylus avellana pollen. The k71s23 antibody (directed to the kinesin heavy chain from bovine brain) [Tiezzi et al., 1992: Cell Motil. Cytoskeleton 21:132-137] localized the polypeptide on the external surface of membrane-bounded organelles. Some 100-kDa-containing vesicles copelleted with microtubules (polymerized from purified bovine brain tubulin) either in presence or absence of 5 mM AMPPNP, but they could be released by 10 mM ATP or 0.5 M KCl. The pollen microtubule-interacting protein, salt-extracted from membranes and partially purified by gel filtration, exhibited an ATPase activity (16.2 nmolPi/mg/min) which could be stimulated about 2-fold (32.5 nmolPi/mg/min) by addition of bovine brain microtubules. We suppose that the 100-kDa polypeptide is part of a molecular complex showing properties of the kinesin class.

Cytoskeletal organisation and modification during pollen tube arrival, gamete delivery and fertilisation in Plumbago zeylanica

The cytoskeletal organisation of the isolated embryo sac and egg cells of Plumbago zeylanica was examined before, during and after pollen tube penetration into the embryo sac to determine the potential involvement of microtubules and actin filaments in fertilisation. Material was singly and triply stained using Hoechst 33258 to localise DNA, fluorescein isothiocyanate (FITC)-labelled anti-alpha-tubulin to detect microtubules and rhodamine-phalloidin to visualise F-actin. Microtubules in the unfertilised egg cell are longitudinally aligned in the micropylar and mid-lateral areas, aggregating into bundles near the filiform apparatus. In the perinuclear cytoplasm of the egg cell, microtubules become more or less randomly aligned. F-actin bundles form a longitudinally aligned mesh in the chalazal cytoplasm of the egg cell. In the central cell, microtubules and F-actin are distributed along transvacuolar strands and are also evident in the perinuclear region and at the periphery of the cell. During pollen tube penetration, sparse microtubule bundles near the pathway of the pollen tube may form an apparent microtubular 'conduit' surrounding the male gametes at the delivery site. Actin aggregates become organised near the pathway of the pollen tube and at the delivery site of the sperm cells. Subsequently, actin aggregates form a 'corona' structure in the intercellular region between the egg and central cell where gametic fusion occurs. The corona may have a role in maintaining the close proximity of the egg and central cell and helping the two sperm cells move and bind to their target cells. The cytoskeleton may also be involved in causing the two nuclei of the egg and central cell to approach one another at the site of gametic fusion and transporting the two sperm nuclei into alignment with their respective female nucleus. The cytoskeleton is reorganised during early embryogenesis.

Ultrastructural changes of Arabidopsis thaliana pollen during final maturation and rehydration

Several ultrastructural changes occur during dehydration and subsequent rehydration of Arabidopsis thaliana pollen. The cytoplasmic channels, present in the outer part of the intine of the mature, dehydrating pollen grain, degenerate and develop into electron-dense inclusions. At the same time a large quantity of electron-dense material is deposited in the cavities of the exine. A large number of vesicles is produced in the vegetative cell, and they become predominantly located in the peripheral region near the intine. Starch of amyloplasts is consumed and the lipid bodies which originally surround the sperm cells become randomly distributed. In addition, the individual lipid bodies become enveloped by single rough endoplasmic reticulum cisterns.

Periodic deposition of arabinogalactan epitopes in the cell wall of pollen tubes of Nicotiana tabacum L

The monoclonal antibodies MAC 207 and JIM 8, recognizing arabinogalactan epitopes, were used to localize the corresponding antigenic sites in pollen tubes of Nicotiana tabacum L. grown in vitro or semi in vivo. The analysis of the immunofluorescence labelling was performed by means of confocal laser scanning microscopy. Most pollen tubes were labelled along their length, with the exception of the tip region, in a ring-like pattern with remarkable periodicity. The diameter of the rings was approx. 12 μm and the distance between two rings was about 6 μm. No labelling of the cytoplasm, the vegetative nucleus or the generative cell was observed. The presence of labelling in the non-apical tube wall after pectinase and cellulase digestion indicates that the epitopes for MAC 207 and JIM 8 are located in the inner callosic sheath of the pollen-tube wall.

An immunoreactive homolog of mammalian kinesin in Nicotiana tabacum pollen tubes

A cytoskeletal apparatus is involved in the movement of vesicles, organelles, and gametes in the pollen tube. The function of microfilaments has been defined quite precisely, but the role of microtubules needs to be further clarified. On the basis of immunological and biochemical investigations, we have identified a polypeptide showing common properties with kinesin, a microtubule-based motor mainly described in nonplant tissues, in the pollen tube of Nicotiana tabacum. Like mammalian kinesin, the kinesin-immunoreactive homolog from Nicotiana tabacum pollen tubes binds to mammalian microtubules in an AMP-PNP dependent manner. The kinesin-like component is likely to be involved in the movement of vesicular material in the growing pollen tube.

Pollen viability and pollen vigor

Investigations were carried out to correlate pollen viability, assessed on the basis of a fluorochromatic reaction (FCR) test, with pollen vigor, assessed on the basis of the time taken for in vitro germination in pollen grains subjected to high humidity (>95% RH) and temperature (38 °C) or storage stress of Nicotiana tabacum, Agave sp., Tradescantia virginiana, and Iris sp. Both high RH and temperature, as well as storage stresses, affected pollen vigor before affecting pollen viability. The results are discussed in the light of available data on the viability and vigor of stressed pollen and of aged seeds. The need for consideration of pollen vigor, particularly in stored pollen, the inadequacy of the methods presently used, and some of the methods suitable to assess pollen vigor are elaborated.

Changes in volume, surface area, and frequency of nuclear pores on the vegetative nucleus of tobacco pollen in fresh, hydrated, and activated conditions

The vegetative nucleus (VN) of Nicotiana tabacum L. has been qualitatively and quantitatively studied in fresh, hydrated, and activated pollen. Techniques included the use of optical sectioning by confocal scanning laser microscopy to obtain volume and surface area measurements, and stereoscopic pairs; and freeze-etch electron microscopy to estimate the frequency of nuclear pores per μm(2) in the vegetative nucleus. Several morphological changes were observed to occur in pollen grain nuclei during the early processes of tube growth. In freshly dehisced pollen grain, the vegetative and generative nuclei were side by side, but following hydration and activation of the grain, the elongated generative nucleus became partially surrounded by the vegetative nucleus. It was found that during hydration, the surface area of the vegetative nucleus increased and there was a decrease in the frequency of nuclear pores. The calculated total number of pores remained similar. After activation and pollen-tube growth, the vegetative nucleus retained the same surface area as in the hydrated state but the frequency of nuclear pores decreased; therefore, the calculated total number of pores was significantly lowered. When considered alongside complementary biochemical data, these morphological results indicate that RNA production in the vegetative nucleus decreases following germination.

The SH-SS exchange reaction between the Ellman's reagent and protein containing SH groups as a method for determining conformational states: tubulin

Reactivity of tubulin SH groups, estimated by the slope to the curve in the SH-SS exchange reaction with 5-5' dithiobis (2-nitrobenzoic acid), was compared with that of bovine serum albumin (BSA) and studied in presence of various ligands. A small part of tubulin SH groups (12%) showed a higher reactivity, while the remaining portion had a smaller reactivity than that of BSA. The SH group reactivity of tubulin but not the total amount (12.8 mu/mole) diminished when assayed with colchicine and MgCl2 (0.1 and 0.2 mM, respectively); 0.2 mM CaCl2 increased the reactivity at the maximum level. On the basis of the biological role of tubulin SH groups and of the opposite effects exerted by Ca++ and Mg++ on the protein, the results presented here seem to indicate a correlation between conformational states of tubulin and its biological functions.

Taxol-induced microtubules from different sources: an ultrastructural comparison

Microtubules from different sources have been isolated using 20 microM taxol. An ultrastructural comparison allowed us to reveal some differences among taxol dependent structures. In tobacco pollen tubes a characteristic system of thin filaments was associated with taxol-induced microtubules. Similar filamentous complexes were not observed in taxol-induced microtubules isolated from other sources.

The pollen calendar of Siena (Central Italy)

The atmospheric pollen count at Siena was carried out for 4 years (1978-1981) to determine the average quantity of airborn pollen and the families, genera or species which contribute to such pollen dispersal. The pollination period stretches from February to mid October. The highest pollination period is from May to July when the most important pollen producers are Poaceae, Urticaceae, Pinus, Quercus, Oleaceae and Castanea sativa. Also in March there are high pollen production values due to Cupressaceae, Corylaceae, Alnus and Ulmus.

Ultrastructure and histochemistry of Citrus limon (L.) stigma

Citrus limon has a "wet" stigma which can be divided in two zones: a glandular superficial one formed by papillae, and a non-glandular one formed by parenchymatic cells. The stigmatic exudate is produced by the papillae after the latter have reached their ultimate size. The papillae of the mature pistil are of varying size and composition. Both the unicellular and multicellular ones are present. The cells at the base of the papillae are rich in cytoplasm, whereas the tip cells are vacuolated. Histochemical analysis has shown that the exudate of Citrus is composed of lipids, polysaccharides, and proteins. Our results indicate that the lipidic component is produced and secreted first, followed by production and secretion of the polysaccharidic component. The lipidic component of the exudate is produced in the basal papillae cells and accumulates as droplets in dilated parts of the smooth endoplasmic reticulum (SER). Subsequently the lipid droplets are transported to the plasma membrane, and transferred by the latter into the cell walls. Then the exudate component is accumulated in the intercellular spaces and in the middle lamellar regions of the walls. Subsequently, the polysaccharidic component of the exudate is produced and secreted by the tip cells of the papillae.

Ultrastructural investigations on Lycopersicum peruvianum pollen activation and pollen tube organization after self-and cross-pollination

No differences have been observed "in vivo" between Lycopersicum peruvianum compatible and incompatible pollen during activation and pollen tube emission and organization, that is until 4 h and 30 min after pollination. During pollen activation the main events are the setting free of rough endoplasmic reticulum (RER) cisterns which were "stacked" in the mature pollen, the increase in the number of polysomes, and a great activity of the dictyosomes. Immediately after germination of the vegetative nucleus and the generative cell move into the tube, the generative cell diviting to form the male gametes; the tube then becomes organized in four zones. This series of changes is similar to what has already been observed "in vitro" except that in vitro the generative cell remains undivided and the whole process from seeding to tube organization takes 3 h instead of 4 h and 30 min after pollination, as it does in vivo. Our findings are compatible with the main models of the tube inhibition mechanism proposed till now.

Ultrastructural aspects of pollen tube growth inhibition after gamma irradiation in Lycopersicum peruvianum

Gamma ray treatments at various doses were applied to flowers after cross-compatible pollinations (S1S4 × S12S13) and self-incompatible pollinations (S1S4 × S1S4). After 200 kR treatment (highest dose) a high percentage of pollen became non-viable, and tube growth of all the germinated pollen was inhibited at the first third, or just before half the length of the style. Observations on the ultrastructural aspects revealed that the pollen tubes were destroyed by a precise degradation process which led to the disappearance of the inner wall, lysis of the tube, accumulation of several bipartite particles and alteration of endoplasmic reticulum into a whorl of concentric circles. These results indicated that the high dose of gamma rays probably interferes with protein synthesis. The ultrastructural aspects of compatible pollen tubes after gamma irradiation were similar to those of self-incompatible tubes and therefore it is suggested that cessation of protein synthesis might also be a result of incompatibility.Electron microscopy observations after treatment with 200 kR gamma rays on flowers after self-incompatible pollination (S1S4 × S1S4) showed that gamma irradiation affects the self-incompatibility reaction; but the results obtained so far are difficult to explain.

Germination and early tube development in vitro of Lycopersicum peruvianum pollen: Ultrastructural features

Morphologic changes occurring during pollen grain activation and ultrastructural features of Lycopersicum peruvianum Mill. pollen tube during the first stages of growth in vitro have been studied. The more evident morphologic changes during activation, in comparison to those already described for mature inactive pollen, concern dictyosomes, rough endoplasmic reticulum (RER), and ribosomes. The dictyosomes are very abundant and produce "large" and "small" vesicles. Near the germinative pores both types of vesicles are present, while all along the remaining cell wall only the large type is observed. These latter react weakly to Thiéry's test and probably contain a callose precursor necessary for the deposition of a callosic layer lining at first only the inner side of the functioning pore and occasionally the other two pores, and subsequently the entire inner surface of the cell wall. The small vesicles, highly positive to Thiéry's test, are present only near the pores and could be involved in the formation of the pectocellulosic layer of the tube wall. The setting free of RER cisterns, which in the mature inactive pollen were aggregated in stacks, coinciding with polysome formation and resumption of protein synthesis, is in accord with the hypothesized role of RER cistern stacks as a reserve of synthesizing machinery. The pollen tube reaches a definitive spatial arrangement soon after the generative cell and vegetative nucleus have moved into it. At this stage four different zones that reflect a functional specialization are present. In the apical and subapical zone two types of dictysosome-originated vesicles, similar to those found in the activated pollen grain, are present. Their role in the formation of the callosic and pectocellulosic wall layers seems to be the same as in the activated pollen grain.

Viral particles in developing pollen grains of Olea europaea

Double-walled tubules containing rows of isodiametric virus particles were observed in developing pollen grains of Olea europaea L. cultivar Correggiolo. Sometimes the tubules are contained in another double-walled tubular structure or in a tubular endoplasmic reticulum cistern. The viruses are present in the cytoplasm from the microspore mother cell stage up to the microspore stage but just before the first haploid mitosis they are to be found only in the pores, inside the evaginations formed by the plasmalemma. During the last phase of pollen grain development, after the germinative pores are completed, the viruses disappear.