FUNCTION OF GOLGI VESICLES IN RELATION TO CELL WALL SYNTHESIS IN GERMINATING PETUNIA POLLEN. II. CHEMICAL COMPOSITION OF GOLGI VESICLES AND POLLEN TUBE WALL

The cell wall of the Arabidopsis pollen tube--spatial distribution, recycling, and network formation of polysaccharides

The pollen tube is a cellular protuberance formed by the pollen grain, or male gametophyte, in flowering plants. Its principal metabolic activity is the synthesis and assembly of cell wall material, which must be precisely coordinated to sustain the characteristic rapid growth rate and to ensure geometrically correct and efficient cellular morphogenesis. Unlike other model species, the cell wall of the Arabidopsis (Arabidopsis thaliana) pollen tube has not been described in detail. We used immunohistochemistry and quantitative image analysis to provide a detailed profile of the spatial distribution of the major cell wall polymers composing the Arabidopsis pollen tube cell wall. Comparison with predictions made by a mechanical model for pollen tube growth revealed the importance of pectin deesterification in determining the cell diameter. Scanning electron microscopy demonstrated that cellulose microfibrils are oriented in near longitudinal orientation in the Arabidopsis pollen tube cell wall, consistent with a linear arrangement of cellulose synthase CESA6 in the plasma membrane. The cellulose label was also found inside cytoplasmic vesicles and might originate from an early activation of cellulose synthases prior to their insertion into the plasma membrane or from recycling of short cellulose polymers by endocytosis. A series of strategic enzymatic treatments also suggests that pectins, cellulose, and callose are highly cross linked to each other.

Microtubule- and Actin Filament-Dependent Motors are Distributed on Pollen Tube Mitochondria and Contribute Differently to Their Movement

The pollen tube exhibits cytoplasmic streaming of organelles, which is dependent on the actin-myosin system. Although microtubule-based motors have also been identified in the pollen tube, many uncertainties exist regarding their role in organelle transport. As part of our attempt to understand the role of microtubule-based movement in the pollen tube of tobacco, we investigated the cooperation between microtubules and actin filaments in the transport of mitochondria and Golgi vesicles, which are distributed differently in the growing pollen tube. The analysis was performed using in vitro motility assays in which organelles move along both microtubules and actin filaments. The results indicated that the movement of mitochondria and Golgi vesicles is slow and continuous along microtubules but fast and irregular along actin filaments. In addition, the presence of microtubules in the motility assays forces organelles to use lower velocities. Actin- and tubulin-binding tests, immunoblotting and immunogold labeling indicated that different organelles bind to identical myosins but associate with specific kinesins. We found that a 90 kDa kinesin (previously known as 90 kDa ATP-MAP) is associated with mitochondria but not with Golgi vesicles, whereas a 170 kDa myosin is distributed on mitochondria and other organelle classes. In vitro and in vivo motility assays indicate that microtubules and kinesins decrease the speed of mitochondria, thus contributing to their positioning in the pollen tube.

Pollen‒pistil interaction inLilium longiflorum: the role of the pistil in controlling pollen tube growth following cross- and self-pollinations

By cytophysiological methods, the self-incompatibility mechanism of the breeding system inLilium longiflorumhas been examined with particular reference to the synthesis, location and nature of the stylar factors involved in the control of pollen tube development. A ‘bioassay’ has been developed by which the effect of stylar extracts on pollen tube elongation may be investigated. With use of this system, a crude fraction of proteins from the stylar fluid has been shown to inhibit pollen tube growth only when protein fractions from ‘self’ styles are used. The proteins of this fraction have been analysed by thin-layer gel electrofocusing. Changes in the profiles thus obtained following selfing and a heat treatment known to inactivate the self-incompatibility response indicate a highly polarized glycoprotein to be an active component of the system. The various ways by which such a glycoprotein could control pollen tube elongation are considered in detail, and these events inLiliumare discussed in the light of our knowledge of other self-incompatibility systems operating in angiosperms.

Immuno-gold localization of α-L-arabinofuranosyl residues in pollen tubes of Nicotiana alata Link et otto

Immuno-gold labelling using a monoclonal antibody (PCBC3) with a primary specificity for α-L-arabinofuranosyl residues was used to locate these residues in pollen tubes of Nicotiana alata grown in vivo. The antibody bound to the outer fibrillar layer of the pollen-tube wall: the inner, non-fibrillar wall layer was not labelled. Cytoplasmic vesicles (0.2 μm diameter) were also labelled. The antibody may bind to an arabinan in the pollen-tube wall.

Composition of the cell walls of Nicotiana alata Link et Otto pollen tubes

Cell walls isolated from pollen of Nicotiana alata germinated in vitro contain glucose and arabinose as the predominant monosaccharides. Methylation analysis and cytochemical studies are consistent with the major polysaccharides being a (1→3)-β-D-glucan (callose) and an arabinan together with small amounts of cellulose. The cell walls contain 2.8% uronic acids. Alcian blue stains the pollen-tube walls intensely at the tip, indicating that acidic polysaccharides are concentrated in the tip. Synthetic aniline-blue fluorochrome is specific primarily for (1→3)-β-D-glucans and stains the pollen-tube walls, except at the tip. Protein (1.5%), containing hydroxyproline (2.4%), is present in the cell wall.

Wall-bound proteins of pollen tubes after self- and cross-pollination in Lilium longiflorum

Wall-bound proteins of Lilium longiflorum pollen tubes grown in vivo constitute 20-27% of the dry matter. Twenty-two-twenty-six percent of these proteins are NaCl soluble. Wall-bound proteins of in vivo pollen tubes are present in amounts 5-7 times that found in tubes grown in vitro. The protein pattern of wall-bound proteins is different between in vitro and in vivo grown pollen tubes. There are two kinds of pollen tube wall proteins: loosely bound and tightly bound. The latter are NaCl insoluble, contain hydroxyproline and are assumed to be covalently bound. No significant differences have been found in the amount of wall-bound proteins present between pollen tubes resulting after self-pollination and those resulting from cross-pollination. However, some band differences between self- and cross pollen tubes have been observed after gel electrophoresis. It can be supposed that some wall-bound proteins of pollen tubes are associated with the incompatibility reaction.

Ultrastructure of cell wall and plugs of tobacco pollen tubes after chemical extraction of polysaccharides

Tobacco pollen tubes grown in vitro and from pollinated tobacco styles were treated by chemical solvents to remove one or more of the following polysaccharides from the tube walls: pectin (ethylenediamine tetraacetic acid); hemicellulose (alkali); callose (alkali; potassium hypochlorite); cellulose (cuprammonium); and all polysaccharides with exception of cellulose (H2O2/glacial acetic acid). Both the inner tube wall, which we had regarded as the secondary wall, and the plugs contained, in addition to callose, microfibrils of cellulose and "non-cellulosic" microfibrils that had "pectin-like" properties. When using the expressions callosic or callose layer and callose plugs in reference to pollen tubes, one should realize that they do not imply the exclusive presence of callose in the inner tube wall layer and its localized thickenings.

β-Glucan synthetase activity in Golgi vesicles ofPetunia hybrida

β-Glucan synthetase activity has been demonstrated in a Golgi vesicle fraction isolated from pollen tubes ofPetunia hybrida. Thisβ-glucan synthetase activity differs from that of most other higher plants in its inability to incorporate [(14)C]glucose from GDP-[(14)C]glucose. UDP-[(14)C]glucose, however, is an appropriate glucose donor for this enzyme. The optimum conditions for thisβ-glucan synthetase activity are: 1 mg Golgi vesicle protein/ml reaction mixture; pH=±8 and a temperature of 25°C. The newly synthesized alkali-insoluble glucan containsβ-1,3- as well as β-1,4-glucosidic linkages.

Ultrastructural localization of acid phosphatase in the pollen tube of Prunus avium L. (sweet cherry)

The pollen tube of Prunus avium (cherry) consists of a growth zone of vesicles at the tip and an assemblage of organelles typical of an actively metabolizing cell. Electron opaque globules are closely associated with the plasma membrane and fibrillar cell wall layer at the tip. Acid phosphatase (EC 3.1.3.2) activity is localized in the membranes of 120 nm vesicles and ER system, the lumen of 50 nm vesicles, the plasma membrane and the tube nucleus.