Origin and growth of cell surface components

Polarized cell growth in higher plants

Pollen tubes and root hairs are highly elongated, cylindrically shaped cells whose polarized growth permits them to explore the environment for the benefit of the entire plant. Root hairs create an enormous surface area for the uptake of water and nutrients, whereas pollen tubes deliver the sperm cells to the ovule for fertilization. These cells grow exclusively at the apex and at prodigious rates (in excess of 200 nm/s for pollen tubes). Underlying this rapid growth are polarized ion gradients and fluxes, turnover of cytoskeletal elements (actin microfilaments), and exocytosis and endocytosis of membrane vesicles. Intracellular gradients of calcium and protons are spatially localized at the growing apex; inward fluxes of these ions are apically directed. These gradients and fluxes oscillate with the same frequency as the oscillations in growth rate but not with the same phase. Actin microfilaments, which together with myosin generate reverse fountain streaming, undergo rapid turnover in the apical domain, possibly being regulated by key actin-binding proteins, e.g., profilin, villin, and ADF/cofilin, in concert with the ion gradients. Exocytosis of vesicles at the apex, also dependent on the ion gradients, provides precursor material for the continuously expanding cell wall of the growing cell. Elucidation of the interactions and of the dynamics of these different components is providing unique insight into the mechanisms of polarized growth.

Pollen tube tip growth

This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

The effects of ruthenium red, lanthanum, fluorescein isothiocyanate and trifluoperazine on vesicle transport, vesicle fusion and tip extension in pollen tubes

The effects of ruthenium red, lanthanum, fluorescein isothiocyanate and trifluoperazine, all antagonists of Ca(2+) function in cells, have been studied in growing pollen tubes of Tradescantia virginiana. All four drugs inhibit pollen-tube growth but bring about different ultrastructural changes at the growing tips and within the cytoplasm. The results strongly support the hypothesis that Ca(2+) plays a vital role in the mechanism of pollen-tube tip growth. The effect of ruthenium red provides evidence that sequestration of Ca(2+) by mitochondria critically adjusts the concentration of these ions at tube tips. Fluorescein isothiocyanate appears to be a potent inhibitor of vesicle fusion at the plasma membrane, with vesicles accumulating in the tip at rates equivalent to those determined previously for their production. Both vesicle fusion and tip extension are regulated by Ca(2+) but appear to be independently controlled processes.

Disoriented growth of pollen tubes of Lilium longiflorum Thunb. induced by prolonged treatment with the calcium-chelating antibiotic, chlorotetracycline

Pollen of Lilium longiflorum Thunb. was germinated for 12 h in growth medium containing 1·10(-4) M chlorotetracycline (CTC), or growing tubes were treated with 1·10(-4) M CTC for up to 2 h. These treatments have drastic effects: In the CTC-containing medium, out-growing tubes form only short tubes. Irregular wall thickenings are visible. Thirty minutes CTC-treatment cause growing tubes to bend and grow back toward the grain. Electron micrographs of CTC-treated tubes show that CTC affects the organelle distribution: The polar zonation of organelles is disturbed. Vesicle-and endoplasmic reticulum-accumulations are found in the wrong places, together with extensive wall thickenings and a very irregular plasma membrane. The structural details of most cell organelles look normal after CTC treatment, but the mitochondria possess unusual cristae, and microtubules are absent. The disoriented growth is interpreted as an effect of the ability of CTC to chelate intracellular calcium ions, to bind them to membranes, and thus to disturb the dynamics of the delicate Ca(2+)-equilibria thought to regulate oriented exocytosis.

Calcium ionophore A 23187 affects localized wall secretion in the tip region of pollen tubes of Lilium longiflorum

The effects of the calcium inonophore A 23187 on growing pollen tubes of Lilium longiflorum Thunb. cv. Ace were investigated with the light and electron microscope. Tip growth is slowed down and stopped within 20 min after application of 5x10(-5) M ionophore A 23187. The main effects are the disappearance of the clear zone at the pollen tube tip and a thickening of the cell wall at the tip and at the pollen tube flanks. This effect on cell wall formation is confirmed under the electron microscope: The vesicular zone in treated pollen tubes is reduced, numerous vesicular contents are irregularly integrated in the pollen tube wall not only in the tip, but over a long distance of the pollen tube wall. In addition, effects on mitochondria and dictyosomes are observed. These results are interpreted as a disorientation of the Ca(2+)-based orientation mechanism of exocytosis after equilibration of the Ca(2+)-gradient.

Reduced external calcium or sodium stimulates calcium influx in Pelvetia eggs

The effect of external calcium and sodium ion concentrations on the calcium fluxes on the Pelvetia fastigiata De Toni egg was measured. Decreasing external [Ca(2+)] greatly increased the permeability of the eggs to Ca(2+); at 1 mM external Ca(2+) this permeability was 60 times as great as it was at the normal [Ca(2+)] of 10 mM. Lowering the external [Na(+)] also increased Ca(2+) influx; at 2 mM Na(+), the Ca(2+) influx was 2-3 times as great as it was at the normal [Na(+)] if choline was used as a Na(+) substitute. Lithium was less effective as a Na(+) substitute in increasing Ca(2+) influx. The extra Ca(2+) influx in low [Na(+)] seemed to be dependent on internal [Na(+)]. The Ca(2+) efflux increased transiently and then declined in low Na(+) media.

The formation of the fibrils in the lorica of Poteriochromonas stipitata: Tip growth, kinetics, site, orientation

The microfibrils of the lorica of Poteriochromonas stipitata Scherffel obviously are formed at the surface of the plasmalemma. They elongate unidirectionally by tip growth. On 1 μm(2) plasmalemma about 1-2×10(6) glycosidic bonds per minute are formed. In the stalk, the primary fibrils are arranged helically. They tend to fasciate; a ribbon-like fibril has the width of about 20 nm and, therewith, nearly that of a microtubule. Every primary fibril coincides precisely with a microtubule. The microtubules lie under the plasmalemma and are connected with it by cross bridges. The secondary fibrils form a more irregular network. They are mainly produced where and when microtubules are absent. They band zipper-like with each other and with the primary fibrils. It is discussed how the different structures which are involved in microfibril formation move and how the formation process is regulated. Microtubules obviously influence morphogenesis of the lorica: they participate in special orientation of microfibrils, but play no role in their synthesis and secretion.