Ripe pollen carbohydrate changes in Trachycarpus fortunei: the effect of relative humidity

Pollen of the palm Trachycarpus fortunei was kept at 25°C and relative humidities (RH) of 20, 55 and 98%. Changes in viability, water content and carbohydrates were measured over 2–17 days. Water content remained almost constant at 20 and 50% RH and increased dramatically at 98%. Pollen viability and germination rate remained almost constant over 14 days at 20% RH and decreased to about 2% after 7–9 days at 55% and to even less at 98% RH. Although the three experimental conditions were constant, qualitative and quantitative variations in pollen carbohydrates were recorded, even after pollen had lost its viability. The quantities of mono-, di- and polysaccharides varied with the period of pollen storage at the various RH. The greatest changes in glucose, fructose and sucrose content were recorded at 55 and 98% RH. At these relative humidities, maximum glucose and fructose content and minimum sucrose content occurred at maximum water content. Starch was not present in mature pollen but appeared and peaked after 7–9 days of pollen storage at 55 and 98%. Appearance of starch coincided with an increase in pectin content. PAS-positive cytoplasmic polysaccharides showed an increasing trend at 20% RH. A relation was found between pollen viability, water content and monosaccharide content. Pollen viability and germination capacity remained high at 20% RH for 14 days. At this relative humidity, pollen water, glucose and fructose contents remained almost constant, while sucrose reached its maximum value. The fluctuations of more complex carbohydrates (starch, pectins and PAS-positive cytoplasmic polysaccharides) were less easy to interpret. Changes observed under experimental conditions could simulate processes occurring in nature during pollen presentation and dispersal.     

The division of the generative nucleus and the formation of callose plugs in pollen tubes of Aechmea fasciata (Bromeliaceae) cultured in vitro

The effect of different external factors on pollen germination and pollen tube growth is well documented for several species. On the other hand the consequences of these factors on the division of the generative nucleus and the formation of callose plugs are less known. In this study we report the effect of medium pH, 2-[N-morpholino]ethanesulfonic acid (MES) buffer, sucrose concentration, partial substitution of sucrose by polyethyleneglycol (PEG) 6000, arginine (Arg), and pollen density on the following parameters: pollen germination, pollen tube length, division of the generative nucleus, and the formation of callose plugs. We also studied the different developmental processes in relation to time. The optimal pH for all parameters tested was 6.7. In particular, the division of the generative nucleus and callose plug deposition were inhibited at lower pH values. MES buffer had a toxic effect; both pollen germination and pollen tube length were lowered. MES buffer also influenced migration of the male germ unit (MGU), the second mitotic division, and the formation of callose plugs. A sucrose concentration of 10% was optimal for pollen germination, pollen tube growth rate and final pollen tube length, as well as for division of the generative nucleus and the production of callose plugs. Partial substitution of sucrose by PEG 6000 had no influence on pollen germination and pollen tube length. However, in these pollen tubes the MGU often did not migrate and no callose plugs were observed. Pollen tube growth was independent of the migration of the MGU and the deposition of callose plugs. In previous experiments Arg proved to be positive for the division of the generative nucleus in pollen tubes cultured in vitro. Here, we found that more pollen tubes had callose plugs and more callose plugs per pollen tube were produced on medium with Arg. After the MGU migrated into the pollen tube (1 h after cultivation), callose plugs were deposited (3 h). After 8 h the first sperm cells were produced. The MGU moved away from the active pollen tube tip until the second pollen mitosis occurred, thereafter the distance from the MGU to the pollen tube tip diminished. Callose plug deposition never started prior to MGU migration into the pollen tube. Pollen tubes without a MGU also lack callose plugs (±30% of the total number of pollen tubes). Furthermore, we found a correlation between the occurrence of sperm cells in pollen tubes and the synthesis of callose plugs.     

Identification of a pollen-specific sucrose transporter-like protein NtSUT3 from tobacco.

Pollen cells are symplasmically isolated during maturation and germination. Pollen therefore needs to take up nutrients via membrane carriers. Physiological measurements on pollen indicate sucrose transport in the pollen tube. A cDNA encoding a pollen-specific sucrose transporter-like protein NtSUT3 was isolated from a tobacco pollen cDNA library. NtSUT3 expression is detected only in pollen and is restricted to late pollen development, pollen germination and pollen tube growth. Altogether these data indicate that pollen is supplied not only with glucose, but also with sucrose through a specific sucrose transporter. The respective contribution of each transport pathway may change during pollen tube growth.     

The Significance of the Obturator in the Control of Pollen Tube Entry into the Ovary in Peach (Prunus persica)

Changes in the obturator of the peach (Prunus persica) havebeen investigated and related to pollen tube growth in thisregion. At anthesis, the cells of the obturator are active andrich in starch reserves. Twelve days after anthesis these cellsproduce a secretion that stains for carbohydrates and for proteins.As the secretion is produced, starch vanishes from these cellsand they degenerate and collapse as callose is accumulated.Secretion is independent of pollination as it takes place ina similar fashion both in pollinated and in unpollinated flowers. Pollen tube growth along the obturator surface depends on thissecretion for, although pollen tubes reach the base of the styleseven days after pollination, they cannot grow on the obturatoruntil five days later, when the secretion is produced. Thisdiscontinuous secretion taking place at the obturator may providea mechanism that controls the entrance of pollen tubes intothe ovary in the peach. Prunus persica, peach, obturator, pollen tube     

Immunolocalisation of arabinogalactan proteins and pectins in Actinidia deliciosa pollen

Summary. The cell wall composition of germinating pollen grains of Actinidia deliciosa was studied by immunolocalization with monoclonal antibodies against arabinogalactan proteins (AGPs) and pectins. In ungerminated pollen, the JIM8 epitope (against a subset of AGPs) was located in the intine and in the cytoplasm, while the MAC207 epitope (against AGPs) was only located in the exine. After germination, the JIM8 and MAC 207 epitopes were located in the cytoplasm and in the pollen tube wall. The Yariv reagent that binds to AGPs was added to the germination medium inducing a reduction or inhibition in pollen germination. This indicates that AGPs are present in the growing pollen tube and play an important role in pollen germination. To identify the nature of the pectins found in pollen grains and tubes, four monoclonal antibodies were used. The JIM5 epitope (against unesterified pectins) was located in the intine, more intensely in the pore region, and along the pollen tube wall, and the JIM7 epitope (against methyl-esterified pectins) was also observed in the cytoplasm. After germination, the JIM5 epitope was located in the pollen tube wall; although, the tube tip was not labelled. The JIM7 epitope was located in the entire pollen tube wall. LM5 (against galactans) showed a labelling pattern similar to that of JIM5 and the pattern of LM6 (against arabinans) was similar to that of JIM7. Pectins show different distribution patterns when the degree of esterification is considered. Pollen tube wall pectins are less esterified than those of the pollen tube tip. The association of AGPs with pectins in the cell wall of the pollen grain and the pollen tube may play an important role in the maintenance of cell shape during pollen growth and development.Correspondence and reprints: Instituto de Biologia Molecular e Celular, Universidade do Porto, Rua do Campo Alegre, 823, 4150-180 Porto, Portugal.     

Effect of Salt Stress on Viability, Germination and Endogenous Levels of Some Metabolites and Ions in Maize (Zea mays L.) Pollen

Maize plants, subjected to 0, 80, 120 and 160 meq l^–1salinity using NaCl, showed adverse effects on viability, germinationand tube growth of pollen, besides enhancing the bursting ofpollen. The endogenous levels of various metabolites in pollenwere also affected. Pollen grains from salinized plants hadmore soluble carbohydrates, free amino acids, especially proline,phenols and DNA and less starch, protein and RNA compared tothe non-saline controls. Salinity also resulted in the accumulationof ions such as Na⁺, K⁺ and Cl^– while it caused a reductionin the boron content of pollen. These metabolic disturbancespossibly lead to decreased viability, germination and tube growthof pollen thereby resulting into a reduction in reproductivecapacity of the plants under salt stress. Zea mays L., maize, pollen, viability, germination, salt stress     

Disruption of actin filaments by latrunculin B affects cell wall construction in Picea meyeri pollen tube by disturbing vesicle trafficking

The involvement of actin filaments (AFs) in vesicle trafficking, cell wall construction and tip growth was investigated during pollen tube development of Picea meyeri. Pollen germination and tube elongation were inhibited in a dose-dependent manner by the latrunculin B (LatB) treatment. The fine AFs were broken down into disorganized fragments showing a tendency to aggregate. FM4-64 labeling revealed that the dynamic balance of vesicle trafficking was perturbed due to F-actin disruption and the fountain-like cytoplasmic pattern changed into disorganized Brownian movement. The configuration and/or distribution of cell wall components, such as pectins, callose and cellulose, as well as arabinogalactan proteins changed in obvious ways after the LatB application. Fourier transform infrared (FTIR) analysis further established significant changes in the chemical composition of the wall material. Our results indicate that depolymerization of AFs affects the distribution and configuration of cell wall components in Picea meyeri pollen tube by disturbing vesicle trafficking.     

Types and meaning of pollen carbohydrate reserves

During pollen development, soluble carbohydrates of sporophytic origin may be consumed immediately, polymerized to form starch reserves or intine, or transformed into other molecules. Disregarding intine, in mature pollen there are three different types of carbohydrates: (1) polysaccharides such as starch in amyloplasts or polysaccharides in cytoplasmic vesicles, (2) disaccharides such as sucrose and (3) monosaccharides such as glucose and fructose. At dispersal, pollen may be partly or slightly dehydrated, or not dehydrated at all. Partly dehydrated pollen has the capacity to lose or acquire water within limits without detriment to its viability. Slightly and non-dehydrated pollen is vulnerable to water loss and quickly becomes inviable. In partly dehydrated of pollen the carbohydrates consist of cytoplasmic polysacharides and sucrose; in slightly and non-dehydrated pollen these are absent or in low concentrations but there may be reserves of cytoplasmic callose. Starch, glucose and fructose are found in both types. It is postulated that cytoplasmic carbohydrates and sucrose are involved in protecting pollen viability during exposure and dispersal.     

The block of intracellular calcium release affects the pollen tube development of Picea wilsonii by changing the deposition of cell wall components

Two potent drugs, neomycin and TMB-8, which can block intracellular calcium release, were used to investigate their influence on pollen tube growth and cell wall deposition in Picea wilsonii. Apart from inhibiting pollen germination and pollen tube growth, the two drugs largely influenced tube morphology. The drugs not only obviously disturbed the generation and maintenance of the tip-localized Ca(2+) gradient but also led to a heavy accumulation of callose at the tip region of P. wilsonii pollen tubes. Fourier transform infrared (FTIR) spectroscopy analysis showed that the deposition of cell wall components, such as carboxylic acid, pectins, and other polysaccharides, in pollen tubes was changed by the two drugs. The results obtained from immunolabeling with different pectin and arabinogalactan protein antibodies agreed well with the FTIR results and further demonstrated that the generation and maintenance of the gradient of cross-linked pectins, as well as the proportional distribution of arabinogalactan proteins in tube cell walls, are essential for pollen tube growth. These results strongly suggest that intracellular calcium release mediates the processes of pollen germination and pollen tube growth in P. wilsonii and its inhibition can lead to abnormal growth by disturbing the deposition of cell wall components in pollen tube tips.     

POLLEN TUBE DISTRIBUTIONS IN NICOTIANA GLAUCA: EVIDENCE FOR DENSITY DEPENDENT GROWTH

I hand sectioned styles of Nicotiana glauca at intervals along their length and counted the number of pollen tubes in each section using fluorescence microscopy. Evidence of density dependent growth was found for three stages of pollen growth. Pollen germination on the stigma increased with increasing pollen population size. Pollen tube penetration in the stigma was unaffected by increasing density from low to moderate levels but was reduced at high densities. Pollen tube penetration in the style was enhanced by increasing density. This enhanced growth in the style was apparently confounded by interference among pollen tubes growing at high densities. In particular, the area of tissue able to support pollen tube growth decreases from the stigma into the lower style, which could cause overcrowding of pollen tubes growing at high densities. Enhanced pollen tube penetration with increasing density combined with interference among pollen tubes growing at high densities resulted in greater mean pollen tube lengths for populations with moderate densities. The shift from density independent growth in the stigma to positively density dependent growth in the style may represent a shift from autotrophic to heterotrophic growth stages of pollen.     

Fast pollen tube growth in Conospermum species

BACKGROUND AND AIMS: An unusual form of pollen tube growth was observed for several Conospermum species (family Proteaceae). The rate of pollen tube growth, the number of tubes to emerge and the ultrastructure of these tubes are given here. METHODS: Pollen was germinated in vitro in different sucrose concentrations and in the presence of calcium channel blockers, and tube emergence and growth were recorded on a VCR. Measurements were taken of the number of tubes to emerge and rate of tube emergence. Pollen behaviour in vivo was also observed. The ultrastructure of germinated and ungerminated pollen was observed using TEM. RESULTS: After 10 s to 3 min in germination medium, up to three pollen tubes emerged and grew at rates of up to 55 micro m s(-1); the rate then slowed to around 2 micro m s(-1), 30 s after the initial growth spurt. Tubes were observed to grow in pulses, and the pulsed growth continued in the presence of calcium channel blockers. Optimal sugar concentration for pollen germination was 300 g L(-1), in which up to 81 % of pollen grains showed fast germination. Germination and emergence of multiple tubes were observed in sucrose concentrations of 100-800 g L(-1). The vegetative and generative nuclei moved into one of the tubes. Multiple tubes from a single grain were observed on the stigma. Under light microscopy, the cytoplasm in the tube showed a clear region at the tip. The ultrastructure of C. amoenum pollen showed a bilayered exine, with the intine being very thick at the pores, and elsewhere having large intrusions into the plasma membrane. The cytoplasm was dense with vesicles packed with inner tube cell wall material. Golgi apparatus producing secretory vesicles, and mitochondria were found throughout the tube. The tube wall was bilayered; both layers being fibrous and loosely packed. CONCLUSIONS: It is proposed that, for Conospermum, initial pollen tube wall constituents are manufactured and stored prior to pollen germination, and that tube extension occurs as described in the literature for other species, but at an exceptionally fast rate.     

Pollen carbohydrates and water content during development, presentation, and dispersal: a short review

Summary. Pollen accumulates starch reserves during development and the final stage of ripening. Before the anther opens, starch is totally or partially converted to pectins, glucose, fructose, sucrose, and to some unknown polysaccharides. Pollen is exposed to dispersing agents in an arrested developmental state which differs according to pollen water content. Pollen is classified as partially dehydrated or partially hydrated. The final water content may be reached before or after anther opening. Especially during exposure and dispersal, partially dehydrated pollen may interconvert soluble and insoluble reserves, modifying internal turgor pressure and hindering water loss or gain. Partially hydrated pollen is commonly devoid of mechanisms to conserve viability in time but has the advantage of quickly emitting pollen tubes on reaching the stigma. Correspondence and reprints: Department of Environmental Sciences, Siena University, 4 Via P.A. Mattioli, 53100 Siena, Italy.     

Occurrence of mono- or disaccharides and polysaccharide reserves in mature pollen grains

 Pollen from 13 species of gymnosperms and angiosperms was studied for soluble and insoluble carbohydrates at dispersal. Starch reserves stored during pollen development give rise to carbohydrates at maturity. Combinations of different types of carbohydrates in mature pollen may depend on the extent of starch hydrolysis. An inverse relationship was found between the extent of starch hydrolysis and sucrose content. If the starch was scarcely de-polymerized, the cytoplasm had very low levels of soluble sugars and none of the periodic acid-Schiff (PAS)-positive material as found in pollen not subject to high dehydration (Cucurbita pepo L., Zea mays L.). After total or partial starch hydrolysis, insoluble PAS-positive oligo/polysaccharides were found in the cytoplasm associated with much soluble sugar, and the pollen grains were dehydrated at dispersal as in Typha latifolia L., Chamaerops humilis L., Trachycarpus excelsa Wendl., and other specimens. Intermediate levels of starch and soluble sugars, together with cytoplasmic PAS-positive material, characterized species with dehydrated pollen such as Pinus halepensis Miller. Carbohydrates may be related to pollen longevity, which largely depends on the abundance of sucrose, which is known to protect membrane integrity. The relationship between PAS-positive material and pollen viability is unclear at present. Received: 30 July 1996 / Revision accepted: 18 December 1996     

Sucrose concentration in the growth medium affects the cell wall composition of tobacco pollen tubes

The cell wall of pollen tubes is organized in both spatial and temporal order to allow the pollen tube to grow according to external conditions. The deposition of methyl-esterified and acid pectins in addition to callose/cellulose occurs according to a series of temporally succeeding events. In this work, we attempted to determine how the composition of the external growth medium (in terms of osmolarity) could affect the deposition of cell wall components. Pollen tubes of tobacco were grown in a hypotonic medium and then analyzed for the distribution of pectins and callose/cellulose [as well as for the distribution of the enzyme callose synthase (CALS)]. The data indicate that pollen tubes grown in a hypotonic medium show changes of the initial growth rate followed by modification of the deposition of acid pectins and, to a lesser extent, of CALS. These observations indicate that, under the osmolarity determined by the growth medium, pollen tubes adapt their cell wall to the changing conditions of growth.     

Sucrose synthase is associated with the cell wall of tobacco pollen tubes

Sucrose synthase (Sus; EC 2.4.1.13) is a key enzyme of sucrose metabolism in plant cells, providing carbon for respiration and for the synthesis of cell wall polymers and starch. Since Sus is important for plant cell growth, insights into its structure, localization, and features are useful for defining the relationships between nutrients, growth, and cell morphogenesis. We used the pollen tube of tobacco (Nicotiana tabacum) as a cell model to characterize the main features of Sus with regard to cell growth and cell wall synthesis. Apart from its role during sexual reproduction, the pollen tube is a typical tip-growing cell, and the proper construction of its cell wall is essential for correct shaping and direction of growth. The outer cell wall layer of pollen tubes consists of pectins, but the inner layer is composed of cellulose and callose; both polymers require metabolic precursors in the form of UDP-glucose, which is synthesized by Sus. We identified an 88-kD polypeptide in the soluble, plasma membrane and Golgi fraction of pollen tubes. The protein was also found in association with the cell wall. After purification, the protein showed an enzyme activity similar to that of maize (Zea mays) Sus. Distribution of Sus was affected by brefeldin A and depended on the nutrition status of the pollen tube, because an absence of metabolic sugars in the growth medium caused Sus to distribute differently during tube elongation. Analysis by bidimensional electrophoresis indicated that Sus exists as two isoforms, one of which is phosphorylated and more abundant in the cytoplasm and cell wall and the other of which is not phosphorylated and is specific to the plasma membrane. Results indicate that the protein has a role in the construction of the extracellular matrix and thus in the morphogenesis of pollen tubes.     

小果短柱茶花粉离体培养系统的研究

山茶的短柱茶组是优良种质资源,有必要对小果短柱茶(Camellia confusa Chang 1941)的花粉萌发和花粉管生长的生理特性进行研究.本文研究了花粉生活力、培养温度及pH对小果短柱茶花粉萌发和花粉管生长的影响.结果表明:最适离体萌发培养基为5%蔗糖、0.003%的硼酸,0.005%的氯化钙和12%的PEG...     

Cytochalasin B Treatment of Apple (<Emphasis Type="Italic">Malus pumila</Emphasis> Mill.) Pollen Tubes Alters the Cytoplasmic Calcium Gradient and Causes Major Changes in the Cell Wall Components

It is well established that the actin cytoskeleton is absolutely essential to pollen germination and tube growth. In this study we investigated the effects of cytochalasin B (CB), which affects actin polymerization by binding to the barbed end of actin filaments, on apple (Malus pumila Mill.) pollen tube growth. Results showed that CB altered the morphology of pollen tubes, which had a larger diameter than control tubes beside inhibiting pollen germination and tube growth. Meantime CB also caused an abnormal distribution of actin filaments in the shank of the treated pollen tubes. Fluo-3/AM labeling indicated that the gradient of cytosolic calcium ([Ca²⁺]c) in the pollen tube tip was abolished by exposure to CB, which induced a much stronger signal in the cytoplasm. Cellulose and callose distribution in the tube apex changed due to the CB treatment. Immunolabeling with different pectin and arabinogalactan protein (AGP) antibodies illustrated that CB induced an accumulation of pectins and AGPs in the tube cytoplasm and apex wall. The above results were further supported by Fourier-transform infrared (FTIR) analysis. The results suggest the disruption of actin can result in abnormal growth by disturbing the [Ca²⁺]c gradient and the distribution of cell wall components at the pollen tube apex.     

From anther and pollen ripening to pollen presentation

The events and processes occurring between pollen maturation, opening of the anther and presentation of pollen to dispersing agents are described. In the final phases of pollen development, starch is always stored; this occurs before the anther opens. Depending on the species, this starch may be totally or partially transformed into: (a) other types of polysaccharides (fructans and rarely callose); (b) disaccharides (sucrose); (c) monosaccharides (glucose and fructose, all situated in the cytoplasm. While awaiting dispersing agents and during dispersal, polysaccharides, especially fructans, and sucrose may be interconverted to control osmotic pressure and prevent loss and uptake of water. Opening of the anther is preceded by disappearance of the locular fluid and in many cases by partial dehydration of the pollen. Pollen generally has a water content between 5 and 50%. Pollen with a high water content may or may not be able to control water retention during pollen exposure and dispersal. Pollen may be dispersed in monads or grouped in pollen dispersing units by the following mechanisms: (i). tangling of filamentous pollen; (ii). adhesion by viscous substances (pollenkitt, tryphine, elastoviscin) derived from the tapetum; (iii). common walls. When the anther opens, the pollen may be dispersed immediately, remain until dispersed (primary presentation), or be presented to pollinators in another part of the flower (secondary presentation).     

Response of in vitro pollen germination and pollen tube growth of almond (Prunus dulcis Mill.) to temperature,polyamines and polyamine synthesis inhibitor

Prunus dulcis L. ‘Mamaei’ is grown widely in souhtwest of Iran. It blooms in early spring when temperatures are still low. Based on our knowledge there are no reports in the literature regarding pollen behavior of this cultivar under specified condition. Thus, the possible factors for low germination percentage in this cultivar have not been reported. The effect of three different temperatures (10, 25, or 35 °C), polyamines (putrescine, spermidine, and spermine) and polyamine synthesis inhibitor, methylglyoxals-bis (guanyl-hydrazone) (MGBG) on in vitro pollen germination and pollen tube growth were investigated in P. dulcis L. ‘Mamaei’. All temperatures and chemicals significantly affected both pollen germination percentage and pollen tube growth. In general, different polyamines stimulated the pollen germination percentage compared to the control at all temperatures, but increasing the temperature, particularly to 35 °C, had demonstrated inhibitory effects on pollen germination. At a concentration of 0.05 mM putrescine and spermidine and 0.005 and 0.025 mM spermine revealed longer pollen tube growth than that of the control at 10 °C, while higher concentrations tended to inhibit pollen tube growth. At 25 °C, most of the treatments had an inhibitory effect on pollen tube growth except for 0.25 mM putrescine and 0.005 mM spermine, which slightly stimulated pollen tube growth. Pollen germination and pollen tube growth were inhibited by MGBG at all temperatures and in all concentrations.     

Boron deficiency alters cytosolic Ca2+ concentration and affects the cell wall components of pollen tubes in Malus domestica

• Boron (B) is essential for normal plant growth, including pollen tube growth. B deficiency influences various physiological and metabolic processes in plants. However, the underlying mechanism of B deficiency in pollen tube growth is not sufficiently understood. In the present research, the influence of B deficiency on apple (Malus domestica) pollen tube growth was studied and the possible regulatory mechanism evaluated.
• Apple pollen grains were cultured under different concentrations of B. Scanning ion‐selective electrode technique, fluorescence labelling and Fourier‐transform infrared (FTIR) analysis were used to detect calcium ion flux, cytosolic Ca²⁺ concentration ([Ca²⁺]cyt), actin filaments and cell wall components of pollen tubes.
• B deficiency inhibited apple pollen germination and induced retardation of tube growth. B deficiency increased extracellular Ca²⁺ influx and thus led to increased [Ca²⁺]cyt in the pollen tube tip. In addition, B deficiency modified actin filament arrangement at the pollen tube apex. B deficiency also altered the deposition of pollen tube wall components. Clear differences were not observed in the distribution patterns of cellulose and callose between control and B deficiency treated pollen tubes. However, B deficiency affected distribution patterns of pectin and arabinogalactan proteins (AGP). Clear ring‐like signals of pectins and AGP on control pollen tubes varied according to B deficiency. B deficiency further decreased acid pectins, esterified pectins and AGP content at the tip of the pollen tube, which were supported by changes in chemical composition of the tube walls.
• B appears to have an active role in pollen tube growth by affecting [Ca²⁺]cyt, actin filament assembly and pectin and AGP deposition in the pollen tube. These findings provide valuable information that enhances our current understanding of the mechanism regulating pollen tube growth.