Persistent Symmetry Frustration in Pollen Tubes

Pollen tubes are extremely rapidly growing plant cells whose morphogenesis is determined by spatial gradients in the biochemical composition of the cell wall. We investigate the hypothesis (MP) that the distribution of the local mechanical properties of the wall, corresponding to the change of the radial symmetry along the axial direction, may lead to growth oscillations in pollen tubes. We claim that the experimentally observed oscillations originate from the symmetry change at the transition zone, where both intervening symmetries (cylindrical and spherical) meet. The characteristic oscillations between resonating symmetries at a given (constant) turgor pressure and a gradient of wall material constants may be identified with the observed growth-cycles in pollen tubes.     

Electrotropism of Nicotiana Pollen Tubes

Electrotropism of tobacco pollen tubes towards the anode wasanalysed. The threshold and saturation values for the electrotropismwere less than 50 mV mm^–1 and 200 mV mm^–1, respectively.The tropic response gradually increased with increasing durationto exposure, but no further increase in the tropic responsewas observed when exposure of the electric field was terminated.Pollen tubes growing towards the cathode had a tendency to burstin a strong electric field. These results suggest that an externallyapplied electric field acts as a motive force for electrotropismbut not as a trigger and that endogenous currents play a rolein tip growth of pollen tubes. Possible mechanisms responsiblefor the electrotropism of pollen tubes are discussed. (Received July 9, 1993; Accepted September 18, 1993)     

扇脉杓兰花粉超微结构及花粉管生长的观察

为明确扇脉杓兰花粉形态结构及雄性生殖特性,利用扫描电镜、透射电镜和荧光显微镜对花粉形态和超微结构及花粉管生长过程进行观察。结果表明:(1)扇脉杓兰单粒花粉长球形,表面光滑无特征纹饰,有少量胶黏物质,一些表面有2个或以上的深凹陷,凹陷内有球形突起的内容物。(2)花粉壁分为由棒状的基柱小单元组成的外壁和纤维素果胶组成的内壁,有覆盖层;生殖细胞近圆形,细胞核大而致密;营养细胞多弧形,核质分散;花粉粒细胞质含有大量的线粒体、质体和小泡等细胞器,淀粉、蛋白质和多糖含量丰富。(3)花粉管萌发后沿子房壁方向伸长,授粉20d花粉管伸长生长并不明显,授粉30d伸长的花粉管中出现大量胼胝质塞,并且伸长方向转为胚珠中间,花粉管逐渐接近胚珠,在授粉后50d基本完成受精作用。研究认为,扇脉杓兰花粉发育正常,不阻碍有性生殖过程。     

Pistil Structure and Pollen Tube Pathways in Leptospermum myrsinoides and L. continentale (Myrtaceae)

Pistil structure and pollen tube growth were investigated inLeptospermum myrsinoides and L. continentale (Myrtaceae). BothL. myrsinoides and L. continentale pistils consist of an ovarywith five locules, a style and a five-lobed dry, papillate stigma.A centrally located stigmatic cleft is present but extends onlyto the base of the stigma. Pollen germinates and grows intercellularlythrough the stigma into the central transmitting tissue of thestyle. Pollen tubes do not grow down the stigmatic cleft. Atthe base of the style the transmitting tissue separates intofive, each tract leading through the placenta to one of thefive locules. The pollen tubes continue to grow intercellularlythrough these five tracts entering the locules between the lobesof the placenta. Pollen tubes are smooth-walled and straightwhilst in the transmitting tissue of the style but produce shortlateral branches at regular intervals when in the locules. Branchingcontinues until pollen tubes enter ovules. It is suggested thatthe observed branching in the locules is a result of pollentubes following a chemotropic or thigmotropic pathway to theovules. This behaviour was consistent in all pistils examinedand no difference was observed in the behaviour of self- orcross-pollen tubes in the style or ovary.Copyright 1994, 1999Academic Press Leptospermum myrsinoides Schldl., Leptospermum continentale J. Thompson, pistil structure, pollen tube pathway, pollen tube branching     

Environmental Effects on Components of Pollen Performance in Faramea occidentalis (L.) A. Rich. (Rubiaceae)1

Studies focusing on gametophytic competition have focused on differences in pollen performance (e.g., pollen germination rate and pollen tube growth rate) among donors in order to examine genetic sources of variation in siring success among them. Donors that produce the fastest growing pollen tubes are expected to fertilize more ovules relative to donors with slow growing pollen tubes under conditions of gametophytic competition. However, the performance of pollen in the field is known to be influenced by environmental conditions in addition to the genotype of donor plants. This field study of Faramea occidentalis was conducted to: (1) determine the effect of environmental conditions during pollination on pollen performance; (2) measure differences among pollen donors in pollen performance; (3) determine if the pollen of different donors responds differently to a variety of environmental conditions surrounding pollinated flowers; and (4) measure differences among pollen recipients in pollen performance. Single-donor crosses were made between four pollen donors and four pollen recipients under a variety of environmental conditions. Pollen performance was then quantified as the growth rate of the fastest pollen tube, the mean pollen tube growth rate, and by a pollen germination index. Pollination environment (the environment surrounding a pollinated flower) and recipient significantly affected all three measures of pollen performance. Pollen donors did not differ overall in pollen performance. However, there was significant among-donor variation in two of the five pollination environment conditions. Future studies of variation in relative siring success may benefit by considering pollination environment in addition to donor and recipient identity.     

Keto Acids in Pollen and Pollen Tubes of Sesbania aegyptica

Pollen and pollen tubes of Sesbania aegyptica Pers. contain α-ketoglutaric acid, oxaloacetic acid and pyruvic acid. Changes in the keto acids have been correlated with their corresponding amino acids during different phases of germination. It is suggested that keto acids were readily turned over during the elongation of pollen tubes.     

Transmitting tissue ECM distribution and composition, and pollen germinability in Sarcandra glabra and Chloranthus japonicus (Chloranthaceae)

BACKGROUND: and Aims Free-flowing surface exudates at the stigmatic (wet versus dry stigma) and adaxial epidermis at the site of angiospermy in carpels of Chloranthaceous species have been proposed to comprise a continuous extracellular matrix (ECM) operating in pollen tube transmission to the ovary. The aim of this research was to establish the spatial distribution and histo/immunochemical composition of the ECM involved in pollen tube growth in Sarcandra glabra and Chloranthus japonicus (Chloranthaceae). METHODS: Following confirmation of the pollen tube pathway, the histo/immunochemical make-up of the ECM was determined with histochemistry on fresh tissue to detect cuticle, esterase, proteins, pectins, and lipids and immunolocalization at the level of the TEM on sections from cryofixed/freeze-substituted tissue to detect molecules recognized by antibodies to homogalacturonans (JIM7, 5), arabinogalactan-proteins (JIM13) and cysteine-rich adhesion (SCA). KEY RESULTS: Pollen germinability is low in both species. When grains germinate, they do so on an ECM comprised of an esterase-positive cuticle proper (dry versus wet stigma). Pollen tubes do not track the surface ECM of stigma or adaxial epidermal cells at the site of angiospermy. Instead, tubes grow between stigmatic cells and subsequently along the inner tangential walls of the stigmatic and adaxial carpel cells at the site of angiospermy. Pollen tubes enter the ovary locule at the base of the funiculus. The stigmatic ECM is distinct by virtue of the presence of anti-JIM5 aggregates, lipids, and a protein recognized by anti-SCA. CONCLUSIONS: The Chloranthaceae joins a growing number of basal angiosperm taxa whereby pollen tubes germinate on a dry versus wet stigma to subsequently grow intercellularly en route to the ovary thereby challenging traditional views that the archetype pollen tube pathway was composed of the surface of stigma and adaxial epidermal cells covered with a free-flowing exudate.     

The Germination of Vinca rosea L. Pollen Grains and the Growth of the Pollen Tubes in vitro

The effect of medium concentration, pollen grain concentration, pH of the media, light and temperature on the germination of Vin ca rosea pollen grains, and the growth of their pollen tubes in vitro have been studied. The pollen grains germinate best at a sucrose concentration between 14.2% and 30%; when the pollen grain concentration exceeds 800 per 0.0234 ml; at near neutral pH (6.5); in darkness and at a temperature close to 30°. Moreover buffering ions affect the growth of the pollen tubes. Pollen grains remain viable in a wide range of temperatures, and the wall of the pollen grain is capable of withstanding severe osmotic imbalance. Low temperature induces spherical swellings at the tips of the pollen tubes, followed by accumulation of a hyaline plug.     

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.     

Polyester and nylon powders used as pollen diluents preserve pollen germination and tube growth in controlled pollinations

Pollen acquisition for seed production, breeding programs and supplemental pollination can be costly and difficult. The identification of dry particulates for use as pollen diluents would facilitate the use of limited amounts of pollen and aid in accurate pollen application and dispersion. Four powders - Rilsan ES, polyester, wheat flour, and Lycopodium spores - were evaluated as pollen diluents using petunia as a model system. Diluents were combined with petunia pollen at a 5:1 (v/v) ratio. Two types of studies were conducted: (1) storage studies evaluated the viability of pollen combined and held with diluent for different durations; and (2) in vivo studies evaluated pollen tube growth in the styles of flowers pollinated with pollen-diluent mixtures. Pollen germination was not affected when stored as pollen-diluent mixtures for 4 days. Slight detrimental effects on pollen germination were observed after 6 days storage with Rilsan ES powders. Pollination with all the pollen-diluent mixtures resulted in fewer pollen tubes growing in the style compared to controls diluted with heat-killed pollen instead of diluent powders. Lycopodium-pollen mixtures were the most inhibitory, providing only 8% of the tube numbers observed in controls. Pollen mixed with polyester powders, Rilsan ES powders or wheat flour had tube numbers ranging from 47 to 61% of the control, but still had 175 or more pollen tubes per style, which would be sufficient for high rates of seed set in petunia. Wheat flour-pollen mixtures tended to clump and degrade pollen flow. Rilsan ES and polyester were identified as two promising pollen diluent powders that can facilitate accurate metering and distribution of pollen, produce large numbers of pollen tubes, and maintain pollen viability under storage.     

Pollen germinates precociously in the anthers of raring-to-go, an Arabidopsis gametophytic mutant

Pollen hydration is usually tightly regulated and occurs in vivo only when desiccated pollen grains acquire water from the female, thus enabling pollen tube growth. Pollen tubes are easily visualized by staining with decolorized aniline blue, a stain specific for callose. We identified a mutant, raring-to-go, in which pollen grains stained for callose before anther dehiscence. When raring-to-go plants are transferred to high humidity, pollen tubes dramatically elongate within the anther. As early as the bicellular stage, affected pollen grains in raring-to-go plants acquire or retain water within the anther, and precociously germinate. Thus, the requirement for contact with the female is circumvented. We used pollen tetrad analysis to show that raring-to-go is a gametophytic mutation, to our knowledge the first gametophytic mutation in Arabidopsis that affects early events in the pollination pathway. To aid in identifying raring-to-go alleles, we devised a new technique for screening pollen in bulk with decolorized aniline blue. We screened a new M(1) mutagenized population and identified several additional mutants with a raring-to-go-like phenotype, demonstrating the usefulness of this technique. Further, we isolated other mutants (gift-wrapped pollen, polka dot pollen, and emotionally fragile pollen) with unexpected patterns of callose staining. We suggest that raring-to-go and these other mutants may help dissect components of the pathway that regulates pollen hydration and pollen tube growth.     

Directional Guidance of Nicotiana alata Pollen Tubes in Vitro and on the Stigma

Pollen tubes navigate the route from stigma to ovule with great accuracy, but the cues that guide them along this route are not known. We reproduced the environment on the stigma of Nicotiana alata by immersing pollen in stigma exudate or oil close to an interface with an aqueous medium. The growth of pollen in this culture system mimicked growth on stigmas: pollen grains hydrated and germinated, and pollen tubes grew toward the aqueous medium. The rate-limiting step in pollen germination was the movement of water through the surrounding exudate or oil. By elimination of other potential guidance cues, we conclude that the directional supply of water probably determined the axis of polarity of pollen tubes and resulted in growth toward the interface. We propose that a gradient of water in exudate is a guidance cue for pollen tubes on the stigma and that the composition of the exudate must be such that it is permeable enough for pollen hydration to occur but not so permeable that the supply of water becomes nondirectional. Pollen tube penetration of the stigma may be the most frequently occurring hydrotropic response of higher plants.     

Pollen morphology and germination inNarcissus

Pollen grains of several species and varieties ofNarcissus were examined with a light microscope and a scanning electron microscope. Normal pollen grains were kidney- or spindle-shaped with a germination furrow and a reticulate structure similar to that of the pollen grains ofAmaryllis. Pollen grains germinated within 2 to 3 hr. Percentage of germination was dependent upon temperature and treatment. Pollen tubes grew in length up to 1,000 μm and branched occasionally or behaved in strange fashion. Fresh pollen grains germinated more in distilled water at lower temperature than in sucrose-aqueous medium. Both in the presence and in the absence of stigmatic exudate calcium increased the percentage of germination. Gibberellic acid, abscisic acid and coumarin inhibited the pollen germination. Plasmoptysis occurred in all species and in all media tested except in a medium containing coumarin without stigmatic exudates. Plasmoptysis did not seem to be induced by hypotonic medium alone. Pollen of high germination capacity showed a high percentage of plasmoptysis. Based on the results obtained, evolution and sterility of theNarcissus plant was discussed.     

Floral Viability and Pollen Tube Growth in Vicia faba L.

Durations of stigmatic receptivity, pollen viability and pollen tube growth were investigated in the largely entomophilous faba bean. Stigmas were examined for deposited and germinated pollen, and growth of pollen tubes was investigated using aniline blue-induced fluorescence. Entry of a pollen tube into a micropyle was found to be a reliable indicator of fertilization. After anthesis, stigmas remained receptive to pollination for six days, and pollen viable for five. Pollen tubes took up to three days to reach the ovules furthest from the stigma. Inspection of ovular development within pods showed that the incidence of fertilization had been accurately determined in the flowers.     

花粉管引导机制的研究进展

花粉管引导是指显花植物在受精过程中,雌蕊组织与花粉管相互作用使花粉管定向生长并最终到达胚囊的过程,其机制颇为复杂。该文基于调控花粉管生长的孢子体引导和配子体细胞引导两个主要过程,阐述雌蕊中不同蛋白分子和其它小分子物质的浓度梯度在花粉管的孢子体组织引导中的作用,以及胚囊中不同类型的细胞及其相关基因与蛋白在花粉管的配子体细胞引导中的作用。同时,该文也对精细胞在花粉管引导中的作用进行了阐述。     

Pollen tube dynamics following half- and fully-compatible pollinations in self-compatible almond cultivars

. Pollen tube growth dynamics along the style at different times after self- and cross-pollination of four heterozygous self-compatible (SfSx) almond cultivars were studied by means of fluorescence microscopy. Results showed a reduction of pollen tube number along the style following both self-pollination (half-compatible reaction) and cross-pollination (fully-compatible). An important decrease in the percentage of pollen tubes present took place in the upper section of the style in both crosses. As expected in a gametophytic incompatibility system, a higher number of pollen tubes were stopped along the style in the case of the half-compatible crosses; however, the percentage of pollen tubes reaching the base of the style was similar in both cases. These results reveal the existence of other mechanisms controlling pollen tube growth along the style.     

花粉管细胞壁结构及胞质运动

花粉管的极性顶端生长是将雄配子体运输到子房的过程,在高等植物有性生殖过程中起着重要的作用。花粉管的生长过程包括许多方面,其中最为重要的是花粉管细胞壁的合成和胞质运动。本文就细胞壁的结构及组成,生殖细胞和营养核的移位,细胞器以及分泌小泡的运动等方面作了较全面论述。