The dynamics of calcium distribution in the synergid cells of wheat after pollination

Summary Sister synergids of wheat were examined for post-pollination calcium distribution using freeze-substitution fixation, scanning electron microscopy, and energy-dispersive X-ray microanalysis. At 15–30 min postpollination (before pollen-tube penetration and discharge), the sister synergids were similar in size and in elemental concentrations. Both synergids measured about the same as the egg and contained relatively high concentrations of calcium, potassium, and magnesium. The sister synergids remained identical even in an aberrant case where a pollen tube had penetrated, but not discharged into a synergid. After 30 min post-pollination (after pollen-tube discharge), typically one of the sister synergids was reduced in size and devoid of high concentrations of elements. The other synergid initially remained unaltered and only later underwent slight size reduction; however, it retained high concentrations of elements. The results indicate that high concentrations of calcium, potassium, and magnesium are rapidly lost from a synergid following pollen-tube discharge. A hypothesis is proposed to explain the regulation of entry by only a single pollen tube into an embryo sac.On Specific Cooperative Agreement 58-6612-8-002 with the Department of Biochemistry, University of Georgia, Athens, GA 30602, USA     

She's the boss: signaling in pollen tube reception

In angiosperms, the sperm cells are carried within the pollen tubes (male gametophytes) to the female gametophyte so that double fertilization can occur. The female gametophyte exerts control over the male, with specialized cells known as synergids guiding the pollen tubes and controlling their behavior when they enter the female gametophyte so that the sperm cells can be delivered to the egg and central cell. Upon pollen tube arrival at the ovule, signal transduction cascades mediated by receptor-like kinases are initiated in both the synergid and the tip of the pollen tube, leading to synergid cell death and pollen tube rupture. In this review, we discuss the role of these receptors and of newly discovered members of the pollen tube reception pathway.     

Guidance in vitro of the pollen tube to the naked embryo sac of torenia fournieri

The precise guidance of the pollen tube to the embryo sac is critical to the successful sexual reproduction of flowering plants. We demonstrate here the guidance of the pollen tube to the embryo sac in vitro by using the naked embryo sac of Torenia fournieri, which protrudes from the micropyle of the ovule. We developed a medium for culture of both the ovule and the pollen tube of T. fournieri and cocultivated them in a thin layer of solid medium. Although pollen tubes that had germinated in vitro passed naked embryo sacs, some pollen tubes that grew semi-in vitro through a cut style arrived precisely at the site of entry into the embryo sac, namely, the filiform apparatus of the synergids. When pollen tubes were unable to enter the embryo sac, they continuously grew toward the same filiform apparatus, forming narrow coils. Pollen tubes selectively arrived at complete, unfertilized embryo sacs but did not arrive at those of heat-treated ovules or those with disrupted synergids. These results convincingly demonstrate that pollen tubes are specifically attracted to the region of the filiform apparatus of living synergids in vitro.     

Calcium changes in ovules and embryo sacs of Plumbago zeylanica L.

Calcium was localized in ovules of Plumbago zeylanica from 1 day before anthesis to 3 days after anthesis using potassium antimonate and transmission electron microscopy in pollinated and emasculated flowers. At 1 day before anthesis, embryo sacs (containing an egg cell, a central cell and zero to three accessory cells) appear mature and contain abundant calcium precipitates (ppts), in contrast to nucellar cells. At anthesis, the vacuoles of nucellar cells have enlarged, and micropylar cells, in particular, are heavily labeled with calcium ppts. As pollen tubes elongate through ovular tissues, ppts diminish in ovular cells and become concentrated in the pollen tube cell wall. After fertilization, the calcium ppts sharply diminish in fertilized ovules; in unfertilized ovules, calcium ppts remain abundant up to 3 days after anthesis (when unfertilized ovules are shed). The distribution of calcium in the ovule changes in apparent response to fertilization, suggesting that calcium content may be related to the attraction and receipt of the pollen tube. In contrast with conventionally-organized embryo sacs with synergids, Plumbago accumulates calcium in the egg cell. Received: 30 December 1999 / Revision accepted: 24 March 2000     

CAPSELLA EMBRYOGENESIS: THE SYNERGIDS BEFORE AND AFTER FERTILIZATION

The ultrastructure and composition of the synergids of Capsella bursa-pastoris were studied before and after fertilization. The synergids in the mature embryo sac contain numerous plastids, mitochondria, dictyosomes and masses of ER and associated ribosomes. Each synergid contains a large chalazal vacuole, a nucleus with a single nucleolus and is surrounded by a wall. This wall is thickest at the micropyle end of the cell where it proliferates into the filiform apparatus. At the chalazal end of the cell the wall thins and may be absent for small distances. The pollen tube grows into one of the two synergids through the filiform apparatus and extends one-third the length of the cell before it discharges. Following discharge of the pollen tube, mitochondria and plastids of the tube can be identified in the synergid as can hundreds of 0.5 μ polysaccharide spheres liberated by the tube. The method by which the sperm or sperm nuclei enter the egg or central cell is not known although an apparent rupture was found in the wall of the egg near the tip of the pollen tube. The second synergid changes at the time the pollen tube enters the first synergid. These changes result in the disorganization of the nucleus and loss of the chalazal wall and plasma membrane. Eventually this synergid loses its identity as its cytoplasm merges with that of the central cell.     

The Arabidopsis mutant feronia disrupts the female gametophytic control of pollen tube reception

Reproduction in angiosperms depends on communication processes of the male gametophyte (pollen) with the female floral organs (pistil, transmitting tissue) and the female gametophyte (embryo sac). Pollen-pistil interactions control pollen hydration, germination and growth through the stylar tissue. The female gametophyte is involved in guiding the growing pollen tube towards the micropyle and embryo sac. One of the two synergids flanking the egg cell starts to degenerate and becomes receptive for pollen tube entry. Pollen tube growth arrests and the tip of the pollen tube ruptures to release the sperm cells. Failures in the mutual interaction between the synergid and the pollen tube necessarily impair fertility. But the control of pollen tube reception is not understood. We isolated a semisterile, female gametophytic mutant from Arabidopsis thaliana, named feronia after the Etruscan goddess of fertility, which impairs this process. In the feronia mutant, embryo sac development and pollen tube guidance were unaffected in all ovules, although one half of the ovules bore mutant female gametophytes. However, when the pollen tube entered the receptive synergid of a feronia mutant female gametophyte, it continued to grow, failed to rupture and release the sperm cells, and invaded the embryo sac. Thus, the feronia mutation disrupts the interaction between the male and female gametophyte required to elicit these processes. Frequently, mutant embryo sacs received supernumerary pollen tubes. We analysed feronia with synergid-specific GUS marker lines, which demonstrated that the specification and differentiation of the synergids was normal. However, GUS expression in mutant gametophytes persisted after pollen tube entry, in contrast to wild-type embryo sacs where it rapidly decreased. Apparently, the failure in pollen tube reception results in the continued expression of synergid-specific genes, probably leading to an extended expression of a potential pollen tube attractant.     

莴苣助细胞发育过程中钙的分布研究

用焦锑酸盐沉淀法对莴苣助细胞中的钙分布进行了观察。结果表明,开花前3天刚形成的助细胞中的钙颗粒很少:开花前2天助细胞壁中的钙颗粒增加;开花前1天助细胞珠孔端细胞壁加厚,其中积累了许多钙颗粒:开花当天助细胞珠孔端的丝状器中聚集了大量的钙颗粒。授粉后1h时两个助细胞的结构和钙分布发生差异,一个呈退化状,其中的钙颗粒明显增多,另一宿存助细胞中的钙分布与授粉前相似。去雄不授粉1天后两个助细胞均保持完好,且两助细胞中的钙分布没有明显差异,表明由花粉管引起一个助细胞中钙含量增加进而导致了助细胞退化。退化助细胞在卵细胞与中央细胞之间形成一薄层。助细胞退化后不同部位的钙颗粒呈现出与受精作用密切有关的变化:授粉后1h时,钙主要聚集在近合点端部位;授粉后2.5h卵细胞即将受精,这时许多细小的钙颗粒主要聚集在卵细胞与中央细胞之间的薄层中;授粉后4h精、卵细胞已融合,这时退化助细胞合点端的钙颗粒明显减少,而在其珠孔端又聚集了较多的钙。上述助细胞中的钙含量变化与吸引花粉管进入胚囊和促使精卵细胞融合密切有关。     

Structural aspects of in vitro pollen tube growth and micropylar penetration inGasteria verrucosa (Mill.) H. Duval andLilium longiflorum Thunb.

Summary In vitro penetration of the micropyle of freshly isolatedGasteria verrucosa ovules by pollen tube was monitored on agar medium. 40–60% of the micropyles were penetrated, comparable with in vivo penetration percentages. When germinated on agar,Gasteria pollen tube elongation lasts for up to 8 h while plasma streaming continues for about 20–24 h. The generative cell divides between 7 and 20 h after germination, and after 20 h the pollen tube arrives at one of the synergids. The sperm cells arrive after 22 h. The whole process takes more time in vitro than in vivo. In fast growing pollen tubes, a pulsed telescope-like growth pattern of tube elongation is observed. The formation of pollen tube wall material precedes tube elongation and probably prevents regular enlargement of the pollen tube tip-zone. Rapid stretching of the new pollen tube wall material follows, probably due to gradually increased osmotic pressure and the use of lateral wall material below the tip. The stretching ceases when the supplies of plasma membrane and excretable wall material are exhausted. Multiple pollen tube penetration of the micropyle occurs in vitro as it does in vivo. Most pollen tube growth ceases within the micropyle but, if it continues, the pollen tubes curl. Inside the micropyle the pollen tube shows haustorial growth. At the ultrastructural level, the wall thickening of in vitro pollen tubes is quite similar to that in vivo. Before transfer of pollen tube cytoplasm a small tube penetrates one of the synergids. Sperm nuclei with condensed chromatin are observed in the pollen tube and the synergid. In vivo prometaphase nuclei are found in the most chalazal part of a synergid, against the egg cell nucleus and nucleus of the central cell at a later stage. Using media forLilium ovule culture,Gasteria ovules were kept alive for at least 6 weeks. Swelling of the ovule depends on pollen tube penetration. The conditions for fertilization to occur after in vitro ovular pollination seem to be present.     

Pre-fertilization degeneration of both synergids in Brassica campestris ovules

Summary In Brassica campestris, both synergids of the ovule degenerate before the arrival of the pollen tube. Synergid degeneration does not depend on pollination. At the non-degenerated stage, the synergids are completely filled with a complexly organized cytoplasm containing numerous mitochondria with many cristae, a large number of dictyosomes with many associated vesicles, and a very extensive rough endoplasmic reticulum. The degenerative changes that occur in the cytoplasm of the synergids are characterized by a loss of visibility of the membranes of the endoplasmic reticulum and the simultaneous formation of dense deposits on the surrounding membranes of the mitochondria. Locally, the plasma membranes of the synergids disappear, and some ground plasma of the synergids penetrates into the space between the plasma membranes of the egg cell and the central cell.     

POLLEN TUBE-SYNERGID INTERACTIONS IN PROBOSCIDEA LOUISIANICA (MARTINEACEAE)

The ultrastructure of the synergids of Proboscidea louisianica was investigated from just before fertilization until 48 hr after pollination. It was found that the cytoplasm of one synergid consistently begins to degenerate before arrival of the pollen tube at the embryo sac, and that it is always this synergid which receives the pollen tube tip and its discharge. The other synergid (persistent synergid) remained unchanged throughout the study period. Polysaccharide vesicles of pollen tube origin were observed fusing with the pollen tube wall as well as contributing to cell wall formation of the degenerate synergid. In one ovule (48 hr after pollination) two pollen tubes had entered and grown the length of the micropyle, but only the first tube penetrated the degenerate synergid and discharged normally. The second pollen tube was abutting against the persistent synergid, but had not entered or discharged. In another exceptional case (18 hr after pollination), a pollen tube had grown the length of the micropyle, but did not discharge, or enter either synergid. Both synergids of this ovule were observed to be completely intact. It is concluded that synergid and pollen tube cytoplasmic degeneration is the result of a very specific interaction between these two cells and that this degeneration is probably a prerequisite for normal pollen tube entrance and discharge into the embryo sac, and for male gamete transfer to the egg and central cell.     

被子植物助细胞的发育和功能

助细胞是被子植物雌配子体的组成细胞之一,是大多数被子植物完成受精作用的关键环节之一。在受精过程中,助细胞吸引花粉管向雌配子体生长,并接受花粉管长入细胞程序死亡助细胞中。接下来的花粉管停止生长和花粉管顶端破裂释放出2个精细胞的过程可能也依赖于助细胞。另外,雄性生殖单位的解体、提供精细胞的运动轨道和启动精细胞合成DNA的生物学事件也可能与助细胞有关。助细胞的形态结构已研究得比较清楚,但有关助细胞的发育机理和它在受精过程中的作用则仍不明确。近年来对助细胞的研究又取得了一些新结果,尤其是一些分子生物学的研究结果为揭示助细胞的功能提供了重要线索。该文总结了这方面的研究成果,并对助细胞的生殖功能进行了分析。     

Dynamics of antimonate-precipitated calcium and degeneration in unpollinated pearl millet synergids after maturity

Pearl millet synergids from unpollinated pistils at 1.5–2 and 2.5–3 days postmaturity (dpm) were examined using transmission electron microscopy following antimonate fixation to precipitate loosely-bound calcium (Ca). With increasing age of synergids, the gap above filiform apparatus (FA) and the coalesced vacuoles in midchalazal core extended and merged. The FAs became compressed and precipitates along their common wall were dispersed. The matrix material in numerous small chalazal vacuoles changed from dense to flocculent. Precipitates in vacuoles appeared mainly as clumps without or with a halo in the dense matrix and mostly finely distributed in the flocculent matrix. Eventually, vacuoles became free of both matrix and precipitates. Precipitates bound to nucleus, nucleolus, and micropylar cytoplasm increased initially, but then seemed to decrease, while the nucleus became disorganized and the nucleolus disappeared. Precipitates in the embryo sac wall and nucellar cells also increased initially, but then decreased. At very late stages, egg apparatus and ES lost structural integrity and lacked precipitates. Differences in degeneration and Ca levels of sister synergids were smaller at 1.5–2 dpm than at 2.5–3 dpm. A logical and correlative scheme of degenerative events and Ca distributional changes occurring in pearl millet synergids from maturity to 2.5–3 dpm is presented. The significance of results in pollen tube/sperm cell interactions with synergids and Ca is addressed.On Specific Cooperative Agreement 58-6612-8-002 with the Department of Biochemistry, University of Georgia, Athens, GA 30602, USA     

Embryogenesis of Polyembryonic Rice ApⅢ: Structural and Histochemical Studies of Egg Apparatus Around Fertilization

The structural and histochemical changes of the egg apparatus in the polyembryonic rice (Oryza sativa L.), ApⅢ with the highest frequence of additional embryos among the polyembryonic rice investigated, before and after fertilization were studied and compared with those of normal and other polyembryonic rices in a similar developmental period. A total of 2 932 ovules were observed and each of them contained only asingle embryo sac with a set of egg apparatus. Among 1 655 embryo sacs, there were 1 643 embryo sacs (99.27%) with one normal egg apparatus in each embryo sac, and only 12 embryo sacs (0.73%) from the remainder with 4 celled egg apparatus, i.e. two eggs and two synergids. Neither the numerous poly egg apparatus and egg like cells, nor the double set of embryo sacs each containing one egg apparatus and other abnormal egg apparatus in single ovary, which were reported by earlier investigators to have high frequency of embryo production in SB 1 and ApⅣ, were observed. The egg cell was located at the subterminal site of the micropylar end of embryo sac. The cytoplasm of egg cell was rich in protein materials and poly saccharide grains, which did not disappear until the division of zygote. The prominent nucleus was closely surrounded by protein and polysaccharide grains, which did not disappear until the division of zygote. No cytological difference was found between egg cells from the normal and abnormal egg apparatus. The two synergids were fully developed and situated at the upper most part of the micropylar end of the mature embryo sac. In most embryo sacs, the synergids were flask shaped with longer necks, and a widened cap shaped top, in close contact with the micropyle. The synergids had a well developed filiform apparatus. The characteristic appearance of the filiform apparatus as well as the cap neck region of synergids before and after pollen tube penetration were easily distinguishable from the egg cell. The structure, the stainability with Coomassie Brilliant Blue and PAS reaction, the process of accumulation, distribution and disapperance of the cytoplasmic protein materials and polysaccharide grains of the two synergids, the persistent and rarely the receptive synergids before and after pollen tube penetration, were closely similar to those of egg cell of the same developmental stage. In comparison with normal and other polyembryonic rice reported, the size of nucleus and nucleolus and their stainability also strongly resembled those of egg cell. Based on the results observed, the main conclusions are summarized as follows: (1) the additional embryos very frequently developed in the young and mature seed of polyembryonic rice ApⅢ were produced by one or two synergids of normal egg apparatus, rarely by 4 celled egg apparatus; (2) during fertilization, the synergids, in addition to the natural specific function of introducing pollen tube and transferring sperms to egg cell and central cell, could be closely associated with the potentiality to breed one or two additional embryos; and (3) as compared with that of normal or other polyembryonic rice it is firstly disclosed that in a few embryo sacs of ApⅢ, the cytoplasmic and nuclear structure, the active anabolism and catabolism of protein and polysaccharide materials and the delayed disorganization at the mid basal region of the receptive and persistent synergid still remained unchanged before the division of zygote. Such salient features could be the predisposition for the origin of additional embryos in ApⅢ.     

被子植物助细胞的发育和功能

助细胞是被子植物雌配子体的组成细胞之一, 是大多数被子植物完成受精作用的关键环节之一。在受精过程中, 助细胞吸引花粉管向雌配子体生长, 并接受花粉管长入细胞程序死亡助细胞中。接下来的花粉管停止生长和花粉管顶端破裂释放出2个精细胞的过程可能也依赖于助细胞。另外, 雄性生殖单位的解体、提供精细胞的运动轨道和启动精细胞合成DNA的生物学事件也可能与助细胞有关。助细胞的形态结构已研究得比较清楚, 但有关助细胞的发育机理和它在受精过程中的作用则仍不明确。近年来对助细胞的研究又取得了一些新结果, 尤其是一些分子生物学的研究结果为揭示助细胞的功能提供了重要线索。该文总结了这方面的研究成果, 并对助细胞的生殖功能进行了分析。     

白头翁的受精及组织化学研究

成熟花粉为二细胞,生殖细胞的蛋白质染色较营养细胞的深,淀粉粒充满营养细胞。花粉管常见穿人正在退化中的助细胞,并释放出两个精子,大量稠浓的蛋白质和淀粉,个别的释放在助细胞与胚囊壁之间。在一些卵细胞核,次生核(或极核)中,受精前后有1—3个小核仁(约1微米);助细胞具丝状器;反足细胞宿存,具多核和多核仁。胚囊中各个细胞的原生质稠浓程度和蛋白质染色深浅不同,其中以反足细胞和助细胞的原生质最稠浓,蛋白质染色最深。淀粉十分贫乏,绝大多数卵细胞,中央细胞和几乎全部助细胞和反足细胞均不含淀粉。双受精属于有丝分裂前类型,两性核融合步骤为:精子核接近和贴附在卵核和次生核上(或极核上),两性核的核膜溶解,其染色质沉入融合核,并随之松解,同时出现雄性核仁,最终,雄性核仁和雌性核仁合并,形成具单核和单核仁的合子和初生胚乳细胞。另外,雄性核仁同卵核仁合并较雄性核仁和次生核的晚,所以卵细胞完成受精较次生核晚。     

Fertilization and Histochemical Investigation on Pulsatilla chinensis (Bung) Regel

Fertilization and variation of protein and starch grains in Pulsatilla chinensis (Bung) Regel have been studied at light microscopic level with histochemical test. Based upon the observations, the main conclusions are summarized as follows: The mature pollen grains are two-celled in which the generative cell shows the stronger protein staining than the vegetative cell. And vegetative cells are full of starch garins. When the pollen tube enters into the embryo sac, one synergid is destroyed, or in a few cases synergids are intact. Occasionally two synergids are disorganized as pollen tube penetrates. However, most of the remaining syuergids break down during fertilization, only in a few cases it remains till early stage of embryo development. The contents discharged by the pollen tube consist of two sperms, which stain intensely blue with protein dyes, a great amount of protein and starch grains. Mature female gametophyte (embryo sac) consists of an egg apparatus, central cell, which has a huge secondary nucleus, and antipodal apparatus which retain in course of fertilization. A few of embryo sac contain two sets of egg apparatus, a central cell with two huge secondary nuclei and two sets of antipodal apparatus. In some nucleoli of the central cell the comb-like structure pattern may be detected clearly. There are 1–2 small nucleoli in some egg cells and central cells. All the cells in embryo sac show protein positive reaction. According to the different shades of the color in cells, its may be arranged in the following order: antipodal cells, synergids, central cell and egg cell. Only a few small starch grains are present near nuclei of central cell and egg cell before fertilization, but no starch grains remain in most of the central cell, the synergids and antipodal cells. The fertilization is of the premitotic type. The fusion of the sexual nuclei progresses in the following order: 1, sperms approach and lie on the egg nucleus and secondary nucleus; 2, sperm chromatin sinks themselves into female nucleus, and male nucleolus emerges with the sperm chromosome; and 3, male nucleoli fuse with the nucleoli of egg nucleus and central cell nucleus, and finally forming the zygote and the primary endosperm cells respectively. Nevertheless, as it is well known, the fertilization completes in central cell obviously earlier than that in egg cell. Though it has been explained in cereals and cotton, in Pulsatilla chinensis the main reason is that nucleolar fusion of the male and female nucleoli in egg nucleus is slower than that in secondary nucleus. And the dormancy of the primary endosperm nucleus is shorter than that of the zygote. In the process of fertilization, histochemical changes are considerably obvious in the following three parts: 1, from the begining of fusion of male and female nuclei to form zygote and primary endosperm cell, Protein staining around female nucleus appears to increase gradually; 2, no starch grains are detected in embryo sac. Though only starch grains are carried in by pollen tube, they are completely exhausted during this period; and 3, near completion of fertilization starch grains appear again in zygote, however, not yet in primary endosperm nucleus till its dividing for the first time. The present study reveals that antipodal cells and synergids seem to play a significant role in nutrition of the embryo sac during the fertilization.     

FINE STRUCTURE AND COMPOSITION OF THE EGG APPARATUS BEFORE AND AFTER FERTILIZATION IN QUERCUS GAMBELII: THE FUNCTIONAL OVULE

A study of the egg apparatus of Quercus gambelii was made at both the light and the electron microscope levels. This investigation was concerned primarily with the changes that occur in these cells before and after the process of fertilization and what role, if any, is played by the synergids in this phenomenon. The synergids before fertilization are, on the basis of ultrastructure, healthy, intact, functional cells. They have numerous mitochondria, dictyosomes, endoplasmic reticulum, ribosomes, and a typical nucleus. A prominent filiform apparatus is present, but the cell wall only extends a short distance around the micropylar end of the cells. Just before fertilization, one of the synergids degenerates. This is the synergid that receives the pollen tube and its discharge, including both male gametes. Dictyosomes increase in number and activity in the other synergid (persistent synergid) after fertilization. Eventually a complete cell wall forms around both of the synergids. No plasmodesmata are present in these walls. The egg has numerous mitochondria, dictyosomes, endoplasmic reticulum, and ribosomes, both free in the cytoplasm and attached to the endoplasmic reticulum. Lipid bodies are characteristic of this cell. A cell wall is present only around the micropylar end of the egg. After fertilization, little change occurs in the zygote. The number and activity of the dictyosomes increase, apparently in correlation with cell wall formation. The number of lipid bodies increases. The zygote is approximately the same size as the egg. Plastids are scarce, and starch grains are typically absent from all cells of the egg apparatus. It is suggested that the synergids function in the secretion of chemotropic substances that guide the growth of the pollen tube. Comparisons are made between the egg apparatus of Quercus gambelii and that of the other plants studied thus far.     

Adhesion and guidance in compatible pollination

The mechanisms of compatible pollination are less studied than those of incompatible pollination and yet most of the angiosperms show self-compatibility. From the release of pollen from anthers to the penetration of the micropyle by the pollen tube tip, there are numerous steps where the interaction between pollen and the pistil can be regulated. Recent studies have documented some diverse ways in which pollen tubes carrying sperm cells are guided to the ovules through the pistil extracellular matrices of the transmitting tract. What is still missing is an understanding of pollen tube cell biology in vivo. A recent finding supports the role of the synergids in the crucial guidance cue for the pollen tube tip at the micropyle, but experimental evidence for other 'guidepost' cells in the pistil is still lacking. The fact that the pollen tube must first travel through the matrices of the stigma and style before it can respond to the cue from the ovule makes it likely that there is a hierarchy of signalling events in pollen-pistil interactions starting at the stigma and ending at the micropyle. On the pistil side, several model systems have been used in the discovery of molecules implicated in either physical or chemical guidance. In lily, which has a hollow style, adhesion molecules (pectin and SCA) are implicated in guidance. SCA alone is also capable of inducing pollen chemotropism in an in vitro assay, suggesting that this peptide plays a dual role in lily pollination: chemotactic in the stigma and haptotactic (adhesion mediated) in the style.     

钙在有花植物受精过程中的作用

钙作为第二信使在植物信号转导中的作用一直是植物生理学、细胞生物学和发育生物学研究的热点。近年已有不少综述和专著从不同角度对此作了详细评论［1～5］。虽然这些文章中只有部分内容涉及本文的主题———钙在植物受精中的作用,但是它们所论述的关于钙信使系统在植物...