The signals to trigger the initiation of ovule enlargement are from the pollen tubes: The direct evidence

In angiosperms, initiation of ovule enlargement represents the start of seed development, the molecular mechanism of which is not yet elucidated.It was previously reported that pollen tube contents,rather than double fertilization, can trigger ovule enlargement. However, it remains unclear whether the signal(s) to trigger the initiation of ovule enlargement are from the sperm cells or from the pollen tubes.Recently, we identified a mutant drop1- drop2-, which produces pollen tubes with no sperm cells. Taking advantage of this special genetic material, we conducted pollination assays, and found that the ovules pollinated with drop1- drop2- pollen could initiate the enlargement and exhibited significant enlarged sizes at 36 h after pollination in comparison with those unpollinated ovules. However, the sizes of the ovules pollinated with drop1- drop2- pollen are significantly smaller than those of the ovules pollinated with wildtype pollen. These results demonstrate that the pollen tube, rather than the sperm cells, release the signal to trigger the initiation of ovule enlargement, and that double fertilization is required for further enlargement of the seeds.     

Fertilization recovery after defective sperm cell release in Arabidopsis

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (～80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.     

Gamete fusion is required to block multiple pollen tubes from entering an Arabidopsis ovule

In double fertilization, a reproductive system unique to flowering plants, two immotile sperm are delivered to an ovule by a pollen tube. One sperm fuses with the egg to generate a zygote, the other with the central cell to produce endosperm. A mechanism preventing multiple pollen tubes from entering an ovule would ensure that only two sperm are delivered to female gametes. We use live-cell imaging and a novel mixed-pollination assay that can detect multiple pollen tubes and multiple sets of sperm within a single ovule to show that Arabidopsis efficiently prevents multiple pollen tubes from entering an ovule. However, when gamete-fusion defective hap2(gcs1) or duo1 sperm are delivered to ovules, as many as three additional pollen tubes are attracted. When gamete fusion fails, one of two pollen tube-attracting synergid cells persists, enabling the ovule to attract more pollen tubes for successful fertilization. This mechanism prevents the delivery of more than one pair of sperm to an ovule, provides a means of salvaging fertilization in ovules that have received defective sperm, and ensures maximum reproductive success by distributing pollen tubes to all ovules.     

CHEMOTROPIC ACTIVITY AND THE PATHWAY OF THE POLLEN TUBE IN LILY,

A study of the pollen tube pathway in Lilium leucanthum var. centifolium and in L. regale reveals that the entire pathway from stigma to ovule is lined with cytologically unique stigmatoid cells. Assays for chemotropic activity of tissues and exudates along the pathway of pollinated or unpollinated pistils showed that onset of chemotropic activity progressed basipetally (and, when pollinated, in advance of the pollen tubes), commencing at the stigma 3-5 days before anthesis and appearing in the ovules 1-2 days after anthesis. Activity persists about 10 days in ovules of pollinated pistils and for 14-16 days in ovules of non-pollinated pistils. Attempts to localize the source of the chemotropic factor showed that gynoecial tissues bearing stigmatoid cells are chemo-tropically active while slices of style or ovary wall lacking stigmatoid cells are inactive. When ovules were sliced transversely and the micropylar and chalazal halves assayed, only the micropylar half showed activity. We suggest that the ovules and the stigmatoid tissue along the pollen tube pathway are the sources of the chemotropic factor responsible for the directional growth of the pollen tube.     

AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube guidance

Reproductive isolation is a prerequisite to form and maintain a new species. Multiple prezygotic and postzygotic reproductive isolation barriers have been reported in plants. In the model plant, Arabidopsis thaliana conspecific pollen tube precedence controlled by AtLURE1/PRK6-mediated signaling has been recently reported as a major prezygotic reproductive isolation barrier. By accelerating emergence of own pollen tubes from the transmitting tract, A. thaliana ovules promote self-fertilization and thus prevent fertilization by a different species. Taking advantage of a septuple atlure1null mutant, we now report on the role of AtLURE1/PRK6-mediated signaling for micropylar pollen tube guidance. Compared with wild-type (WT) ovules, atlure1null ovules displayed remarkably reduced micropylar pollen tube attraction efficiencies in modified semi-in vivo A. thaliana ovule targeting assays. However, when prk6 mutant pollen tubes were applied, atlure1null ovules showed micropylar attraction efficiencies comparable to that of WT ovules. These findings indicate that AtLURE1/PRK6-mediated signaling regulates micropylar pollen tube attraction in addition to promoting emergence of own pollen tubes from the transmitting tract. Moreover, semi-in vivo ovule targeting competition assays with the same amount of pollen grains from both A. thaliana and Arabidopsis lyrata showed that A. thaliana WT and xiuqiu mutant ovules are mainly targeted by own pollen tubes and that atlure1null mutant ovules are also entered to a large extent by A. lyrata pollen tubes. Taken together, we report that AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube attraction representing an additional prezygotic isolation barrier.

A modified ovule targeting assay revealed that AtLURE1/PRK6-mediated signaling promotes micropylar guidance of Arabidopsis thaliana pollen tubes while discriminating tubes of related Arabidopsis lyrata.     

Pollen tubes enter neighbouring ovules by way of receptacle tissue,resulting in increased fruit-set in Sagittaria potamogetifolia Merr

Using fluorescence microscopy, deposition of pollen on stigmas and pollen tube growth in the gynoecium of Sagittaria potamogetifolia Merr., a monoecious species with an apocarpous gynoecium, were observed. The maximum rate of pollination averaged 83.9 +/- 4.7 %, and the number of pollen grains per stigma ranged from zero to 30. Pollen tubes grew through one stigma to the base of the ovary at almost the same speed, but generally only one of the pollen tubes then turned towards the ovule and finally entered the nucellus through the micropyle. The other pollen tubes grew through the ovary base and the receptacle tissue into ovules of adjacent carpels whose stigmas were not pollinated or which had been pollinated later. This phenomenon is termed pollen tube 'reallocation' by the authors. To verify the direct effect of the phenomenon on fruit set, artificial pollination experiments were conducted in which two or more pollen grains were placed onto only one stigma in each gynoecium; frequently more than one fruitlet was obtained from each flower treated. The reallocation of pollen tubes among pistils in the gynoecium could effect fertilization of ovules of unpollinated pistils and lead to an increase in sexual reproduction efficiency. It would, to some extent, also increase pollen tube competition among pistils of the whole gynoecium.     

Pollen tube reallocation in two preanthesis cleistogamous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae)

Pollen deposition on stigmas and pollen tube growth in two apocarpous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae), were examined with fluorescence microscopy. The reallocation of pollen tubes among pistils was observed in both species. The percentage of pollinated stigmas per flower was only 22.0% in R. rostratum and 51.0% in S. guyanensis, though the seed/ovule ratios are higher than 65% in both species. The number of pollen grains on each single stigma ranged from 0 to 96 in R. rostratum, and from 0 to 125 in S. guyanensis. When more than one pollen grain deposited on a stigma, all pollen tubes grew to the ovary, but only one of them turned towards the ovule and finally entered the nucleus. The other tubes grew through the receptacle tissue into ovules of adjacent carpels whose stigmas were unpollinated or pollinated later. The intercarpellary growth of pollen tubes could be a mechanism to increase the efficiency of sexual reproduction in an apocarpous gynoecium with low pollination on the pistils.     

Potassium flux in the pollen tubes was essential in plant sexual reproduction

Potassium channels are controlling K⁺ transport across plasma membrane and thus playing a central role in all aspects of osmolarity as well as numerous other functions in plants, including in sexual reproduction. We have used whole-cell and single-channel patch-clamp recording techniques investigated the regulation of intracellular free Ca²⁺-activated outward K⁺ channels in Pyrus pyrifolia pollen tube protoplasts. We have also showed the channels could be inhibited by heme and activated carbon monoxide (CO). In the presence of oxygen and NADPH, hemoxygenases catalyzes heme degradation, producing biliverdin, iron and CO. Considered the oxygen concentration approaching zero in the ovary, the heme will inhibit the K⁺ outward flux from the intracellular of pollen tube, increasing the pollen tubes osmolarity, inducing pollen tube burst. Here we discuss the putative role of K⁺ channels in plant sexual reproduction.Key words: pear, pollen, K⁺ channels, heme, carbon monoxideIon channels in the pollen tube play critical roles in mediation pollen germination and pollen tube growth.^1–3 Early studies were focus on the plasma membrane calcium channel regulation and cytosolic free calcium concentration variation in the pollen tube reason by which was one of the most important second messengers in plants.^3–7 However, reports have also showed that the potassium channels in the pollen tubes were also involved in several important steps of plant sexual reproduction.^8–19 Recently, more reports further demonstrated this phenomena.^20–24 In the report by Lu et al. they demonstrated that two cation/proton exchangers (CHX), CHX21 and CHX23, are essential for pollen tube growth guidance in Arabidopsis.²² chx21 chx23 double mutant induces the fertility impaired, but which is unchanged in both single chx21 or chx23 mutants. They have also found that the double mutant pollen grains germination and pollen tube growth in the transmitting tract were not difference with the wild-type, however, the double mutant pollen tubes fail to turn toward ovules.²² Protein localization experiments show CHX23 is expressed in the endoplasmic reticulum of pollen tubes; functional analysis results showed that CHX23 as a K⁺ transporter mediates K⁺ uptake in a pH-dependent manner. So, these protein affect the signal transduction pathway of pollen tube growth toward to the ovule by controlling the cation balance and pH in the pollen tube.²² Amien et al. identified a signaling ligand of defensin-like (DEFL) protein, ZmES4, which expressed in maize synergid. ZmES4 activates the maize pollen tube tip plasma membrane K⁺ Shaker channel KZM1.²⁰ This finding is also very interesting. Pollen tube bursting suggested to be based on the osmotic stress; the influx of K⁺ mediated by ZmES4-activated KZM1 will trigger rapid plasma membrane depolarization, which induced the pollen tube tip burst.²⁰ Furthermore, the osmotic increasing induced by too much K⁺ in the cytosolic of pollen tube was not only resulted by inward K⁺ channel activation, but also resulted by outward K⁺ channel inhibition in the pollen tube plasma membrane. In our report, we find a intracellular Ca²⁺-sensitive outward K⁺ channel in pear pollen tube plasma membrane, which could be inhibited by heme and activated by heme oxidative production, carbon monoxide (CO), may play a functional role in the pollen tube brusting.²³In the presence of oxygen and NADPH, hemoxygenases catalyzes heme degradation, producing biliverdin, iron and CO.²⁵ Early reports showed that oxygen plays an important role in plant sexual reproduction. Pollen tubes grow through the style toward the ovary with high speed, a process that consumes tremendous amounts of energy and requires rapid oxygen uptake by pollen tubes.²⁶ Pollen grains have roughly 20 times the level of mitochondria and respire 10 times faster than vegetative tissue.^12,27–29 Furthermore, oxygen has been proposed as a possible cue for pollen-tube guidance.³⁰ Indeed, the existence of an oxygen gradient in the unpollinated style has been shown in some species such as Hipeastrum hybridum. Oxygen pressure is high in the stigma and style but suddenly decreases at the base of the style, approaching zero in the ovary. Moreover, pollen-tube growth itself creates hypoxic regions within the style.³¹ Therefore, we suggest that the outward K⁺ channel inhibited by heme is dominant compared with which activated by CO when pollen tubes reach the ovary, based on where the hypoxic condition (Fig. 1). However, the gene encode the outward K⁺ channel in the pear pollen tube remains to be determined in the further study.Open in a separate windowFigure 1Reciprocal regulation of heme and carbon monoxide in putative Ca²⁺-activated outward K⁺ channel. Under normal condition, in the presence of NADPH, heme is metabolized by hemeoxygenase to generate carbon monoxide (CO), which activates outward K⁺ channel. However, without the oxygen, heme cannot be metabolized. The accumulated heme acts as an inhibitor of outward K⁺ channel, even in the presence of NADPH. The accumulated K⁺ in the cytosolic of pollen will induced the pollen tube depolarized, then burst.     

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     

Ovary and Gametophyte Development Are Coordinately Regulated by Auxin and Ethylene following Pollination

The differentiation and development of ovules in orchid flowers are pollination dependent. To define the developmental signals and timing of critical events associated with ovule differentiation, we have examined factors that regulate the initial events in megasporogenesis and female gametophyte development and characterized its progression toward maturity and fertilization. Two days after pollination, ovary wall epidermal cells begin to elongate and form hair cells; this is the earliest visible morphological change, and it occurs at least 3 days prior to pollen germination, indicating that signals associated with pollination itself trigger these early events. The effects of inhibitors of ethylene biosynthesis on early morphological changes indicated that ethylene, in the presence of auxin, is required to initiate ovary development and, indirectly, subsequent ovule differentiation. Surprisingly, pollen germination and growth were also strongly inhibited by inhibitors of ethylene biosynthesis, indicating that male gametophyte development is also regulated by ethylene. Detailed characterization of the development of both the female and male gametophyte in pollinated orchid flowers indicated that pollen tubes entered the ovary and grew along the ovary wall for 10 to 35 days, at which time growth was arrested. Approximately 40 days after pollination, coincident with ovule differentiation as indicated by the presence of a single archesporial cell, the direction of pollen tube growth became redirected toward the ovule, suggesting a chemical signaling between the developing ovule and male gametophyte. Taken together, these results indicate that both auxin and ethylene contribute to the regulation of both ovary and ovule development and to the coordination of development of male and female gametophytes.     

Silencing gene expression of the ethylene-forming enzyme results in a reversible inhibition of ovule development in transgenic tobacco plants

To study the role of ethylene in plant reproduction, we constructed transgenic tobacco plants in which the expression of a pistil-specific gene coding for the ethylene-forming enzyme 1-aminocyclopropane-1-carboxylate oxidase was inhibited. Flowers from transgenic plants showed female sterility due to an arrest in ovule development. Megasporogenesis did not occur, and ovules did not reach maturity. When pollinated, pollen tubes were able to reach the ovary but did not penetrate into the immature ovule in transgenic plants. Flower treatment with an ethylene source resulted in a functional recovery of ovule development and restored guidance of the pollen tube tip into the ovule micropyle that resulted in seed set. The recovery was abolished if inhibitors of ethylene action were present. These results demonstrate that the plant hormone ethylene is required during the very early stages of female sporogenesis and ultimately to enable fertilization.     

DUF784基因在花粉管导向中的功能分析

花粉管导向是高等植物完成双受精过程的重要环节,是受多重信号调控的复杂过程.最近的研究揭示,配子体阶段花粉管导向的诱导信号分子是一类具多态性的富含半胱氨酸的防卫素类似蛋白,如来自玉米的ZmEA1和蓝猪耳草中的LUREs在吸引花粉管进入珠孔起重要作用.但是拟南芥及其它植物中此类信号未知.转录组学分析表明,一组DUF784基因可能在花粉管导向中起到重要作用.通过RNAi技术降低一组DUF784基因的表达,分析发现在RNAi转基因植株中,出现胚珠败育现象,花粉管导向出现异常,一部分花粉管不能进入珠孔.另外,用MYB98基因的启动子携带1个DUF基因的编码区,然后转化ccg突变体,发现ccg转基因株系中胚胎败育率下降,即DUF基因能部分互补ccg突变体的表型;从这两方面证实了DUF784基因在花粉管定向导入过程中的作用.     

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.     

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.     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance in Arabidopsis

The fertilization process of plants is governed by different kinds of cell-cell interactions. In higher plants, these interactions are required both for recognition of the pollen grain by the female reproductive system and to direct the growth of the pollen tube inside the ovary. Despite many years of study, the signaling mechanisms that guide the pollen tube toward its target, the ovule, are largely unknown. Two distinct types of principles, mechanical and chemotropic, have been suggested to account for the directed growth of the pollen tube. The first of these two types of models implies that the guidance of the pollen tube depends on the architecture and chemical properties of the female reproductive tissues, whereas the latter suggests that the ovule provides a signal for the target-directed growth of the pollen tube. To examine such a role for the ovules, we analyzed the growth path of pollen tubes in mutants defective in ovule development in Arabidopsis. The results presented here provide unique in vivo evidence for an ovule-derived, long-range activity controlling pollen tube guidance. A morphological comparison of the ovule mutants used in this study indicates that within the ovule, the haploid embryo sac plays an important role in this long-range signaling process.     

Regulation of LAT52 promoter activity during pollen tube growth through the pistil of Nicotiana alata.

The pollen-specific promoter of the LAT52 gene is known to direct expression of marker proteins during the last stages of pollen maturation and in very early pollen tube growth.We have examined the expression of LAT52-GUS during later stages of pollen tube growth in style and ovary of the relatively long-styled species Nicotiana alata. GUS activity was detected histochemically and found to be present in germinating pollen grains of N. alata and in tubes growing through the upper part of the style. No GUS activity was detected in 99% of the pollen tubes growing through the lower part of the style, but activity was present in tubes within the ovary. This finding indicates that the LAT52 promoter is regulated in growing pollen tubes, and is most active during the earliest and latest stages of pollen tube growth. GUS activity was also detected in some ovules, where it presumably marked the release of pollen tube cytoplasm into the ovule. The distribution of ovules with GUS activity within the ovary is not consistent with high-precision pollen tube guidance to the ovule. Received: 16 August 1999 / Revision accepted: 20 December 1999     

鹤顶兰花粉管在子房中的生长途径

运用扫描电镜对鹤顶兰(Phaiustankervilliae(Aiton)Bl.)花粉管在子房内的生长途径进行了观察。结果表明:花粉管在子房中的生长途径可以分为3个阶段:(1)沿子房壁轴向生长阶段,从授粉开始至大孢子母细胞四分体时期,花粉管经过合蕊柱到达子房,经由胎座基部沿子房壁轴向生长;(2)沿子房径向生长阶段,二核胚囊之后,花粉管在胚珠之间穿梭,以径向生长为主;(3)朝珠孔定向生长阶段,胚囊成熟时,花粉管朝珠孔定向生长进入胚囊。实验结果说明花粉管的定向生长受胚珠的分子信号调控。