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Pollen-Pistil Interaction and Fertilization
Abstract
Pollen-pistil interaction covers the sequential events from pollination until the pollen tubes enter the ovules. During this interaction the pistil screens the pollen grains. Pollen of other species are inhibited at the level of pollen germination or pollen tube growth in the style. Conspecific pollen grains are recognized by the stigma and activate physiological processes to facilitate their germination and pollen tube growth. In self-incompatible species, even self-pollen are recognized and inhibited. Only compatible pollen grains which are most vigorous are selected for fertilization. Thus, there is a close dialogue between the gametophytic partners (pollen grains and pollen tubes) and the sporophytic partner (the pistil) during pollen-pistil interaction. Extracellular components present in and on the pollen wall and on the stigma and along the path of the pollen tube in the style are involved in pollen recognition and their subsequent promotion/inhibition. Some of the genes/gene products involved in these interactions have been identified in recent years in a few model systems.
... Thus, the pollen-pistil interface is a critical area that involves intricate interactions between the pollen and the pistil. This interface involves biochemical signalling, physical interaction, and genetic recognition that ensure successful pollination and fertilization (Zheng et al. 2018;Shivanna 2020). On the other hand, the pollen-pistil interface is highly influenced by external environment. ...
Premise of the study. Biological factors and environmental stresses are predominant elements affecting the progamic phase of sexual plant reproduction. Thus, new knowledge is needed to select and develop cultivars adapted to these changes. This work was conducted to evaluate the pollen growth dynamics and the stigmatic receptivity at normal and elevated temperatures, in 12 Tunisian plum genotypes selected based on six different biological factors. Methods. The pistils were hand‐pollinated using their own pollen, and then the dynamics of pollen tube growth and stigma receptivity of Tunisian plum genotypes were surveyed under ambient temperature and under 35 °C. Key results. Results showed that the differences in flower size, geographical origin and drought tolerance did neither affect pollen growth dynamics nor stigma receptivity. However, the polyploidization enlarged the stigmatic receptivity but does not affect the pollen dynamics. Flowering time and wild plantation fasten the pollen growth and enlarge the stigmatic receptivity. We assume that, in particular case, the flowering plants seem to opt for an enlarged stigmatic receptivity and a rapid pollen growth as reproductive strategies to ‘ hold on for dear life’ . Experiments conducted at 35 °C showed that the temperature elevation accelerates pollen growth dynamics, while it shortens the stigmatic receptivity period. In particular, two accessions: the wild plum ‘Sauvage’ and the late flowering ‘Tasstour Hamra Tardive’ showed faster pollen growth and enlarged stigmatic receptivity, compared to other accessions. Conclusion. The main relevant conclusion of this work resides in the identification of the best adapted Tunisian genotypes that could be selected as pollen sources to extend the pollination period. Especially, accessions presenting a tolerance to high‐temperature deserve serious attention.
... This is the first report in L. sativus and L. cicera species pollen viability assessment using acetocarmine, fluorescein diacetate test and in vitro pollen germination in BK medium with 15% sucrose (Figs. 9, 10). The use of acetocarmine to stain pollen grains is a standard method for determining pollen fertility (Shivanna 2003). In the present study, four accessions namely, Ratan, Prateek, Mahateora and Narayangaon were used. ...
The advancement of Lathyrus through genetic enhancement and cultivar development faces challenges due to limited knowledge about their reproductive biology. Thus, the present study systematically investigates the reproductive biology, phenology, receptivity of stigma, germination and viability of pollen in Lathyrus sativus L. and L. cicera L. Experiments were carried out using 20 accessions of L. sativus and three accessions of L. cicera. Anther dehiscence and stigma receptivity were determined through microscopic observation and hydrogen peroxide test, respectively. Pollen viability was assessed through in vitro pollen germination and fluorescein diacetate (FDA) test. Flowers were found to be bisexual and protogynous, with self-pollination favored by the cleistogamous nature of flowers. Five flower-development and three pod-development stages were classified in both species. The timing of all stages differed significantly among the accessions studied. Stigma receptivity was observed to occur 2–4 days before anther dehiscence and persisted until flower wilting. The study reveals that L. sativus and L. cicera are primarily self-pollinated, underscoring the critical timing of emasculation for successful manual crossing in breeding, ultimately enhancing pod and seed set, and facilitating plant breeders in optimizing hybridization programs.
... Canal cells contain abundant amounts of smooth and rough endoplasmic reticulum, plastids, mitochondria, active dictyosomes and ribosomes which are metabolically active (Raghavan, 1997). Tilton & Horner (1980) made a detailed study of the structure of the dry stigma and hollow style of O. caudatum and divided the secretory phase in the style into periods. ...
The stigma of Ornithogalum sigmoideum is of dry and papillate type. The papillae are covered by a cuticle-pellicle layer, as revealed by staining. The activity of nonspecific esterase, acid phosphatase and peroxidase increases in the pellicle during the receptivity period. The style of O. sigmoideum is of the hollow type. Ultrastructural study of the cells lining the canal indicated that they are secretory cells and contain abundant endoplasmic reticulum, dictyosomes, mitochondria, plastids and ribosomes. After anthesis these organdies show degeneration at the end of the secretory phase. In canal cells, cytochemical tests showed the presence of acidic polyanions, insoluble and acidic polysaccharides, proteins and lipids. Before anthesis the canal cells are rich in polysaccharides, proteins and lipids. At maturity the cuticle is ruptured and secretory materials from the canal cells are released Into the canal. In the unpollinated style of O. sigmoideum the exudates accumulated in the center of the canal: In pollinated pistils the same secretion materials were dispersed through the canal, which became wider.
Investigating reproductive biology is relevant in evaluating the invasive capacity of alien plants. Low quality of pollen grains of an exotic plant, Oxalis corymbosa, in Taiwan has been found in a previous study. The objectives of this study were to evaluate the female fertility and to examine the morphology and ontogeny of ovules of O. corymbosa, in Taiwan. Cleared whole-mount, resin embedded sections and paraffin sections of pistils and ovules of flowers of two different morphs of the plant were made and observed. The observation revealed that O. corymbosa has anatropous and bitegmic ovules. Most of the observed megasporangia in ovules of flowers during anthesis were developmentally arrested. A 4-celullar, 5-nucleate female gametophyte (embryo sac), containing one egg cell, two synergids, and one central cell with two polar nuclei, was rarely (about 2%) observed in ovules of fully-open flowers. Observations of flower buds of different lengths revealed that multiple abnormalities occurred in the development of megasporangia and in cells of integuments. Consequently, most of mature ovules observed contained no or develop-arrested embryo sacs (female gametophyte). The result confirmed female sterility of O. corymbosa in Taiwan. The ecological implications of the absence of sexual reproduction of the exotic plant in Taiwan is discussed.
Many plants can produce seeds via multiple reproductive modes, such as selfing and outcrossing. Having multiple reproductive modes can be advantageous if it assures seed production when outcrossing fails, which is important for species inhabiting environments where pollinators are scarce or variable. However, it can also be disadvantageous due to the fitness costs associated to selfing. Consequently, plants have mechanisms to reduce the incidence of selfing. Here we examined the breeding system of Myrcianthes coquimbensis; this threatened Atacama Desert shrub is the last species to bloom in the community and exhibits low visitation rates per flower because pollinators are less abundant. Our aim was to determine whether this plant can produce fruits by modes other than outcrossing, and whether it possesses floral traits to prevent sexual interference. We conducted experimental flower treatments in two localities to determine whether fruits were produced by outcrossing, selfing, autonomous selfing and agamospermy. We also evaluated stigma receptivity and pollen viability during a flower’s lifespan. M. coquimbensis developed fruits and seeds by all the reproductive modes assessed, including selfing and agamospermy. Flowers presented partial segregation of sexual functions, with the peak of pollen viability occurring before the peak of stigma receptivity. Selfing is unavoidable in M. coquimbensis and likely interferes with outcrossing. Coupled with possible early inbreeding depression, it probably results in a cost for seed production. Our results suggest that this species may be vulnerable in scenarios where pollinators are scarce; however, agamospermy may provide an alternative route of seed production in these scenarios.
Floral features. All or nearly all species are dioecious. Knuth (1904), however, cites a report of trees being male in their sex expression when young and female later. Perianth of one whorl of three largely united segments. Stamens two to thirty, partly or wholly united. Ovary superior, consisting of a single uniovulate carpel. Floral development has been described by Armstrong & Tucker (1986). Type of pollen presentation. ‘Pollenhaufen’, using anther appendages and perianth.
Racing to fertilization
Pollen tubes, which carry plant sperm, need to grow from where they land in the flower to where the ovule is. Zhong et al. now show how pollen from related plant species race to reach the ovule first. One set of fast-evolving peptide signals is tuned to speed up growth of conspecific pollen tubes. A related set of evolutionarily ancient peptides is tuned to attract all pollen tubes. Thus, fertilization is more likely to happen through conspecific pollen tubes, but a fail-safe system encourages even the laggards to get where they need to go.
Science , this issue p. eaau9564
In non-cleistogamous plants, the male gametophyte, the pollen grain is immotile and exploits various agents, such as pollinators, wind, and even water, to arrive to a receptive stigma. The complex process of pollination involves a tubular structure, i.e., the pollen tube, which delivers the two sperm cells to the female gametophyte to enable double fertilization. The pollen tube has to penetrate the stigma, grow in the style tissues, pass through the septum, grow along the funiculus, and navigate to the micropyle of the ovule. It is a long journey for the pollen tube and its two sperm cells before they meet the female gametophyte, and it requires very accurate regulation to perform successful fertilization. In this review, we update the knowledge of molecular dialogues of pollen-pistil interaction, especially the progress of pollen tube activation and guidance, and give perspectives for future research.
Pollen tube guidance is a unique navigating system that is required for the successful sexual reproduction of plants. As plant sperm cells are non-motile and egg cells are embedded deep inside the female tissues, a pollen tube delivers the two sperm cells that it contains by growing towards the ovule, in which the egg cell resides. Pollen tube growth towards the ovule is precisely controlled and divided into two stages, preovular and ovular guidance. In this Cell Science at a Glance article and accompanying poster, we provide a comprehensive overview of pollen tube guidance and highlight some of the attractant peptides used during ovular guidance. We further discuss the precise one-to-one guidance system that exists in multi-ovular plants. The pollen tube-blocking system, which is mediated by male-female crosstalk communication, to avoid attraction of multiple pollen tubes, is also reviewed.
Over the last decades, the female gametophyte of flowering plants has attracted much attention. This is on one side due to the fact that this minute structure generates the female gametes, represents the site of fertilization and marks the inception of new life. In addition, it has become clear, that the female gametophyte serves as an attractive model system for studies of cell specification, cell-cell communication and programmed cell death (PCD) (Sprunck and Gross-Hardt, 2011; Heydlauff and Groß-Hardt, 2014; Susaki et al., 2015). The knowledge that we generate is to a large extent dictated by the available techniques and resources. Accordingly, several of the new insights that we gained on development and function of the female gametophyte have been triggered by the establishment or adaptation of tools for the analysis of reproductive tissue. In light of these technical advances, we here discuss recent results and concepts that shed light on the complex development and function of flowering plant embryo sacs with an explicit focus on mechanisms underlying differentiation, patterning, and cell-cell communication.
Pollen tube guidance in flowering plants is a unique and critical process for successful sexual reproduction. The pollen tube that grows from pollen, which is the male gametophyte, precisely navigates to the embryo sac, which is the female gametophyte, within the pistil. Recent advances have clarified the molecular framework of gametophytic pollen tube guidance. Multiple species-specific attractant peptides are secreted from synergid cells, the proper development and function of which are regulated by female gametes. Multiple receptor-like kinases on the pollen tube tip are involved in sensing species-specific attractant peptides. In this Update article, recent progress in our understanding of the mechanism of gametophytic pollen tube guidance is reviewed, including attraction by synergid cells, control of pollen tube guidance by female gametes, and directional growth of the pollen tube by directional cue sensing. Future directions in the study of pollen tube guidance also are discussed. © 2017 American Society of Plant Biologists. All rights reserved.
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.
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.
In angiosperms, the conversion of an egg cell into a zygote involves two sequential gametic processes: plasmogamy, the fusion of the plasma membranes of male and female gametes, and karyogamy, the fusion of the gametic nuclei. In this study, the nuclei and nuclear membranes of rice (Oryza sativa L.) gametes were fluorescently labeled using histone H2B-GFP/RFP and SUN2-GFP, respectively, which were heterologously expressed. These gametes were fused in vitro to produce zygotes, and the nuclei and nuclear membranes in the zygotes were observed during karyogamy. The results indicated that the sperm nucleus migrates adjacent to the egg nucleus 5 to 10 min after plasmogamy via an actin cytoskelton, and the egg chromatin then appears to move unidirectionally into the sperm nucleus through a possible nuclear connection. The enlargement of sperm nucleus accompanies this possible chromatin remodeling. Then, 30 to 70 min after fusion, the sperm chromatin begins to decondense with the completion of karyogamy. Based on the present observations, the development of early rice zygotes from plasmogamy to karyogamy was divided into eight stages, and, using RT-PCR analyses, paternal and de novo synthesized transcripts were separately detected in zygotes at early and late karyogamy stages, respectively.
Our knowledge of pollen, the gold dust that carries the male germ line of flowering plants and is vital for sexual reproduction and seed formation, has 'come of age' with the publication of this book. Here, for the first time in a single volume, are all the ideas and techniques developed in the last two decades concerning the manipulation of pollen and pollen tubes in plant breeding and biotechnology. Pollen has never been an easy topic to come to grips with, with its variable and often inexplicable terminology that has made it a more difficult field in which to work. This book will remedy that, with its overview of pollen biology and pollen-pistil interactions that explains terms and concepts of the male function of pollen in a way that is readily understandable to the student and professional plant scientist and plant breeder.
Bouncer keeps fertilization specific
Fertilization needs to be highly efficient while remaining species-specific. However, despite decades of research, it is still unclear how these two requirements are met. Herberg et al. report the discovery of the Ly6/uPAR-type protein Bouncer as a species-specific fertilization factor in zebrafish (see the Perspective by Lehmann). Bouncer localizes to the egg membrane and is required for sperm entry. Remarkably, expression of Bouncer from another fish species (medaka) in zebrafish allowed for cross-species fertilization.
Science , this issue p. 1029 ; see also p. 974
Studies of the cytology of angiosperm pollen which extend our knowledge of the distribution of binucleate and trinucleate pollen to almost 2,000 species of flowering plants are summarized. Approximately 70% of the species studied release pollen in a binucleate stage, and none of them shed both types of pollen as a constant varietal trait. All phylogenetically primitive taxa are binucleate. The trinucleate trait evidently has originated independently at many times during angiosperm evolution. In no instance must one infer the origin of binucleate taxa from trinucleate ancestors. Most genera are monotypic with respect to pollen cytology, and only 10 genera are known to include both binucleate and trinucleate species. Among the 265 families studied, 179 include only binucleate genera and 54 include only trinucleate genera, while 32 include both types. Nearly all aquatic species with submersed flowers shed pollen in a trinucleate stage. A group of physiological differences which distinguish binucleate and trinucleate pollen is reviewed in relation to the survival and possible selective advantage of trinucleate mutants, and the significance of the binucleate pollen grain in the origin and evolution of flowering plants is discussed.
Pollen grains placed within longitudinal cuts in styles germinate and produce pollen tubes which grow equally well towards the stigma or the ovary. If there are simultaneous stigmatic pollinations, the growth of the intrastylar pollen tubes toward the stigma is significantly impeded. This observation indicates the presence of an influence which may or may not be related to pollen tube tropism, one which is inducible rather than constitutive.
Fertilization in flowering plants appears simple when compared to that in higher animals. In reality all pre-fertilization events involved in screening and selection of the partners, so familiar in animals, take place in a subtle way in flowering plants also. As plants lack mobility, they cannot perform, on their own, the most important and primary requirement of bringing the male (pollen grain) and the female (pistil) partners together. This process, termed pollination, is effectively outsourced largely to animal agents. Both plants and animals have evolved fascinating adaptations to do this, which is vital not only for their sustenance but also for crop productivity.
In flowers with dry stigmas, pollen development, pollination and pollen tube growth require spatial and temporal regulation of water and nutrient transport. To better understand the molecular mechanisms involved in reproductive processes, we characterized NIP4;1 and NIP4;2, two pollen-specific aquaporins (AQPs) of Arabidopsis thaliana. NIP4;1 and NIP4;2 are paralogs found exclusively in the angiosperm lineage. Although they have 84% amino acid identity, they displayed different expression patterns. NIP4;1 has low expression levels in mature pollen, while NIP4;2 expression peaks during pollen tube growth. Additionally, NIP4;1pro:GUS flowers showed GUS activity in mature pollen and pollen tubes, whereas NIP4;2pro:GUS flowers only in pollen tubes. Single T-DNA mutants and double amiRNA knockdowns had fewer seeds per silique and reduced pollen germination and pollen tube length. Functional assays in oocytes showed NIP4;1 and NIP4;2 function as water and non-ionic channels. We also found that NIP4;1 and NIP4;2 C-termini are phosphorylated by a pollen-specific CPK that modifies their water permeability. Transport assays in yeasts indicated that NIP4;1 also transports ammonia, urea, boric acid and H2O2. Thus, we propose that aquaporins NIP4;1 and NIP4;2 are exclusive components of the reproductive apparatus of angiosperms with partially redundant roles in pollen development and pollination.
Precise directional control of pollen-tube growth by pistil tissue is critical for successful fertilization of flowering plants [1-3]. Ovular attractant peptides, which are secreted from two synergid cells on the side of the egg cell, have been identified [4-6]. Emerging evidence suggests that the ovular directional cue is not sufficient for successful guidance but that competency control by the pistil is critical for the response of pollen tubes to the attraction signal [1, 3, 7]. However, the female molecule for this competency induction has not been reported. Here we report that ovular methyl-glucuronosyl arabinogalactan (AMOR) induces competency of the pollen tube to respond to ovular attractant LURE peptides in Torenia fournieri. We developed a method for assaying the response capability of a pollen tube by micromanipulating an ovule. Using this method, we showed that pollen tubes growing through a cut style acquired a response capability in the medium by receiving a sufficient amount of a factor derived from mature ovules of Torenia. This factor, named AMOR, was identified as an arabinogalactan polysaccharide, the terminal 4-O-methyl-glucuronosyl residue of which was necessary for its activity. Moreover, a chemically synthesized disaccharide, the β isomer of methyl-glucuronosyl galactose (4-Me-GlcA-β-(1→6)-Gal), showed AMOR activity. No specific sugar-chain structure of plant extracellular matrix has been identified as a bioactive molecule involved in intercellular communication. We suggest that the AMOR sugar chain in the ovary renders the pollen tube competent to the chemotropic response prior to final guidance by LURE peptides.
In flowering plants, sperm-containing pollen tubes are guided towards ovules by attractants from the female reproductive organ. Receptors for the attractant molecule AtLURE1 have now been found. See Letters p.241 & p.245
Directional control of tip-growing cells is essential for proper tissue organization and cell-to-cell communication in animals and plants. In the sexual reproduction of flowering plants, the tip growth of the male gametophyte, the pollen tube, is precisely guided by female cues to achieve fertilization. Several female-secreted peptides have recently been identified as species-specific attractants that directly control the direction of pollen tube growth. However, the method by which pollen tubes precisely and promptly respond to the guidance signal from their own species is unknown. Here we show that tip-localized pollen-specific receptor-like kinase 6 (PRK6) with an extracellular leucine-rich repeat domain is an essential receptor for sensing of the LURE1 attractant peptide in Arabidopsis thaliana under semi-in-vivo conditions, and is important for ovule targeting in the pistil. PRK6 interacted with pollen-expressed ROPGEFs (Rho of plant guanine nucleotide-exchange factors), which are important for pollen tube growth through activation of the signalling switch Rho GTPase ROP1 (refs 7, 8). PRK6 conferred responsiveness to AtLURE1 in pollen tubes of the related species Capsella rubella. Furthermore, our genetic and physiological data suggest that PRK6 signalling through ROPGEFs and sensing of AtLURE1 are achieved in cooperation with the other PRK family receptors, PRK1, PRK3 and PRK8. Notably, the tip-focused PRK6 accumulated asymmetrically towards an external AtLURE1 source before reorientation of pollen tube tip growth. These results demonstrate that PRK6 acts as a key membrane receptor for external AtLURE1 attractants, and recruits the core tip-growth machinery, including ROP signalling proteins. This work provides insights into the orchestration of efficient pollen tube growth and species-specific pollen tube attraction by multiple receptors during male-female communication.
Sexual reproduction requires recognition between the male and female gametes. In flowering plants, the immobile sperms are delivered to the ovule-enclosed female gametophyte by guided pollen tube growth. Although the female gametophyte-secreted peptides have been identified to be the chemotactic attractant to the pollen tube, the male receptor(s) is still unknown. Here we identify a cell-surface receptor heteromer, MDIS1-MIK, on the pollen tube that perceives female attractant LURE1 in Arabidopsis thaliana. MDIS1, MIK1 and MIK2 are plasma-membrane-localized receptor-like kinases with extracellular leucine-rich repeats and an intracellular kinase domain. LURE1 specifically binds the extracellular domains of MDIS1, MIK1 and MIK2, whereas mdis1 and mik1 mik2 mutant pollen tubes respond less sensitively to LURE1. Furthermore, LURE1 triggers dimerization of the receptors and activates the kinase activity of MIK1. Importantly, transformation of AtMDIS1 to the sister species Capsella rubella can partially break down the reproductive isolation barrier. Our findings reveal a new mechanism of the male perception of the female attracting signals.
Pollen grains of Pinus wallichiana, P. mugo, P. ponderosa and Ephedra distachya germinated and 72 hr after pollination produced fully developed tubes on the in vitro cultured placentae of some species of Brassicaceae, Caryophyllaceae, Fabaceae, Liliaceae and Solanaceae families. Occasionally, pollen tubes were found entering the micropyles. Pollen grains of the same gymnosperm species did not form tubes on stigmas of pistils cultured under the same conditions as were the placentae. Distant pollination of placentae among some species of Araceae, Asclepiadaceae, Brassicaceae, Caryophyllaceae, Fabaceae, Liliaceae and Solanaceae families showed the ability of pollen to germinate and form tubes several hours after pollination. In several combinations of crosses, pollen tubes were found in the micropyle. Embryological analysis of such ovules performed 48-72 hr after pollination demonstrated the presence of pollen tubes inside the embryo sacs and, in some of them the development of globular proembryos.
Sexual reproduction in flowering plants occurs by the processes of pollination and fertilization. Pollination involves the transfer of the pollen grain (the male gametophyte) to the stigma, its adhesion, hydration, and production of a pollen tube. The pollen tube extends through the gynecium (stigma, style, and ovary), carrying the sperm cells that are delivered ultimately to the embryo sac (the female gametophyte) within the ovule. The extension of the pollen tube follows a defined secretory path within the gynecium called the “transmitting tract” or “stylar matrix.” Pollen tube extension is typically restricted to this specialized extracellular matrix (ECM). The stylar ECM that supports pollen tube growth is produced on the surface of the cells along which the pollen tube travels. The secretions produced by transmitting tract cells are frequently described as mucilage or as a fluid composed of various ingredients. In general, these include sugars, amino acids, peptides, phenolic compounds, fatty acids, lipids, and glycolipids.
Flowering plants show great variation in their size, longevity and morphology. They may be herbs or shrubs or trees or climbers or creepers or epiphytes. Their life span is also variable; they may be annuals or biennials or perennials. Annuals complete their life cycle in 1 year; they produce seeds towards the end of their life cycle and die. Biennials devote the first year to vegetative growth building up the reserve of resources and enter reproductive phase in the second year; after seed production they die. Perennials live for many years. When once they reach a required age (depending on the species), perennials enter reproductive phase repeatedly. In some perennial species, particularly the tree species, flowering does not occur every year; they flower in alternate years or at intervals of several years. Also the extent of flowering and fruiting may not be the same in each flowering season; some years they produce large seed crops alternating with years of lean or no seed production. In some such species, all the individuals of a population tend to synchronize their flowering event resulting in a bumper crop (masting) in years of large seed production (Kelly 1994; Fenner and Thompson 2005; Sakai et al. 2005).
Inhibition of flower pigmentation in transgenic petunia plants was previously accomplished by expressing an antisense chalcone synthase (chs) gene under the control of the cauliflower mosaic virus (CaMV) 35S promoter. This chimeric gene was not effective in inhibiting pigmentation in anthers, presumably because the viral CaMV 35S promoter was insufficiently expressed in cell types of this organ in which the pigments are produced. Insertion of the anther box, a homologous sequence found in other genes expressed in anthers, resulted in a modified expression pattern driven by this promoter, as monitored by the beta-glucuronidase (gus) gene. In addition to the basic CaMV 35S expression pattern in anthers, GUS activity was observed in tapetum cells when the modified promoter was fused to the gus gene. This promoter construct was subsequently used to drive an antisense chs gene in transgenic petunia, which led to the inhibition of pigment synthesis in anthers of five of 35 transformants. Transgenic plants with white anthers were male sterile due to an arrest in male gametophyte development. This finding indicated that flavonoids play an essential role in male gametophyte development.
In flowering plants, fertilization-dependent degeneration of the persistent synergid cell ensures one-on-one pairings of male and female gametes. Here, we report that the fusion of the persistent synergid cell and the endosperm selectively inactivates the persistent synergid cell in Arabidopsis thaliana. The synergid-endosperm fusion causes rapid dilution of pre-secreted pollen tube attractant in the persistent synergid cell and selective disorganization of the synergid nucleus during the endosperm proliferation, preventing attractions of excess number of pollen tubes (polytubey). The synergid-endosperm fusion is induced by fertilization of the central cell, while the egg cell fertilization predominantly activates ethylene signaling, an inducer of the synergid nuclear disorganization. Therefore, two female gametes (the egg and the central cell) control independent pathways yet coordinately accomplish the elimination of the persistent synergid cell by double fertilization.
Copyright © 2015 Elsevier Inc. All rights reserved.
The ultrastructural organization of the megagametophyte of Plumbago zeylanica, which lacks synergids, was examined in chemically and physically fixed ovules after entry of the pollen tube. Similar to angiosperms with conventionally organized megagametophytes, the pollen tube enters the ovule through a micropyle, formed by the inner integument, and approaches the female gametophyte by growing between nucellar cells. Unlike other described female gametophytes, however, continued pollen tube growth results in direct penetration of the base of the egg through cell wall projections forming a filiform apparatus and is completed between the egg and central cell without disrupting either of these cells' plasma membranes. A terminal pollen tube aperture forms when the pollen tube reaches an area of strong curvature near the summit of the egg; this results in the release of two sperm cells, the vegetative nucleus, and a limited amount of pollen cytoplasm. The formerly continuous chalazal egg cell wall is locally disrupted near the tip of the pollen tube and apparently is thus modified for reception of male gametes. Discharged pollen cytoplasm rapidly degenerates between the egg and central cell, but unlike pollen tube discharge in conventionally organized megagametophytes, it is unassociated with the degeneraton of any receptor cell within the female gametophyte. Sperm nuclei are transmitted, one to the egg and the other to the central cell, to effect double fertilization by nuclear fusion with their respective female reproductive nuclei. The vegetative nucleus and discharged pollen cytoplasm degenerate between the developing embryo and endosperm during early embryogenesis. The emerging concept that the egg of Plumbago possesses combined egg and synergid functions is supported by the present study and suggests that the megagametophyte of this plant displays a highly specialized egg apparatus composed exclusively of a single, modified egg cell.
The female gametophytes of many flowering plants contain one egg cell, one central cell, two synergid cells, and three antipodal cells with respective morphological characteristics and functions. These cells are formed by cellularization of a multinuclear female gametophyte. However, the dynamics and mechanisms of female gametophyte development remain largely unknown due to the lack of a system to directly visualize and manipulate female gametophytes in living material. Here, we established an in vitro ovule culture system to examine female gametophyte development in Torenia fournieri, a unique plant species with a protruding female gametophyte. The four-nucleate female gametophyte became eight nucleate by the final (third) mitosis and successively cellularized and matured to attract a pollen tube. The duration of final mitosis was 28 ± 6.5 min, and cellularization was completed in 54 ± 20 min after the end of the third mitosis. Fusion of polar nuclei in the central cell occurred in 13.1 ± 1.1 h, and onset of LURE2 gene expression, a pollen tube attractant gene, was visualized by a green fluorescent protein reporter 10.7 ± 2.3 h after cellularization. Laser disruption analysis demonstrated that the egg and central cells were required for synergid cells to acquire the pollen tube attraction function. Moreover, aberrant nuclear positioning and downregulation of LURE2 were observed in one of the two synergid cells after disrupting an immature egg cell, suggesting that cell specification was affected. Our system provides insights into the precise dynamics and mechanisms of female gametophyte development in T. fournieri.
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During sexual reproduction of flowering plants, pollen tube guidance by pistil tissue is critical for the delivery of nonmotile sperm cells to female gametes. Multistep controls of pollen tube guidance can be divided into two phases: preovular guidance and ovular guidance. During preovular guidance, various female molecules, including stimulants for pollen germination and pollen tube growth, are provided to support tube growth toward the ovary, where the ovules are located. After entering the ovary, pollen tubes receive directional cues from their respective target ovules, including attractant peptides for precise, species-preferential attraction. Successful pollen tube guidance in the pistil requires not only nutritional and directional controls but also competency controls to make pollen tubes responsive to guidance cues, regulation to terminate growth once a pollen tube arrives at the target, and strategies to stop ovular attraction depending on the fertilization of female gametes. Expected final online publication date for the Annual Review of Plant Biology Volume 66 is April 29, 2015. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Premise of the study:
Factors affecting floral receptivity in angiosperms remain opaque, but recent studies suggest that the acquisition of stigmatic receptivity associated with cell-wall-related arabinogalactan proteins (AGPs) may be a widespread feature of flowering plants. Here, the time during which a stigma is receptive is evaluated and related to the secretion of AGPs in Magnolia virginiana, a protogynous member of an early-divergent angiosperm clade (magnoliids) with a clearly discernible female receptive phase.
Methods:
Magnolia virginiana flower phenology was documented, and histochemical changes in the stigma before and after pollination were examined. Stigmatic receptivity was evaluated in relation to the secretion of AGPs detected in whole mounts and immunolocalized in sectioned stigmas.
Key results:
Protogynous Magnolia flowers had a precise window of stigmatic receptivity, which is concomitant with the secretion of two AGPs labeled for different epitopes. After pollen germination and tube growth, these two AGPs could no longer be detected in the stigmas, suggesting that these AGPs interact with the growing male gametophytes and could be markers of stigmatic receptivity.
Conclusions:
These results show that the period of stigmatic receptivity is finely coordinated with the secretion of two arabinogalactan proteins on stigmas of flowers of M. virginiana. This first report of AGP presence in stigmatic tissues in a member of the magnoliids, together with recently described similar patterns in eudicots, monocots, and members of early-divergent lineages of flowering plants, suggests an ancient and widespread role for AGPs on stigmatic receptivity in angiosperms.
By use of chlorotetracycline and X-ray microanalysis it is demonstrated that the receptive surface of the stigma of Ruscus aculeatus is rich in calcium. The high level of calcium is found in the epidermal cells and in the exudate covering the stigma. These results indicate that in vivo, as in vitro, calcium takes part in the regulation of pollen grain germination.
Generally, the life of sexual organisms starts from a single cell, the fertilized egg cell or zygote. By cell division and growth this single cell finally gives rise to the mature organism, which contains different cell types, tissues and organs. The zygote, the progenitor cell is the origin for the formation of cells of different developmental fates and the main body axes. In flowering plants (angiosperms) little is known about the underlying mechanisms of these processes and, in spite of its fundamental importance, regulation of early embryonic development is only poorly understood. New data suggest that the asymmetric division of the zygote separates determinants of apical and basal cell fates and that programmes of transcription are initiated in the domains of single cells of the early embryo. In this context, we describe results obtained by the use of powerful tools of in vitro fertilization and micromanipulation techniques for the elucidation of mechanisms of early embryonic patterning in higher plants.
The grasses are unusual among flowering plants in having rapid pollen germination on the stigma and a subsequent rapid rate of tube growth. This, and the structural simplicity of the stigma tissues, makes them apt subjects for the investigation of pollen-pistil interactions. Several aspects of these interactions are reviewed, including (a) the hydrodynamics of pollen in the period immediately preceding germination; (b) the responses of stigma cells to pollen contact; (c) the function of intine-held proteins during germination and the organisation of the pollen tube tip; (d) cuticle penetration; (e) tube growth, wall synthesis, and enzyme emission; and (0 the cytology and cytochemistry of the transmitting tract.Some features of the physiology of incompatibility systems in wild and cultivated grasses are discussed.
Summary The substitution of sucrose in the nutrient medium for in vitro pollinated placentae by glucose and fructose favours seed formation and considerably enhances the germination of seeds just attaining maturity. This effect of invert sugar was wholly suppressed in the mixture with two parts of sucrose.
This paper reports upon a study of the characteristics of the receptive surfaces of angiosperm stigmas, covering almost 1000 species of about 900 genera of some 250 families. In both monocotyledons and dicotyledons the major subdivision is into those stigmas which are ‘dry’ at maturity, having a hydrated, proteinaceous extracuticular layer or pellicle but no free-flowing secretion, and those which are ‘wet’ bearing such a fluid secretion when in the receptive state. Further subdivision may be made talcing into account the nature of the surface cells—whether or not the receptive surface is papillate or smooth, and if papillate, whether the papillae are unicellular or multicellular. When the genera are classified according to these criteria, various taxonotnic regularities emerge. Many families are homogeneous in stigma type, but some prove to be diverse. Among these are families such as the Liliaceae, a fact that may have phylogenetic significance.
The physiological importance of stigma type is shown by the correlations that exist between the characteristics of the receptive surface and self-incompatibility system. Sporophytic self-incompatibility systems are associated with dry, papillate stigmas; most gametophytic systems with wet stigmas. Further relationships exist with pollen type and physiology. Trinucleate pollen, not readily germinated in vitro, tends to be associated with dry stigmas, while wet-stigma forms tend to have binucleate pollen, easily germinated in liquid or semi-solid media; binucleate pollen, however, occurs with both wet and dry stigmas.
Angiosperm pollen grains possess walls of remarkable structural complexity, and the architectural forms encountered are often sufficiently specific and consistent to be useful taxonomically. Lindley, von Mohl and others appreciated this systematic potential almost a century and a half ago, and today a comprehensive pollen taxonomy is taking shape with the publication of the World pollen flora founded by the late Professor Gunnar Erdtman. Surprisingly, until quite recently this interest in the taxonomic aspects of pollen wall morphology has not been matched by any great concern for the functional significance of even the most conspicuous features, such for example as the deep sculpturing so commonly found in the outer layer of the wall. When Erdtman spent a period in my laboratory in Belfast in the mid-1950s we talked at length about pollen and spore morphology and morphogenesis, but I do not recall that we seriously touched upon the adaptive meaning of the wall architecture. We need not have heen so blind, for there were already many suggestive leads. Wodehouse in his distinguished book of 1935 addressed himself not only to the problems of pollen wall morphogenesis, but noted also relations between structure and dispersal agency. German authors had thought along similar lines, and had already pointed to the functional importance of one type of pollen surface material, that which in some species acts as a binding agent ( Polenkitt ), holding grains together in groups and facilitating attachment to pollen vectors. Much earlier had appeared the work of Green (1894), who seems to have been the first to study the nature of pollen emissions. In an early application of substrate film methods, Green showed that intact moistened pollen grains released various hydrolytic enzymes, and surmised that these played some part in the pollen-stigma interaction, perhaps in germination and pollen-tube growth and nutrition. When Green wrote, Blackley’s famous work on the causes of hay fever was already 20 years old; and by the 1950s it was well established that the active constituents released by pollen were non-dialysable and predominantly protein in nature. Various authors commented on the rapidity with which protein exudates are released from pollen grains, and there was speculation on the sources of such mobile fractions; indeed the facts suggested the possibility that these, too, were held like Pollenkitt in sites at or near the surface.
Both compatible and incompatible pollen tubes growing from the ovarian to the stigmatic end of Lilium longiflorum style sections reached lengths not significantly different from similar tubes growing from the stigmatic to the ovarian end. Similarly, there was no significant difference in pollen tube lengths following interspecific pollinations of ovarian versus stigmatic ends of style sections. Neither the self- nor the interspecific-incompatibility reactions appear to be due to localized stylar barriers to pollen tube growth in L. longiflorum.
Despite the great importance for forest tree breeding, very limited knowledge is yet available about the breeding systems of forest trees. Where incompatibility has been studied in the hardwoods; patterns have been observed which confirm the general rules detected for other angiosperms. Self- and interspecific incompatibility at the level of pollen tube growth has been reported for example in Betula and Alnus. In Alnus one case of unilateral interspecific incompatibility has been found. Self- incompatibility has, so far, not been reported from the conifers. Interspecific incompatibility in the form of the arrestment of the pollen tube growth in the nucellus tissue has been observed in Picea and is particularly clear in Pinus crosses between the subgenera Haploxylon and Diploxylon, but also within the Diploxylon-group. The nature of the incompatibility mechanism is still unknown, but serological differences related to the behaviour in the crosses has been detected in birch and pine pollen. It is suggested that the complex polysaccharidic composition of the cell walls and membranes might form a specific stereochemical basis for the incompatibility reaction. The presence of a combination of self- pollination, polyembryony and genetic load is discussed as an alternative mechanism favouring outbreeding in the Gymnosperms.
Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of cross-talk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review we focus on inter-cellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), ROS/NO signaling and the second messenger Ca(2+) play center stage in most of these processes.
Proteins in pollen grain walls are essential for germination in poplars;
they also affect the incompatibility systems on which pollen tube growth
is dependent.
Double Delivery During Plant Fertilization
Double fertilization is a defining feature of flowering plants and involves two nonmotile male gametes (sperm cells) and two female gametes (egg cell and central cell). Both fertilization events are necessary for reproductive success. It is not clear how flowering plants ensure the reliable and on-time fusion of the two pairs of gametes, while preventing polyspermy. Sprunck et al. (p. 1093 ; see the Perspective by Snell ) now show that gamete interactions in Arabidopsis depend on small cysteine-rich EGG CELL 1 (EC1) proteins that accumulate in storage vesicles of the egg cell and that are released during sperm-egg interaction. EC1 peptides trigger the delivery of a fusogen to the sperm cell surface. An intercellular link connects the two sperm cells throughout the gamete fusion process and could play a role in preventing the spontaneous fusion of activated sperm cells.