Mature female gametophyte
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The above image is cropped, reduced, and labeled from gopher: Four celled stage of the female gametophyte of Lilium. The above image is reduced and cropped from gopher: Follow that link to view a larger image. Lilium eight-celled female gametophyte. Double Fertilization Back to Top The process of pollination being accomplished, the pollen tube grows Mature female gametophyte the stigma and style toward the ovules in the ovary. The germ cell in the pollen grain divides and releases two sperm cells which move down the pollen tube. Once the tip of the tube reaches the micropyle end of the embryo sac, the tube grows through into the embryo sac through one of the synergids which flank the egg.
One sperm cell fuses with the egg, producing the zygote which will later develope into the next-generation sporophyte. The second sperm fuses with the two polar bodies located in the center of the sac, producing the nutritive triploid endosperm tissue that will provide energy for the embryo's growth and development.
Pollen tube with haploid male gametophyte nuclei. The above image is cropped from http: Structure of the female gametophyte enbryo sac and the events approaching fertilization. Image from Purves et al. The events of "double fertilization" of the egg and polar nuclei by the two sperm cells. Stages of growth and development of the embryo. The protein has been associated with translation repression by microRNAs miRNAs 2021 and is also important to repress biosynthesis of cytokinins Crosstalk between cytokinin and auxin affects developmental modules in many parts of the plant; in many cases, the balance between these hormones is essential for correct patterning of cell types reviewed in It is possible that a specific ratio of auxin and cytokinin activity is needed for correct micropylar patterning and that AMP1 is necessary to maintain this balance Mature female gametophyte 2.
Detailed molecular characterization of AMP1 function in the ovule may shed light on the mechanism controlling synergid specification in gametophyte development. After cellularization, the synergid and egg cells presumably have acquired cell identity information. Despite this, cell ablation experiments in Arabidopsis and Torenia consistently show that loss of the egg cell causes morphological and marker line changes in at least one synergid, which takes on features of an egg cell and may even be fertilized 11 Similarly, mutants in which important cellular functions of the egg cell are disrupted also cause at least partial alteration of synergid identity 25 This suggests that the egg cell prevents its synergid neighbors from acquiring egg cell fate later in development through cell-to-cell communication Figure 2.
In contrast, in the ablation experiments, the central cell is not disrupted, can be fertilized, and does not take on aspects of egg cell morphology 24indicating that polar nuclei and the central cell are not subject to the same interaction. CKI1 expression, initially present at both poles through FG3, is quickly restricted to the two nuclei of the chalazal end at FG4. This polarity of expression is maintained through the next nuclear division FG5 stage. After the chalazal polar nucleus and its associated ER migrate toward the micropylar polar nucleus at stage FG5, CKI1 expression continues in the resulting diploid cell as well as in the antipodal cells at the chalazal end.
Ectopic expression of CKI1 is sufficient to induce central cell fate in the egg cell and synergids and produce seeds with multiple ectopic endosperms but lacking embryos. Therefore, CKI1 appears to specify central cell identity while restricting micropylar cell fates Figure 2.
Similar femalw the cytokinin Matuge, CKI1 acts through activation of a two-component signaling cascade, involving phosphorylation of Arabidopsis phosphotransfer proteins AHPswhich then activate downstream transcription factors. It is likely that the absence of cell membranes allows expansion of central cell and antipodal cell identity factors and suggests that CKI1 not only promotes chalazal identity but also a mechanism to limit that identity to the appropriate space. CKI1 is expressed in antipodal cells Marure is required for antipodal cell specification, as antipodal cells acquire egg cell attributes in cki1 mutants. At the same time, CKI1 does not alter antipodal cell fate when overexpressed; that is, antipodal cells are not re-specified as central cells This suggests that CKI1 action must be redirected in these cells femake an additional Maturr specification factor acting at the chalazal end Figure 2.
Such a factor could be supplied by chalazal sporophytic cells, as the antipodal cells are in close contact with this tissue. Gamrtophyte of a fluorescent protein ZsYellow from antipodals to neighboring maternal cells has been demonstrated 24suggesting a feemale connection between these cells that may allow movement of an identity signal. In Arabidopsis, the antipodal cells become inconspicuous and eventually degenerate after fertilization 8but gametlphyte maize and other grasses, the antipodal cells proliferate instead of diminishing, perhaps to facilitate nutrient transfer from the sporophyte to developing endosperm and embryo 23.
Proliferation of the antipodals in maize has been proposed to involve auxin signaling Maintenance of antipodal identity in the proliferating antipodal cells requires a secreted, grass-specific factor, ZmEAL1, that is synthesized in the egg cell; without this factor, the antipodals acquire central cell characteristics at low frequency Orthologs of ZmEAL1 can be found in other grasses but not in eudicots. In summary, in both Arabidopsis and maize, specification of antipodals requires additional factors but these factors are likely to be different, as suggested by the very different fate of the antipodal cells in grasses. Egg cell as the default state? Egg cell fate predominates in the absence of CKI1, as cki1 mutants fail to specify antipodals and central cells, and nuclei at the chalazal positions express egg cell markers instead Similarly, at the micropylar positions, there are a number of different mutants—such as amp1, eostre, lachesis, and yucca1 yucca2—in which synergids are not correctly specified and acquire egg cell fates instead 141825 To investigate cell type-specific processes in higher plants, root hairs and trichomes have been used as models, both for their physiological importance and their accessibility at the epidermal surface for details see below; Ishida et al.
In addition, starting with only a few examples at the beginning of the twenty-first century Kehr,cell type-specific transcriptional profiling has become a robust and frequently used method. In the model plant Arabidopsis thaliana, novel insights into plant development and cellular responses to environmental stimuli were for example gained through studies on individual cell types of the root, root hairs, trichomes, and guard cells, and by transcriptional profiling during male and female gametogenesis reviewed in Taylor-Teeples et al. These examples clearly illustrate the importance of cell type-specific investigations for a detailed understanding of differentiation processes and environmental responses of distinct cell types.
However, depending on the cell type under investigation, the currently available methods for cell isolation may be challenging, time-consuming, or limited to a subset of omics approaches e. While studies focusing on specific cell types, which can be isolated in quantities high enough for the full set of omics approaches, can serve as initial models for cell type-specific systems biology in plants Libault et al. Cell and tissue types frequently used for cell type-specific systems biology and omics studies in plants.
For the germlines, only the mature gametophytes are shown. Methods for the Acquisition of Large-scale Quantitative Data from Specific Cell Types Large-scale profiling of distinct cell types critically depends on the possibility to isolate these cells in sufficient purity and quantity, as well as the sensitivity and accuracy of the profiling methods. Despite the rapid improvements of established and novel tools for systems biology, the demand for fast and easily applicable methodologies for cell type-specific analyses is not yet satisfied. Further challenges are associated with the requirement for normalization and integration of different data types, and the increasing demand for platforms allowing storage and sharing of the rapidly growing amount of large-scale datasets reviewed by Chuang et al.
In brief, three steps are of great importance for cell type-specific systems biology: In the following sections, we will present current methods to acquire large-scale quantitative data required for systems biology. We will focus on methods allowing genome-wide cell type-specific analyses and present representative examples.
Gametophyte Mature female
For a discussion on the computational challenges in systems biology, the reader is referred to several recent reviews Ahrens et al. Methods for the Isolation of Specific Cell Types A few cell types in plants are exposed on the surfaces of tissues and can be collected gametopyte abrasion or mechanical detachment. Depending on the species, relatively simple Matuure isolation procedures for trichomes and root hairs enabled a large spectrum of gametopbyte. Mechanical isolation femalw trichomes allowed transcriptomics and metabolomics femzle various species for an integrated database see Dai et al. Another example for an exposed gaametophyte type are root hairs, for which gajetophyte simple isolation procedures gameto;hyte transcriptomics Libault et al.
Certain other cell types can be Mature female gametophyte by tissue disruption, followed by Mature female gametophyte methods or manual isolation of the dissociated cells femqle a microscope using a micropipette eventually with a marker for the cell type of gametophyye. Examples include specific cell types from fenale male or female reproductive lineages, plant mesophyll cells, and Matyre cells reviewed by Matture and Chen, ; Schmidt et al. Proteomic profiling has, for example, been performed on Brassica napus guard cells and mesophyll cells that could be purified as protoplasts Zhu et al. However, for most cell types these methods are not applicable.
Several methods for the isolation of specific cell types embedded in differentiated tissues have been established. Fluorescent Activated Cell Sorting FACS can be used to sort fluorescent cells based on their light scattering characteristics and fluorescence reviewed by Hu et al. This method allowed high resolution transcriptional profiling of different cell types in the Arabidopsis root, and, more recently, proteomics Petricka et al. They can only be applied if transgenic lines carrying cell type-specific fluorescent markers can be established, and they are thus not suitable for most non-model species. In addition, depending on the tissue type, longer enzymatic incubations are required to digest the cell walls and to release the protoplasted cells prior to sorting Evrard et al.
Consequently, changes in, for example, the transcriptome or metabolome cannot be fully excluded. This method is suitable to study epigenetic modifications DNA methylation of histone modifications and to profile the RNA within the nucleus. To study actively translated mRNAs bound to ribosomes translatomesmall epitope tags can be fused to a ribosomal protein to allow immunopurification of the ribosomes containing the mRNAs with a method named TRAP Translating Ribosome Affinity Purification; reviewed in Bailey-Serres, It has to be noted that the analyses of transcriptome and translatome abundance will not give the same results, because not all mRNAs present in a cell are actively translated at a given time point.
In this respect, profiling of mRNAs bound to ribosomes gives complementary results to transcriptome profiling as the readouts are closer to the synthesis of proteins Bailey-Serres, An alternative method not requiring any molecular knowledge is LAM Kerk et al. Plant tissues are thereby typically fixed and embedded in paraffin wax reviewed in Schmidt et al. Alternatively, the tissue may also be embedded in optimal cutting temperature compound for cryosectioning, followed by on-slide tissue dehydration and LAM Kelliher and Walbot, The main constraint of LAM is that harvesting sufficient material for downstream omics methods can be very time-consuming.
Furthermore, the suitability for single cell isolation depends on the optical resolution in sectioned tissues and the recognizability of the cell type of interest. In addition, the physical properties of the laser beam of the instrument used can impose limitations on which cell types can be isolated Schmidt et al. Thus, the time required for collecting enough material for one sample is largely dependent on the cell type of interest. So far, the applications of LAM for cell type-specific omics have been restricted to transcriptional profiling, e. However, other applications, such as genome wide profiling of DNA methylation, are likely feasible see below.
In rebound to spice, a plant tabloids much fewer suspect gametophytes, which are not blocked in the flyer floral tissue e. Garage Full Vision.
Methods for Data Acquisition 2. In Isoetes and Selaginellawhich are heterosporousthe megaspore remains attached to the parent sporophyte gamteophyte a highly reduced megagametophyte develops inside. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside. The gametophytes of Isoetes appear to be similar in this respect to those of gamettophyte extinct Carboniferous giant arborescent clubmosses, Lepidodendron and Lepidostrobus. The gametophytes develop into multicellular organisms while still enclosed within the spore wall, and the megaspores are retained within the sporangium.
Dioicous and Heterospory In plants with heteromorphic gametophytes, there are two distinct kinds of gametophytes. Because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg producing gametophyte is known as a megagametophyte, because it is typically larger, and the sperm producing gametophyte is known as a microgametophyte. The angiosperm gametophytes are composed of few cells and are embedded within the sexual organs of the flower. The female gametophyte develops within the ovule and generally consists of three antipodal cells, one central cell, two synergid cells, and one egg cell Figures 1A and 1B.
The female gametophyte is also commonly called the embryo sac or megagametophyte. The male gametophyte, also called the pollen grain or microgametophyte, develops within the anther and consists of two sperm cells encased within a vegetative cell Gifford and Foster,