The Egg Cell: Development and Role in Fertilization and Early Embryogenesis

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

F-actin regulates the polarized secretion of pollen tube attractants in Arabidopsis synergid cells

Pollen tube attraction is a key event of sexual reproduction in flowering plants. In the ovule, two synergid cells neighboring the egg cell control pollen tube arrival via the active secretion of attractant peptides such as AtLURE1 and XIUQIU from the filiform apparatus (FA) facing toward the micropyle. Distinctive cell polarity together with longitudinal F-actin and microtubules are hallmarks of the synergid cell in various species, though the functions of these cellular structures are unclear. In this study, we used genetic and pharmacological approaches to indicate the roles of cytoskeletal components in FA formation and pollen tube guidance in Arabidopsis thaliana. Genetic inhibition of microtubule formation reduced invaginations of the plasma membrane but did not abolish micropylar AtLURE1.2 accumulation. By contrast, the expression of a dominant-negative form of ACTIN8 induced disorganization of the FA and loss of polar AtLURE1.2 distribution toward the FA. Interestingly, after pollen tube reception, F-actin became unclear for a few hours in the persistent synergid cell, which may be involved in pausing and resuming pollen tube attraction during early polytubey block. Our data suggest that F-actin plays a central role in maintaining cell polarity and in mediating male-female communication in the synergid cell.

Plant egg cell fate determination depends on its exact position in female gametophyte

Plant fertilization involves both an egg cell, which fuses with a sperm cell, and synergid cells, which guide pollen tubes for sperm cell delivery. Therefore, egg and synergid cell functional specifications are prerequisites for successful fertilization. However, how the egg and synergid cells, referred to as the "egg apparatus," derived from one mother cell develop into distinct cell types remains an unanswered question. In this report, we show that the final position of the nuclei in female gametophyte determines the cell fate of the egg apparatus. We established a live imaging system to visualize the dynamics of nuclear positioning and cell identity establishment in the female gametophyte. We observed that free nuclei should migrate to a specific position before egg apparatus specialization. Artificial changing in the nuclear position on disturbance of the actin cytoskeleton, either in vitro or in vivo, could reset the cell fate of the egg apparatus. We also found that nuclei of the same origin moved to different positions and then showed different cell identities, whereas nuclei of different origins moved to the same position showed the same cell identity, indicating that the final positions of the nuclei, rather than specific nucleus lineage, play critical roles in the egg apparatus specification. Furthermore, the active auxin level was higher in the egg cell than in synergid cells. Auxin transport inhibitor could decrease the auxin level in egg cells and impair egg cell identity, suggesting that directional and accurate auxin distribution likely acts as a positional cue for egg apparatus specialization.

Female gametophyte and embryo development in Helleborus bocconei Ten. (Ranunculaceae)

In this study, we investigated cytohistochemistry, cycle progression, and relative DNA content of the female gametophyte cells of Helleborus bocconei Ten. before and after fertilization process. The early stages of embryo development were also investigated. H. bocconei possesses a monosporic seven-celled/eight-nucleate Polygonum type female gametophyte, characterized by a morpho-functional polarity. The cells of the embryo sac showed abundant reserves of polysaccharides, strongly increasing in the egg cell just before fertilization. With different timing in DNA replication during cell cycle progression, synergids, egg cells, and polar nuclei showed a haploid DNA content at the end of their differentiation, while antipodes underwent three DNA endoreduplication cycles. Programmed cell death symptoms were detectable in synergid and antipodal cells. After double fertilization, the central cell quickly underwent many mitotic cycles forming the endosperm, which exhibited a progressive increase in protein bodies and starch grains. Close to the developing embryo, the endosperm differentiated a well-defined region rich in a fibrillar carbohydrate matrix. The zygote, that does not start immediately to divide after double fertilization, developed in to an embryo that reached the heart stage at fruit maturation time. A weakly differentiated embryo at this time indicates a morpho-physiological dormancy of seeds, as a survival strategy imposed by the life cycle of this plant with seed dispersal in spring and their germination in the following winter.

KATANIN 1 Is Essential for Embryogenesis and Seed Formation in Arabidopsis

Cytoskeletal remodeling has a fundamental role, especially during transitional developmental stages when cells rapidly adopt new forms and roles, like gametogenesis, fertilization and concomitant embryogenesis and seed formation. KATANIN 1, a microtubule severing protein, fulfills a major regulatory mechanism of dynamic microtubule turnover in eukaryotes. Herein, we show that three well-established KATANIN 1 mutants, fra2, lue1 and ktn1-2 collectively display lower fertility and seed set in Arabidopsis. These lower fertility and seed set rates of fra2, lue1 and ktn1-2 mutants were correlated to abnormalities in the development of embryo proper and seed. Such phenotypes were rescued by transformation of mutants with functional pKTN1::GFP:KTN1 construct. This study significantly expands the already broad functional repertoire of KATANIN 1 and unravels its new role in embryo and seed development. Thus, KATANIN 1 significantly contributes to the fertility and proper embryo and seed formation in Arabidopsis.

Embryogenesis in Polianthes tuberosa L var. Simple: from megasporogenesis to early embryo development

The genus Polianthes belongs to the subfamily Agavoideae of the Asparagaceae family formerly known as Agavaceae. The genus is endemic to México and comprises about 15 species, among them is Polianthes tuberosa L. The aim of this work was to study and characterize the embryo sac and early embryo development of this species in order to generate basic knowledge for its use in taxonomy, in vitro fertilization and production of haploid plants and to complement studies already performed in other genera and species belonging to the Agavoideae sub-family. It was found that the normal development of the P. tuberosa var. Simple embryo sac follows a monosporic pattern of the Polygonum type and starts its development from the chalazal megaspore. At maturity, the embryo sac is of a pyriform shape with a chalazal haustorial tube where the antipodals are located, just below the hypostase, which connects the embryo sac with the nucellar tissue of the ovule. The central cell nucleus shows a high polarity, being located at the chalazal extreme of the embryo sac. The position of cells inside the P. tuberosa embryo sac may be useful for in depth studies about the double fertilization. Furthermore, it was possible to make a chronological description of the events that happen from fertilization and early embryo development to the initial development of the endosperm which was classified as of the helobial type.

TRANSLOCASE OF THE INNER MEMBRANE9 and 10 are essential for maintaining mitochondrial function during early embryo cell and endosperm free nucleus divisions in Arabidopsis

In the life cycle of flowering plants, the sporophytic generation takes up most of the time and plays a dominant role in influencing plant growth and development. The embryo cell and endosperm free nucleus divisions establish the critical initiation phase of early sporophyte development, which forms mature seeds through a series of cell growth and differentiation events. Here, we report on the biological functions of two Arabidopsis (Arabidopsis thaliana) mitochondrial proteins, TRANSLOCASE OF THE INNER MEMBRANE9 (TIM9) and TIM10. We found that dysfunction of either AtTIM9 or AtTIM10 led to an early sporophyte-lethal phenotype; the embryo and endosperm both arrest division when the embryo proper developed to 16 to 32 cells. The abortion of tim9-1 and tim10 embryos at the 16/32-cell stage was caused by the loss of cell viability and the cessation of division in the embryo proper region, and this inactivation was due to the collapse of the mitochondrial structure and activity. Our characterization of tim9-1 and tim10 showed that mitochondrial membrane permeability increased and that cytochrome c was released from mitochondria into the cytoplasm in the 16/32-cell embryo proper, indicating that mitochondrial dysfunction occurred in the early sporophytic cells, and thus caused the initiation of a necrosis-like programmed cell death, which was further proved by the evidence of reactive oxygen species and DNA fragmentation tests. Consequently, we verified that AtTIM9 and AtTIM10 are nonredundantly essential for maintaining the mitochondrial function of early embryo proper cells and endosperm-free nuclei; these proteins play critically important roles during sporophyte initiation and development in Arabidopsis.

Embryo sac formation and early embryo development in Agave tequilana (Asparagaceae)

Agave tequilana is an angiosperm species that belongs to the family Asparagaceae (formerly Agavaceae). Even though there is information regarding to some aspects related to the megagametogenesis of A. tequilana, this is the first report describing the complete process of megasporogenesis, megagametogenesis, the early embryo and endosperm development process in detail. The objective of this work was to study and characterize all the above processes and the distinctive morphological changes of the micropylar and chalazal extremes after fertilization in this species. The agave plant material for the present study was collected from commercial plantations in the state of Jalisco, Mexico. Ovules and immature seeds, previously fixed in FAA and kept in ethanol 70%, were stained based on a tissue clarification technique by using a Mayer’s-Hematoxylin solution. The tissue clarification technique was successfully used for the characterization of the megasporogenesis, megagametogenesis, mature embryo sac formation, the early embryo and endosperm development processes by studying intact cells. The embryo sac of A. tequilana was confirmed to be of the monosporic Polygonum-type and an helobial endosperm formation. Also, the time-lapse of the developmental processes studied was recorded.

NtProRP1, a novel proline-rich protein, is an osmotic stress-responsive factor and specifically functions in pollen tube growth and early embryogenesis in Nicotiana tabacum

Proline-rich proteins (PRPs) are known to play important roles in sexual plant reproduction. Most of the known proteins in the family were found in styles or pollen and modulate pollen tube growth. Here, we identified a novel member of the gene family, NtProRP1, which is preferentially expressed in tobacco pollen grains, pollen tubes and zygotes. NtProRP1 could be secreted into the extracellular space including the cell wall, and the predicted N-terminal signal peptide is crucial for its secretion. In NtProRP1-RNAi plants, pollen germination and pollen tube growth were significantly slower and showed zigzag or swell morphology in vitro. Early embryogenesis also exhibited aberrant development, indicative of its critical role in both pollen tube growth and early embryogenesis. Further investigation revealed that NtProRP1 plays a crucial role in osmotic stress response during pollen tube growth and is likely regulated by Tsi, a stress-responsive gene, suggesting that the regulatory mechanism is also involved in the stress response during sexual plant reproduction. These data provide evidence that NtProRP1 functions as a downstream factor of Tsi1 in the stress response and converges the stress signal into the modulation of pollen tube growth and early embryogenesis.

Pollen tube entry into the synergid cell of Arabidopsis is observed at a site distinct from the filiform apparatus

In higher plants, the double-fertilization process begins with the successful delivery of two sperm cells to the female gametophyte. The sperms cells are carried by a pollen tube that upon arrival at the micropylar end of the female gametophyte, bursts, and discharges its content into one of two specialized cells called the synergid cells. At their micropylar ends, both synergid cells form a thickened cell wall with a unique structure called the filiform apparatus. The filiform apparatus is believed to play a major role in pollen tube guidance and reception. It has also been assumed that the pollen tube enters the receptive synergid cell through the filiform apparatus. Here, we show that in Arabidopsis ovules, the arriving pollen tube appears to grow beyond the filiform apparatus to enter the synergid cell at a more distant site, where the tube bursts to release its contents. Thus, fertilization in Arabidopsis might involve two spatially and temporally separable stages, recognition and entry, with the latter apparently not requiring the filiform apparatus.

Identification of genes preferentially expressed in wheat egg cells and zygotes

KEY MESSAGE : Wheat genes differentially expressed in the egg cell before and after fertilization were identified. The data support zygotic gene activation before the first cell division in wheat. To have an insight into fertilization-induced gene expression, cDNA libraries have been prepared from isolated wheat egg cells and one-celled zygotes. Two-hundred and twenty-six egg cell and 253 zygote-expressed EST sequences were determined. Most of the represented transcripts were detected in the wheat egg cell or zygote transcriptome at the first time. Expression analysis of fourteen of the identified genes and three controls was carried out by real-time quantitative PCR. The preferential expression of all investigated genes in the female gametophyte-derived samples (egg cells, zygotes, two-celled proembryos, and basal ovule parts with synergids) in comparison to the anthers, and the leaves were verified. Three genes with putative signaling/regulatory functions were expressed at a low level in the egg cell but exhibited increased (2-to-33-fold) relative expression in the zygote and the proembryo. Genes with high EST abundance in cDNA libraries exhibited strong expression in the egg cell and the zygote, while the ones coding for unknown or hypothetical proteins exhibited differential expression patterns with preferential transcript accumulation in egg cells and/or zygotes. The obtained data support the activation of the zygotic genome before the first cell division in wheat.

Cytokinesis in plant male meiosis

In somatic cell division, cytokinesis is the final step of the cell cycle and physically divides the mother cytoplasm into two daughter cells. In the meiotic cell division, however, pollen mother cells (PMCs) undergo two successive nuclear divisions without an intervening S-phase and consequently generate four haploid daughter nuclei out of one parental cell. In line with this, the physical separation of meiotic nuclei does not follow the conventional cytokinesis pathway, but instead is mediated by alternative processes, including polar-based phragmoplast outgrowth and RMA-mediated cell wall positioning. In this review, we outline the different cytological mechanisms of cell plate formation operating in different types of PMCs and additionally focus on some important features associated with male meiotic cytokinesis, including cytoskeletal dynamics and callose deposition. We also provide an up-to-date overview of the main molecular actors involved in PMC wall formation and additionally highlight some recent advances on the effect of cold stress on meiotic cytokinesis in plants.

Mitochondrial GCD1 dysfunction reveals reciprocal cell-to-cell signaling during the maturation of Arabidopsis female gametes

Cell-to-cell communication in embryo sacs is thought to regulate the development of female gametes in flowering plants, but the details remain poorly understood. Here, we report a mitochondrial protein, GAMETE CELL DEFECTIVE 1 (GCD1), enriched in gametophytes that is essential for final maturation of female gametes. Using Arabidopsis gcd1 mutants, we found that final maturation of the egg and central cells is not required for double fertilization but is necessary for embryogenesis initiation and endosperm development. Furthermore, nonautonomous effects, observed when GCD1 or AAC2 function is disrupted, suggest that mitochondrial function influences reciprocal signaling between central and egg cells to regulate maturation of the partner (egg or central) cell. Our findings confirm that cell-to-cell communication is important in functional maturation of female gametic cells and suggest that both egg and central cells sense and transmit their mitochondrial metabolic status as an important cue that regulates the coordination of gamete maturation.

Molecular characterization of the glauce mutant: a central cell-specific function is required for double fertilization in Arabidopsis

Double fertilization of the egg cell and the central cell by two sperm cells, resulting in the formation of the embryo and the endosperm, respectively, is a defining characteristic of flowering plants. The Arabidopsis thaliana female gametophytic mutant glauce (glc) can exhibit embryo development without any endosperm. Here, we show that in glc mutant embryo sacs one sperm cell successfully fuses with the egg cell but the second sperm cell fails to fuse with the central cell, resulting in single fertilization. Complementation studies using genes from the glc deletion interval identified an unusual genomic locus having homology to BAHD (for BEAT, AHCT, HCBT, and DAT) acyl-transferases with dual transcription units and alternative splicing that could rescue the sterility defect of glc. Expression of these transcripts appears restricted to the central cell, and expression within the central cell is sufficient to restore fertility. We conclude that the central cell actively promotes its own fertilization by the sperm cell through a signaling mechanism involving products of At1g65450. Successful fertilization of the egg cell is not blocked in the glc mutant, suggesting that evolution of double fertilization in flowering plants involved acquisition of specific functions by the central cell to enable its role as a second female gamete.

Ribosomal RNA of Hyacinthus orientalis L. female gametophyte cells before and after fertilization

The nucleolar activity of Hyacinthus orientalis L. embryo sac cells was investigated. The distributions of nascent pre-rRNA (ITS1), 26S rRNA and of the 5S rRNA and U3 snoRNA were determined using fluorescence in situ hybridization (FISH). Our results indicated the different rRNA metabolism of the H. orientalis female gametophyte cells before and after fertilization. In the target cells for the male gamete, i.e., the egg cell and the central cell whose activity is silenced in the mature embryo sac (Pięciński et al. in Sex Plant Reprod 21:247-257, 2008; Niedojadło et al. in Planta doi: 10.1007/s00425-012-1599-9 , 2011), rRNA metabolism is directed at the accumulation of rRNPs in the cytoplasm and immature transcripts in the nucleolus. In both cells, fertilization initiates the maturation of the maternal pre-rRNA and the expression of zygotic rDNA. The resumption of rRNA transcription observed in the hyacinth zygote indicates that in plants, there is a different mechanism for the regulation of RNA Pol I activity than in animals. In synergids and antipodal cells, which have somatic functions, the nucleolar activity is correlated with the metabolic activity of these cells and changes in successive stages of embryo sac development.

The MADS29 transcription factor regulates the degradation of the nucellus and the nucellar projection during rice seed development

The MADS box transcription factors are critical regulators of rice (Oryza sativa) reproductive development. Here, we here report the functional characterization of a rice MADS box family member, MADS29, which is preferentially expressed in the nucellus and the nucellar projection. Suppressed expression of MADS29 resulted in abnormal seed development; the seeds were shrunken, displayed a low grain-filling rate and suppressed starch biosynthesis, and contained abnormal starch granules. Detailed analysis indicated that the abnormal seed development is due to defective programmed cell death (PCD) of the nucellus and nucellar projection, which was confirmed by a TUNEL assay and transcriptome analysis. Further studies showed that expression of MADS29 is induced by auxin and MADS29 protein binds directly to the putative promoter regions of genes that encode a Cys protease and nucleotide binding site-Leu-rich repeat proteins, thereby stimulating the PCD. This study identifies MADS29 as a key regulator of early rice seed development by regulating the PCD of maternal tissues. It provides informative clues to elucidate the regulatory mechanism of maternal tissue degradation after fertilization and to facilitate the studies of endosperm development and seed filling.

The MADS29 transcription factor regulates the degradation of the nucellus and the nucellar projection during rice seed development

The MADS box transcription factors are critical regulators of rice (Oryza sativa) reproductive development. Here, we here report the functional characterization of a rice MADS box family member, MADS29, which is preferentially expressed in the nucellus and the nucellar projection. Suppressed expression of MADS29 resulted in abnormal seed development; the seeds were shrunken, displayed a low grain-filling rate and suppressed starch biosynthesis, and contained abnormal starch granules. Detailed analysis indicated that the abnormal seed development is due to defective programmed cell death (PCD) of the nucellus and nucellar projection, which was confirmed by a TUNEL assay and transcriptome analysis. Further studies showed that expression of MADS29 is induced by auxin and MADS29 protein binds directly to the putative promoter regions of genes that encode a Cys protease and nucleotide binding site-Leu-rich repeat proteins, thereby stimulating the PCD. This study identifies MADS29 as a key regulator of early rice seed development by regulating the PCD of maternal tissues. It provides informative clues to elucidate the regulatory mechanism of maternal tissue degradation after fertilization and to facilitate the studies of endosperm development and seed filling.

Abnormal endosperm development causes female sterility in rice insertional mutant OsAPC6

A T-DNA insertional mutant OsAPC6 of rice, with gibberellic acid insensitivity and reduced height, had up to 45% reduced seed set. The insertion occurred on chromosome 3 of rice in the gene encoding one of the subunits of anaphase promoting complex/Cyclosome APC6. The primary mother cells of the mutant plants had normal meiosis, male gametophyte development and pollen viability. Confocal laser scanning microscopic (CLSM) studies of megagametophyte development showed abnormal mitotic divisions with reduced number or total absence of polar nuclei in about 30-35% megagametophytes of OsAPC6 mutant leading to failure of endosperm and hence embryo and seed development. Abnormal female gametophyte development, high sterility and segregation of tall and gibberellic acid sensitive plants without selectable marker Hpt in the selfed progeny of OsAPC6 mutant plants indicate that the mutant could be maintained in heterozygous condition. The abnormal mitotic divisions during megagametogenesis could be attributed to the inactivation of the APC6/CDC16 of anaphase promoting complex of rice responsible for cell cycle progression during megagametogenesis. Functional validation of the candidate gene through transcriptome profiling and RNAi is in progress.

Nuclear behavior, cell polarity, and cell specification in the female gametophyte

In flowering plants, the haploid gamete-forming generation comprises only a few cells and develops within the reproductive organs of the flower. The female gametophyte has become an attractive model system to study the genetic and molecular mechanisms involved in pattern formation and gamete specification. It originates from a single haploid spore through three free nuclear division cycles, giving rise to four different cell types. Research over recent years has allowed to catch a glimpse of the mechanisms that establish the distinct cell identities and suggests dynamic cell-cell communication to orchestrate not only development among the cells of the female gametophyte but also the interaction between male and female gametophytes. Additionally, cytological observations and mutant studies have highlighted the importance of nuclei migration- and positioning for patterning the female gametophyte. Here we review current knowledge on the mechanisms of cell specification in the female gametophyte, emphasizing the importance of positional cues for the establishment of distinct molecular profiles.

Germline specification and function in plants

The flowering plant germline is produced during the haploid gametophytic stage. Defining the germline is complicated by the extreme reduction of the male and female gametophytes, also referred to as pollen and embryo sac, respectively. Both male and female gamete progenitors are segregated by an asymmetric cell division, as is the case for the germline in animals. Genetic studies and access to the transcriptome of isolated gametes have provided a regulatory framework for the mechanisms that define the male germline. What specifies female germline identity remains unknown. Recent evidence indicates that an auxin gradient provides positional information and plays a role in defining the identity of the female gamete lineage. The animal germline is also marked by production of small RNAs, and recent evidence indicates that this trait might be shared with the plant gamete lineage.

Dynamic changes of transcript profiles after fertilization are associated with de novo transcription and maternal elimination in tobacco zygote, and mark the onset of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is characterized by the turnover of zygote development from maternal to zygotic control, and has been extensively studied in animals. A majority of studies have suggested that early embryogenesis is maternally controlled and that the zygotic genome remains transcriptionally inactive prior to the MZT. However, little is known about the MZT in higher plants, and its timing and impact remain uncharacterized. Here, we constructed cDNA libraries from tobacco (Nicotiana tabacum) egg cells, zygotes and two-celled embryos for gene expression profiling analysis, followed by RT-PCR confirmation. These analyses, together with experiments using zygote microculture coupled with transcription inhibition, revealed that a marked change in transcript profiles occurs approximately 50 h after fertilization, and that the MZT is initiated prior to zygotic division in tobacco. Although maternal transcripts deposited in egg cells support several early developmental processes, they appear to be insufficient for zygotic polar growth and subsequent cell divisions. Thus, we propose that de novo transcripts are probably required to trigger embryogenesis in later zygotes in tobacco.

Gamete fusion site on the egg cell and autonomous establishment of cell polarity in the zygote

Gamete fusion activates the egg in animals and plants, and the gamete fusion site on the zygote might provide a possible cue for zygotic development and/or embryonic patterning. In angiosperms, a zygote generally divides into a two-celled proembryo consisting of an apical and a basal cell with different cell fates. This is a putative step in the formation of the apical-basal axis of the proembryo. We observed the positional relationship between the gamete fusion site and the division plane formed by zygotic cleavage using an in vitro fertilization system with rice gametes. There was no relationship between the gamete fusion site and the division plane leading to the two-celled proembryo. Thus, the gamete fusion site on the rice zygote does not appear to function as a determinant for positioning the zygote division plane, and the zygote apparently possesses autonomous potential to establish cell polarity along the apical-basal axis for its first cleavage.

What we have learned from transcript profile analyses of male and female gametes in flowering plants

Double fertilization is one of the predominant features of sexual reproduction in flowering plants but, because of the physical inaccessibility of gametes, the essential molecular mechanisms in these processes are largely unknown. Based on the techniques for isolating highly purified gametes from several species and well-developed methods for manipulating RNA from limited quantities of gametes, genome-wide investigations of gamete transcription profiles were recently conducted in flowering plants. In this review, we survey the accumulated knowledge on gamete collection and purification, cDNA library construction, and transcript profile analysis to assess our understanding of the molecular mechanisms of gamete specialization and fertilization.

Positional relationship between the gamete fusion site and the first division plane in the rice zygote

In angiosperms, a zygote generally divides into a two-celled proembryo consisting of an apical and a basal cell that possess different cell fates. This first division of the zygote is a putative step in the formation of the apical-basal axis of the proembryo. The gamete fusion activates the egg, and the gamete fusion site on the zygote has been reported to provide a possible cue for subsequent zygotic development and/or embryonic patterning in animals and plants. In this study, the gamete fusion site on the rice zygote was labelled by in vitro fertilization of a rice egg cell with a fluorescence-stained sperm cell. The positional relationship between the gamete fusion site and the division plane formed by zygotic cleavage was monitored using a fixed culture of the fusion site-labelled zygote until the two-celled proembryo stage. The results indicate that gamete fusion sites exist on two-celled proembryos with no relation to the position of the first division plane, and that the gamete fusion site on the rice zygote does not function as a determinant for positioning the zygote division plane.

Sex-biased lethality or transmission of defective transcription machinery in Arabidopsis

Unlike animals, whose gametes are direct products of meiosis, plant meiotic products undergo additional rounds of mitosis, developing into multicellular haploid gametophytes that produce egg or sperm cells. The complex development of gametophytes requires extensive expression of the genome, with DNA-dependent RNA polymerases I, II, and III being the key enzymes for nuclear gene expression. We show that loss-of-function mutations in genes encoding key subunits of RNA polymerases I, II, or III are not transmitted maternally due to the failure of female megaspores to complete the three rounds of mitosis required for the development of mature gametophytes. However, male microspores bearing defective polymerase alleles develop into mature gametophytes (pollen) that germinate, grow pollen tubes, fertilize wild-type female gametophytes, and transmit the mutant genes to the next generation at moderate frequency. These results indicate that female gametophytes are autonomous with regard to gene expression, relying on transcription machinery encoded by their haploid nuclei. By contrast, male gametophytes make extensive use of transcription machinery that is synthesized by the diploid parent plant (sporophyte) and persists in mature pollen. As a result, the expected stringent selection against nonfunctional essential genes in the haploid state occurs in the female lineage but is relaxed in the male lineage.

How females become complex: cell differentiation in the gametophyte

In contrast to animals, gametes in plants form a separate haploid generation, the gametophyte. The female gametophyte of flowering plants consists of just four different cell types that play distinct roles in the reproductive process. Differentiation of the distinct cell fates is tightly controlled and appears to follow regional cues that are arranged along a polar axis. Mutant analysis suggests that important aspects of gametophyte patterning are gametophytically regulated. Additionally, structural and molecular changes following misspecification indicate that the female gametophyte is a remarkably versatile structure with enormous respecification potential. Recently, new tools have been developed that open fascinating possibilities to access and analyze those processes that ultimately ensure successful fertilization.

Cell-fate switch of synergid to egg cell in Arabidopsis eostre mutant embryo sacs arises from misexpression of the BEL1-like homeodomain gene BLH1

In Arabidopsis thaliana, the female gametophyte is a highly polarized structure consisting of four cell types: one egg cell and two synergids, one central cell, and three antipodal cells. In this report, we describe the characterization of a novel female gametophyte mutant, eostre, which affects establishment of cell fates in the mature embryo sac. The eostre phenotype is caused by misexpression of the homeodomain gene BEL1-like homeodomain 1 (BLH1) in the embryo sac. It is known that BELL-KNAT proteins function as heterodimers whose activities are regulated by the Arabidopsis ovate family proteins (OFPs). We show that the phenotypic effect of BLH1 overexpression is dependent upon the class II knox gene KNAT3, suggesting that KNAT3 must be expressed and functional during megagametogenesis. Moreover, disruption of At OFP5, a known interactor of KNAT3 and BLH1, partially phenocopies the eostre mutation. Our study indicates that suppression of ectopic activity of BELL-KNOX TALE complexes, which might be mediated by At OFP5, is essential for normal development and cell specification in the Arabidopsis embryo sac. As eostre-1 embryo sacs also show nuclear migration abnormalities, this study suggests that a positional mechanism might be directing establishment of cell fates in early megagametophyte development.

LACHESIS restricts gametic cell fate in the female gametophyte of Arabidopsis

In flowering plants, the egg and sperm cells form within haploid gametophytes. The female gametophyte of Arabidopsis consists of two gametic cells, the egg cell and the central cell, which are flanked by five accessory cells. Both gametic and accessory cells are vital for fertilization; however, the mechanisms that underlie the formation of accessory versus gametic cell fate are unknown. In a screen for regulators of egg cell fate, we isolated the lachesis (lis) mutant which forms supernumerary egg cells. In lis mutants, accessory cells differentiate gametic cell fate, indicating that LIS is involved in a mechanism that prevents accessory cells from adopting gametic cell fate. The temporal and spatial pattern of LIS expression suggests that this mechanism is generated in gametic cells. LIS is homologous to the yeast splicing factor PRP4, indicating that components of the splice apparatus participate in cell fate decisions.

The selection and specification of the egg cell determine the number of eggs produced by an animal or plant, which in turn dictates how many offspring that organism can produce. In most higher plants, the egg cell forms in a specialized structure consisting of four different cell types. Two cells, the egg cell and the central cell, are fertilized by sperm cells and develop into the embryo proper and the nutritive tissue (endosperm), respectively. These two gametic cells are flanked by accessory cells; but why do some cells become gametic while others differentiate into accessory cells? To answer this question, we looked for mutants in which this process is disturbed. In the lachesis mutant, accessory cells become extra egg cells. Interestingly, it seems that the misspecification of these accessory cells results from defects in the gametic cells. This suggests that accessory cells monitor the state of the gametic cells to act as a backup if required, ensuring the formation of the key reproductive cells.

In plant egg cells, gametophytes differentiate into both gametic and accessory cells; here the authors characterize a mutant that turns accessory cells into gametic cells.

Prunus necrotic ringspot virus Early Invasion and Its Effects on Apricot Pollen Grain Performance

ABSTRACT The route of infection and the pattern of distribution of Prunus necrotic ringspot virus (PNRSV) in apricot pollen were studied. PNRSV was detected both within and on the surface of infected pollen grains. The virus invaded pollen during its early developmental stages, being detected in pollen mother cells. It was distributed uniformly within the cytoplasm of uni- and bicellular pollen grains and infected the generative cell. In mature pollen grains, characterized by their triangular shape, the virus was located mainly at the apertures, suggesting that PNRSV distribution follows the same pattern as the cellular components required for pollen tube germination and cell wall tube synthesis. PNRSV also was localized inside pollen tubes, especially in the growth zone. In vitro experiments demonstrated that infection with PNRSV decreases the germination percentage of pollen grains by more than half and delays the growth of pollen tubes by approximately 24 h. However, although PNRSV infection affected apricot pollen grain performance during germination, the presence of the virus did not completely prevent fertilization, because the infected apricot pollen tubes, once germinated, were able to reach the apricot embryo sacs, which, in the climatic conditions of southeastern Spain, mature later than in other climates. Thus, infected pollen still could play an important role in the vertical transmission of PNRSV in apricot.

Synergid cell death in Arabidopsis is triggered following direct interaction with the pollen tube

During angiosperm reproduction, one of the two synergid cells within the female gametophyte undergoes cell death prior to fertilization. The pollen tube enters the female gametophyte by growing into the synergid cell that undergoes cell death and releases its two sperm cells within the degenerating synergid cytoplasm to effect double fertilization. In Arabidopsis (Arabidopsis thaliana) and many other species, synergid cell death is dependent upon pollination. However, the mechanism by which the pollen tube causes synergid cell death is not understood. As a first step toward understanding this mechanism, we defined the temporal relationship between pollen tube arrival at the female gametophyte and synergid cell death in Arabidopsis. Using confocal laser scanning microscopy, light microscopy, transmission electron microscopy, and real-time observation of these two events in vitro, we demonstrate that synergid cell death initiates after the pollen tube arrives at the female gametophyte but before pollen tube discharge. Our results support a model in which a signaling cascade triggered by pollen tube-synergid cell contact induces synergid cell death in Arabidopsis.

In situ localization of three cDNA sequences associated with the later stages of aposporic embryo sac development of Brachiaria brizantha

Brachiaria brizantha is a forage grass of African origin, highly cultivated in the Brazilian tropics for beef cattle production. We have analyzed the temporal and spatial expression of cDNA sequences by in situ hybridization in ovaries of apomictic and sexual plants. The studied sequences share molecular identity with myosin, aquaporin, and mitogen-activated protein kinase and were named BbrizMYO, BbrizAQP, and BbrizMAPK, respectively. BbrizMYO was expressed in apomictic and sexual embryo sacs, but somewhat later in the Polygonum type embryo sacs of sexual plants. BbrizAQP and BbrizMAPK transcripts were restricted to the Panicum type embryo sacs of apomictic plants; BbrizMAPK, in synergids; and BbrizAQP, also in different ovular cells during development. The common feature that arose from the analysis of the expression patterns of these three sequences was significant expression in the synergids. Their putative role in the maturation of Panicum type embryo sacs of apomictic plants and embryo development is discussed in view of the characteristics of apomictic reproduction.

MYB98 is required for pollen tube guidance and synergid cell differentiation in Arabidopsis

The synergid cells of the female gametophyte play a role in many steps of the angiosperm fertilization process, including guidance of pollen tube growth to the female gametophyte. However, the mechanisms by which the synergid cells become specified and develop their unique features during female gametophyte development are not understood. We identified MYB98 in a screen for Arabidopsis thaliana genes expressed in the female gametophyte. MYB98 is a member of the R2R3-MYB gene family, the members of which likely encode transcription factors. In the context of the ovule, MYB98 is expressed exclusively in the synergid cells, and mutations in this gene affect the female gametophyte specifically. myb98 female gametophytes are affected in two unique features of the synergid cell, pollen tube guidance and the filiform apparatus, but are otherwise normal. MYB98 also is expressed in trichomes and endosperm. Homozygous myb98 mutants exhibit no sporophytic defects, including trichome and endosperm defects. Together, these data suggest that MYB98 controls the development of specific features within the synergid cell during female gametophyte development.

Seed development and genomic imprinting in plants

Genomic imprinting refers to an epigenetic phenomenon where the activity of an allele depends on its parental origin. Imprinting at individual genes has only been described in mammals and seed plants. We will discuss the role imprinted genes play in seed development and compare the situation in plants with that in mammals. Interestingly, many imprinted genes appear to control cell proliferation and growth in both groups of organisms although imprinting in plants may also be involved in the cellular differentiation of the two pairs of gametes involved in double fertilization. DNA methylation plays some role in the control of parent-of-origin-specific expression in both mammals and plants. Thus, although imprinting evolved independently in mammals and plants, there are striking similarities at the phenotypic and possibly also mechanistic level.

Confocal microscopy of whole ovules for analysis of reproductive development: the elongate1 mutant affects meiosis II

Analysis of female meiosis (megasporogenesis) and embryo sac development (megagametogenesis) in angiosperms is technically challenging because the cells are enclosed within the nucellus and ovule tissues of the female flower. This is in contrast to male sporogenesis and gametogenesis where development can readily be observed through the easily dissectable developing anthers. Observation of embryo sac development is a particular problem in crassinucellate ovules such as those of maize. To overcome the problems in observing reproductive development, we developed a simple Feulgen staining procedure optimized for use with confocal microscopy to observe reproductive progression in the crassinucellate ovules of maize. The procedure greatly facilitates the observation of nuclei and cell structures of all stages of megasporogenesis and embryo sac development. The high resolution obtained using the technique enabled us to readily visualize chromosomes from individual cells within ovule tissue samples of maize. A propidium iodide staining technique was also used and compared with the Feulgen-based technique. Static cytometry of relative DNA content of individual nuclei was possible using Imaris software on both Feulgen and propidium iodide-stained samples. The techniques also proved successful for the observation of Arabidopsis and Hieracium aurantiacum female gametophyte and seed development, demonstrating the general applicability of the techniques. Using both staining methods, we analysed the maize meiotic mutant elongate1, which produces functional diploid instead of haploid embryo sacs. The precise defect in meiosis from which diploid embryo sacs arise in elongate1 has not previously been reported. We used confocal microscopy followed by static cytometry using Imaris software to show that the defect by which diploid embryo sacs arise in the maize mutant elongate1 is the absence of meiosis II with one of the dyad cells directly initiating megagametogenesis.

Eukaryotic cells and their cell bodies: Cell Theory revised

BACKGROUND

Cell Theory, also known as cell doctrine, states that all eukaryotic organisms are composed of cells, and that cells are the smallest independent units of life. This Cell Theory has been influential in shaping the biological sciences ever since, in 1838/1839, the botanist Matthias Schleiden and the zoologist Theodore Schwann stated the principle that cells represent the elements from which all plant and animal tissues are constructed. Some 20 years later, in a famous aphorism Omnis cellula e cellula, Rudolf Virchow annunciated that all cells arise only from pre-existing cells. General acceptance of Cell Theory was finally possible only when the cellular nature of brain tissues was confirmed at the end of the 20th century. Cell Theory then rapidly turned into a more dogmatic cell doctrine, and in this form survives up to the present day. In its current version, however, the generalized Cell Theory developed for both animals and plants is unable to accommodate the supracellular nature of higher plants, which is founded upon a super-symplasm of interconnected cells into which is woven apoplasm, symplasm and super-apoplasm. Furthermore, there are numerous examples of multinucleate coenocytes and syncytia found throughout the eukaryote superkingdom posing serious problems for the current version of Cell Theory.

SCOPE

To cope with these problems, we here review data which conform to the original proposal of Daniel Mazia that the eukaryotic cell is composed of an elemental Cell Body whose structure is smaller than the cell and which is endowed with all the basic attributes of a living entity. A complement to the Cell Body is the Cell Periphery Apparatus, which consists of the plasma membrane associated with other periphery structures. Importantly, boundary structures of the Cell Periphery Apparatus, although capable of some self-assembly, are largely produced and maintained by Cell Body activities and can be produced from it de novo. These boundary structures serve not only as mechanical support for the Cell Bodies but they also protect them from the hostile external environment and from inappropriate interactions with adjacent Cell Bodies within the organism.

CONCLUSIONS

From the evolutionary perspective, Cell Bodies of eukaryotes are proposed to represent vestiges of hypothetical, tubulin-based 'guest' proto-cells. After penetrating the equally hypothetical actin-based 'host' proto-cells, tubulin-based 'guests' became specialized for transcribing, storing and partitioning DNA molecules via the organization of microtubules. The Cell Periphery Apparatus, on the other hand, represents vestiges of the actin-based 'host' proto-cells which have become specialized for Cell Body protection, shape control, motility and for actin-mediated signalling across the plasma membrane.

gemini pollen 2, a male and female gametophytic cytokinesis defective mutation

Gametophytic cytokinesis is essential for the development and function of the male and female gametophytes. We have previously described the isolation and characterisation of the gemini pollen 1 (gem1) that acts gametophytically to disturb asymmetric division and cytokinesis at pollen mitosis I in Arabidopsis. Here we describe the genetic and cytological analysis of an independent gametophytic mutant, gem2, with similar characteristics to gem1, but which maps to a different genetic locus. gem2 shows reduced genetic transmission through both male and female gametes and leads to the production of divided or twin-celled pollen. Developmental analysis revealed that gem2 does not affect karyokinesis at pollen mitosis I, but leads to repositioning of the cell plate and partial or complete failure of cytokinesis, resulting in symmetrical divisions or binucleate pollen grains respectively. Symmetrical divisions lead to altered pollen cell fate with both sister cells displaying vegetative cell fate. Moreover, we demonstrate that the predominant female defect in gem2 is a lack of cellularization of the embryo sac during megagametogenesis. GEM2 therefore defines an independent genetic locus that is involved in the correct specification of both male and female gametophytic cytokinesis.

Development and function of the angiosperm female gametophyte

The plant life cycle alternates between a diploid sporophyte generation and a haploid gametophyte generation. The angiosperm female gametophyte is critical to the reproductive process. It is the structure within which egg cell production and fertilization take place. In addition, the female gametophyte plays a role in pollen tube guidance, the induction of seed development, and the maternal control of seed development. Genetic analysis in Arabidopsis has uncovered mutations that affect female gametophyte development and function. Mutants defective in almost all stages of development have been identified, and analysis of these mutants is beginning to reveal features of the female gametophyte developmental program. Other mutations that affect female gametophyte function have uncovered regulatory genes required for the induction of endosperm development. From these studies, we are beginning to understand the regulatory networks involved in female gametophyte development and function. Further investigation of the female gametophyte will require complementary approaches including expression-based approaches to obtain a complete profile of the genes functioning within this critical structure.

Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants

The monosporic seven-celled/eight-nucleate Polygonum-type female gametophyte has long served as a focal point for discussion of the origin and subsequent evolution of the angiosperm female gametophyte. In Polygonum-type female gametophytes, two haploid female nuclei are incorporated into the central cell, and fusion of a sperm cell with the binucleate central cell produces a triploid endosperm with a complement of two maternal and one paternal genomes, characteristic of most angiosperms. We document the development of a four-celled/four-nucleate female gametophyte in Nuphar polysepala (Engelm.) and infer its presence in many other ancient lineages of angiosperms. The central cell of the female gametophyte in these taxa contains only one haploid nucleus; thus endosperm is diploid and has a ratio of one maternal to one paternal genome. Based on comparisons among flowering plants, we conclude that the angiosperm female gametophyte is constructed of modular developmental subunits. Each module is characterized by a common developmental pattern: (1) positioning of a single nucleus within a cytoplasmic domain (pole) of the female gametophyte; (2) two free-nuclear mitoses to yield four nuclei within that domain; and (3) partitioning of three uninucleate cells adjacent to the pole such that the fourth nucleus is confined to the central region of the female gametophyte (central cell). Within the basal angiosperm lineages Nymphaeales and Illiciales, female gametophytes are characterized by a single developmental module that produces a four-celled/four-nucleate structure with a haploid uninucleate central cell. A second pattern, typical of Amborella and the overwhelming majority of eumagnoliids, monocots, and eudicots, involves the early establishment of two developmental modules that produce a seven-celled/eight-nucleate female gametophyte with two haploid nuclei in the central cell. Comparative analysis of ontogenetic sequences suggests that the seven-celled female gametophyte (two modules) evolved by duplication and ectopic expression of an ancestral Nuphar-like developmental module within the chalazal domain of the female gametophyte. These analyses indicate that the first angiosperm female gametophytes were composed of a single developmental module, which upon double fertilization yielded a diploid endosperm. Early in angiosperm history this basic module was duplicated, and resulted in a seven-celled/eight-nucleate female gametophyte, which yielded a triploid endosperm with the characteristic 2:1 maternal to paternal genome ratio.

Apomixis: a developmental perspective

The term apomixis encompasses a suite of processes whereby seeds form asexually in plants. In contrast to sexual reproduction, seedlings arising from apomixis retain the genotype of the maternal parent. The transfer of apomixis and its effective utilization in crop plants (where it is largely absent) has major advantages in agriculture. The hallmark components of apomixis include female gamete formation without meiosis (apomeiosis), fertilization-independent embryo development (parthenogenesis), and developmental adaptations to ensure functional endosperm formation. Understanding the molecular mechanisms underlying apomixis, a developmentally fascinating phenomenon in plants, is critical for the successful induction and utilization of apomixis in crop plants. This review draws together knowledge gained from analyzing ovule, embryo, and endosperm development in sexual and apomictic plants. It consolidates the view that apomixis and sexuality are closely interrelated developmental pathways where apomixis can be viewed as a deregulation of the sexual process in both time and space.

Fertilization in Arabidopsis thaliana wild type: developmental stages and time course

We describe some previously uncharacterised stages of fertilization in Arabidopsis thaliana and provide for the first time a precise time course of the fertilization process. We hand-pollinated wild type pistils with wild type pollen (Columbia ecotype), fixed them at various times after pollination, and analysed 600 embryo sacs using Confocal Laser Scanning Microscopy. Degeneration of one of the synergid cells starts at 5 Hours After Pollination (HAP). Polarity of the egg changes rapidly after this synergid degeneration. Karyogamy is then detected by the presence of two nucleoli of different diameters in both the egg and central cell nuclei, 7-8 HAP. Within the next hour, first nuclear division takes place in the fertilized central cell and two nucleoli can then be seen transiently in each nucleus produced. In a second set of experiments, we hand-pollinated wild type pistils with pollen from a transgenic promLAT52::EGFP line that expresses EGFP in its pollen vegetative cell. Release of the pollen tube contents into the synergid cell could be detected in living material. We show that the timing of synergid degeneration and pollen tube release correlate well, suggesting that either the synergid cell degenerates at the time of pollen tube discharge or very shortly before it. These observations and protocols constitute an important basis for the further phenotypic analysis of mutants affected in fertilization.

Control of early seed development

Seed development requires coordinated expression of embryo and endosperm and has contributions from both sporophytic and male and female gametophytic genes. Genetic and molecular analyses in recent years have started to illuminate how products of these multiple genes interact to initiate seed development. Imprinting or differential expression of paternal and maternal genes seems to be involved in controlling seed development, presumably by controlling gene expression in developing endosperm. Epigenetic processes such as chromatin remodeling and DNA methylation affect imprinting of key seed-specific genes; however, the identity of many of these genes remains unknown. The discovery of FIS genes has illuminated control of autonomous endosperm development, a component of apomixis, which is an important developmental and agronomic trait. FIS genes are targets of imprinting, and the genes they control in developing endosperm are also regulated by DNA methylation and chromatin remodeling genes. These results define some exciting future areas of research in seed development.

Choosing sides: establishment of polarity in zygotes of fucoid algae

The acquisition and expression of polarity during early embryogenesis underlies developmental pattern. In many multicellular organisms an initial asymmetric division of the zygote is critical to the determination of different cell fates of the early embryonic cells. Zygotes of the marine fucoid algae are initially apolar and become polarized in response to external cues. This results in an initial asymmetric division of the zygote. Subsequent divisions occur in a highly ordered spatial and temporal pattern. A combination of cell biological and biochemical studies is providing new details, and some controversies concerning the mechanisms by which zygotic polarity is acquired and amplified. Here, we discuss some of the more recent studies that are allowing improved understanding of polarization in this system.

Double fertilization in flowering plants: discovery, study methods and mechanisms

The double fertilization of flowering plants was discovered a century ago. The cytology of the gametes is now well known. However the description of the fertilization steps is still poor and most of the cellular and molecular mechanisms involved are unknown. Recent research using in vitro fertilization demonstrated that the early steps of fertilization share some homology with those in animal species. In particular, gamete fusion is followed by a cytosolic calcium increase in the fertilized egg as well as a calcium influx. Further understanding of fertilization also comes from the analysis of mutants isolated in Arabidopsis thaliana. Important new ideas have already emerged from these studies such as the importance of the female gametophyte in embryo development, and an early silencing of the male genome during the first days following gamete fusion.