Polyspermy produces tri-parental seeds in maize

Deep imaging reveals dynamics and signaling in one-to-one pollen tube guidance

In the pistil of flowering plants, each ovule usually associates with a single pollen tube for fertilization. This one-to-one pollen tube guidance, which contributes to polyspermy blocking and efficient seed production, is largely different from animal chemotaxis of many sperms to one egg. However, the functional mechanisms underlying the directional cues and polytubey blocks in the depths of the pistil remain unknown. Here, we develop a two-photon live imaging method to directly observe pollen tube guidance in the pistil of Arabidopsis thaliana, clarifying signaling and cellular behaviors in the one-to-one guidance. Ovules are suggested to emit multiple signals for pollen tubes, including an integument-dependent directional signal that reaches the inner surface of the septum and adhesion signals for emerged pollen tubes on the septum. Not only FERONIA in the septum but ovular gametophytic FERONIA and LORELEI, as well as FERONIA- and LORELEI-independent repulsion signal, are involved in polytubey blocks on the ovular funiculus. However, these funicular blocks are not strictly maintained in the first 45 min, explaining previous reports of polyspermy in flowering plants.

EGG CELL 1 contributes to egg-cell-dependent preferential fertilization in Arabidopsis

During double fertilization in angiosperms, the pollen tube delivers two sperm cells into an embryo sac; one sperm cell fuses with an egg cell, and the other sperm cell fuses with the central cell. It has long been proposed that the preference for fusion with one or another female gamete cell depends on the sperm cells and occurs during gamete recognition. However, up to now, sperm-dependent preferential fertilization has not been demonstrated, and results on preferred fusion with either female gamete have remained conflicting. To investigate this topic, we generated Arabidopsis thaliana mutants that produce single sperm-like cells or whose egg cells are eliminated; we found that although the three different types of sperm-like cell are functionally equivalent in their ability to fertilize the egg and the central cell, each type of sperm-like cell fuses predominantly with the egg cell. This indicates that it is the egg cell that controls its preferential fertilization. We also found that sperm-activating small secreted EGG CELL 1 proteins are involved in the regulation of egg-cell-dependent preferential fertilization, revealing another important role for this protein family during double fertilization.

Exploring Novel Polytubey Reproduction Pathways Utilizing Cumulative Genetic Tools

In the anthers and ovaries of flowers, pollen grains and embryo sacs are produced with uniform cell compositions. This stable gametogenesis enables elaborate interactions between male and female gametophytes after pollination, forming the highly successful sexual reproduction system in flowering plants. As most ovules are fertilized with a single pollen tube, the resulting genome set in the embryo and endosperm is determined in a single pattern by independent fertilization of the egg cell and central cell by two sperm cells. However, if ovules receive four sperm cells from two pollen tubes, the expected options for genome sets in the developing seeds would more than double. In wild-type Arabidopsis thaliana plants, around 5% of ovules receive two pollen tubes. Recent studies have elucidated the abnormal fertilization in supernumerary pollen tubes and sperm cells related to polytubey, polyspermy, heterofertilization and fertilization recovery. Analyses of model plants have begun to uncover the mechanisms underlying this new pollen tube biology. Here, we review unusual fertilization phenomena and propose several breeding applications for flowering plants. These arguments contribute to the remodeling of plant reproduction, a challenging concept that alters typical plant fertilization by utilizing the current genetic toolbox.

Plant biomacromolecule delivery methods in the 21st century

The 21st century witnessed a boom in plant genomics and gene characterization studies through RNA interference and site-directed mutagenesis. Specifically, the last 15 years marked a rapid increase in discovering and implementing different genome editing techniques. Methods to deliver gene editing reagents have also attempted to keep pace with the discovery and implementation of gene editing tools in plants. As a result, various transient/stable, quick/lengthy, expensive (requiring specialized equipment)/inexpensive, and versatile/specific (species, developmental stage, or tissue) methods were developed. A brief account of these methods with emphasis on recent developments is provided in this review article. Additionally, the strengths and limitations of each method are listed to allow the reader to select the most appropriate method for their specific studies. Finally, a perspective for future developments and needs in this research area is presented.

Conflict over fertilization underlies the transient evolution of reinforcement

When two species meet in secondary contact, the production of low fitness hybrids may be prevented by the adaptive evolution of increased prezygotic isolation, a process known as reinforcement. Theoretical challenges to the evolution of reinforcement are generally cast as a coordination problem, i.e., "how can statistical associations between traits and preferences be maintained in the face of recombination?" However, the evolution of reinforcement also poses a potential conflict between mates. For example, the opportunity costs to hybridization may differ between the sexes or species. This is particularly likely for reinforcement based on postmating prezygotic (PMPZ) incompatibilities, as the ability to fertilize both conspecific and heterospecific eggs is beneficial to male gametes, but heterospecific mating may incur a cost for female gametes. We develop a population genetic model of interspecific conflict over reinforcement inspired by "gametophytic factors", which act as PMPZ barriers among Zea mays subspecies. We demonstrate that this conflict results in the transient evolution of reinforcement-after females adaptively evolve to reject gametes lacking a signal common in conspecific gametes, this gamete signal adaptively introgresses into the other population. Ultimately, the male gamete signal fixes in both species, and isolation returns to pre-reinforcement levels. We interpret geographic patterns of isolation among Z. mays subspecies considering these findings and suggest when and how this conflict can be resolved. Our results suggest that sexual conflict over fertilization may pose an understudied obstacle to the evolution of reinforcement.

A Plastid-Bound Ankyrin Repeat Protein Controls Gametophyte and Early Embryo Development in Arabidopsis thaliana

Proplastids are essential precursors for multi-fate plastid biogenesis, including chloroplast differentiation, a powerhouse for photosynthesis in plants. Arabidopsis ankyrin repeat protein (AKRP, AT5G66055) is a plastid-localized protein with a putative function in plastid differentiation and morphogenesis. Loss of function of akrp leads to embryo developmental arrest. Whether AKRP is critical pre-fertilization has remained unresolved. Here, using reverse genetics, we report a new allele, akrp-3, that exhibited a reduced frequency of mutant embryos (<13%) compared to previously reported alleles. akrp-3 affected both male and female gametophytes resulting in reduced viability, incompetence in pollen tube attraction, altered gametic cell fate, and embryo arrest that were depleted of chlorophyll. AKRP is widely expressed, and the AKRP-GFP fusion localized to plastids of both gametophytes, in isolated chloroplast and co-localized with a plastid marker in pollen and pollen tubes. Cell-type-specific complementation of akrp-3 hinted at the developmental timing at which AKRP might play an essential role. Our findings provide a plausible insight into the crucial role of AKRP in the differentiation of both gametophytes and coupling embryo development with chlorophyll synthesis.

Consequences of whole genome duplication for 2n pollen performance

The vegetative cell of the angiosperm male gametophyte (pollen) functions as a free-living, single-celled organism that both produces and transports sperm to egg. Whole-genome duplication (WGD) should have strong effects on pollen because of the haploid to diploid transition and because of both genetic and epigenetic effects on cell-level phenotypes. To disentangle historical effects of WGD on pollen performance, studies can compare 1n pollen from diploids to neo-2n pollen from diploids and synthetic autotetraploids to older 2n pollen from established neo-autotetraploids. WGD doubles both gene number and bulk nuclear DNA mass, and a substantial proportion of diploid and autotetraploid heterozygosity can be transmitted to 2n pollen. Relative to 1n pollen, 2n pollen can exhibit heterosis due to higher gene dosage, higher heterozygosity and new allelic interactions. Doubled genome size also has consequences for gene regulation and expression as well as epigenetic effects on cell architecture. Pollen volume doubling is a universal effect of WGD, whereas an increase in aperture number is common among taxa with simultaneous microsporogenesis and pored apertures, mostly eudicots. WGD instantly affects numerous evolved compromises among mature pollen functional traits and these are rapidly shaped by highly diverse tissue interactions and pollen competitive environments in the early post-WGD generations. 2n pollen phenotypes generally incur higher performance costs, and the degree to which these are met or evolve by scaling up provisioning and metabolic vigor needs further study.

Comparative Embryogenesis in Angiosperms: Activation and Patterning of Embryonic Cell Lineages

Following fertilization in flowering plants (angiosperms), egg and sperm cells unite to form the zygote, which generates an entire new organism through a process called embryogenesis. In this review, we provide a comparative perspective on early zygotic embryogenesis in flowering plants by using the Poaceae maize and rice as monocot grass and crop models as well as Arabidopsis as a eudicot model of the Brassicaceae family. Beginning with the activation of the egg cell, we summarize and discuss the process of maternal-to-zygotic transition in plants, also taking recent work on parthenogenesis and haploid induction into consideration. Aspects like imprinting, which is mainly associated with endosperm development and somatic embryogenesis, are not considered. Controversial findings about the timing of zygotic genome activation as well as maternal versus paternal contribution to zygote and early embryo development are highlighted. The establishment of zygotic polarity, asymmetric division, and apical and basal cell lineages represents another chapter in which we also examine and compare the role of major signaling pathways, cell fate genes, and hormones in early embryogenesis. Except for the model Arabidopsis, little is known about embryopatterning and the establishment of the basic body plan in angiosperms. Using available in situ hybridization, RNA-sequencing, and marker data, we try to compare how and when stem cell niches are established. Finally, evolutionary aspects of plant embryo development are discussed.

Postzygotic reproductive isolation established in the endosperm: mechanisms, drivers and relevance

The endosperm is a developmental innovation of angiosperms that supports embryo growth and germination. Aside from this essential reproductive function, the endosperm fuels angiosperm evolution by rapidly establishing reproductive barriers between incipient species. Specifically, the endosperm prevents hybridization of newly formed polyploids with their non-polyploid progenitors, a phenomenon termed the triploid block. Furthermore, recently diverged diploid species are frequently reproductively isolated by endosperm-based hybridization barriers. Current genetic approaches have revealed a prominent role for epigenetic processes establishing these barriers. In particular, imprinted genes, which are expressed in a parent-of-origin-specific manner, underpin the interploidy barrier in the model species Arabidopsis. We will discuss the mechanisms establishing hybridization barriers in the endosperm, the driving forces for these barriers and their impact for angiosperm evolution. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Fertilized egg cells secrete endopeptidases to avoid polytubey

Upon gamete fusion, animal egg cells secrete proteases from cortical granules to establish a fertilization envelope as a block to polyspermy^1-4. Fertilization in flowering plants is more complex and involves the delivery of two non-motile sperm cells by pollen tubes^5,6. Simultaneous penetration of ovules by multiple pollen tubes (polytubey) is usually avoided, thus indirectly preventing polyspermy^7,8. How plant egg cells regulate the rejection of extra tubes after successful fertilization is not known. Here we report that the aspartic endopeptidases ECS1 and ECS2 are secreted to the extracellular space from a cortical network located at the apical domain of the Arabidopsis egg cell. This reaction is triggered only after successful fertilization. ECS1 and ECS2 are exclusively expressed in the egg cell and transcripts are degraded immediately after gamete fusion. ECS1 and ESC2 specifically cleave the pollen tube attractor LURE1. As a consequence, polytubey is frequent in ecs1 ecs2 double mutants. Ectopic secretion of these endopeptidases from synergid cells led to a decrease in the levels of LURE1 and reduced the rate of pollen tube attraction. Together, these findings demonstrate that plant egg cells sense successful fertilization and elucidate a mechanism as to how a relatively fast post-fertilization block to polytubey is established by fertilization-induced degradation of attraction factors.

Polyspermy Block in the Central Cell During Double Fertilization of Arabidopsis thaliana

During double fertilization in angiosperms, two male gametes (sperm cells), are released from a pollen tube into the receptive region between two female gametes; the egg cell and the central cell of the ovule. The sperm cells fertilize the egg cell and the central cell in a one-to-one manner to yield a zygote and an endosperm, respectively. The one-to-one distribution of the sperm cells to the two female gametes is strictly regulated, possibly via communication among the four gametes. Polyspermy block is the mechanism by which fertilized female gametes prevent fertilization by a secondary sperm cell, and has been suggested to operate in the egg cell rather than the central cell. However, whether the central cell also has the ability to avoid polyspermy during double fertilization remains unclear. Here, we assessed the one-to-one fertilization mechanism of the central cell by laser irradiation of the female gametes and live cell imaging of the fertilization process in Arabidopsis thaliana. We successfully disrupted an egg cell within the ovules by irradiation using a femtosecond pulse laser. In the egg-disrupted ovules, the central cell predominantly showed single fertilization by one sperm cell, suggesting that neither the egg cell nor its fusion with one sperm cell is necessary for one-to-one fertilization (i.e., monospermy) of the central cell. In addition, using tetraspore mutants possessing multiple sperm cell pairs in one pollen, we demonstrated that normal double fertilization was observed even when excess sperm cells were released into the receptive region between the female gametes. In ovules accepting four sperm cells, the egg cell never fused with more than one sperm cell, whereas half of the central cells fused with more than one sperm cell (i.e., polyspermy) even 1 h later. Our results suggest that the central cell can block polyspermy during double fertilization, although the central cell is more permissive to polyspermy than the egg cell. The potential contribution of polyspermy block by the central cell is discussed in terms of how it is involved in the one-to-one distribution of the sperm cells to two distinct female gametes.

Central Cell in Flowering Plants: Specification, Signaling, and Evolution

During the reproduction of animals and lower plants, one sperm cell usually outcompetes the rivals to fertilize a single egg cell. But in flowering plants, two sperm cells fertilize the two adjacent dimorphic female gametes, the egg and central cell, respectively, to initiate the embryo and endosperm within a seed. The endosperm nourishes the embryo development and is also the major source of nutrition in cereals for humankind. Central cell as one of the key innovations of flowering plants is the biggest cell in the multicellular haploid female gametophyte (embryo sac). The embryo sac differentiates from the meiotic products through successive events of nuclear divisions, cellularization, and cell specification. Nowadays, accumulating lines of evidence are raveling multiple roles of the central cell rather than only the endosperm precursor. In this review, we summarize the current understanding on its cell fate specification, intercellular communication, and evolution. We also highlight some key unsolved questions for the further studies in this field.

Cellular dynamics of double fertilization and early embryogenesis in flowering plants

Flowering plants (angiosperms) perform a unique double fertilization in which two sperm cells fuse with two female gamete cells in the embryo sac to develop a seed. Furthermore, during land plant evolution, the mode of sexual reproduction has been modified dramatically from motile sperm in the early-diverging land plants, such as mosses and ferns as well as some gymnosperms (Ginkgo and cycads) to nonmotile sperm that are delivered to female gametes by the pollen tube in flowering plants. Recent studies have revealed the cellular dynamics and molecular mechanisms for the complex series of double fertilization processes and elucidated differences and similarities between animals and plants. Here, together with a brief comparison with animals, we review the current understanding of flowering plant zygote dynamics, covering from gamete nuclear migration, karyogamy, and polyspermy block, to zygotic genome activation as well as asymmetrical division of the zygote. Further analyses of the detailed molecular and cellular mechanisms of flowering plant fertilization should shed light on the evolution of the unique sexual reproduction of flowering plants.

Fertilization in flowering plants: an odyssey of sperm cell delivery

In light of the available discoveries in the field, this review manuscript discusses on plant reproduction mechanism and molecular players involved in the process. Sperm cells in angiosperms are immotile and are physically distant to the female gametophytes (FG). To secure the production of the next generation, plants have devised a clever approach by which the two sperm cells in each pollen are safely delivered to the female gametophyte where two fertilization events occur (by each sperm cell fertilizing an egg cell and central cell) to give rise to embryo and endosperm. Each of the successfully fertilized ovules later develops into a seed. Sets of macromolecules play roles in pollen tube (PT) guidance, from the stigma, through the transmitting tract and funiculus to the micropylar end of the ovule. Other sets of genetic players are involved in PT reception and in its rupture after it enters the ovule, and yet other sets of genes function in gametic fusion. Angiosperms have come long way from primitive reproductive structure development to today's sophisticated, diverse, and in most cases flamboyant organ. In this review, we will be discussing on the intricate yet complex molecular mechanism of double fertilization and how it might have been shaped by the evolutionary forces focusing particularly on the model plant Arabidopsis.

Reproductive Multitasking: The Female Gametophyte

Fertilization of flowering plants requires the organization of complex tasks, many of which become integrated by the female gametophyte (FG). The FG is a few-celled haploid structure that orchestrates division of labor to coordinate successful interaction with the sperm cells and their transport vehicle, the pollen tube. As reproductive outcome is directly coupled to evolutionary success, the underlying mechanisms are under robust molecular control, including integrity check and repair mechanisms. Here, we review progress on understanding the development and function of the FG, starting with the functional megaspore, which represents the haploid founder cell of the FG. We highlight recent achievements that have greatly advanced our understanding of pollen tube attraction strategies and the mechanisms that regulate plant hybridization and gamete fusion. In addition, we discuss novel insights into plant polyploidization strategies that expand current concepts on the evolution of flowering plants.

Selective egg cell polyspermy bypasses the triploid block

Polyploidization, the increase in genome copies, is considered a major driving force for speciation. We have recently provided the first direct in planta evidence for polyspermy induced polyploidization. Capitalizing on a novel sco1-based polyspermy assay, we here show that polyspermy can selectively polyploidize the egg cell, while rendering the genome size of the ploidy-sensitive central cell unaffected. This unprecedented result indicates that polyspermy can bypass the triploid block, which is an established postzygotic polyploidization barrier. In fact, we here show that most polyspermy-derived seeds are insensitive to the triploid block suppressor admetos. The robustness of polyspermy-derived plants is evidenced by the first transcript profiling of triparental plants and our observation that these idiosyncratic organisms segregate tetraploid offspring within a single generation. Polyspermy-derived triparental plants are thus comparable to triploids recovered from interploidy crosses. Our results expand current polyploidization concepts and have important implications for plant breeding.

Shaping the genome of plants

Fertilization of an egg cell by more than one sperm cell can produce viable progeny in a flowering plant.

The molecular foundations of zygosis

Zygosis is the generation of new biological individuals by the sexual fusion of gamete cells. Our current understanding of eukaryotic phylogeny indicates that sex is ancestral to all extant eukaryotes. Although sexual development is extremely diverse, common molecular elements have been retained. HAP2-GCS1, a protein that promotes the fusion of gamete cell membranes that is related in structure to certain viral fusogens, is conserved in many eukaryotic lineages, even though gametes vary considerably in form and behaviour between species. Similarly, although zygotes have dramatically different forms and fates in different organisms, diverse eukaryotes share a common developmental programme in which homeodomain-containing transcription factors play a central role. These common mechanistic elements suggest possible common evolutionary histories that, if correct, would have profound implications for our understanding of eukaryogenesis.

Polyspermy in angiosperms: Its contribution to polyploid formation and speciation

Polyploidization has played a major role in the long-term diversification and evolutionary success of angiosperms. Triploid formation among diploid plants, which is generally considered to be achieved by fertilization of an unreduced gamete with a reduced one, has been accepted as a means of polyploid production. In addition, it has been supposed that polyspermy also contributes to the triploid formation in maize, wheat, and some orchids; however, such a mechanism has been considered uncommon because reproducing the polyspermic situation and unambiguously investigating developmental profiles of polyspermic zygotes are difficult. To overcome these problems, rice polyspermic zygotes have been successfully produced by electrofusion of an egg cell with two sperm cells, and their developmental profiles have been monitored. The triploid zygotes progress through karyogamy and divide into two-celled embryos via a typical bipolar mitotic division; the two-celled embryos further develop into triploid plants, indicating that polyspermic plant zygotes, unlike those of animals, can develop normally. Furthermore, progenies consisting of triparental genetic materials have been successfully obtained in Arabidopsis through the pollination of two different kinds of male parents with a female parent. These different pieces of evidence for development and emergence of polyspermic zygotes in vitro and in planta suggest that polyspermy is a key event in polyploidization and species diversification.

A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization

In flowering plants, pollen tubes undergo tip growth to deliver two nonmotile sperm to the ovule where they fuse with an egg and central cell to achieve double fertilization. This extended journey involves rapid growth and changes in gene activity that manage compatible interactions with at least seven different cell types. Nearly half of the genome is expressed in haploid pollen, which facilitates genetic analysis, even of essential genes. These unique attributes make pollen an ideal system with which to study plant cell-cell interactions, tip growth, cell migration, the modulation of cell wall integrity, and gene expression networks. We highlight the signaling systems required for pollen tube navigation and the potential roles of Ca²⁺ signals. The dynamics of pollen development make sexual reproduction highly sensitive to heat stress. Understanding this vulnerability may generate strategies to improve seed crop yields that are under threat from climate change.

Triparental inheritance in Dictyostelium

Sex promotes the recombination and reassortment of genetic material and is prevalent across eukaryotes, although our knowledge of the molecular details of sexual inheritance is scant in several major lineages. In social amoebae, sex involves a promiscuous mixing of cytoplasm before zygotes consume the majority of cells, but for technical reasons, sexual progeny have been difficult to obtain and study. We report here genome-wide characterization of meiotic progeny in Dictyostelium discoideum We find that recombination occurs at high frequency in pairwise crosses between all three mating types, despite the absence of the Spo11 enzyme that is normally required to initiate crossover formation. Fusions of more than two gametes to form transient syncytia lead to frequent triparental inheritance, with haploid meiotic progeny bearing recombined nuclear haplotypes from two parents and the mitochondrial genome from a third. Cells that do not contribute genetically to the Dictyostelium zygote nucleus thereby have a stake in the next haploid generation. D. discoideum mitochondrial genomes are polymorphic, and our findings raise the possibility that some of this variation might be a result of sexual selection on genes that can promote the spread of individual organelle genomes during sex. This kind of self-interested mitochondrial behavior may have had important consequences during eukaryogenesis and the initial evolution of sex.

Seeds-An evolutionary innovation underlying reproductive success in flowering plants

"Seeds nourish, seeds unite, seeds endure, seeds defend, seeds travel," explains the science writer Thor Hanson in his book The Triumph of Seeds (2015). The seed is an ultimate product of land plant evolution. The nursing and protective organization of the seed enable a unique parental care of the progeny that has fueled seed plant radiation. Seeds promote dispersal and optimize offspring production and thus reproductive fitness through biological adaptations that integrate environmental and developmental cues. The composite structure of seeds, uniting tissues that originate from three distinct organisms, enables the partitioning of tasks during development, maturation, and storage, while a sophisticated interplay between the compartments allows the fine-tuning of embryonic growth, as well as seed maturation, dormancy, and germination. In this review, we will highlight peculiarities in the development and evolution of the different seed compartments and focus on the molecular mechanisms underlying the interactions between them.

Friend or foe: Signaling mechanisms during double fertilization in flowering seed plants

Since the first description of double fertilization 120 years ago, the processes of pollen tube growth and guidance, sperm cell release inside the receptive synergid cell, as well as fusion of two sperm cells to the female gametes (egg and central cell) have been well documented in many flowering plants. Especially microscopic techniques, including live cell imaging, were used to visualize these processes. Molecular as well as genetic methods were applied to identify key players involved. However, compared to the first 11 decades since its discovery, the past decade has seen a tremendous advancement in our understanding of the molecular mechanisms regulating angiosperm fertilization. Whole signaling networks were elucidated including secreted ligands, corresponding receptors, intracellular interaction partners, and further downstream signaling events involved in the cross-talk between pollen tubes and their cargo with female reproductive cells. Biochemical and structural biological approaches are now increasingly contributing to our understanding of the different signaling processes required to distinguish between compatible and incompatible interaction partners. Here, we review the current knowledge about signaling mechanisms during above processes with a focus on the model plants Arabidopsis thaliana and Zea mays (maize). The analogy that many of the identified "reproductive signaling mechanisms" also act partly or fully in defense responses and/or cell death is also discussed.