Why is fertilization in a flower double fertilization

The fertilized central cell forms the endosperm while the fertilized egg cell, the zygote, will form the actual embryo and suspensor. The latter structure connects the embryo with the sporophytic maternal tissues of the developing seed. The underlying mechanisms of double fertilization are tightly regulated to ensure delivery of functional sperm cells and the formation of both, a functional zygote and endosperm. In this review we will discuss the current state of knowledge about the processes of directed pollen tube growth and its communication with the synergid cells resulting in pollen tube burst, the interaction of the four gametes leading to cell fusion and finally discuss mechanisms how flowering plants prevent multiple sperm cell entry polyspermy to maximize their reproductive success.

High crop yield strongly depends on efficient formation of numerous ovules, which after successful fertilization, develop into seeds comprising seed coat, embryo, and endosperm. In angiosperms, the haploid gametophytic generations produce the male and female gametes required to execute double fertilization. Both gametophytes are reduced to only a few cells. The female gametophyte is deeply embedded and thus protected by the maternal sporophytic tissues of the pistil Figure 1.

It harbors the female gametes egg and central cell and is surrounded by the nucellus tissue as well as the inner and outer integuments. After fertilization these different tissues form the seed coat. The female gametophyte arises from a megaspore mother cell though processes known as megasporogenesis and megagametogenesis for review see Evans and Grossniklaus, ; Drews and Koltunow, The functional megaspore undergoes three mitotic divisions resulting in a syncytium containing eight nuclei.

After nuclei migration and cellularization seven cells are differentiated: the haploid egg cell and its two adjoining synergid cells are located at the micropylar pole forming the egg apparatus.

The homodiploid central cell containing two fused or attached nuclei is located more centrally, whereas three antipodal cells are found at the chalazal pole of the ovule opposite to the egg apparatus.

While synergid cells are essential for pollen tube attraction, burst and sperm cell release see below , the function of antipodal cells is so far unknown. During female gametophyte maturation antipodal cells are degenerating in the ovule of the eudicot model plant Arabidopsis Mansfield et al.

Figure 1. The female gametophyte is deeply imbedded inside the female flower organs. A Dissected and reconstructed Arabidopsis flower. One of four petals P and one of six stamina SA are shown. They surround the pistil, which represents the female flower organ. It can be dissected into three parts. The upper part contains the papilla cells and forms the stigma S , which is connected to the ovary OY by the style ST. The ovary is formed by two fused carpels C , which harbor two rows of ovules OV.

A side view B and front view C of a 3D-remodeled ovule reconstructed from toluidine blue stained single, successive ultra-thin sections of a dissected pistil. See Supplemental Movie 1 for whole series of sections.

The ovule is connected to the septum SE, yellow containing the transmitting tract TT, blue by the funiculus F, petrol and surrounded by the carpel tissue C green. A 3D-model of a dissected ovule shown from various angles is shown in Supplemental Movie 2. The vacuole and nucleus of the different female gametophyte cells showed highest contrast and are therefore shown individually.

Near to the micropyle MY , the two nuclei of the two synergid cells SY are shown in red and green. The egg cell, indicated by EC in D , has a comparably large vacuole light blue and its nucleus blue is located at its chalazal pole.

The center of the female gametophyte is filled by the vacuole light yellow of the central cell, indicated by CC in D , and its homo-diploid nucleus yellow. The three degenerating antipodal cells, indicated by AP in turquoise color in D at the chalazal pole are not highlighted. D DIC microscopic image of a mature female gametophyte surrounded by the maternal sporophytic tissues of the ovule.

The cell types and tissues are artificially colored as shown in B,C. At full maturity the nucellus cell NC layer surrounding the developing embryo sac is flattened between inner integument II and female gametophyte cells.

The haploid male gametophyte pollen grain is formed during the processes of microsporogenesis and microgametogenesis from the microspore mother cell by meiosis and two successive mitotic divisions resulting in the formation of a tricellulate pollen grain.

The vegetative cell encases the two sperm cells, which are connected with the vegetative cell nucleus by the generative cell plasma membrane, forming the male germ unit MGU. MGU formation ensures the simultaneous delivery of both gametes to the ovule for review see McCue et al. The major task of the vegetative cell is to deliver the sperm cells through the maternal tissues of the style and ovary to an unfertilized ovule. After pollen germination, the vegetative cell forms a tube and grows by tip-based-growth mechanism along papillae cells of the stigma into the style toward the transmitting tract.

Inside the transmitting tract, pollen tubes are guided toward the ovules by mechanical and chemotactic cues involving numerous interactions with the sporophytic style tissues. In many eudicots pollen tubes exit the transmitting tract and grow along the septum, the funiculus and the outer integument toward the micropyle of unfertilized ovules.

In grasses the ovary contains a single ovule and the pollen tube is directly guided toward its surface after leaving the blind ending transmitting tract. The pollen tube continues to grow along its surface toward the micropylar region for review see Lausser and Dresselhaus, Finally, the pollen tube enters the micropyle, an opening between the inner and outer integuments, and grows toward the two synergid cells.

The pollen tube bursts and sperm cells are released. This process is associated with the degeneration of the receptive synergid cell due to programmed cell death. Subsequently, both sperm cells arrive at the gamete fusion site and fertilize the egg and central cell Hamamura et al. Its extended growth inside the female flower tissue is regulated by many different guidance, attraction and support mechanisms. After sperm cell release all gametes are activated, followed by fusion of their membranes and nuclei by processes known as plasma- and karyogamy, respectively.

After successful double fertilization further signaling events are activated to prevent polyspermy. In this review we will summarize and discuss the cell—cell communication processes, which are essential to successfully accomplish double fertilization and to initiate seed development in angiosperms. Pollen tube growth and guidance toward the female gametes are controlled at various stages by chemotactic signals and growth support molecules derived from the sporophytic and gametophytic tissues of the female flower organs.

Pollen grains placed on the stigma Figure 1A by contact, wind or different pollinators stick to the papilla cells and start to hydrate followed by their germination. The efficient adhesion of the pollen grain to the papilla cell is regulated by interaction events between these cells and may activate thereby inter- and intra-species barriers to prevent unsuccessful pollination and fertilization events already at this early time point during reproduction. Angiosperms possess different strategies to recognize self from alien pollen and evolved independent self-incompatibility SI mechanisms to prevent self-fertilization.

Early SI mechanisms are based on cell-cell communication events between the papilla cells and the pollen grains, whereas later SI mechanisms occur while the growing pollen tube interacts with the cells of the transmitting tract.

Species of the Solanaceae , for example, use a pistil-expressed S-RNase, which penetrates the pollen tube McClure et al. Their successful interaction leads to proteasome dependent degradation of Exo70A1, an essential component of the exocyst complex. It is thought to be involved in secretion of essential pollen germination factors necessary for pollen hydration Synek et al.

Rejection of pollen in Brassicaceae thus occurs already during pollen hydration and germination at the surface of papilla cells. Little is known about SI in the economically important grasses reviewed in Dresselhaus et al. Pollen hydration and germination appear not to be affected, although only grass pollen tubes are capable of penetrating the style and reach the transmitting tract.

This indicates that SI in the grasses depends on successful interaction of the pollen tube with the sporophytic cells of the style and transmitting tract. The signaling events involved in this recognition process still await their discovery. After adhesion and hydration, compatible pollen germinates, penetrates the style and grows through the extracellular space of stylar cells toward the transmitting tract Figure 1B.

The water flow during hydration forms an external gradient specifying the site of pollen tube outgrowth and was shown to be controlled by triacylglyceride Lush et al. D-serine is produced by Serine-Racemase1 SR1 , which shows an expression peak in the style indicating D-serine availability. During pollen tube growth the tip needs to modulate the surrounding cell wall of stylar cells enabling its penetration through the extra-cellular space, most likely by interaction with extensin-like proteins and arabinogalactan proteins as well as the secretion of cell wall softening enzymes and inhibitors such as polygalacturonases and pectin methylesterase inhibitors Cosgrove et al.

The transmitting tract is composed of small cylindrical cells that are surrounded by an extracellular matrix ECM , which contains a mixture of glycoproteins, glycolipids, and polysaccharides Lennon et al.

The ECM provides essential nutrients as well as components for an accelerated, extended and guided pollen tube growth Palanivelu and Preuss, The transmitting tract-specific arabinogalactan glycoproteins TTS1 and TTS2 have a positive effect on in vitro grown tobacco pollen and show a gradient of increased glycosylation correlating with pollen tube growth direction inside the transmitting tract Cheung et al.

Another factor which has a positive effect on pollen tube growth and guidance is chemocyanin, a small secreted peptide in the style of lily Kim et al. The different sporophyte-derived signals do not only guide or increase pollen tube growth rate, but rather lead to a change in the pollen transcriptome and thereby activate the pollen for female gametophyte-derived attraction signals Higashiyama et al.

Recently, de novo expression of closely related MYB transcription factors and other genes were reported to be induced during pollen tube growth through the style regulating themselves a number of downstream genes.

Hence pollen tubes maturate during their growth through the sporophytic tissue and thereby become competent for fertilization Leydon et al. The signaling events that control pollen tube exit from the transmitting tract and guidance toward the ovule are not known. In Arabidopsis this process was shown to be tightly regulated and usually only a single pollen tube exits the transmitting tract in proximity of an unfertilized ovule.

The pollen tube grows on the septum surface toward the funiculus, the tissue connecting the ovule with the septum Figures 1B—D ; Supplemental Movies 1, 2. At the funiculus the pollen tube is directed through the micropyle inside the ovule by a mechanism known as micropylar guidance Shimizu and Okada, At moderate concentrations GABA stimulates pollen tube growth and thus likely supports growth toward the ovule Palanivelu et al.

Another candidate involved in micropylar guidance is D-serine, which was already described above. Its synthesizing enzyme gene SR1 is also expressed in the ovule indicating the presence of D-serine Michard et al. In summary, our current understanding of ovular pollen tube guidance is very limited, but a whole orchestra of small molecules derived from the ovule seem to be involved in pollen tube growth support and attraction, and multiple signaling networks are required in pollen tubes to respond to the diverse set of signals and to direct their growth behavior.

After arrival at the surface of the ovule, the pollen tube reaches the last phase of its journey, which is known as micropylar pollen tube guidance. It enters the micropyle, an opening between the two integuments, and directly grows toward the egg apparatus in species such as Arabidopsis Figure 2A.

It was believed for a long time that the pollen tube grows through the filiform apparatus to enter one synergid cell, leading to pollen tube burst and cell death of the receptive synergid cell. Recently, it was shown that the pollen tube is repelled by the filiform apparatus and instead grows along the cell wall of the synergid cells until it reaches a certain point after the filiform apparatus where its growth is arrested and burst occurs explosively Leshem et al.

Pollen tube burst results in the discharge of its cytoplasmic contents including the two sperm cells. The synergid cells represent the main source for chemo-attractants required for micropylar pollen tube guidance. Moreover, laser ablation experiments in Torenia fournieri have demonstrated that a single synergid cell is sufficient and necessary to attract pollen tubes Higashiyama et al. The major function of the filiform apparatus may thus be to considerably increase the micropylar surface of the synergid cells, which represent glandular cells of the egg apparatus.

Many known components required for pollen tube growth and guidance are membrane-associated and accumulate at the filiform apparatus, which gives it the additional role of a signaling platform. In Arabidopsis the formation of the filiform apparatus as well as the expression of different attractants in the synergid cells depend on the activity of the R2R3-type Myb transcription factor MYB98 Kasahara et al.

Figure 2. Model of signaling events during micropylar pollen tube attraction and double fertilization in Arabidopsis. A The micropylar opening of the ovule is formed by the inner and outer integuments.

The central cell surrounds the egg apparatus. The synergid cells are the main sources of pollen tube attractants. Calcium transporters are involved in pollen tube growth control. The plasma membranes of synergid cells harbor a high concentration of receptors like FER and LRE, especially in the region of the filiform apparatus. B The pollen tube bursts when it reaches a certain point beyond the filiform apparatus and releases its cytoplasmic contents including the two sperm cells.

Released sperm cells are located at the gamete fusion side, between the two female gametes. The two sperm cells are connected to each other, likely involving tetraspanins.

The male gametes adhere to female gametes by GEX2 located at their surface. Unknown egg and central cell-specific fusogenic proteins as well as EC1 receptor are indicated by question marks in green, black, and purple, respectively. More puzzling is the role of the central cell in micropylar guidance of the pollen tube. For example magatama maa mutants show defects in central cell maturation; both haploid nuclei are smaller and often fail to fuse.

Pollen tubes grow in the direction of an unfertilized maa ovule but loose their way just before entering the micropyle. Moreover, mutant female gametophytes attracted two pollen tubes at a high frequency Shimizu and Okada, MAA3 was recently shown to encode a helicase required for general RNA metabolism, which could explain the central cell maturation defect but not the defect in pollen tube guidance Shimizu et al.

These guidance defects may be indirect and caused by non-functional or immature central cells influencing maturation of egg apparatus cells and thus the generation of guidance components in these cells. It might also be possible that molecules generated by the MAA3 and CCG pathways directly regulate the generation of guidance molecules in the neighboring cells.

Also the egg cell seems to be involved in micropylar guidance. Until recently, male factors and signaling pathways reacting to attractants secreted from the egg apparatus were unknown. Both proteins show membrane localization due to a palmitoylation site and are involved in the AtLURE1-dependent guidance mechanism.

After fertilization, the fertilized ovule forms the seed while the tissues of the ovary become the fruit. In the first stage of embryonic development, the zygote divides to form two cells; one will develop into a suspensor, while the other gives rise to a proembryo. In the second stage of embryonic development in eudicots , the developing embryo has a heart shape due to the presence of cotyledons. As the embryo grows, it begins to bend as it fills the seed; at this point, the seed is ready for dispersal.

Key Terms double fertilization : a complex fertilization mechanism that has evolved in flowering plants; involves the joining of a female gametophyte with two male gametes sperm suspensor : found in plant zygotes in angiosperms; connects the endosperm to the embryo and provides a route for nutrition from the mother plant to the growing embryo proembryo : a cluster of cells in the ovule of a fertilized flowering plant that has not yet formed into an embryo.

Double Fertilization After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. This is called a double fertilization. After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. Orchids are highly-valued flowers, with many rare varieties. They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25, species of orchids have been identified.

Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard; they have evolved different ways to attract the desired pollinators.

They use a method known as food deception, in which bright colors and perfumes are offered, but no food. Anacamptis morio , commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent, which usually indicates food for a bee. In the process, the bee picks up the pollen to be transported to another flower.

Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and, in the process, transfers pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps.

The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, but instead picks up pollen, which it then transfers to the next counterfeit mate.

Pollination by deception in orchids : Certain orchids use food deception or sexual deception to attract pollinators. Shown here is a bee orchid Ophrys apifera. After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels.

The germination of the pollen tube requires water, oxygen, and certain chemical signals. During this process, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac; it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm.

Together, these two fertilization events in angiosperms are known as double fertilization. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed. Double fertilization : In angiosperms, one sperm fertilizes the egg to form the 2n zygote, while the other sperm fuses with two polar nuclei to form the 3n endosperm.

This is called a double fertilization. After fertilization, embryonic development begins. The zygote divides to form two cells: the upper cell terminal cell and the lower cell basal cell.

The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo. In dicots eudicots , the developing embryo has a heart shape due to the presence of the two rudimentary cotyledons. In non-endospermic dicots, such as Capsella bursa , the endosperm develops initially, but is then digested.

In this case, the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they become crowded inside the developing seed and are forced to bend.

Ultimately, the embryo and cotyledons fill the seed, at which point, the seed is ready for dispersal. Embryonic development is suspended after some time; growth resumes only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.

After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell. The basal cell also divides, giving rise to the suspensor. Monocot and dicot seeds develop in differing ways, but both contain seeds with a seed coat, cotyledons, endosperm, and a single embryo. The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat, known as the testa, and inner coat, known as the tegmen.

The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl.

The embryonic axis terminates in a radicle, which is the region from which the root will develop. In angiosperms, the process of seed development begins with double fertilization and involves the fusion of the egg and sperm nuclei into a zygote. The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue.

This tissue becomes the food the young plant will consume until the roots have developed after germination. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.

In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds.

The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue. Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids. These products are absorbed by the scutellum and transported via a vasculature strand to the developing embryo.

Monocots and dicots : The structures of dicot and monocot seeds are shown. Dicots left have two cotyledons. Monocots, such as corn right , have one cotyledon, called the scutellum, which channels nutrition to the growing embryo. Both monocot and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle that forms the root.

The embryonic axis comprises everything between the plumule and the radicle, not including the cotyledon s. The pollen tube cell grows into a pollen tube through which the generative cell travels.

The germination of the pollen tube requires water, oxygen, and certain chemical signals. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm.

Together, these two fertilization events in angiosperms are known as double fertilization Figure 1. After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.

Figure 1.