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Double Fertilization

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Double Fertilization
Seeds are an important part of the human diet. Beans, peas, corn, wheat, rice, almonds, etc. -seeds are highly nutritious containing varying amounts of starch and other carbohydrates, as well as proteins and oils. Almost all seeds that we eat come from flowering plants (angiosperms) because they produce a large tissue that stores all these nutrients for the plant embryo development. We'll describe the process of double fertilization and its advantages, unique to flowering plants, that originate this food-storage tissue.

Double fertilization in plants

Angiosperms and gymnosperms are groups of plants that produce seeds. You now know that gymnosperms’ seeds are “naked” (exposed on the surface of cones’ bracts or scales) while angiosperms have seeds enclosed by a fruit. However, their seeds are quite different as the processes that form them have some differences.

Both types of seeds contain the developing embryo and a nutritious tissue. Both types of embryos result from typical fertilization where one sperm cell fertilizes an egg cell. The nutritious tissue in gymnosperm is haploid, derived from the female gametophyte haploid tissue. On the other hand, the nutritious tissue of angiosperm seeds is triploid, originated from a second fertilization. This is why angiosperms are said to have double fertilization, while gymnosperms have a single fertilization.

Haploid: the organism has just one set of chromosomes (unpaired).

Triploid: the organism has three homologous copies of each chromosome.

Double fertilization occurs in the flower

In angiosperms (flowering plants) sexual reproduction occurs in the flower. The flower produces and bears the plant’s reproductive structures. Fertilization occurs with the fusion of two gametes, male (sperm cell) and female (egg cell). Gametes are produced by the gametophytes which develop inside the male and female reproductive structures. The androecium is the male reproductive part composed of the stamens. The male gametophyte (pollen grain) is produced in the anther of the stamen. The gynoecium is the female reproductive part composed of carpels or pistils. The female gametophyte (embryo sac) is produced in the ovule enclosed by the ovary.

Gamete: a sexual reproductive cell that is haploid and fuses with another gamete during fertilization resulting in a diploid zygote. Gametes are formed from meiosis or from cells originated by meiosis.

Gametophyte: the multicellular sexual phase that is haploid and produces gametes through mitosis, in the alternation of generations of plants and some algae life cycle.

We have previously described gametophyte development and gamete production in flowering plants. Here we focus on the fertilization process that occurs after the male gametophyte reaches the pistil of another flower. Pollen grains are transported to the female part of another flower through pollination. In angiosperms, most plants are pollinated by animals (mostly insects and birds) that transfer the pollen when they move from flower to flower. A smaller fraction of flowering plants are wind-pollinated.

Pollination is the transfer of pollen from the male reproductive structure to the female reproductive structure in seed plants.

Double fertilization diagram

Before fertilization, the female and male gametes have been produced in the gametophytes (although male gametes are sometimes produced after the pollen grain reaches a flower pistil). Let's have a quick recap of the gametophytes' structure.

The ovary of a flower contains one or more ovules, which in turn contain the embryo sac. Remember that the embryo sac (haploid female gametophyte) is composed of eight nuclei and seven cells (Figure 1) that have specific locations:

  • Three cells stay near the micropyle, one being the egg cell (female gamete). The other two, called synergid cells, seats one on each side of the egg and are thought to attract the pollen tube.
  • Other three cells, called the antipodal cells, remain on the opposite side of the micropyle. Their function is unknown and they disintegrate near the time of fertilization.
  • The remaining two nuclei, called polar nuclei, stay at the center of the large cell. These also participate in the fertilization process.

Double Fertilization Embryo sac structure Studysmarter

Figure 1. The angiosperm female gametophyte, or embryo sac, and its components. The embryo sac is contained by the ovule surrounded by two layers of integuments. Source: modified from Tameeria at English Wikipedia, Public domain, via Wikimedia Commons.

A pollen grain (haploid male gametophyte) has two cells inside, a tube cell and a generative cell. The diagram below shows the process of double fertilization (Figure 2).

Double Fertilization Pollen germination and fertilization diagram Studysmarter

Figure 2. Pollen germination (1) and fertilization (2). Source: modified from LadyofHats, Public domain, via Wikimedia Commons.

Description of the process of double fertilization in flowering plants

Once the pollen grain is transferred from the anther to the stigma of a pistil, the pollen grain germinates, and the tube cell produces a pollen tube (Figure 2). In order to be able to germinate, the pollen grain absorbs water from the stigma, and it has to be recognized by the stigma through molecular signaling. This recognition prevents the germination of the pollen if it lands on the pistil of a different species, and avoids self-fertilization in bisexual flowers of some plants. The pollen tube is a long and slender tube that goes all the way down through the style and reaches the interior of the ovary. The generative cell moves through this tube while it divides once by mitosis, producing two haploid cells (two sperms).

Inside the ovary, the pollen tube gets to an ovule and enters through the micropyle (an opening in the integuments or layers that surrounds the embryo sac). Evidence suggests that the synergids secrete chemical signals to guide the pollen tube near the egg cell. The two sperm cells are discharged into the embryo sac. One sperm fertilizes the egg cell, producing a diploid zygote. The other sperm fuses with the two polar nuclei, resulting in a triploid cell. Hence, a double fertilization occurs in angiosperms.

The diploid zygote grows into an embryo, dividing through mitosis. The triploid cell forms the nutritious tissue of the seed, also dividing through mitosis, called the endosperm. Thus, after the double fertilization, the ovule develops into a seed that contains an embryo and the endosperm, surrounded by the integuments that become the seed coat. The ovary develops into a fruit that contains one or more seeds.

Depending on the plant species, the endosperm might be transient. Angiosperms are divided into two big groups based on morphological traits, including the embryo and seed. Embryos develop elongated structures similar to leaves called cotyledons (sometimes called seed leaves). Monocotyledon species (like rice, corn, and most cereal grains) have embryos with only one thin cotyledon and the endosperm usually keeps its nutrients until the seed matures. Thus, the endosperm is the direct nutrient source for monocotyledons’ embryos.

On the other hand, dicotyledon species (like beans, peanuts, and peas) have embryos with two cotyledons that take over the storage of nutrients. The nutrients are transferred from the endosperm to the growing cotyledons and the mature seed does not have an endosperm anymore. The cotyledons nurture the developing embryo in these plants. Dicotyledon seeds are easy to recognize as you can split them into the two cotyledons (as in beans) that are full of nutrients.

Advantages of double fertilization

Double fertilization is unique to angiosperms’ sexual reproduction. The product of the second fertilization, the endosperm, represents a highly nutritious source for the developing embryo that increases its survival probabilities. The multicellular endosperm accumulates proteins, oils, and carbohydrates, in different compositions depending on the plant species.

Remember that the gametophyte of gymnosperms is also a pollen grain that produces two sperm cells. However, one of the two sperm cells usually degenerates in gymnosperms before fertilization. Only some gymnosperm species in the phylum Gnetophyta present a different type of double fertilization. In those species, the two sperms fertilize two egg nuclei (egg cells in these species are binucleate), only that the second fertilization results in a second diploid zygote, which degenerates later.

Besides the formation of a highly nutritious endosperm, double fertilization is thought to avoid the formation of a nutritious tissue if fertilization does not occur. Gymnosperms seeds also have a nutritious tissue (haploid in this case), derived from the larger and multicellular female gametophyte, and it forms before fertilization of the egg cell. Thus, in gymnosperms, if fertilization does not occur, the resources used for the growth of the female gametophyte might be wasted. Double fertilization in angiosperms may avoid this waste of resources by forming the nutritious tissue only when the egg cell is fertilized.

Double Fertilization - Key takeaways

  • Double fertilization occurs in flowering plants only, gymnosperms have a simple fertilization.
  • Fertilization in angiosperms occurs in the flower, specifically inside the embryo sac (female gametophyte) contained by an ovule enclosed by the ovary.
  • The products of double fertilization are:
    • a diploid zygote, through the fusion of one haploid sperm cell with the haploid egg cell, and
    • a triploid endosperm, through the fusion of another sperm cell with the two polar nuclei of the female gametophyte.
  • The diploid zygote grows into an embryo while the endosperm provides nutrients to the developing embryo (in most monocotyledons), or it transfers the nutrients to the embryo cotyledons (in most dicotyledons).
  • Double fertilization is thought to avoid the waste of resources to form a nutritious tissue if fertilization does not occur since the endosperm is formed only when the egg cell is fertilized.
  • After the double fertilization in flowering plants, the ovule develops into a seed that contains an embryo and the endosperm, surrounded by the integuments that become the seed coat. The ovary develops into a fruit that contains one or more seeds.

Frequently Asked Questions about Double Fertilization

Double fertilization in flowering plants occurs when one haploid sperm cell fertilizes the haploid egg cell (forming a diploid zygote), and another sperm cell fertilizes the two polar nuclei of the female gametophyte (forming a triploid endosperm). This occurs inside the ovule that is enclosed by the ovary (in the flower).

The significance of double fertilization in flowering plants is the increased survival probability of the embryo provided by a highly nutritious tissue (endosperm), and to avoid the waste of resources by forming the endosperm only if the egg is fertilized.

Double fertilization in plants is the fusion of two sperm cells with an egg cell (resulting in a zygote) and two polar nuclei (resulting in the endosperm), respectively. Double fertilization occurs only in angiosperms (flowering plants) inside the female structures in the flower.

Double fertilization has a couple of advantages. Besides the formation of a highly nutritious endosperm that increases the survival probability of the embryo, double fertilization is thought to avoid the waste of resources to form a nutritious tissue if fertilization does not occur. Therefore, the endosperm is formed only when the egg cell is fertilized.

The result of double fertilization is a diploid zygote, through the fusion of one haploid sperm cell with the haploid egg cell, and a triploid endosperm, through the fusion of another sperm cell with the two polar nuclei of the female gametophyte.

Final Double Fertilization Quiz

Question

The role of double fertilization in angiosperms, in contrast to single fertilization, is to produce:

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Answer

an endosperm

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The products of double fertilization in angiosperms are:

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Answer

A diploid zygote and a triploid endosperm. 

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Question

Gymnosperms do not present double fertilization; therefore, they do not produce: 


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Answer

a zygote

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Question

Both gymnosperms and angiosperms seeds contain:


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Answer

an embryo

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Question

The nutritious tissue originates from the ___ in gymnosperms, and from the ___ in angiosperms:


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Answer

female gametophyte, fusion of sperm cell and two female nuclei

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Question

The endosperm in flowering plants is formed by the fusion of a sperm cell with:


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Answer

two polar nuclei

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Question

Which ones of the following eventually disintegrate?


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antipodal cells

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Question

The ___ secrete chemicals signals to guide the pollen tube:


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antipodal cells

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Question

The zygote becomes the ___, the endosperm is the ___, and the integument becomes the ___. 


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Answer

embryo, nutrient storage, seed coat

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The ovule becomes the __ and the ovary becomes the ___. 


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Answer

seed, fruit

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Question

Which of the following are advantages of double fertilization in angiosperms?


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Answer

the formation of a zygote

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Question

The ___ is usually the direct nutrient source for embryos of monocotyledon species. 


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triploid endosperm

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The ___ is usually the direct nutrient source for embryos of dicotyledon species.


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Answer

triploid endosperm

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Question

In angiosperms, unlike gymnosperms, the endosperm is formed: 


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Answer

before fertilization

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Question

Which of the following is necessary for a pollen grain to germinate? 


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Answer

the absorption of water from the stigma

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