Double Fertilization

Double fertilization in plants

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.

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.

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.

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 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.

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.

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. Pollen germination (1) and fertilization (2).

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. 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 signalling. 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.

Advantages of double fertilization in plants

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.

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.