Double fertilization is a process that occurs exclusively in angiosperms and is characterized by the production of an embryo and a triploid structure called endosperm.
→ Pollen grain and embryo sac
Before describing this process, we need to understand the characteristics of the pollen grain and the embryo sac. Next, we will describe these two structures:
- Pollen grain: Contrary to what many think, the pollen grain is not the male gamete. When released from the anther, it is usually formed by a vegetative cell (tube cell) and a generative cell and, at this stage, it is known as an immature microgametophyte. Subsequently, the generative cell divides and then gives rise to male gametes, called sperm cells.
- Embryonic bag:At the beginning of development, the ovule is just a nucellus, and this structure is later surrounded by the integuments, which form an opening called a micropyle at the end of the ovule. A single megasporocyte arises in the nucellus and then divides by meiosis, forming four haploid megaspores. Three of the four megaspores degenerate, and the functional megaspore begins to divide its nucleus mitotically. At the end of the third mitotic cycle, we have eight nuclei. Two nuclei are located in the center of the octonucleated cell and are called polar nuclei. Three nuclei are located in the region close to the micropyle and constitute the oospheric apparatus with two synergids and an egg. Another three nuclei are located in the region opposite the micropyle and are called antipodes. At the antipodes, the formation of a cell wall is observed, as well as synergistics. We therefore have a structure with eight nuclei and seven cells: the embryo sac or mature megagametophyte.
For double fertilization to occur, the so-called pollination must initially take place . In this process, the pollen grain is deposited on the stigma of the flower , that is, the female part. The pollen grain absorbs water from the stigma and germinates, forming the pollen tube. The tube then travels through the stigma and style until it reaches the entrance of the micropyle.
Upon entering the micropyle and reaching the embryo sac, the pollen tube, which carries the two male gametes and the vegetative nucleus, enters a synergid. That cell degenerates and gametes are discharged. One nucleus of a male gamete unites with the egg nucleus and another with the polar nuclei. It is for this reason that we say that a double fertilization occurs.
→ Result of double fertilization
As a result of double fertilization, the embryo and endosperm will form . The union of the male gamete with the egg will produce the embryo, while the union of the gamete with the polar nuclei will lead to the formation of the endosperm. The latter stands out for being a nutrient-rich triploid tissue that will provide food for the developing embryo.