In angiosperms, the female gamete (egg) is formed and remains fixed at the micropylar end of the embryo sac deeply embedded in the sporophytic tissues of the ovule, which is enclosed in the ovary well removed from the stigma. The male gametes (sperms) are enclosed in the pollen grain. To effect fertilization, the pollen germinate on the stigma to form a pollen tube that transports the two non-motile sperms to the embryo sac and delivers them in the vicinity of the egg. Whereas one of the sperms fertilizes the egg (syngamy), the other fertilizes the central cell (triple fusion). The fertilized egg (zygote) develops into an embryo, the progenitor of the next generation, and the fertilized central cell forms the endosperm tissue, the main source of nutrition for the developing and germinating embryo. Thus, in the angiosperms the gametes, the process of double fertilization, zygote, early stages of embryo and endosperm development are not readily accessible to study the cellular and molecular aspects of fertilization and embryogenesis. Therefore, for almost 100 years since the discovery of double fertilization in angiosperms, by Nawaschin (1898), not much progress could be made in this area. Whatever little information is known is based mainly on mutant analysis in Arabidopsis. Sexual incompatibility is a serious handicap in developing desirable hybrids. In this the pollen fails to germinate on the stigma or the pollen tube gets arrested or bursts before reaching the ovary. Two in vitro techniques developed to overcome this problem are in vivo pollination (IVP) and In vitro fertilization (IVF). IVF, developed in nineties involved isolation of male and female gametes and their in vitro fusion (fertilization) culture of the in vitro zygote to regenerate full plants. Complete IVF technique has been developed in only two plants, namely maize and rice. This technique is proving to be an invaluable aid to directly observe and analyse fertilization and post-fertilization process in flowering plants which is not possible under in vivo conditions.
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Suggested Further Reading
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