Pollen-Pistil Interaction and Fertilization

Following pollination pollen grains have to complete a series of sequential events in the pistil, termed pollen-pistil interaction, before they can discharge male gametes into the embryo sac for fertilization. Successful completion of pollen-pistil interaction is essential for development of fruits and seeds, the end products of sexual reproduction. Pollen-pistil interaction is dynamic and there is a continuous dialogue between the pollen grain and later the pollen tube, and the saprophytic tissues of the pistil. During this interaction, pollen grains are recognized; if compatible, the pistil facilitates post-pollination events and if incompatible, it inhibits pollen germination or pollen tube growth. Pollen-pistil interaction also creates intense competition amongst even the compatible pollen and only the most vigorous pollen grains are able to achieve fertilization. Less vigorous pollen grains are eliminated from this competition. Pollen competition has been shown to improve the fitness of the progeny. Considerable progress has been made in identifying the pollen and pistil components involved in this interaction, their nature and location. Extracellular components present in the pollen wall, and on the surface of the stigma and in the intercellular spaces in the style have been shown to be involved in pollen-pistil dialogue. In a few species some of the components involved in this dialogue have been identified. Impressive advances have been made in understanding the components responsible for guiding the pollen tube from the placenta in the ovary into the micropyle of the ovule. Synergids and the filiform apparatus present at the micropylar tip of the synergids play a crucial role in this guidance. In a few species the genes and gene products involved in this guidance have been identified. Double fertilization is unique to flowering plants; of the two sperm cells discharged in one of the synergids, one of them fuses with the egg to give rise to the zygote and the 191 other fuses with the central cell. The zygote develops into the embryo and the central cell into the endosperm. As fertilization occurs deep inside the ovule, accessibility has been a major problem for effective experimentation and has remained an enigma since long. In recent years the success in achieving in vitro fertilization using isolated egg and sperm cells combined with the application of the techniques of molecular genetics and high resolution imaging to study fertilization in living cells have started unrevealing the details of double fertilization. With continued application of these advanced approaches to study pollen-pistil interaction and fertilization more rapid progress are likely to be made in the coming years leading not only to a better understanding of these processes but also in their effective manipulation for practical benefits.