Flavonoids and genetic modification of male fertility.

For their reproductive success higher plants depend on the functionality and efficacy of a specialized structure, the flower. This contains specialized organs (anther and carpel) in which the male and female reproductive cells are produced and non-reproductive organs (sepal and petal) which attract pollinators and protect the developing seed. In the anther the microspores develop which after maturation become pollen grains. A pollen grain can be regarded as a free living haploid organism. It contains one vegetative and one generative cell in bicellular pollen or one vegetative cell and two sperm cells in tricellular pollen. Its main task is to deliver the two sperm nuclei through the female reproductive tissues to the egg cell. The pollen can be transported to the female organs by a variety of mediators such as wind or insect pollinators. After arrival on a compatible and receptive stigma the pollen grains germinate by the extrusion of a tube through a germ pore in the pollen wall. The pollen tube grows through the transmitting tissue of the style to reach the ovary. After the ovary has been entered, the tube grows through the micropyle of the ovule towards the embryo sac. The tube enters one of the synergids of the embryo sac and ruptures. The sperm cells are then released into the synergid. In a process known as double fertilization one sperm cell fuses with the egg cell to form the diploid zygote while the other sperm cell fuses with the central cell to form the triploid endosperm. For most angiosperms, pollen germination, tube growth and fertilization are rapid events which are completed within 1 to 48 hours. At the morphological level a wealth of knowledge about the formation of the reproductive organs and cells, pollen germination, and the fertilization process has been gained during the last 50–100 years. This is in sharp contrast with the availability of data describing molecular processes regulating the different steps of plant reproduction. Only limited data exist describing the role of gene products or signal molecules which interact in and control the development of male and female reproductive cells and the fertilization process.