18 From Pollination to Fertilization in Arabidopsis

Abstract

The life cycle of plants comprises two alternating forms of the organism, one which is diploid (the sporophyte) and one which is haploid (the gametophyte). In higher plants, the gametophytic form of the organism has been greatly reduced. On the male side it includes the pollen grain, the pollen tube grown from it, and the two sperm cells that migrate down the pollen tube to participate in fertilization. The female gametophyte is usually a seven-celled structure, the embryo sac, embedded within the sporophytic tissue of the ovule. Although the size and developmental complexity of these gametophytes have been greatly reduced relative to the lower vascular plants, the function remains the same: to bring about the union of haploid gametes to form a sporophytic zygote and thus reinitiate the cycle. The term fertilization is usually defined as the final events of this process, that is, gamete and nuclear fusion. We define the term “fertilization process” to include all interactions between the male gametophyte and the sporophytic or gametophytic female tissues necessary to achieve the successful production of a zygote.

During the fertilization process, the male gametophyte must grow and develop in a precise manner to deliver the sperm cells to the embryo sac. The process begins with the arrival of the pollen grain on the surface of the stigma (pollination), where it undergoes hydration and germinates to grow a pollen tube. This tube penetrates the surface of the stigmatic cell and grows basally toward, and ultimately into, the ovary. During the...