Molecular and biochemical events during the induction of microspore embryogenesis

Abstract

The life cycle of higher plants alternates between a diploid sporophytic phase and a haploid gametophytic phase. The gametophytic generation begins after meiosis with two types of haploid gametophytes or spores: microspores, which develop into pollen grains within the anther (male gametogenesis), and megaspores, which form the embryo sac within the ovule (female gametogenesis). During male gametogenesis microspores undergo a precisely defined and often synchronous sequence of cell divisions that lead to the formation of highly specialized cells: a large vegetative cell and two sperm cells. Although this tightly controlled developmental pathway consists of only two cell divisions, under certain experimental conditions, immature male gametophytes can be induced to undergo an altered development leading to the production of haploid embryos. During this process, known as androgenesis or microspore embryogenesis, the male gametophyte develops directly into an embryo, without an intervening fertilization. In this review, several recent findings on microspore embryogenesis will be described. A central issue will be what triggers microspores or pollen to change their developmental fate to produce sporophytes. The emphasis will be on molecular events that occur during the initial stages of microspore embryogenesis. Two main questions that still remain to be answered are: what is the genetic basis for embryogenic potential, and how is embryogenic development induced?