Summary

Under the appropriate in vitro culture conditions, anthers and isolated microspores of higher plants develop haploid embryos by a process referred to as androgenesis. Embryo development can be a direct recapitulation of the developmental stages characteristic of zygotic embryos, or it can be preceded by a callus stage.

There are a number of factors governing pollen embryogenesis, but genotype, donor plant physiology, stage of microspore development, and in vitro culture conditions are the most important. Some characteristics of embryogenie microspores have been identified, and late uninucleate to early binucleate cells are the most responsive. Under inductive culture conditions, uninucleate microspores divide symmetrically to initiate embryogenesis, whereas binucleate pollen exhibits sustained cell division of the vegetative or generative cell. High carbohydrate levels and an initial period at high temperature are conducive to pollen embryogenesis in some species.

Pollen embryos are developmentally similar to zygotic embryos and, under the appropriate culture conditions, they can mature and accumulate seed-specific storage products in a comparable manner. Such embryos develop directly into plants, although plants may arise from secondary structures. Double haploid plants are produced through chromosome doubling techniques. These homozygous plants are useful in plant breeding and genetic studies. In addition, haploid embryos are used in mutant isolation, gene transfer, studies of storage product biochemistry, and physiological aspects of embryo maturation.

Introduction

The occurrence of haploid embryos in plants was first reported by Blakeslee et al. in 1922. Other reports indicated the recovery of haploids, probably pollen derived, as a consequence of interspecific hybridization and embryo development without fertilization (Kostoff 1934).