Insights into a Key Developmental Switch and Its Importance for Efficient Plant Breeding

Sexual reproduction of plants starts with the for- mation of gametes and a 2-fold reduction in the number of chromosomes (ploidy) of these cells. Fer- tilization, the union of sperm and egg cell restores the ploidy level to its normal value again. During the formation of pollen grains, the structures that contain the male gametes, the reduction in the number of chromosomes happens when the vegetative cells un- dergo meiosis and give rise to microspores. In a diploid plant with 2n chromosomes the microspores are haploid with n chromosomes. Microspores nor- mally develop into pollen grains, but as a result of an unusual event, microspores can also undergo a de- velopmental process that leads to the formation of a haploid embryo. This androgenesis pathway is also called microspore embryogenesis or regeneration. During androgenesis the haploid microspore divides and develops into an embryo and subsequently into a new haploid plant. Haploid plants are not fertile because they cannot make gametes again since this would require another halving of the chromosome number. If chromosome doubling occurs at some stage during androgenesis, the regenerated plants from these microspores are completely homozygous (doubled haploid) fertile individuals. Such doubled haploid plants from haploid microspores provide ex- cellent material for research, plant breeding, and plant transformation. However, androgenesis is not a naturally occurring event in angiosperms and only seems to be induced as a result of certain chemical and physical stimuli. Since androgenesis involves the control and repro- gramming of developmental switches, it provides opportunities to investigate key elements in develop- mental control. Moreover, via androgenesis, fertile homozygous progeny from a heterozygous parent can be obtained in a single generation, thus signifi- cantly reducing time required in breeding programs and providing a major advantage in preparing F1 hybrid seeds as well. Both the possibilities for study- ing basic processes in plant development and the economic importance have motivated numerous re- search groups to investigate androgenesis in differ- ent crops. Here, the basic steps in androgenesis in- duction protocols, different processes and the signals involved in reprogramming of the pollen develop- ment pathway, the possible molecular markers, and the evolutionary perspective are considered.