Androgenesis, the process by which pseudoembryos (embryoids) able to germinate into plants are produced from microspores (pollen embryogenesis), is of significant interest for development and genetic research as well as for plant breeding and biotechnology, since the process is a means for producing genetically true-breeding, doubled-haploid (DH) plants. By producing DH progeny, the number of possible gene combinations for inherited traits is more manageable. An efficient DH technology can greatly reduce the time and the cost of cultivar development.
Low efficiency in DH production previously has limited exploitation of this potentially powerful method for crop improvement. Several methods of haploid production have been investigated and reported in the literature, including microspore and/or another culture (androgenesis), ovule culture (gynogenesis), Hordeum bulbosum L. or maize (Zea mays L.) pollination methods (alien species chromosome elimination), and an alien cytoplasm system (Dunwell, 1985; Kasha, 1989). Microspore and other culture methods have the potential to produce more than a thousand haploid plants per cultured anther; all other methods are limited to one haploid plant per floret (Devaux, 1988). Androgenesis induction in microspores may be affected by various factors which cause low induction efficiency and by genotype dependence (Dunwell, 1985). Most advances toward improving anther–microspore culture methods have been focused primarily on the concept of using “stress” treatments to induce androgenesis from the preprogrammed gametophytic to the sporophytic pathway (Touraev et al., 1996, 1997; Hu and Kasha, 1999; Zhou and Konzak, 1997; Zheng and Konzak, 1999; Simonson et al., 1997; Reynolds, 1997). Those culture systems have been effective only for a narrow range of responsive genotypes, and other genotypes remain recalcitrant. Thus, more effective methods are needed for inducing androgenesis in large populations of microspores for a wide range of genotypes.
A relationship between microspore embryogenesis and chemical treatment was observed in our experiments and by others (Konzak et al., 2000; Bennett and Hughes, 1972; Rowell and Miller, 1971; Picard et al., 1987). Although Picard et al. (1987) described improvements in androgenesis with wheat (T. aestivum) anther cultures, their treatments were less effective than those in use for another culture at that time (Zhou and Konzak, 1989). We envisioned that according to the signal system concept of Ryan and Balls (1962) and Constabel et al. (1995) some chemical formulations could effectively induce a large proportion of microspores to become embryogenic, if the correct formulations were developed. We recognized, however, that after embryogenesis was induced, the induced microspores require an optimal physiological environment to develop further into embryoids able to germinate and develop into green plants.
The objectives of this work were to develop a method for efficiently initiating microspore embryogenesis by a chemical inducer formulation, and for producing large quantities of microspore-derived green plants from a wide spectrum of genotypes under optimal culture conditions.
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