Ming Y. Zheng
Professor of Biology, Gordon College, Wenham, MA 01984, USA
Production of doubled haploids (DH) via microspores embryogenesis is a proven method to obtain homozygous individuals in a single step, thus the method is very useful in genetic engineering and crop breeding. The instant homozygosity saves plant breeders multiple years of inbreeding necessary in conventional cross breeding and substantially reduce the population sizes required for effective selection of superior trait combinations. An efficient doubled haploid platform is also highly sought after in generating transgenic plants. Large number of embryogenic microspores are ideal target for genetic transformation. Dominant or recessive target gene(s) to be transferred into gametic cells can be made homozygous in a single generation. In addition, doubled haploids are important tools in plant genome mapping and many areas of basic research, including in vitro embryogenesis, signal transduction and developmental biology. Since the initial success in reprogramming microspores for embryogenesis in our laboratory, many factors associated with microspore embryogenesis were investigated in our research, including stress variables, isolation protocol, media components, nuclear divisions, embryogenic abortion and chromosome doubling. An artificial manipulation, in the form of physical stress, and /or chemical treatment, is employed to reprogram microspores from gametophytic to sporophytic development. Induced embryogenic microspores, characterized by unique morphological features, undergo an interlocked cascades of cell divisions and differentiation that lead to the formation of embryoids. These embryoids “germinate” to give rise to haploid or doubled haploid plants. We also investigated “nursing factors”, released by live ovaries co-cultured with microspores, and found their effects in promoting embryogenesis and reducing the abortion rate among developing microspores. In addition, nuclear fusion inside embryogenic microspores within the first several days of culture is a leading cause for chromosome doubling. These studies have yielded useful information for understanding the developmental process of embryogenesis. As a result, a more efficient doubled haploid system is in place to accelerate crop breeding aiming at rapid responses to climate change, sudden outbreak of pests, diseases and other environmental distresses.