Fertilization-Independent Embryo Development As a Component of Apomixis

Asexual reproduction through seeds, also known as apomixis, is widespread among angiosperm families, but rare among crop species. Pennisetum squamulatum (syn. Cenchrus squamulatus) is a wild relative of the grain crop species P. glaucum (pearl millet, syn. C. americanus) that reproduces by obligate apospory. Aposporous embryo sacs form from nucellar cells via mitosis, and the unreduced egg undergoes parthenogenesis to develop into an embryo. A gene linked with parthenogenesis was cloned from P. squamulatum and introduced into sexual pearl millet, rice, and maize where its expression led to the formation of haploid embryos. In the most extensively studied transgenic lines of pearl millet, the frequency of haploid embryos was greater than the frequency of haploid plants recovered, likely due to the variability in timing and extent of embryo development along with lack of, or asynchrony with, endosperm formation. Central cell fertilization is required in P. squamulatum for pseudogamous endosperm formation. Fine-tuning temporal regulation of seed developmental processes will enhance progress in installation of apomixis or the utilization of this haploid-inducing technology in grain crops.