Abstract
In plants, embryogenesis and reproduction are not strictly dependent on fertilization. Several species can produce embryos in seeds asexually, a process known as apomixis. Apomixis is defined as clonal asexual reproduction through seeds, whereby the progeny is identical to the maternal genotype, and provides valuable opportunities for developing superior cultivars, as its induction in agricultural crops can facilitate the development and maintenance of elite hybrid genotypes. In this review, we summarize the current understanding of apomixis and highlight the successful introduction of apomixis methods into sexual crops. In addition, we discuss several genes whose overexpression can induce somatic embryogenesis as candidate genes to induce parthenogenesis, a unique reproductive method of gametophytic apomixis. We also summarize three schemes to achieve engineered apomixis, which will offer more opportunities for the realization of apomictic reproduction.
Highlights
This result highlights the significant role of PsASGR-BBML in parthenogenesis and might be valuable for engineering apomixis in crops
The heterologous expression of the ToPAR gene can induce embryo-like structures under the control of its own promoter in egg cells of sexual lettuce in the absence of fertilization. These findings show that PsASGR-BBML, OsBBM1, and ToPAR are ideal genes for inducing parthenogenesis
Asexual propagation through engineered apomixis can be achieved by combining the mitosis instead of meiosis (MiMe) triple mutant with the ectopic expression of rice OsBBM1 in egg cells (Khanday et al, 2019)
Summary
Reproduction is a fundamental, essential process in plant biology and has great practical significance as much of the food supply is seed-based (Fei et al, 2019). CRISPR/Cas9-mediated genome editing was used to synchronously knockout three genes in rice, PAIR1, OsREC8, and OsOSD1 (Khanday et al, 2019) or OsSPO11-1, OsREC8, and OsOSD1 (Xie et al, 2019), to produce MiMe phenotypes and achieve apomixis through additional parthenogenesis or an inducer without fertilization. To achieve apomixis, another pivotal step is the autonomous development of an embryo from an egg cell by parthenogenesis. When BBML is expressed under the control of its own promoter in sexual pearl millet, haploid embryos are produced (Conner et al, 2015) This result highlights the significant role of PsASGR-BBML in parthenogenesis and might be valuable for engineering apomixis in crops.
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