Research advances on plant synthetic apomixis

Heterosis, a phenomenon in which hybrid offspring shows better performance in many aspects than either of their homozygous parents, has been widely applied for improving crop productivity and adaptability. However, progenies generated via sexual reproduction are diverse owing to the genetic recombination during meiosis, which is not conducive to the application of heterosis. Apomixis is a type of asexual reproduction through which plants can produce clonal seeds without undergoing normal meiosis and fertilization. Therefore, apomixis has been a research hotspot for a long time because of its potential in fixing heterosis. Over 400 plant species exhibit apomictic phenomena in nature, and many genes linked with apomictic loci have been discovered in various species. These genes are involved in distinct regulatory mechanisms of apomixis. Even after decades of effort, these genes have not been successfully applied for crop apomixis because of complexity of the underlying mechanisms and lack of apomictic crops in nature. Due to the apomixis of plants also can be induced artificially by allowing plants to undergo unusual meiosis and fertilization, synthetic apomixis based on reproductive mechanisms has been proposed to fix heterosis. In MiMe ( mitosis instead of meiosis ), meiotic division is completely replaced by a mitotic-like division and diploid gametes that are genetically identical to their mother are produced. MiMe is induced by the absence of genes required for the pairing and recombination of homologous chromosomes, maintenance of sister chromatid cohesionas well as for the transition of germ cells from meiosis I to II. Following normal fertilization of clonal diploid gametes, the ploidy of offspring is doubled in each generation. To achieve synthetic apomixis and obtain clonal seeds, chromosome doubling following normal fertilization must be prevented. An effective way for this prevention is the elimination of one set of chromosomes from either parent. When a plant with altered CENH3 (centromere-specific histone H3) protein is crossed with a wild-type plant, haploid progenies are induced through the elimination of chromosomes from the cenh3 mutant. Furthermore, combining MiMe plants with a genome elimination line via crossing is an effective strategy to produce clonal seeds. In addition, upon expression of BBM1 (BABY BOOM 1), which plays an important role in the initiation of embryogenesis, in the egg cell, haploid progenies are generated that produce seeds with haploid embryos following self-fertilization. Moreover, MTL (matrilineal) disruption in plants using CRISPR/Cas9 gene editing technology also can generate haploid progenies. Therefore, the combination of MiMe with BBM1 - or MTL -induced production of haploid progenies can achieve synthetic apomixis in plants and produce clonal seeds. In this review, we summarize advances in synthetic apomixis at the molecular level and discuss the improvement of synthetic apomixis as well as its potential application in crop breeding.