Autonomous development and regeneration of isolated rice egg cells in a fertilization-independent manner

Parthenogenesis is suppressed in rice egg cells to avoid precocious development before fertilization. We found that cold treatment released cell cycle arrest in egg cells and triggered mitosis. Egg cells isolated from japonica (Nipponbare; NB) and aus (Kasalath; KS) subspecies divided and regenerated into mature plants after cold treatment. The egg-derived plants showed variety of ploidy levels, including haploid (n), diploid (2n), and tetraploid (4n). Nuclear DNA quantification showed that genome duplication occurred during early parthenogenetic development. Owing to the presence of single nucleotide polymorphisms (SNPs) between NB and KS, inter-subspecific hybrid plants (NB-KS hybrids) were created via electrofusion. Egg cells from the NB-KS hybrid developed parthenogenetically into polyploid plants. 2n and 4n plants originating from the same NB-KS egg cell displayed the same homozygous SNP patterns throughout the genome, indicating that these plants were doubled and quadrupled haploids. Transcriptome analyses of cold-treated egg cells demonstrated that parthenogenesis-related candidate genes, including OsBBML1, were upregulated.