Engineering BBM cis-regulatory elements releases epigenetic repression to enable highly efficient parthenogenesis and...

In angiosperms, unfertilized egg cells rarely develop into embryos, suggesting that a suppressive mechanism has been established within the egg cell to prevent parthenogenesis. However, this suppressing machinery has remained largely unknown. Here, we report that parthenogenesis in Arabidopsis is inhibited by H3K27me3 methylation on BBM, a trigger for embryogenesis. We found that either the promoter or coding region of AtBBM is transcriptional inhibited in Arabidopsis egg cell. The repression of BBM expression in the egg cell can be relieved either through mutation of H3K27me3 writer CLF and SWN or by overexpression of H3K27me3 eraser JMJ12. Mutation of four putative polycomb response elements (pPREs) in BBM coding region (BBM-m1234) decreased H3K27me3 methylation on BBM and increased its expression. Ectopic expression BBM-m1234 in the egg cell leading to 88.7% egg cell division without fertilization, and produced an average of 9.0% haploid and 60.4% doubled haploid plants after fertilization. We further showed that the number of pPREs in BBM in sexually reproductive species is much higher than that of parthenogenetic species. This study elucidates a fine mechanism that inhibits parthenogenesis and introduces an applicable strategy for highly efficient parthenogenesis and haploid induction.