Synergistic induction of fertilization-independent embryogenesis in rice egg cells by paternal-genome-expressed transcription factors

In flowering plants, rapid activation of the zygotic genome occurs after fertilization1–3, but there is limited knowledge of the molecular pathways underlying embryo initiation4. In rice, a key role is played by the transcription factor BABY BOOM 1 (OsBBM1), initially expressed from the paternal genome1. Ectopic OsBBM1 expression in the egg cell can override the fertilization requirement, giving rise to parthenogenetic progeny5. Here we show that the WOX-family transcription factor DWARF TILLER1 (OsDWT1)/WUSCHEL-LIKE HOMEODOMAIN 9 (OsWOX9A)6, another gene paternally expressed in zygotes, is a strong enhancer of embryo initiation by OsBBM1. Co-expression of OsWOX9A and OsBBM1 in egg cells results in 86–91% parthenogenesis, representing 4- to 15-fold increases over OsBBM1 alone. These results suggest that embryo initiation is promoted by the synergistic action of paternal-genome-expressed transcription factors in the fertilized egg cell. These findings can be utilized for the efficient production of haploids, as well as clonal hybrid seeds in crop plants7,8. The efficient induction of embryos without fertilization, which is important for crop breeding and hybrid seed production, can be achieved by combined expression in the egg of BBM1 and WOX9A, two rice transcription factors normally expressed from the male genome.