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 fertilization, but there is limited knowledge of the molecular pathways underlying embryo initiation. In rice, a key role is played by the transcription factor BABY BOOM 1 (OsBBM1), initially expressed from the paternal genome. Ectopic OsBBM1 expression in the egg cell can override the fertilization requirement, giving rise to parthenogenetic progeny. Here we show that the WOX-family transcription factor DWARF TILLER1 (OsDWT1)/WUSCHEL-LIKE HOMEODOMAIN 9 (OsWOX9A), 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 plants.