Transcriptional Activation of Arabidopsis Axis Patterning Genes WOX8/9 Links Zygote Polarity to Embryo Development

In most flowering plants, the apical-basal body axis is established by an asymmetric division of the polarized zygote. In Arabidopsis, early embryo patterning is regulated by WOX homeobox genes, which are coexpressed in the zygote but become restricted to apical (WOX2) and basal (WOX8/9) cells. How the asymmetry of zygote division is regulated and connected to the daughter cell fates is largely unknown. Here, we show that expression of WOX8 is independent of the axis patterning signal auxin, but, together with the redundant gene WOX9, is activated in the zygote, its basal daughter cell, and the hypophysis by the zinc-finger transcription factor WRKY2. In wrky2 mutants, egg cells polarize normally but zygotes fail to reestablish polar organelle positioning from a transient symmetric state, resulting in equal cell division and distorted embryo development. Both defects are rescued by overexpressing WOX8, indicating that WRKY2-dependent WOX8 transcription links zygote polarization with embryo patterning.