Independent parental contributions initiate zygote polarization in Arabidopsis thaliana

Embryogenesis of flowering plants is initiated by polarization of the zygote, a prerequisite for correct axis formation in the embryo. Zygote polarity and the decision between embryonic and non-embryonic development in the daughter cells is controlled by a MITOGEN-ACTIVATING PROTEIN (MAP) kinase signaling pathway including the MAPKK kinase YODA (YDA). Upstream of YDA act two members of the BRASINOSTEROID SIGNALING KINASE (BSK) family, BSK1 and BSK2. These membrane-associated proteins serve as signaling relay linking receptor kinase complexes with intracellular signaling cascades. The receptor kinases acting upstream of BSK1 and BSK2 in the zygote, however, have so far not been identified. Instead, YDA can in part be activated by the non-canonical BSK family member SHORT SUSPENSOR (SSP) that is contributed by the sperm cell during fertilization. Here, we show that the receptor kinase ERECTA plays a crucial role in zygote polarization as maternally contributed part of the embryonic YDA pathway. SSP on the other hand provides an independent, paternal input to YDA activation, outlining a Y-shaped pathway. We conclude that two independent parental contributions initiate zygote polarization and control suspensor formation and embryonic development.