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. The daughter cells of the asymmetric zygote division form the pro-embryo and the mostly extra-embryonic suspensor.1 The suspensor plays a pivotal role in nutrient and hormone transport and rapid growth of the embryo.2,3 Zygote polarization is controlled by a MITOGEN-ACTIVATING PROTEIN (MAP) kinase signaling pathway including the MAPKK kinase (MAP3K) YODA (YDA)4 and the upstream membrane-associated proteins BRASINOSTEROID SIGNALING KINASE 1 (BSK1) and BSK2.5,6 Furthermore, suspensor development is controlled by cysteine-rich peptides of the EMBRYO SURROUNDING FACTOR 1 (ESF1) family.7 While they act genetically upstream of YDA, the corresponding receptor to perceive these potential ligands is unknown. In other developmental processes, such as stomata development, YDA activity is controlled by receptor kinases of the ERECTA family (ERf).8-12 While the receptor kinases upstream of BSK1/2 in the embryo have so far not been identified,1 YDA is in part activated by the sperm cell-derived BSK family member SHORT SUSPENSOR (SSP) that represents a naturally occurring, constitutively active variant of BSK1.5,13 It has been speculated that SSP might be a paternal component of a parental tug-of-war controlling resource allocation toward the embryo.2,13 Here, we show that in addition to SSP, the receptor kinase ERECTA plays a crucial role in zygote polarization as a maternally contributed part of the embryonic YDA pathway. We conclude that two independent parental contributions initiate zygote polarization and control embryo development.