Abstract
Plant embryogenesis is initiated by an asymmetric cell division of the zygote. However, how different cell identities are achieved in the daughter cells of this asymmetric cell division on a molecular, mechanistic level is poorly understood. In Arabidopsis thaliana, the cellular identity of the resulting daughter cells is controlled by a MAP kinase signaling cascade including the MAP3K YODA (YDA). Polar activation of YDA suppresses embryo formation in the basal daughter cell and ultimately promotes suspensor formation. In Brassicaceae, the YODA pathway can be activated by distinct parental contributions: This includes canonical receptor-mediated signaling by a maternally provided receptor complex including ERECTA and BRASSINOSTEROID SIGNALING KINASE1 (BSK1) and BSK2. In addition, the paternally provided pseudokinase SHORT SUSPENSOR (SSP/BSK12), a Brassicaceae-specific member of the BSK family, activates YODA in a receptor-independent fashion. We present new data on the impact of polar YODA activation on early embryonic patterning and shed light on the mechanism and evolution of distinct modes of YDA activation in the zygote on a molecular and structural level. We furthermore discuss possible benefits of different modes of YDA activation and their distinct parent-of-origin effects.
