Significance In flowering plants, membrane-associated kinases of the BRASSINOSTEROID SIGNALING KINASE (BSK) family are ubiquitous, receptor-associated signaling partners in various receptor kinase pathways, where they function in signaling relay. The Brassicaceae-specific BSK family member SHORT SUSPENSOR (SSP), however, acts as a patterning cue in the zygote, initiating the apical-basal patterning process in a signal-like manner. The SSP protein has lost a regulatory, intramolecular interaction and activates the MAPKKK YODA signaling pathway constitutively, in principle, enabling the protein to initiate embryonic patterning without receptor activation. We further show that the BSK family members BSK1 and BSK2, both conserved in flowering plants, activate the same signaling pathway in parallel to SSP and might constitute remnants of an older, canonical signaling pathway still active in Arabidopsis .