Involved in the main abscisic acid-mediated (ABA) signaling pathway and in early ABA perception. Together with RPK2, required for pattern formation along the radial axis (e.g. the apical embryonic domain cell types that generate cotyledon primordia), and the apical-basal axis (e.g. differentiation of the basal pole during early embryogenesis). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed in roots, stems, leaves, and flowers. First detected in the suspensor cells of octant-stage embryos, in the basal plasma membrane of the basal-most cell of the suspensor, which is in direct contact with maternal tissue. Expressed in the central domain protodermal cells when cotyledon primordia become recognizable. Later observed throughout the central domain and basal domain of the embryo proper, as well as the suspensor. By ABA (at protein level), dehydration, high salt levels, and low temperature. Decreased sensitivity to ABA during germination, growth, and stomatal closure. Impaired central domain protoderm patterning defects, and defective cotyledon primordia cell types. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.