Functions in a MAP kinase cascade that acts as a molecular switch to regulate the first cell fate decisions in the zygote and the early embryo. Promotes elongation of the zygote and development of its basal daughter cell into the extra-embryonic suspensor. In stomatal development, acts downstream of the LRR receptor TMM, but upstream of the MKK4/MKK5-MPK3/MPK6 module to regulate stomatal cell fate before the guard mother cell (GMC) is specified. Plays a central role in both guard cell identity and pattern formation. This MAPK cascade also functions downstream of the ER receptor in regulating coordinated local cell proliferation, which shapes the morphology of plant organs. Upon brassinosteroid signaling, is inhibited by phosphorylation of its auto-inhibitory N-terminal domain by the GSK3-like kinase ASK7. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Contains an N-terminal autoinhibitory domain. Interacts with ASK7 (PubMed:22307275). Interacts with BSK12/SSP (PubMed:28821747). Binds to BASL and MPK6 (PubMed:25843888). Recruited by BASL at the cell cortex in a polarized manner. Expressed in roots, leaves, guard cells, stems, flowers and siliques. Copolarizes with BASL and MPK3/MPK6 in stomatal asymmetric cell division (ACD) cells. Severely dwarf plants that rarely survive on soil, small rosettes, compact leaves, extremely compressed shoots, and short, sterile flowers. No proper elongation of the zygote and differentiation of the extra-embryonic suspensor. Excess of stomata in leaves. N-terminal deletions of YDA results in gain-of-function alleles with phenotypes (no stomata and exaggerated suspensor growth) opposite to loss-of-function phenotypes. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.