Involved in the second phase of hydrogen peroxide generation during hypersensitive response-like cell death. Involved in the innate immune MAP kinase signaling cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) downstream of bacterial flagellin receptor FLS2. Activates by phosphorylation the downstream MPK3 and MPK6. YDA-MKK4/MKK5-MPK3/MPK6 module regulates stomatal cell fate before the guard mother cell (GMC) is specified. This MAPK cascade also functions downstream of the ER receptor in regulating coordinated local cell proliferation, which shapes the morphology of plant organs. MKK4 and MKK5 participate in the regulation of floral organ abscission. Mediates osmotic-stress response via its regulation of MPK3 activity. Target of the Pseudomonas syringae type III effector HopF2. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Activated through serine and threonine phosphorylation by MEKK1. Inhibited through phosphorylation by GSK3/Shaggy-like kinase ASKs. Interacts with ASK7/BIN2 (PubMed:23341468). Interacts with P.syringae type III effector HopF2 (PubMed:20571112). Interacts with MPK6 (PubMed:19513235). Interacts with RACK1A, RACK1B and RACK1C (PubMed:25731164). Interacts with MAPKKK5 mainly in the cytosol (PubMed:27679653). Expressed higher in stems and leaves than in flowers and roots. Phosphorylation at Thr-224 and Ser-230 by MAP kinase kinase kinases positively regulates kinase activity. Phosphorylation at Ser-230 and Thr-234 by GSK3/Shaggy-like kinase ASKs negatively regulates kinase activity. Phosphorylated by MAPKKK5 (PubMed:27679653). RNAi double mutants MKK4 and MKK5 have a strong abscission defect. Higher sensitivity to high salt and drought stresses. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.