Mitogen-activated protein kinase kinase (MAPKK) which regulates abscisic acid (ABA) responses in a MAPKKK20-MKK5-MPK6 cascade involved in root growth (e.g. root cell division and elongation) and stomatal response, probably via MAPK6 activation by protein phosphorylation (PubMed:27913741). 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. Target of the Pseudomonas syringae type III effector HopF2, that inhibits the activation of the downstream MPK6 and PAMP-triggered immunity. Plays a critical role in high light stress tolerance by the mediation of the Cu/Zn SODs CSD1 and CSD2 gene expression. Phosphorylates BZR1 in vitro. 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 and MAPKKK20 in response to abscisic acid (ABA). Inhibited through phosphorylation by GSK3/Shaggy-like kinase ASKs. Inhibited through ADP-Ribosylation by P.syringae HopF2. Activated after high light stress. Interacts with P.syringae type III effector HopF2 (PubMed:20571112). Interacts with BZR1 (PubMed:24019147). Interacts with MPK6 and MPK3 (PubMed:19513235, PubMed:27913741). Interacts with RACK1A, RACK1B and RACK1C (PubMed:25731164). Interacts with MAPKKK5 mainly in the cytosol (PubMed:27679653). Binds to BASL (PubMed:25843888). Interacts with MAPKKK20 (PubMed:27913741). Expressed higher in stems and leaves than in flowers and roots. Phosphorylation at Thr-215 and Ser-221 by MAP kinase kinase kinases positively regulates kinase activity (PubMed:11875555). Phosphorylated by MAPKKK5 and MAPKKK20 in response to abscisic acid (ABA) (PubMed:27679653, PubMed:27913741). ADP-ribosylation at Arg-313 by P.syringae type III effector HopF2 reduces the ability of the protein to phosphorylate downstream MPK6. RNAi double mutants MKK4 and MKK5 have a strong abscission defect. RNAi mutant MKK5 shows super-sensitivity to high light stress. Insensitivity to abscisic acid (ABA) in terms of root growth inhibition (e.g. root cell division and elongation) and stomatal response leading to increased water loss under dehydrated conditions (PubMed:27913741). Impaired ABA-mediated increased activity of MPK6 (PubMed:27913741). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily. Truncated N-terminus.