MEKK1, MKK1/MKK2 and MPK4/MPK6 function in a signaling pathway that modulates the expression of genes responding to biotic and abiotic stresses and also plays an important role in pathogen defense by negatively regulating innate immunity. Activates by phosphorylation the downstream MPK4. Acts redundantly with MKK2. MKK1-MPK6 module mediates abscisic acid (ABA)-dependent CAT1 expression with H(2)O(2) production and response to drought and salt stress. MKK1-MPK6 module is also involved in sugar signaling during the process of seed germination. 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 in response to wounding, cold, drought, salt stresses, abscisic acid (ABA), hydrogen peroxide, bacterial flagellin and laminarin beta-glucan. Interacts with MEKK1 and MPK4. May form a ternary complex composed of MEKK1 and MKK1/MKK2 and MPK4. Interacts with P.syringae type III effector HopF2. Interacts with MPK11. Expressed in roots, stem, flowers and siliques. By wounding. Phosphorylation at Thr-218 and Ser-224 by MAP kinase kinase kinases positively regulates kinase activity. No obvious developmental defects under normal growth conditions. Compromised in resistance to both virulent and avirulent Pseudomonas syringae strains. Reduced sensitivity to abscisic acid (ABA) during germination and reduced drought tolerance of seedlings. Simultaneous knockdown of MKK1 and MKK2 results in dwarf and small plants exhibiting a seedling-lethality phenotype. May be due to an intron retention. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.