MEKK1, MKK1/MKK2 and MPK4 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. Plays a role in abiotic stress tolerance and plant disease resistance through activation of MPK4 and MPK6 by phosphorylation. Acts redundantly with MKK1. 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 in response to cold and salt stresses through serine and threonine phosphorylation by MEKK1. Interacts with MEKK1, MPK4 and MPK6. May form a ternary complex composed of MEKK1 and MKK1/MKK2 and MPK4. Interacts with MPK10 and MPK11. Interacts with MAPKKK5 mainly in the cytosol (PubMed:27679653). Phosphorylation at Thr-220 and Thr-226 by MAP kinase kinase kinases positively regulates kinase activity. Phosphorylated by MEKK1 in response to cold (PubMed:23857079). Phosphorylated by MAPKKK5 (PubMed:27679653). No obvious developmental defects under normal growth conditions. Simultaneous knockdown of MKK1 and MKK2 results in dwarf and small plants exhibiting a seedling-lethality phenotype. May be due to intron retention. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily.