Mitogen-activated protein kinase kinase (MAPKK) that phosphorylates both MKK3 and MPK18 and regulate two separate signaling pathways involved in root microtubule functions (PubMed:28848569). 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 MKK5 activation by protein phosphorylation and subsequent activation of MAPK6 by MKK5 (PubMed:27913741). Involved in various abiotic stresses (e.g. osmotic stress, cold and hydrogen peroxide) responses by phosphorylating and thus regulating MPK6 activity, in an ABA-independent manner (PubMed:21969089). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated through serine, threonine and tyrosine phosphorylation, especially upon abscisic acid (ABA) treatment (PubMed:28848569, PubMed:27913741). Restricted activity by ABI1-mediated dephosphorylation (PubMed:27913741). Interacts with MKK3 and MPK18 via its C-terminal domain (PubMed:28848569, PubMed:30081740). Binds to MKK5 (PubMed:27913741). Expressed in roots, seedlings, leaves, flower buds, flowers and siliques. During flower development, strongly expressed in pollen grains with a progressive increase from young buds to later stages of maturation (PubMed:28848569). To a lesser extent, present in the gynoecium, particularly in the style and carpel (PubMed:28848569). Also expressed in nectaries, petals, sepals and pollen tubes (PubMed:28848569). Accumulates progressively during seedlings development (PubMed:21969089). In mature plants, confined to floral clusters (PubMed:21969089). In roots, present in meristematic and elongation zones (PubMed:27913741). Induced by salt (NaCl), mannitol, methyl viologen (MV), sorbitol and cold (PubMed:21969089). Triggered by abscisic acid (ABA) (PubMed:27913741). Autophosphorylates; active in phosphorylated state (PubMed:28848569). Dephosphorylated by ABI1 (PubMed:27913741). Short roots with abnormal twisting (e.g. leftward skewing) in media containing microtubule-disrupting drugs (e.g. oryzalin) (PubMed:28848569). Stronger sensitivity to high salt concentration and higher water loss rates under dehydration conditions (PubMed:21969089). Increased accumulation of superoxide under high salt condition (PubMed:21969089). All these phenotypes are associated with reduced MPK6 activity (PubMed:21969089). 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. Ser/Thr protein kinase family.