The ANPs-MKK6-MPK4 module is involved in the regulation of plant cytokinesis during meiosis and mitosis. Essential to promote the progression of cytokinesis and for cellularization (formation of the cell plate) during male-specific meiosis. Involved in cortical microtubules organization and stabilization by regulating the phosphorylation state of microtubule-associated proteins such as MAP65-1. Involved in root hair development process. Negative regulator of systemic acquired resistance (SAR) and salicylic acid- (SA) mediated defense response. Required for jasmonic acid- (JA) mediated defense gene expression. May regulate activity of transcription factor controlling pathogenesis-related (PR) gene expression. Seems to act independently of the SAR regulatory protein NPR1 (Nonexpresser of PR1). Phosphorylates MKS1 and transcription factors WRKY25 and WRKY33. The 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 (PubMed:11163186, PubMed:15225555, PubMed:15358537, PubMed:15990873, PubMed:18982020, PubMed:20215588, PubMed:20802223, PubMed:21098735, PubMed:21575092, PubMed:25770109). Phosphorylates MEKK2 upon treatment with flg22 (PubMed:22643122). Involved in stomatal movement regulation by repressing HT1 and HT1-mediated GHR1 phosphorylation (PubMed:27694184, PubMed:27923039). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by threonine and tyrosine phosphorylation. Activated by the MAP kinase kinases MKK1 and MKK2. Activated in response to touch, wounding, low temperature, low humidity, salt stress and the bacterial elicitors flagellin and harpin. Activated upon Pseudomonas syringae pv. tomato DC3000 infection. Repressed by the protein phosphatase 2C AP2C1. Repressed by DSPTP1-mediated dephosphorylation. Activated by the MAP kinase kinase MKK6 in vitro. Interacts with MEKK1, MKK1, MKK2 and MKK6. May form a ternary complex composed of MEKK1 and MKK1/MKK2 and MPK4. Interacts with MKS1 and AP2C1. May form a ternary or larger complex with MKS1 and WRKY25 and/or WRKY33. Interacts with MAP65-1 (PubMed:15225555, PubMed:15990873, PubMed:17630279, PubMed:18982020, PubMed:19513235, PubMed:20215588, PubMed:20802223, PubMed:21575092, PubMed:9878570). No interactions with RACK1A, RACK1B or RACK1C (PubMed:25731164). Interacts directly with ASR3 and mediates its phosphorylation (PubMed:25770109). Binds to MEKK2 (PubMed:22643122). Interacts with PAT1 (PubMed:25603932). Binds to HT1 (PubMed:27694184). Translocated into the nucleus in response to phosphorylation (Probable). Localized to the cell plate. Ubiquitous. Expressed in the veins and stomatal guard cells of leaf plates, petioles, stem, roots and flowers. Observed in root epidermal cells and root hairs, especially in the root hair formation zone. During root hair development, both observed in root hair bulges and in young outgrowing root hairs. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-201 and Tyr-203, which activates the enzyme. Autophosphorylated on serine and tyrosine residues. Dephosphorylated by DSPTP1. Phosphorylated by MKK6 in vitro. Dwarf plants with cytokinetic defects in leaf epidermal cells. Abnormally developed and branched root hairs. Retarded root growth with the protrusion of many epidermal cells from roots. Heavily bundled and disoriented cortical microtubules resistant to oryzalin. Exhibits a seedling-lethality phenotype. Defects in the formation of the cell plate. Abnormal mature pollen grains. Abolished CO(2)-mediated stomatal closure (PubMed:27694184). Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MAP kinase subfamily.