Mitogen-activated protein kinase (MAPK) 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 (PubMed:27913741). Involved in oxidative stress-mediated signaling cascade (such as ozone). Involved in the innate immune MAP kinase signaling cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) downstream of bacterial flagellin receptor FLS2. May be involved in hypersensitive response (HR)-mediated signaling cascade by modulating LIP5 phosphorylation and subsequent multivesicular bodies (MVBs) trafficking. May phosphorylate regulators of WRKY transcription factors. Phosphorylates 1-aminocyclopropane-1-carboxylic acid synthases (ACS2 and ACS6) and may be involved in the regulation of bacterial elicitor flagellin-induced ethylene production. Regulates locally gene-mediated and basal resistance response to certain pathogens. May be involved in the cold and salinity stress-mediated MAP kinase signaling cascade (MEKK1, MKK1/MKK2 and MPK4/MPK6). 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. MKK3-MPK6 module plays an important role in the jasmonate signal transduction pathway through the negative regulation of MYC2/JIN1 expression. MKK9-MPK3/MPK6 module phosphorylates and activates EIN3, leading to the promotion of EIN3-mediated transcription in ethylene signaling. MPK3/MPK6 cascade regulates camalexin synthesis through transcriptional regulation of the biosynthetic genes after pathogen infection. MKK9-MPK6 module positively regulates leaf senescence. YDA-MKK4/MKK5-MPK3/MPK6 module regulates stomatal cell fate before the guard mother cell (GMC) is specified. When activated, reinforces the feedback loop by phosphorylating BASL, and inhibits stomatal fate by phosphorylating SPCH (PubMed:25843888). This MAPK cascade also functions downstream of the ER receptor in regulating coordinated local cell proliferation, which shapes the morphology of plant organs. 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 MKK2, MKK3, MKK4, MKK5, MKK7 and MKK9. Activated in response to touch, wounding, low temperature, low humidity, salt stress, hydrogen peroxide, ozone, ACC (an ethylene precursor), jasmonic acid (JA), mastoparan and UVC. Activated in response to elicitors: oligogalacturonides, hexameric chitin fragments, fungal xylanase, and the bacterial flagellin and harpin. Activated upon Pseudomonas syringae pv. tomato DC3000 infection. Repressed by the protein phosphatase 2C AP2C1 and the protein-tyrosine-phosphatases MKP1 and PTP1. Repressed by DSPTP1B/MKP2-mediated dephosphorylation. Activated by polarized BASL (PubMed:27746029). Triggered by MKKK20 in response to various abiotic stresses, including osmotic stress, cold and reactive oxygen species (ROS) (PubMed:21969089). Activated by MKK5 in response to abscisic acid (ABA) (PubMed:27913741). Interacts with MEKK1, MKK1 and MKK2. May form a ternary complex with MEKK1 and MKK1 or MKK2. Interacts with NDPK2, AP2C1, MKP1 and PTP1. Interacts with DSPTP1B/MKP2, especially during HR-like responses triggered by fungal elicitors. Interacts with MKK4, MKK5 and MKK6 (PubMed:12506203, PubMed:15225555, PubMed:17630279, PubMed:19513235, PubMed:19789277, PubMed:20626661, PubMed:21057191, PubMed:21575092, PubMed:27913741). Binds to LIP5 (PubMed:25010425). Interacts with VQ4 and IKU1/VQ14 (PubMed:24750137). Interacts with RACK1A, RACK1B and RACK1C (PubMed:25731164). Interacts with PTP1 (PubMed:27029354). Interacts with FLZ9 (Ref.48). Binds to BASL and YDA (PubMed:25843888). Translocated into the nucleus in response to phosphorylation (Probable). Recruited by BASL at the cell cortex in a polarized manner (PubMed:25843888). Mobility in stomatal lineage ground cells (SLGCs) is triggered by BASL, increased in response to hydrogen peroxide H(2)O(2), but repressed by U0126 (PubMed:27746029). Copolarizes with BASL, YDA and MPK3 in stomatal asymmetric cell division (ACD) cells. By Alternaria brassicae pathogen infection. The TXY motif contains the threonine and tyrosine residues whose phosphorylation activates the MAP kinases. Dually phosphorylated on Thr-221 and Tyr-223, which activates the enzyme. Dephosphorylated by DSPTP1B/MKP2. Reduced sensitivity to abscisic acid (ABA) during germination. Insensitivity to 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). Delayed senescence phenotype. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. MAP kinase subfamily.