Triggers SUMM2-mediated immune responses, including cell death and defense responses. Probably inhibited by the MEKK1-MKK1/ MKK2-MPK4 kinase cascade to adjust plant defense (PubMed:22643122, PubMed:23695980). Seems to contribute in transducing external glutamate (L-Glu) signal that elicits large-scale changes in root architecture (PubMed:23574009). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with MPK4. Expressed at least in rosette leaves (at protein level). Down-regulated by MPK4. Phosphorylated by MPK4 upon treatment with flg22. No visible phenotype (PubMed:20668060, PubMed:23695980). Suppresses the cell death and defense responses not only in mkk1 mkk2 but also in mekk1 and mpk4 mutants (PubMed:22643122, PubMed:23695980). In the triple mutant mekk1 mekk2 mekk3, no apparent phenotype, but meek1-like dwarf phenotype when complemented by MEKK2 (PubMed:23695980). The triple mutant mekk1 mekk2 mekk3 is almost insensitive to external glutamate (L-Glu) on root growth (PubMed:23574009). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.