May function as a negative regulator of polar auxin transport. Positively regulates plant basal and systemic acquired resistance (SAR). Activates MPK3 and MPK6 in vitro. 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] Interacts with MPK15. Expressed in all tissues, with a relatively higher level in leaves and lower level in roots and flowers. By Pseudomonas syringae pv. tomato (Pst) DC3000/avrRpt2 infection. Phosphorylation at Ser-193 and Ser-199 by MAP kinase kinase kinases positively regulates kinase activity. Overexpression of MKK5 (mutant bud1) causes deficiency in polar auxin transport and leads to plant architectural abnormality (PubMed:16377756). The bud1 mutant accumulates elevated levels of SA, and exhibits constitutive PR gene expression and enhanced resistance to both bacterial and oomycete pathogens (PubMed:17908155). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase subfamily. Extended C-terminus.