Functions as a link between plant defense pathways, stress responses and potassium homeostasis. Promotes osmotic stress sensitivity, responses to the bacterial-derived pathogen-associated molecular pattern (PAMP) flg22, and resistance to bacterial pathogens. Promotes the accumulation of POT5/HAK5, a potassium transporter that mediates high-affinity uptake during potassium deficiency. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Kinase activity is suppressed by interaction with CML9. Interacts with CML9 and POT5/HAK5. Induced by mannitol and the pathogen-associated molecular pattern (PAMP) flg22. Autophosphorylated at Ser-17 and Ser-26. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.