Downstream effector of TOR signaling pathway involved in osmotic stress response (PubMed:20683442). Could be involved in the control of plant growth and development (PubMed:20683442). Phosphorylates the ribosomal proteins P14, P16 and S6 (PubMed:20683442). Functions as a repressor of cell proliferation and required for maintenance of chromosome stability and ploidy levels through the RBR1-E2F pathway (PubMed:20683442). Mediates the phosphorylation of MRFs (e.g. MRF1) (PubMed:29084871). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by PDK1. Repressed during osmotic stress. Interacts with RAPTOR1 (PubMed:16377759). Interacts with RBR1-E2FB complex through its LVxCxE motif (PubMed:20683442). Interacts with TAP46 (PubMed:25399018). Binds to MRF1 (PubMed:29084871). Expressed in all tissues. Predominates during high metabolic activity in growing buds, root tips, leaf margins and germinating seeds. The activation loop within the kinase domain is the target of phosphorylation. Undergoes serine-specific autophosphorylation. Phosphorylated at Thr-449 by TOR. Plants overexpressing KPK1 are hypersensitive to osmotic stress. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.