Downstream effector of TOR signaling pathway. May be involved in adaptation of plant to cold or high-salt conditions. 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. Interacts with TAP46 (PubMed:25399018). Binds to MRF1 (PubMed:29084871). The activation loop within the kinase domain is the target of phosphorylation. Undergoes serine-specific autophosphorylation (By similarity). Phosphorylated at Thr-455 by TOR. Belongs to the protein kinase superfamily. AGC Ser/Thr protein kinase family. S6 kinase subfamily.