Casein kinases are operationally defined by their preferential utilization of acidic proteins such as caseins as substrates. Can phosphorylate casein in vitro. Required for normal root development through modulation of cell elongation. Plants silencing CKI1 show abnormal root development, with reduced number of lateral and adventitious roots, and shortened primary roots as a result of reduced cell elongation. May be involved in abscisic acid (ABA) and brassinosteroid (BR) signaling pathways (PubMed:14535884). Plays an important role in the adaptive growth and fitness under low temperature (LT) conditions. May confer tolerance to LT through an auxin-dependent process (PubMed:24635058). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inhibited by N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide (CKI-7). Monomer. Expressed in leaves, stems, panicles and seeds (PubMed:14535884). Expressed in root tissues and lamina joints (PubMed:16827922). Induced by 24-epi-brassinolide (24-eBL) and down-regulated by abscisic acid (ABA). Autophosphorylated. Belongs to the protein kinase superfamily. CK1 Ser/Thr protein kinase family. Casein kinase I subfamily.