CIPK serine-threonine protein kinases interact with CBL proteins. Binding of a CBL protein to the regulatory NAF domain of CIPK protein leads to activation of the kinase in a calcium-dependent manner. Downstream of CBL1, CBL2, CBL3 and CBL9, regulates by phosphorylation the K(+) conductance and uptake of AKT1 in low K(+) condition, in response to calcium signaling and during the stomatal opening regulation by monitoring the turgor pressure in guard cells. In response to low nitrate concentration, phosphorylates NRT1.1, switching it from a low-affinity nitrate transporter to a high-affinity transporter. Confers tolerance to low potassium conditions. Involved in drought sensitivity and leaf transpiration. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Part of a K(+)-channel calcium-sensing kinase/phosphatase complex composed by a calcium sensor CBL (CBL1, CBL2, CBL3 or CBL9), a kinase CIPK (CIPK6, CIPK16 or CIPK23), a phosphatase PP2C (AIP1) and a K(+)-channel (AKT1). Interacts with AKT1, CBL1, CBL2, CBL3, CBL5, CBL8, CBL9 and NRT1.1. Associated to the plasma membrane when associated with AKT1, CBL1 and CBL9. In seedlings, mostly in vascular bundles, and in roots, especially in cortex and endodermis cells. In adult plants, mostly expressed in flowers, and, to a lower extent, in roots, leaves, stems and siliques, particularly in vascular tissues. Also detected in guard cells and root hairs. In roots under low K(+) conditions and transiently by nitrate. The activation loop within the kinase domain is the target of phosphorylation/activation by upstream protein kinases. The PPI motif mediates the interaction with the ABI (abscisic acid-insensitive) phosphatases (By similarity). Autophosphorylated. Plants exhibit an increased drought tolerance and an enhanced sensitivity to abscisic acid (ABA) during the stomatal regulation. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. SNF1 subfamily.