Potassium nutrition, sodium toxicity, and calcium signaling: connections through the CBL–CIPK network

Plant roots take up numerous minerals from the soil. Some minerals (e.g., K+) are essential nutrients and others (e.g., Na+) are toxic for plant growth and development. In addition to the absolute level, the balance among the minerals is critical for their physiological functions. For instance, [K+]/[Na+] ratio and homeostasis often determine plant growth rate. Either low-K or high-Na in the soil represents a stress condition that severely affects plant life and agricultural production. Earlier observations indicated that higher soil Ca2+ improve plants growth under low-K or high-Na condition, implying functional interaction among the three cations. Recent studies have begun to delineate the signaling mechanisms underlying such interactions. Either low-K+ or high-Na+ can trigger cellular Ca2+ changes that lead to activation of complex signaling networks. One such network consists of Ca2+ sensor proteins (e.g., CBLs) interacting with their target kinases (CIPKs). The CBL-CIPK signaling modules interact with and regulate the activity of a number of transporting proteins involved in the uptake and translocation of K+ and Na+, maintaining the “balance” of these cations in plants under stress conditions.