Polyamines improve K+/Na+ homeostasis in barley seedlings by regulating root ion channel activities

Polyamines are known to increase in plant cells in response to a variety of stress conditions. However, the physiological roles of elevated polyamines are not understood well. Here we investigated the effects of polyamines on ion channel activities by applying patch-clamp techniques to protoplasts derived from barley (Hordeum vulgare) seedling root cells. Extracellular application of polyamines significantly blocked the inward Na+ and K+ currents (especially Na+ currents) in root epidermal and cortical cells. These blocking effects of polyamines were increased with increasing polycation charge. In root xylem parenchyma, the inward K+ currents were blocked by extracellular spermidine, while the outward K+ currents were enhanced. At the whole-plant level, the root K+ content, as well as the root and shoot Na+ levels, was decreased significantly by exogenous spermidine. Together, by restricting Na+ influx into roots and by preventing K+ loss from shoots, polyamines were shown to improve K+/Na+ homeostasis in barley seedlings. It is reasonable to propose that, therefore, elevated polyamines under salt stress should be a self-protecting response for plants to combat detrimental consequences resulted from imbalance of Na+ and K+.