Polyamines Improve K+/Na+ Homeostasis in Barley Seedlings by Regulating Root Ion Channel Activities

Abstract

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⁺.