High-Affinity Potassium Transport in Barley Roots. Ammonium-Sensitive and -Insensitive Pathways

Abstract

In an attempt to understand the process mediating K+transport into roots, we examined the contribution of the NH4 +-sensitive and NH4 +-insensitive components of Rb+transport to the uptake of Rb+ in barley (Hordeum vulgare L.) plants grown in different ionic environments. We found that at low external Rb+ concentrations, an NH4 +-sensitive component dominates Rb+ uptake in plants grown in the absence of NH4 +, while Rb+ uptake preferentially occurs through an NH4 +-insensitive pathway in plants grown at high external NH4 + concentrations. A comparison of the Rb+-uptake properties observed in roots with those found in heterologous studies with yeast cells indicated that the recently cloned HvHAK1 K+ transporter may provide a major route for the NH4 +-sensitive component. HvHAK1 failed to complement the growth of a yeast strain defective in NH4 + transport, suggesting that it could not act as an NH4 + transporter. Heterologous studies also showed that the HKT1 K+/Na+-cotransporter may act as a pathway for high-affinity Rb+ transport sensitive to NH4 +. However, we found no evidence of an enhancement of Rb+ uptake into roots due to Na+addition. The possible identity of the systems contributing to the NH4 +-insensitive component in barley plants is discussed.