High-affinity potassium transport in barley roots.: Ammonium-sensitive and -insensitive pathways

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 tho systems contributing to the NH4+-insensitive component in barley plants is discussed.