Rapid Up-Regulation of HKT1, a High-Affinity Potassium Transporter Gene, in Roots of Barley and Wheat following Withdrawal of Potassium

High-affinity K(+) uptake in plant roots is rapidly up-regulated when K(+) is withheld and down-regulated when K(+) is resupplied. These processes make important contributions to plant K(+) homeostasis. A cDNA coding for a high-affinity K(+) transporter, HKT1, was earlier cloned from wheat (Triticum aestivum L.) roots and functionally characterized. We demonstrate here that in both barley (Hordeum vulgare L.) and wheat roots, a rapid and large up-regulation of HKT1 mRNA levels resulted when K(+) was withdrawn from growth media. This effect was specific for K(+); withholding N caused a modest reduction of HKT1 mRNA levels. Up-regulation of HKT1 transcript levels in barley roots occurred within 4 h of removing K(+), which corresponds to the documented increase of high-affinity K(+) uptake in roots following removal of K(+). Increased expression of HKT1 mRNA was evident before a decline in total root K(+) concentration could be detected. Resupply of 1 mm K(+) was sufficient to strongly reduce HKT1 transcript levels. In wheat root cortical cells, both membrane depolarizations in response to 100 μm K(+), Cs(+), and Rb(+), and high-affinity K(+) uptake were enhanced by K(+) deprivation. Thus, in both plant systems the observed physiological changes associated with manipulating external K(+) supply were correlated with levels of HKT1 mRNA expression. Implications of these findings for K(+) sensing and regulation of the HKT1 mRNA levels in plant roots are discussed.