Plant Physiology Preview
Published on October 28, 2005; 10.1104/pp.105.067553
Received June 22, 2005
Returned for revision July 13, 2005
Accepted July 13, 2005
HKT1 Mediates Sodium Uniport in Roots. Pitfalls in the Expression of HKT1 in Yeast
Rosario Haro , María A. Bañuelos , María E. Senn , Javier Barrero-Gil , and Alonso Rodríguez-Navarro *
Departamento de Biotecnología, Universidad Politécnica de Madrid, 28040 Madrid, Spain
* Corresponding author; email: alonso.rodriguez{at}upm.es.
The function of HKT1 in roots is controversial. We tackled this controversy by studying Na+ uptake in barley (Hordeum vulgare) roots, cloning the HvHKT1 gene, and expressing the HvHKT1 cDNA in yeast (Saccharomyces cerevisiae) cells. High-affinity Na+ uptake was not detected in plants growing at high K+ but appeared soon after exposing the plants to a K+-free medium. It was a uniport, insensitive to external K+ at the beginning of K+ starvation and inhibitable by K+ several hours later. The expression of HvHKT1 in yeast was Na+ (or K+) uniport, Na+-K+ symport, or a mix of both, depending on the construct from which the transporter was expressed. The Na+ uniport function was insensitive to external K+ and mimicked the Na+ uptake carried out by the roots at the beginning of K+ starvation. The K+ uniport function only took place in yeast cells that were completely K+ starved and disappeared when internal K+ increased, which makes it unlikely that HvHKT1 mediates K+ uptake in roots. Mutation of the first in-frame AUG codon of HvHKT1 to CUC changed the uniport function into symport. The expression of the symport from either mutants or constructs keeping the first in-frame AUG took place only in K+-starved cells, while the uniport was expressed in all conditions. We discuss here that the symport occurs only in heterologous expression. It is most likely related to the K+ inhibitable Na+ uptake process of roots that heterologous systems fail to reproduce.
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