Sodium transport and HKT transporters: the rice model.
Na+ uptake in the roots of K+-starved seedlings of barley, rice, and wheat was found to exhibit fast rate, low Km, and high sensitivity to K+. Sunflower plants responded in a similar manner but the uptake was not K+ sensitive. Ba2+ inhibited Na+ uptake, but not K+ uptake in rice roots. This demonstrated that Na+ and K+ uptake are mediated by different transporters, and that K+ blocked but was not transported by the Na+ transporter. The genome of rice cv. Nipponbare contains seven HKT genes, which may encode Na+ transporters, plus two HKT pseudogenes. Yeast expressions of OsHKT1 and OsHKT4 proved that they are Na+ transporters of high and low affinity, respectively, which are sensitive to K+ and Ba2+. Parallel experiments of K+ and Na+ uptake in yeast expressing the wheat or rice HKT1 transporters proved that they were very different; TaHKT1 transported K+ and Na+, and OsHKT1 only Na+. Transcript expressions in shoots of the OsHKT genes were fairly constant and insensitive to changes in the K+ and Na+ concentrations of the nutrient solution. In roots, the expressions were much lower than in shoots, except for OsHKT4 and OsHKT1 in K+-starved plants. We propose that OsHKT transporters are involved in Na+ movements in rice, and that OsHKT1 specifically mediates Na+ uptake in rice roots when the plants are K+ deficient. The incidence of HKT ESTs in several plant species suggests that the rice model with many HKT genes applies to other plants.
Garciadeblás
Blanca
B
Departamento de Biotecnología, Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Politécnica de Madrid, 28040 Madrid, Spain.
Senn
María E
ME
Bañuelos
María A
MA
Rodríguez-Navarro
Alonso
A
D000595:Amino Acid Sequence; D027682:Cation Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D017124:Conserved Sequence; D020224:Expressed Sequence Tags; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D017136:Ion Transport; D008969:Molecular Sequence Data; D012275:Oryza / Q000235:genetics / Q000378:metabolism*; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism; D011188:Potassium / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D016415:Sequence Alignment; D012964:Sodium / Q000378:metabolism*
D027682:Cation Transport Proteins; D012964:Sodium; D011188:Potassium