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.
K+饑餓的大麥、水稻和小麥幼苗中都發(fā)現(xiàn)根中的Na+吸收速率很快， Km低且對K+敏感。向日葵也類似，但對K+不敏感。 Ba2+可以抑制水稻根吸收Na+，但不抑制吸收K+。這表明Na+和K+是通過不同的轉(zhuǎn)運蛋白吸收的。水稻日本晴基因組含有7個編碼Na+轉(zhuǎn)運蛋白的HKT基因和兩個HKT假基因。在酵母中表達OsHKT1和OsHKT4證明了它們分別是高親和Na+轉(zhuǎn)運蛋白和低親和Na+轉(zhuǎn)運蛋白且都對K+和Ba2+敏感。在酵母中表達小麥和水稻的HKT1轉(zhuǎn)運蛋白做K+和Na+吸收的平行實驗證明了他們是不同的：TaHKT1可以轉(zhuǎn)運K+和Na+而OsHKT1只轉(zhuǎn)運Na+。OsHKT基因在地上部的表達相對穩(wěn)定，對營養(yǎng)液中的K+和Na+濃度的變化不敏感。OsHKT基因在根中的表達量要比地上部的表達量低，除了 OsHKT4和OsHKT1在K+饑餓的植株根中表達量高外。我們認為OsHKT轉(zhuǎn)運蛋白參與水稻中的Na+運輸，在K+缺乏的條件下OsHKT1特異的介導(dǎo)水稻根吸收Na+。其他幾個植物物種中存在幾個HKT基因的EST暗示了這些植物也和水稻一樣含有許多HKT基因。