Wang R, Liu D, Crawford N M
Department of Biology, University of California at San Diego, La Jolla, CA 92093-0116, USA.
Proc Natl Acad Sci U S A. 1998 Dec 8;95(25):15134-9. doi: 10.1073/pnas.95.25.15134.
The CHL1 (NRT1) gene of Arabidopsis encodes a nitrate-inducible nitrate transporter that is thought to be a component of the low-affinity (mechanism II) nitrate-uptake system in plants. A search was performed to find high-affinity (mechanism I) uptake mutants by using chlorate selections on plants containing Tag1 transposable elements. Chlorate-resistant mutants defective in high-affinity nitrate uptake were identified, and one had a Tag1 insertion in chl1, which was responsible for the phenotype. Further analysis showed that chl1 mutants have reduced high-affinity uptake in induced plants and are missing a saturable component of the constitutive, high-affinity uptake system in addition to reduced low-affinity uptake. The contribution of CHL1 to constitutive high-affinity uptake is higher when plants are grown at more acidic pH, conditions that increase the level of CHL1 mRNA. chl1 mutants show reduced membrane depolarization in root epidermal cells in response to low (250 microM) and high (10 mM) concentrations of nitrate. Low levels of nitrate (100 microM) induce a rapid increase in CHL1 mRNA. These results show that CHL1 is an important component of both the high-affinity and the low-affinity nitrate-uptake systems and indicate that CHL1 may be a dual-affinity nitrate transporter.
拟南芥的CHL1（NRT1）基因编码一种硝酸盐诱导型硝酸盐转运蛋白，被认为是植物低亲和力（机制II）硝酸盐吸收系统的一个组成部分。利用含有Tag1转座元件的植物进行氯酸盐筛选，以寻找高亲和力（机制I）吸收突变体。鉴定出在高亲和力硝酸盐吸收方面存在缺陷的抗氯酸盐突变体，其中一个突变体的chl1中有一个Tag1插入，这导致了该表型。进一步分析表明，chl1突变体在诱导植物中高亲和力吸收降低，除了低亲和力吸收降低外，还缺少组成型高亲和力吸收系统的一个可饱和成分。当植物在酸性更强的pH值下生长时，CHL1对组成型高亲和力吸收的贡献更高，这种条件会增加CHL1 mRNA的水平。chl1突变体在根表皮细胞中对低（250微摩尔）和高（10毫摩尔）浓度硝酸盐的膜去极化反应降低。低水平的硝酸盐（100微摩尔）会诱导CHL1 mRNA迅速增加。这些结果表明，CHL1是高亲和力和低亲和力硝酸盐吸收系统的重要组成部分，并表明CHL1可能是一种双亲和力硝酸盐转运蛋白。
