Huang N C, Chiang C S, Crawford N M, Tsay Y F
Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan, Republic of China.
Plant Cell. 1996 Dec;8(12):2183-91. doi: 10.1105/tpc.8.12.2183.
The Arabidopsis CHL1 (AtNRT1) gene confers sensitivity to the herbicide chlorate and encodes a nitrate-regulated nitrate transporter. However, how CHL1 participates in nitrate uptake in plants is not yet clear. In this study, we examined the in vivo function of CHL1 with in vivo uptake measurements and in situ hybridization experiments. Under most conditions tested, the amount of nitrate uptake by a chl1 deletion mutant was found to be significantly less than that of the wild type. This uptake deficiency was reversed when a CHL1 cDNA clone driven by the cauliflower mosaic virus 35S promoter was expressed in transgenic chl1 plants. Furthermore, tissue-specific expression patterns showed that near the root tip, CHL1 mRNA is found primarily in the epidermis, but further from the root tip, the mRNA is found in the cortex or endodermis. These results are consistent with the involvement of CHL1 in nitrate uptake at different stages of root cell development. A functional analysis in Xenopus oocytes indicated that CHL1 is a low-affinity nitrate transporter with a K(m) value of approximately 8.5 mM for nitrate. This finding is consistent with the chlorate resistance phenotype of chl1 mutants. However, these results do not fit the current model of a single, constitutive component for the low-affinity uptake system. To reconcile this discrepancy and the complex uptake behavior observed, we propose a "two-gene" model for the low-affinity nitrate uptake system of Arabidopsis.
拟南芥CHL1（AtNRT1）基因赋予对除草剂氯酸盐的敏感性，并编码一种受硝酸盐调控的硝酸盐转运蛋白。然而，CHL1如何参与植物对硝酸盐的吸收尚不清楚。在本研究中，我们通过体内吸收测量和原位杂交实验研究了CHL1的体内功能。在大多数测试条件下，发现chl1缺失突变体对硝酸盐的吸收量明显低于野生型。当由花椰菜花叶病毒35S启动子驱动的CHL1 cDNA克隆在转基因chl1植物中表达时，这种吸收缺陷得到了逆转。此外，组织特异性表达模式表明，在根尖附近，CHL1 mRNA主要存在于表皮中，但在离根尖较远的地方，mRNA存在于皮层或内皮层中。这些结果与CHL1参与根细胞发育不同阶段的硝酸盐吸收一致。在非洲爪蟾卵母细胞中的功能分析表明，CHL1是一种低亲和力硝酸盐转运蛋白，对硝酸盐的K(m)值约为8.5 mM。这一发现与chl1突变体的氯酸盐抗性表型一致。然而，这些结果与当前低亲和力吸收系统单一组成成分的模型不符。为了调和这一差异以及观察到的复杂吸收行为，我们提出了拟南芥低亲和力硝酸盐吸收系统的“双基因”模型。
