近期植物学研究权威期刊Plant Cell在线发表了植物生理生态研究所植物分子遗传国家重点实验室龚继明研究组最新研究成果：拟南芥NRT1.8基因介导的NO3-再分配在植物逆境胁迫耐受机理中起着重要的调节作用。
硝酸根(NO3-)是陆生植物最重要的氮源之一，它不仅是重要的营养物质，而且还作为信号分子在调控植物的生长发育过程中发挥了重要作用。高等植物通过根系吸收NO3-后，一般会长途转运到植物的地上部位进行同化。但是，逆境条件会促使更多的NO3-留在根部，这个生理现象有什么重要的生物学功能，其调控机制是什么，目前尚不清楚。
龚继明研究组通过高通量表达组技术和电生理技术，从拟南芥基因组中克隆到一个受逆境因子（Cd2＋）和营养信号（NO3-）强烈诱导的基因NRT1.8，该基因编码一个pH依赖的内向型NO3-低亲和转运蛋白，其基本作用在于将木质部导管中的NO3-跨木质部薄壁细胞膜转运到木质部薄壁细胞中，从而卸载木质部导管中运输的NO3-，实现对NO3-长途转运的调控。在镉胁迫下，该基因在根中被强烈诱导，导致NO3-留存在植物根中，使得NO3-在植物地上和地下部位的分配比例发生变化。在该基因功能缺失突变体中，这种NO3-的分配方式受到破坏，并由此导致植物对镉胁迫表现出高度的敏感性；过量表达该基因显着提高了植物对多种逆境的耐性，进一步的研究表明可能是通过重建氮素代谢途径来实现的。这些事实表明NRT1.8介导的NO3-再分配在植物逆境胁迫耐受机理中起着重要的调节作用。该成果对于指导培育矿质营养高效，尤其是逆境条件下矿质营养高效的农作物具有重要的科学意义和潜在的应用价值。
该项工作得到了国家科技部、国家自然科学基金委、中国科学院及上海市的经费支持。
原文出处：
The Plant Cell doi：10.1105/tpc.110.075242
The Arabidopsis Nitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance
Jian-Yong Lia,1, Yan-Lei Fua,1, Sharon M. Pikeb, Juan Baoa, Wang Tianc, Yu Zhanga, Chun-Zhu Chena, Yi Zhanga, Hong-Mei Lia, Jing Huanga, Le-Gong Lic, Julian I. Schroederd, Walter Gassmannb and Ji-Ming Gonga,2
a National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, People's Republic of China
b Division of Plant Sciences, C.S. Bond Life Sciences Center and Interdisciplinary Plant Group, University of Missouri, Columbia, Missouri 65211-7310
c College of Life Sciences, Capital Normal University, Beijing 100037, People's Republic of China
d Division of Biological Sciences and Center for Molecular Genetics, Cell and Developmental Biology Section, University of California, San Diego, California 92093-0116
Long-distance transport of nitrate requires xylem loading and unloading, a successive process that determines nitrate distribution and subsequent assimilation efficiency. Here, we report the functional characterization of NRT1.8, a member of the nitrate transporter (NRT1) family in Arabidopsis thaliana. NRT1.8 is upregulated by nitrate. Histochemical analysis using promoter-β-glucuronidase fusions, as well as in situ hybridization, showed that NRT1.8 is expressed predominantly in xylem parenchyma cells within the vasculature. Transient expression of the NRT1.8:enhanced green fluorescent protein fusion in onion epidermal cells and Arabidopsis protoplasts indicated that NRT1.8 is plasma membrane localized. Electrophysiological and nitrate uptake analyses using Xenopus laevis oocytes showed that NRT1.8 mediates low-affinity nitrate uptake. Functional disruption of NRT1.8 significantly increased the nitrate concentration in xylem sap. These data together suggest that NRT1.8 functions to remove nitrate from xylem vessels. Interestingly, NRT1.8 was the only nitrate assimilatory pathway gene that was strongly upregulated by cadmium (Cd2+) stress in roots, and the nrt1.8-1 mutant showed a nitrate-dependent Cd2+-sensitive phenotype. Further analyses showed that Cd2+ stress increases the proportion of nitrate allocated to wild-type roots compared with the nrt1.8-1 mutant. These data suggest that NRT1.8-regulated nitrate distribution plays an important role in Cd2+ tolerance.
