The Arabidopsis nitrate transporter NRT1.8 functions in nitrate removal from the xylem sap and mediates cadmium tolerance.
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-beta-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 (Cd(2+)) stress in roots, and the nrt1.8-1 mutant showed a nitrate-dependent Cd(2+)-sensitive phenotype. Further analyses showed that Cd(2+) 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 Cd(2+) tolerance.
Li
Jian-Yong
JY
National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, People's Republic of China.
Fu
Yan-Lei
YL
Pike
Sharon M
SM
Bao
Juan
J
Tian
Wang
W
Zhang
Yu
Y
Chen
Chun-Zhu
CZ
Zhang
Yi
Y
Li
Hong-Mei
HM
Huang
Jing
J
Li
Le-Gong
LG
Schroeder
Julian I
JI
Gassmann
Walter
W
Gong
Ji-Ming
JM
D000222:Adaptation, Physiological / Q000187:drug effects* / Q000235:genetics; D027321:Anion Transport Proteins / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000166:cytology / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002104:Cadmium / Q000633:toxicity*; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D009154:Mutation / Q000235:genetics; D000091285:Nitrate Transporters; D009566:Nitrates / Q000378:metabolism*; D053147:Plant Exudates / Q000235:genetics / Q000378:metabolism*; D021381:Protein Transport / Q000187:drug effects; D013312:Stress, Physiological / Q000187:drug effects / Q000235:genetics; D013347:Subcellular Fractions / Q000187:drug effects / Q000201:enzymology; D015854:Up-Regulation / Q000187:drug effects; D052584:Xylem / Q000166:cytology / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics
D027321:Anion Transport Proteins; D029681:Arabidopsis Proteins; C552677:NRT1.8 protein, Arabidopsis; D000091285:Nitrate Transporters; D009566:Nitrates; D053147:Plant Exudates; D002104:Cadmium