Sodium (Na+) at high millimolar concentrations in soils is toxic to most higher plants and severely reduces agricultural production worldwide. However, the molecular mechanisms for plant Na+ uptake remain unknown. Here, the wheat root high-affinity potassium (K+) uptake transporter HKT1 was shown to function as a high-affinity K(+)-Na+ cotransporter. High-affinity K+ uptake was activated by micromolar Na+ concentrations; moreover, high-affinity Na+ uptake was activated by K+ (half-activation constant, 2.8 microM K+). However, at physiologically detrimental concentrations of Na+, K+ accumulation mediated by HKT1 was blocked and low-affinity Na+ uptake occurred (Michaelis constant, approximately 16 mM Na+), which correlated to Na+ toxicity in plants. Point mutations in the sixth putative transmembrane domain of HKT1 that increase Na+ tolerance were isolated with the use of yeast as a screening system. Na+ uptake and Na+ inhibition of K+ accumulation indicate a possible role for HKT1 in physiological Na+ toxicity in plants.

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Iron, despite being an essential micronutrient, becomes toxic if present at high levels. As a result, plants possess carefully regulated mechanisms to acquire iron from the soil. The ferric reductase defective3 (frd3) mutant of Arabidopsis (Arabidopsis thaliana) is chlorotic and exhibits constitutive expression of its iron uptake responses. Consequently, frd3 mutants overaccumulate iron; yet, paradoxically, the frd3 phenotypes are due to a reduction in the amount of iron present inside frd3 leaf cells. The FRD3 protein belongs to the multidrug and toxin efflux family, members of which are known to export low-M r organic molecules. We therefore hypothesized that FRD3 loads an iron chelator necessary for the correct distribution of iron throughout the plant into the xylem. One such potential chelator is citrate. Xylem exudate from frd3 plants contains significantly less citrate and iron than the exudate from wild-type plants. Additionally, supplementation of growth media with citrate rescues the frd3 phenotypes. The ectopic expression of FRD3-GFP results in enhanced tolerance to aluminum in Arabidopsis roots, a hallmark of organic acid exudation. Consistent with this result, approximately 3 times more citrate was detected in root exudate from plants ectopically expressing FRD3-GFP. Finally, heterologous studies in Xenopus laevis oocytes reveal that FRD3 mediates the transport of citrate. These results all strongly support the hypothesis that FRD3 effluxes citrate into the root vasculature, a process important for the translocation of iron to the leaves, as well as confirm previous reports suggesting that iron moves through the xylem as a ferric-citrate complex. Our results provide additional answers to longstanding questions about iron chelation in the vasculature and organic acid transport.

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.