Excessive iron deposition in root apoplast is involved in growth arrest of roots in response to low pH.

The rhizotoxicity of protons (H+) in acidic soils is a fundamental constraint that results in serious yield losses. However, the mechanisms underlying H+-mediated inhibition of root growth are poorly understood. In this study, we revealed that H+-induced root growth inhibition in Arabidopsis depends considerably on excessive iron deposition in the root apoplast. Reducing such aberrant iron deposition by decreasing the iron supply or disrupting the ferroxidases LOW PHOSPHATE ROOT 1 (LPR) and LPR2 attenuates the inhibitory effect of H+ on primary root growth efficiently. Further analysis showed that excessive iron deposition triggers a burst of highly reactive oxygen species, consequently impairing normal root development. Our study uncovered a valuable strategy for improving the ability of plants to tolerate H+ toxicity by manipulating iron availability.

NRT1.1 Dual-Affinity Nitrate Transport/Signalling and its Roles in Plant Abiotic Stress Resistance.

NRT1.1 is the first nitrate transport protein cloned in plants and has both high- and low-affinity functions. It imports and senses nitrate, which is modulated by the phosphorylation on Thr101 (T101). Structural studies have revealed that the phosphorylation of T101 either induces dimer decoupling or increases structural flexibility within the membrane, thereby switching the NRT1.1 protein from a low- to high-affinity state. Further studies on the adaptive regulation of NRT1.1 in fluctuating nitrate conditions have shown that, at low nitrate concentrations, nitrate binding only at the high-affinity monomer initiates NRT1.1 dimer decoupling and priming of the T101 site for phosphorylation activated by CIPK23, which functions as a high-affinity nitrate transceptor. However, nitrate binding in both monomers retains the unmodified NRT1.1, maintaining the low-affinity mode. This NRT1.1-mediated nitrate signalling and transport may provide a key to improving the efficiency of plant nitrogen use. However, recent studies have revealed that NRT1.1 is extensively involved in plant tolerance of several adverse environmental conditions. In this context, we summarise the recent progress in the molecular mechanisms of NRT1.1 dual-affinity nitrate transport/signalling and focus on its expected and unexpected roles in plant abiotic stress resistance and their regulation processes.

The K+ and NO3 - Interaction Mediated by NITRATE TRANSPORTER1.1 Ensures Better Plant Growth under K+-Limiting Conditions.

K+ and NO3 - are the major forms of potassium and nitrogen that are absorbed by the roots of most terrestrial plants. In this study, we observed that a close relationship between NO3 - and K+ in Arabidopsis (Arabidopsis thaliana) is mediated by NITRATE TRANSPORTER1.1 (NRT1.1). The nrt1.1 knockout mutants showed disturbed K+ uptake and root-to-shoot allocation, and were characterized by growth arrest under K+-limiting conditions. The K+ uptake and root-to-shoot allocation of these mutants were partially recovered by expressing NRT1.1 in the root epidermis-cortex and central vasculature using SULFATE TRANSPORTER1;2 and PHOSPHATE1 promoters, respectively. Two-way analysis of variance based on the K+ contents in nrt1.1-1/K + transporter1, nrt1.1-1/high-affinity K + transporter5-3, nrt1.1-1/K + uptake permease7, and nrt1.1-1/stelar K + outward rectifier-2 double mutants and the corresponding single mutants and wild-type plants revealed physiological interactions between NRT1.1 and K+ channels/transporters located in the root epidermis-cortex and central vasculature. Further study revealed that these K+ uptake-related interactions are dependent on an H+-consuming mechanism associated with the H+/NO3 - symport mediated by NRT1.1. Collectively, these data indicate that patterns of NRT1.1 expression in the root epidermis-cortex and central vasculature are coordinated with K+ channels/transporters to improve K+ uptake and root-to-shoot allocation, respectively, which in turn ensures better growth under K+-limiting conditions.

Ammonium aggravates salt stress in plants by entrapping them in a chloride over-accumulation state in an NRT1.1-dependent manner.

Global climate change has exacerbated flooding in coastal areas affected by soil salinization. Ammonium (NH4+) is the predominant form of nitrogen in flooded soils, but the role played by NH4+ in the plant response to salt stress has not been fully clarified. We investigated the responses of Arabidopsis thaliana, Oryza sativa, and Nicotiana benthamiana plants fed with NH4+. All species were hypersensitive to NaCl stress and accumulated more Cl- and less Na+ than those fed with NO3-. Further investigation of A. thaliana indicated that salt hypersensitivity induced by the presence of NH4+ was abolished by removing the Cl- but was not affected by the removal of Na+, suggesting that excess accumulation of Cl- rather than Na+ is involved in NH4+-conferred salt hypersensitivity. The expression of nitrate transporter NRT1.1 protein was also up-regulated by NH4+ treatment, which increased root Cl- uptake due to the Cl- uptake activity of NRT1.1 and the absence of uptake competition from NO3-. Knockout of NRT1.1 in plants decreased their root Cl- uptake and retracted the NH4+-conferred salt hypersensitivity. Our findings revealed that NH4+-aggravated salt stress in plants is associated with Cl- over-accumulation through the up-regulation of NRT1.1-mediated Cl- uptake. These findings suggest the significant impact of Cl- toxicity in flooded coastal areas, an issue of ecological significance.

Knockdown of BTS may provide a new strategy to improve cadmium-phytoremediation efficiency by improving iron status in plants.

The identification of the key genes related to cadmium (Cd) tolerance and accumulation is a major element in genetically engineering improved plants for Cd phytoremediation. Owing to the similarity between the ionic hydrated radius of Cd2+ and Fe2+, this study investigated how the Cd tolerance and accumulation of Arabidopsis plants was affected by the knockdown of BTS, a gene that negatively regulates Fe nutrition. After exposure to 40 µM Cd, the BTS-knockdown mutant, bts-1, exhibited greater Fe nutrition and better growth than wild-type plants. In addition, the Cd concentration in both roots and shoots was approximately 50% higher in the bts-1 mutant than in wild-type plants. Consequently, the bts-1 mutant accumulated approximately 100% and 150% more Cd in the roots and shoots, respectively, than wild-type plants. Further study showed that Fe removal from the growth medium and inhibition of the Fe transporter gene, IRT1, removed the differences observed in the growth and Cd concentration of the bts-1 and wild-type plants, respectively. These results demonstrated that BTS knockdown improved Cd tolerance and accumulation in plants by improving Fe nutrition; thus, the knockdown of BTS via biotechnological pathways may represent a valuable strategy for the improvement in the efficiency of Cd phytoremediation.

Nitrate transporter 1.1 alleviates lead toxicity in Arabidopsis by preventing rhizosphere acidification.

Identification of the mechanisms that control lead (Pb) concentration in plants is a prerequisite for minimizing dietary uptake of Pb from contaminated crops. This study examines how nitrate uptake by roots affects Pb uptake and reveals a new resistance strategy for plants to cope with Pb contamination. We investigated the interaction between nitrate transporter (NRT)-mediated NO3- uptake and exposure to Pb in Arabidopsis using NRT-related mutants. Exposure to Pb specifically stimulated NRT1.1-mediated nitrate uptake. Loss of function of NRT1.1 in nrt1.1-knockout mutants resulted in greater Pb toxicity and higher Pb accumulation in nitrate-sufficient growth medium, whereas no difference was seen between wild-type plants and null-mutants for NRT1.2, NRT2.1, NRT2.2, NRT2.4, and NRT2.5. These results indicate that only NRT1.1-mediated NO3- uptake alleviated Pb toxicity in the plants. Further examination indicated that rhizosphere acidification, which favors Pb entry to roots by increasing its availability, is prevented when NRT1.1 is functional and both NO3- and NH4+ are present in the medium.

Effects of split applications of nitrogen fertilizers on the Cd level and nutritional quality of Chinese cabbage.

Cadmium (Cd) contamination in soil is an increasingly serious problem. Management of plant nutrients has been proposed as a potentially promising strategy for minimizing Cd accumulation in crops grown in contaminated soil. This study investigated the effects of split applications of nitrogen (N) fertilizers on the Cd concentration in Chinese cabbage (Brassica chinensis L.) plants grown in Cd-contaminated soil. Compared with single applications, split applications of ammonium or urea resulted in significantly lower Cd concentrations, and higher biomass production and antioxidant-associated nutritional quality in the edible plant parts. However, when nitrate was used as the N fertilizer, there were no significant differences between the split and single applications for the same parameters. We conclude that a split application could be more beneficial than a single application method when ammonium or urea is used as the N fertilizer for vegetable cultivation in Cd-contaminated soil.

Alleviation of proton toxicity by nitrate uptake specifically depends on nitrate transporter 1.1 in Arabidopsis.

Protons in acid soil are highly rhizotoxic to plants, but the mechanism of tolerance of plants to protons is largely unknown. Nitrate uptake by root cells is accompanied by the uptake of protons. Therefore, nitrate uptake transporters (NRTs) may be involved in plant tolerance to proton toxicity. We investigated the root nitrate uptake response to proton stress in Arabidopsis and its association with proton tolerance using NRT-related mutants and pharmacological methods. Lack of NRT1.1 in knockout nrt1.1 mutants led to impaired proton tolerance in nitrate-sufficient growth medium, whereas no difference was seen between wild-type plants and NRT1.2-, NRT2.1-, NRT2.2-, and NRT2.4-null mutants. Another nrt1.1 point mutant, which is defective in nitrate uptake but has a normal nitrate-sensing function, also had impaired proton tolerance compared with the wild-type plant. Furthermore, proton stress induced NRT1.1-mediated nitrate uptake. These results indicate that NRT1.1-conferred proton tolerance depends on nitrate uptake activity. In addition, the rooting medium was alkalified by wild-type plants, but not by knockout nrt1.1 mutants, and in pH-buffered medium, there were no differences in proton tolerance between wild-type plants and knockout nrt1.1 mutants. We conclude that NRT1.1-mediated nitrate uptake plays a crucial role in plant proton tolerance by alkalifying the rhizosphere.

Modification of nitrate uptake pathway in plants affects the cadmium uptake by roots.

NRT1.1 is a dual-affinity nitrate (NO3(-)) transporter involved in both high- and low-affinity NO3(-) uptake in Arabidopsis plants. In a recent study, we showed that, under cadmium (Cd) exposure, blocking the NRT1.1-mediated NO3(-) uptake reduces Cd entry into roots, thus lowing Cd levels in plants and improving plant growth. In addition, we also found that the Cd levels in edible parts of 11 Chinese cabbage (Brassica rapa L. ssp. pekinensis) cultivars correlated well with the NO3(-) uptake rates of their roots. These results suggested that the NO3(-) uptake of roots negatively regulate Cd uptake. Modification of NO3(-) uptake in crops by modulating NO3(-) uptake pathway might provide a biological engineering approach to reducing Cd accumulation in edible organs, thus improving food safety.

Exogenous abscisic acid application decreases cadmium accumulation in Arabidopsis plants, which is associated with the inhibition of IRT1-mediated cadmium uptake.

Cadmium (Cd) contamination of agricultural soils is an increasingly serious problem. Measures need to be developed to minimize Cd entering the human food chain from contaminated soils. We report here that, under Cd exposure condition, application with low doses of (0.1-0.5 µM) abscisic acid (ABA) clearly inhibited Cd uptake by roots and decreased Cd level in Arabidopsis wild-type plants (Col-0). Expression of IRT1 in roots was also strongly inhibited by ABA treatment. Decrease in Cd uptake and the inhibition of IRT1 expression were clearly lesser pronounced in an ABA-insensitive double mutant snrk2.2/2.3 than in the Col-0 in response to ABA application. The ABA-decreased Cd uptake was found to correlate with the ABA-inhibited IRT1 expression in the roots of Col-0 plants fed two different levels of iron. Furthermore, the Cd uptake of irt1 mutants was barely affected by ABA application. These results indicated that inhibition of IRT1 expression is involved in the decrease of Cd uptake in response to exogenous ABA application. Interestingly, ABA application increased the iron level in both Col-0 plants and irt1 mutants, suggesting that ABA-increased Fe acquisition does not depend on the IRT1 function, but on the contrary, the ABA-mediated inhibition of IRT1 expression may be due to the elevation of iron level in plants. From our results, we concluded that ABA application might increase iron acquisition, followed by the decrease in Cd uptake by inhibition of IRT1 activity. Thus, for crop production in Cd contaminated soils, developing techniques based on ABA application potentially is a promising approach for reducing Cd accumulation in edible organs in plants.