The nitrate uptake and growth of wheat were more inhibited under single-layer graphene oxide stress compared to multi-layer graphene oxide

Although the phytotoxicity of graphene-based materials has been investigated extensively, the effects of different graphene-based materials on nutrient uptake in plants remain unclear. Here, we analyzed the differences in phytotoxicity between single-layer graphene oxide (sGO) and multi-layer graphene oxide (mGO) by analyzing the growth status and nitrate (NO₃^-) accumulation in wheat plants at 0, 100, 200, 400, and 800 mg L^-1 graphene oxide supply. Both sGO and mGO displayed concentration-dependent inhibitory effects on biomass, root length, number of lateral roots, and nitrogen (N) nutrient status. Treatment with 400 mg L^-1 sGO caused 0.9-, 1.3-, and 1-fold higher reductions in NO₃^--N, assimilated N, and total N concentrations in roots, respectively, than mGO treatment. Analysis of root oxidative stress and in situ NO₃^- uptake revealed that sGO caused more significant damage to the root tip and a lower NO₃^- net influx rate than mGO. In addition, the expression of NO₃^- transporter (NRT) genes in roots, including NRT1.5, NRT2.1, NRT2.2, NRT2.3, and NRT2.4, under sGO treatment were lower than those under mGO treatment. Overall, sGO treatment induced a more severe inhibitory effect on root growth and NO₃^- uptake and accumulation than mGO treatment, accompanied by significant suppression of the expression of NRTs in sGO-treated roots. This study provides a physiological and molecular basis for studying the phytotoxic effects of various sizes of graphene oxide.

[Oxidative stress-related signals and their regulation under Cd stress: A review.]

Contamination of heavy metal in soil causes several severe environmental problems. Cadmium is one of the most toxic metals to organisms. In plants, Cd toxicity often results in over-accumulation of reactive oxygen species (ROS), which causes oxidative damage to the plant organs. In this review, we discussed the processes of how Cd stress causes over-accumulation of ROS in plant and how plant recovers from the Cd-induced oxdative stress. We especially focused on the roles of signal, including nitric oxide (NO), calcium (Ca), plant hormones (e.g. auxin and abscisic acid) and mitogen-activated protein kinases (MAPKs), in above two processes. Our review may provide theoretical basis for future research on the mechanism of plant tolerating Cd stress.

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.

Inhibition of nitrate transporter 1.1-controlled nitrate uptake reduces cadmium uptake in Arabidopsis

Identification of mechanisms that decrease cadmium accumulation in plants is a prerequisite for minimizing dietary uptake of cadmium from contaminated crops. Here, we show that cadmium inhibits nitrate transporter 1.1 (NRT1.1)-mediated nitrate (NO3 (-)) uptake in Arabidopsis (Arabidopsis thaliana) and impairs NO3 (-) homeostasis in roots. In NO3 (-)-containing medium, loss of NRT1.1 function in nrt1.1 mutants leads to decreased levels of cadmium and several other metals in both roots and shoots and results in better biomass production in the presence of cadmium, whereas in NO3 (-)-free medium, no difference is seen between nrt1.1 mutants and wild-type plants. These results suggest that inhibition of NRT1.1 activity reduces cadmium uptake, thus enhancing cadmium tolerance in an NO3 (-) uptake-dependent manner. Furthermore, using a treatment rotation system allowing synchronous uptake of NO3 (-) and nutrient cations and asynchronous uptake of cadmium, the nrt1.1 mutants had similar cadmium levels to wild-type plants but lower levels of nutrient metals, whereas the opposite effect was seen using treatment rotation allowing synchronous uptake of NO3 (-) and cadmium and asynchronous uptake of nutrient cations. We conclude that, although inhibition of NRT1.1-mediated NO3 (-) uptake by cadmium might have negative effects on nitrogen nutrition in plants, it has a positive effect on cadmium detoxification by reducing cadmium entry into roots. NRT1.1 may regulate the uptake of cadmium and other cations by a common mechanism.

Iron uptake system mediates nitrate-facilitated cadmium accumulation in tomato (Solanum lycopersicum) plants

Nitrogen (N) management is a promising agronomic strategy to minimize cadmium (Cd) contamination in crops. However, it is unclear how N affects Cd uptake by plants. Wild-type and iron uptake-inefficient tomato (Solanum lycopersicum) mutant (T3238fer) plants were grown in pH-buffered hydroponic culture to investigate the direct effect of N-form on Cd uptake. Wild-type plants fed NO₃⁻ accumulated more Cd than plants fed NH₄⁺. Iron uptake and LeIRT1 expression in roots were also greater in plants fed NO₃⁻. However, in mutant T3238fer which loses FER function, LeIRT1 expression in roots was almost completely terminated, and the difference between NO₃⁻ and NH₄⁺ treatments vanished. As a result, the N-form had no effect on Cd uptake in this mutant. Furthermore, suppression of LeIRT1 expression by NO synthesis inhibition with either tungstate or L-NAME, also substantially inhibited Cd uptake in roots, and the difference between N-form treatments was diminished. Considering all of these findings, it was concluded that the up-regulation of the Fe uptake system was responsible for NO₃⁻-facilitated Cd accumulation in plants.

Facilitation of phosphorus adsorption onto sediment by aquatic plant debris

Aquatic plant debris in lakes or rivers may affect phosphorus flux in water-sediment systems. In this study, either aquatic plant debris or typical plant components (cellulose or glucose), were added into a system of sediment (50 g) and overlying water (2L) with different initial SRP (soluble reactive phosphorus) concentrations to investigate the impact. After 18 days of treatment with 4 g of plant debris, the SRP in the overlying water for 0.5 and 2 mg L(-1) initial SRP tests at 30°C decreased by 41 and 53%, respectively, compared to the treatments without plant debris. Cellulose and glucose treatments gave similar results as plant debris treatment. When the water-sediment system was sterilized, the cellulose- or glucose-facilitated decrease in SRP vanished. Additionally, in the non-sterilized system, the glucose treatment significantly increased both the microbial biomass carbon and the microbial biomass phosphorous in the sediment. Although total phosphorous in the sediment increased with glucose treatment, its water soluble and iron associated inorganic fractions, two labile phosphorus fractions, were clearly reduced. Our results suggest that the short-term retention of plant debris in water systems facilitates a decrease in overlying water SRP through microbe-mediated mechanisms of phosphorus adsorption and stabilization in sediment.

NO synthase-generated NO acts downstream of auxin in regulating Fe-deficiency-induced root branching that enhances Fe-deficiency tolerance in tomato plants

In response to Fe-deficiency, various dicots increase their root branching which contributes to the enhancement of ferric-chelate reductase activity. Whether this Fe-deficiency-induced response eventually enhances the ability of the plant to tolerate Fe-deficiency or not is still unclear and evidence is also scarce about the signals triggering it. In this study, it was found that the SPAD-chlorophyll meter values of newly developed leaves of four tomato (Solanum lycocarpum) lines, namely line227/1 and Roza and their two reciprocal F(1) hybrid lines, were positively correlated with their root branching under Fe-deficient conditions. It indicates that Fe-deficiency-induced root branching is critical for plant tolerance to Fe-deficiency. In another tomato line, Micro-Tom, the increased root branching in Fe-deficient plants was accompanied by the elevation of endogenous auxin and nitric oxide (NO) levels, and was suppressed either by the auxin transport inhibitors NPA and TIBA or the NO scavenger cPTIO. On the other hand, root branching in Fe-sufficient plants was induced either by the auxin analogues NAA and 2,4-D or the NO donors NONOate or SNP. Further, in Fe-deficient plants, NONOate restored the NPA-terminated root branching, but NAA did not affect the cPTIO-terminated root branching. Fe-deficiency-induced root branching was inhibited by the NO-synthase (NOS) inhibitor L-NAME, but was not affected by the nitrate reductase (NR) inhibitor NH(4)(+), tungstate or glycine. Taking all of these findings together, a novel function and signalling pathway of Fe-deficiency-induced root branching is presented where NOS-generated rather than NR-generated NO acts downstream of auxin in regulating this Fe-deficiency-induced response, which enhances the plant tolerance to Fe-deficiency.

Differential regulatory role of nitric oxide in mediating nitrate reductase activity in roots of tomato (Solanum lycocarpum)

BACKGROUND AND AIMS

Nitric oxide (NO) has been demonstrated to stimulate the activity of nitrate reductase (NR) in plant roots supplied with a low level of nitrate, and to affect proteins differently, depending on the ratio of NO to the level of protein. Nitrate has been suggested to regulate the level of NO in plants. This present study examined interactive effects of NO and nitrate level on NR activity in roots of tomato (Solanum lycocarpum).

METHODS

NR activity, mRNA level of NR gene and concentration of NR protein in roots fed with 0.5 mM or 5 mM nitrate and treated with the NO donors, sodium nitroprusside (SNP) and diethylamine NONOate sodium (NONOate), and the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO), were measured in 25-d-old seedlings.

KEY RESULTS

Addition of SNP and NONOate enhanced but cPTIO decreased NR activity in the roots fed with 0.5 mm nitrate. The opposite was true for the roots fed with 5 mM nitrate. However, the mRNA level of the NR gene and the protein concentration of NR enzyme in the roots were not affected by SNP treatment, irrespective of nitrate pre-treatment. Nevertheless, a low rate of NO gas increased while cPTIO decreased the NR activities of the enzyme extracts from the roots at both nitrate levels. Increasing the rate of NO gas further increased NR activity in the enzyme extracts of the roots fed with 0.5 mM nitrate but decreased it when 5 mM nitrate was supplied. Interestingly, the stimulative effect of NO gas on NR activity could be reversed by NO removal through N(2) flushing in the enzyme extracts from the roots fed with 0.5 mM nitrate but not from those with 5 mM nitrate.

CONCLUSIONS

The effects of NO on NR activity in tomato roots depend on levels of nitrate supply, and probably result from direct interactions between NO and NR protein.

Elevated carbon dioxide improves plant iron nutrition through enhancing the iron-deficiency-induced responses under iron-limited conditions in tomato

The increases in atmospheric carbon dioxide (CO(2)) concentrations can enhance plant growth and change their nutrient demands. We report that when tomato (Lycopersicon esculentum 'Zheza 809') plants were grown in iron (Fe)-limited medium (with hydrous ferric iron oxide) and elevated CO(2) (800 microL L(-1)), their biomass and root-to-shoot ratio were greater than plants grown in ambient CO(2) (350 microL L(-1)). Furthermore, the associated increase in Fe concentrations in the shoots and roots alleviated Fe-deficiency-induced chlorosis. Despite the improved nutrient status of plants grown in Fe-limited medium under elevated CO(2), the Fe-deficiency-induced responses in roots, including ferric chelate reductase activity, proton secretion, subapical root hair development, and the expression of FER, FRO1, and IRT genes, were all greater than plants grown in the ambient CO(2). The biomass of plants grown in Fe-sufficient medium was also increased by the elevated CO(2) treatment, but changes in tissue Fe concentrations and Fe deficiency responses were not observed. These results suggest that the improved Fe nutrition and induction of Fe-deficient-induced responses in plants grown in Fe-limited medium under elevated CO(2) are caused by interactions between elevated CO(2) and Fe deprivation. Elevated CO(2) also increased the nitric oxide (NO) levels in roots, but treatment with the NO scavenger cPTIO inhibited ferric chelate reductase activity and prevented the accumulation of LeFRO1, LeIRT1, and FER transcripts in roots of the Fe-limited plants. These results implicate some involvement of NO in enhancing Fe-deficiency-induced responses when Fe limitation and elevated CO(2) occur together. We propose that the combination of elevated CO(2) and Fe limitation induces morphological, physiological, and molecular responses that enhance the capacity for plants to access and utilize Fe from sparingly soluble sources, such as Fe(III)-oxide.