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Enhancing nitrogen use efficiency (NUE) in crop plants is an important breeding target to reduce excessive use of chemical fertilizers, with substantial benefits to farmers and the environment. In Arabidopsis (Arabidopsis thaliana), allocation of more NO₃⁻ to shoots was associated with higher NUE; however, the commonality of this process across plant species have not been sufficiently studied. Two Brassica napus genotypes were identified with high and low NUE. We found that activities of V-ATPase and V-PPase, the two tonoplast proton-pumps, were significantly lower in roots of the high-NUE genotype (Xiangyou15) than in the low-NUE genotype (814); and consequently, less vacuolar NO₃⁻ was retained in roots of Xiangyou15. Moreover, NO₃⁻ concentration in xylem sap, [¹⁵N] shoot:root (S:R) and [NO₃⁻] S:R ratios were significantly higher in Xiangyou15. BnNRT1.5 expression was higher in roots of Xiangyou15 compared with 814, while BnNRT1.8 expression was lower. In both B. napus treated with proton pump inhibitors or Arabidopsis mutants impaired in proton pump activity, vacuolar sequestration capacity (VSC) of NO⁻⁻ in roots substantially decreased. Expression of NRT1.5 was up-regulated, but NRT1.8 was down-regulated, driving greater NO₃⁻ long-distance transport fromroots to shoots. NUE in Arabidopsis mutants impaired in proton pumps was also significantly higher than in the wild type col-0. Taken together, these data suggest that decrease in VSC of NO₃⁻ in roots will enhance transport to shoot and essentially contribute to higher NUE by promoting NO₃⁻ allocation to aerial parts, likely through coordinated regulation of NRT1.5 and NRT1.8.

A high concentration of ammonium (NH ) as the sole source of nitrogen in the growth medium often is toxic to plants. The nitrate transporter NRT1.1 is involved in mediating the effects of NH toxicity; however, the mechanism remains undefined. In this study, wild-type Arabidopsis ( Columbia-0 [Col-0]) and mutants ( and ) were grown hydroponically in NH NO and (NH ) SO media to assess the function of NRT1.1 in NH stress responses. All the plants grew normally in medium containing mixed nitrogen sources, but Col-0 displayed more chlorosis and lower biomass and photosynthesis than the mutants in (NH ) SO medium. Grafting experiments between Col-0 and further confirmed that NH toxicity is influenced by NRT1.1. In (NH ) SO medium, NRT1.1 induced the expression of NH transporters, increasing NH uptake. Additionally, the activities of glutamine synthetase and glutamate synthetase in roots of Col-0 plants decreased and soluble sugar accumulated significantly, whereas pyruvate kinase-mediated glycolysis was not affected, all of which contributed to NH accumulation. By contrast, the mutants showed reduced NH accumulation and enhanced NH assimilation through glutamine synthetase, glutamate synthetase, and glutamate dehydrogenase. Moreover, the up-regulation of genes involved in ethylene synthesis and senescence in Col-0 plants treated with (NH ) SO suggests that ethylene is involved in NH toxicity responses. This study showed that NH toxicity is related to a nitrate-independent signaling function of in Arabidopsis, characterized by enhanced NH accumulation and altered NH metabolism, which stimulates ethylene synthesis, leading to plant senescence.

A high concentration of ammonium (NH₄⁺) as the sole source of nitrogen in the growth medium often is toxic to plants. The nitrate transporter NRT1.1 is involved in mediating the effects of NH₄⁺ toxicity; however, the mechanism remains undefined. In this study, wild-type Arabidopsis (Arabidopsis thaliana Columbia-0 [Col-0]) and NRT1.1 mutants (chl1-1 and chl1-5) were grown hydroponically in NH₄NO₃ and (NH₄)₂SO₄ media to assess the function of NRT1.1 in NH₄⁺ stress responses. All the plants grew normally in medium containing mixed nitrogen sources, but Col-0 displayed more chlorosis and lower biomass and photosynthesis than the NRT1.1 mutants in (NH₄)₂SO₄ medium. Grafting experiments between Col-0 and chl1-5 further confirmed that NH₄⁺ toxicity is influenced by NRT1.1. In (NH₄)₂SO₄ medium, NRT1.1 induced the expression of NH₄⁺ transporters, increasing NH₄⁺ uptake. Additionally, the activities of glutamine synthetase and glutamate synthetase in roots of Col-0 plants decreased and soluble sugar accumulated significantly, whereas pyruvate kinase-mediated glycolysis was not affected, all of which contributed to NH₄⁺ accumulation. By contrast, the NRT1.1 mutants showed reduced NH₄⁺ accumulation and enhanced NH₄⁺ assimilation through glutamine synthetase, glutamate synthetase, and glutamate dehydrogenase. Moreover, the up-regulation of genes involved in ethylene synthesis and senescence in Col-0 plants treated with (NH₄)₂SO₄ suggests that ethylene is involved in NH₄⁺ toxicity responses. This study showed that NH₄⁺ toxicity is related to a nitrate-independent signaling function of NRT1.1 in Arabidopsis, characterized by enhanced NH₄⁺ accumulation and altered NH₄⁺ metabolism, which stimulates ethylene synthesis, leading to plant senescence.

Aims We examined differences in soil metabolites from the rice root rhizosphere of long-term rice-rice-fallow (RRF) and rice-rice-rape (RRR) rotations, and examined the effects of 1,2-benzenediol on nitrogen-use efficiency (NUE) and rice growth. Methods The metabolite composition of rice rhizospheres was analyzed using the gas chromatography-mass spectrometry (GC-MS). A range of 0.2, 2.0 and 200 μmol L −1 concentrations of external 1,2-benzenediol were applied to examine their effects on rice growth, nitrate reductase (NR) and glutamine synthetase (GS) activities, and physiological nitrogen-use efficiency (PNUE). Results The metabolite composition of rhizospheres differed significantly between RRR and RRF. Soil total N and 1,2-benzenediol concentrations during the early rice season were significantly lower under RRR than RRF. Rice growth and NUE significantly enhanced at 0.20 μmol 1,2-benzenediol L −1 , but inhibited at 2.0 μmol L −1 or higher. Changes in root morphology and uptake associated with 1,2-benzenediol possibly had contributed to a higher NUE of the early season rice under RRR. The NR and GS activities in rice roots were significantly higher with 0.2 μmol L −1 1,2-benzenediol than without 1,2-benzenediol treatment. Conclusions Crop rotation significantly affected rice rhizosphere metabolites. An optimal soil 1,2-benzenediol concentration under long-term RRR rotation may be associated with an enhanced NUE and root N uptake and assimilation, resulting in an increased rice growth and yield.

Soil in short-term crop rotation systems (STCR) is still in the initial development stage of farmland soil, whereas after long-term crop rotation treatment (LTCR), soil properties are significantly different. This study compares STCR (4 years) and LTCR (30 years) rice-rice-fallow, rice-rice-rape rotation practices under the same soil type background and management system. To reveal ecosystem mechanisms within soils and their effects on rice yield following LTCR, we analyzed the physical, chemical, and microbiological properties of soils with different rotations and rotation times. Relative to STCR, LTCR significantly reduced soil water-stable aggregate (WSA) content in the < 0.053-mm range, while > 2 mm WSA content significantly increased. Soil organic matter increased in fields under LTCR, mainly in > 2 mm, 2–0.25 mm, and < 0.053 mm soil WSA in 0–10 cm soil layer. LTCR was associated with significantly increased total soil organic matter, at the same time being associated with increasing the amount of active organic matter in the 0–20 cm soil layer. The two crop rotation regimes significantly differed in soil aggregate composition as well as in soil N and P, microbial biomass, and community composition. Relative to STCR, LTCR field soils had significantly higher soil organic matter, active organic matter content, soil enzyme activities, and overall microbial biomass, while soil WSA and microbial community composition was significantly different. Our results demonstrate that LTCR could significantly improve soil quality and rice yield and suggest that length of rotation time and rice-rice-rape rotation are critical factors for the development of green agriculture.

To assess the pollution situation and human health risks, the concentrations of heavy metals Pb, Cd, Cu, and Zn in paddy soils and white rice around seven mining-affected areas in Hunan Province were analyzed. The ranges of concentrations of Pb (23.9–1595.8 mg kg−1), Cd (0.3–9.5 mg kg−1), Cu (31.2–321.5 mg kg−1) and Zn (56.1–3478.9 mg kg−1) in all paddy soils were significantly higher than Hunan background values and even exceeded the maximum permissible concentrations for paddy soil quality recommended by the Ministry of Environmental Protection of China. The geoaccumulation index (I-geo) showed that Cd (1.42–6.33) was the predominant pollutant in all paddy soils, while Zn was the least important element. The concentrations of Pb, Cd, Cu, and Zn in white rice ranged from 0.18 to 0.72 mg kg−1, 0.10 to 1.32 mg kg−1, 3.83 to 5.95 mg kg−1, and 8.64 to 18.18 mg kg−1, respectively. Human health risk, associated with these heavy metals, was assessed based on hazard quotients (HQ) and hazard indexes (HI) for adults through consumption of white rice. HQ values of heavy metals (except for Cd) in most of mining-affected areas were below 1.0, while the HI of all heavy metals in all mine areas was higher than 1.0, the maximum acceptable level, suggesting that consumption of such contaminated white rice was a health risk. Except for the Leng Shui-Jiang mine area, Cd was the major contributor to the risk in the mine areas through white rice consumption, amounting to over 67 % of the HI, while Zn was a minimal contributor compared to the other metals.

Nitrate, once taken up by plants, can either be stored in vacuoles or reduced by nitrate reductase in the cytoplasm. High accumulation of NO 3 − in the vacuole occurs when assimilation into the cytoplasm is saturated. This study elucidates how proton pumps at the tonoplast (V-ATPase and V-PPase) affect the NO 3 − content of Brassica napus by controlling the distribution of NO 3 − between the cytoplasm and vacuole. Pot experiments were conducted in a greenhouse under normal N (15.0 mM nitrate) conditions using B. napus genotypes that demonstrated either high (Xiangyou15) or low (814) nitrogen use efficiency (NUE). The NO 3 − content of the high NUE genotype was significantly lower than that of the low NUE genotype, whereas the total N per plant of the two genotypes was almost the same, suggesting that the different NUE between the two genotypes is not due to the difference of NO 3 − uptake. The relative expression levels of V-ATPase ( vha - a3 ) and V-PPase ( avp1 ) genes in the high NUE genotype were significantly lower than in the low NUE genotype, resulting in lower V-ATPase and V-PPase activities in the high NUE genotype. The transport of NO 3 − and protons from the cytoplasm to the vacuole is powered by V-ATPase and V-PPase, so their lower activities increase H + efflux from and reduce NO 3 − influx into the vacuoles of the high NUE genotype. We conclude that the lower activity of proton pumps at the tonoplast is the main reason the high NUE genotype possesses lower NO 3 − content and higher N-use efficiency.

Nitrate (NO₃⁻) can accumulate in high concentrations in plant cell vacuoles if it is not reduced, reutilized or transported into the cytoplasm. Such accumulation of NO₃⁻in the vacuole occurs when mechanisms for NO₃⁻assimilation in the cytoplasm are saturated. Moreover, other processes such as efflux across the plasma membrane might affect NO₃⁻accumulation in the vacuole. These are the main reasons limiting nitrogen use efficiency (NUE) in plants. This study elucidates mechanisms for NO₃⁻transport from the cytoplasm to vacuoles by the V-proton pump (V-ATPase and V-PPase) and their relationship with different NUE in four Brassica napus genotypes. Pot experiments were conducted in a greenhouse under normal (15.0 mmol L⁻¹) and limited N (7.5 mmol L⁻¹) concentrations of nitrate using B. napus genotypes that demonstrated either high (742 and Xiangyou 15) or low (814 and H8) NUE (g g⁻¹). Specific inhibitors of V-ATPase and V-PPase increased nitrate reductase (NR) activity, resulting in greatly decreased NO₃⁻in plant tissues. Nitrate reductase activity and NO₃⁻content correlated more highly to V-PPase activity than they did to V-ATPase activity, and correlation between V-PPase activity and NO₃⁻content was significantly higher than it was to V-ATPase. Genotypes with high NUE had significantly lower activities of V-ATPase and V-PPase than those with low NUE. In the high-NUE plants, lower activities of V-proton pump underlie mechanisms that result in significantly lower NO₃⁻content in plant tissues of the high-NUE genotypes than those found in plant tissues of the low-NUE genotypes. Our results show that the tonoplast proton pumps V-PPase and V-ATPase strongly negatively affect NR activity and positively affect NO₃⁻content. V-PPase contributed more to this regulatory mechanism than did V-ATPase.

Campylobacter spp. are the leading cause of gastroenteritis worldwide. Most human infections result from contaminated food; however, infections are also caused by recreational waterway contamination. Campylobacter culture is technically challenging and enumeration by culture-based methods is onerous. Thus, we employed qPCR to quantify Campylobacter spp. in fresh- and marine-water samples, raw sewage and animal feces. Multiplex PCR determined whether Campylobacter jejuni or C. coli, most commonly associated with human disease, were present in qPCR-positive samples. Campylobacters were detected in raw sewage, and in feces of all avian and mammalian species tested. Campylobacter-positive concentrations ranged from 68 to 2.3 × 106 cells per 500 mL. Although C. jejuni and C. coli were rare in waterways, they were prevalent in sewage and feces. Campylobacter-specific qPCR screening of environmental waters did not correlate with the regulatory EPA method 1600 (Enterococcus culture), nor with culture-independent, molecular-based microbial source tracking indicators, such as human polyomavirus, human Bacteroidales and Methanobrevibacter smithii. Our results suggest that neither the standard EPA method nor the newly proposed culture-independent methods are appropriate surrogates for Campylobacter contamination in water. Thus, assays for specific pathogens may be necessary to protect human health, especially in waters that are contaminated with sewage and animal feces.

Nitrate, once taken up by plants, can either be stored in vacuoles or reduced by nitrate reductase in the cytoplasm. High accumulation of NO3 ^sup -^ in the vacuole occurs when assimilation into the cytoplasm is saturated. This study elucidates how proton pumps at the tonoplast (V-ATPase and V-PPase) affect the NO3 ^sup -^ content of Brassica napus by controlling the distribution of NO3 ^sup -^ between the cytoplasm and vacuole. Pot experiments were conducted in a greenhouse under normal N (15.0 mM nitrate) conditions using B. napus genotypes that demonstrated either high (Xiangyou15) or low (814) nitrogen use efficiency (NUE). The NO3 ^sup -^ content of the high NUE genotype was significantly lower than that of the low NUE genotype, whereas the total N per plant of the two genotypes was almost the same, suggesting that the different NUE between the two genotypes is not due to the difference of NO3 ^sup -^ uptake. The relative expression levels of V-ATPase (vha-a3) and V-PPase (avp1) genes in the high NUE genotype were significantly lower than in the low NUE genotype, resulting in lower V-ATPase and V-PPase activities in the high NUE genotype. The transport of NO3 ^sup -^ and protons from the cytoplasm to the vacuole is powered by V-ATPase and V-PPase, so their lower activities increase H^sup +^ efflux from and reduce NO3 ^sup -^ influx into the vacuoles of the high NUE genotype. We conclude that the lower activity of proton pumps at the tonoplast is the main reason the high NUE genotype possesses lower NO3 ^sup -^ content and higher N-use efficiency.