Zn stress facilitates nitrate transporter 1.1-mediated nitrate uptake aggravating Zn accumulation in Arabidopsis plants.

Describing the mechanisms of zinc (Zn) accumulation in plants is essential to counteract the effects of excessive Zn uptake in crops grown in contaminated soils. Increasing evidence suggests that there is a positive correlation between nitrate supply and Zn accumulation in plants. However, the role of the primary nitrate transporter NRT1.1 in Zn accumulation in plants remains unknown. In this study, a Zn stress-induced increase in nitrate uptake and an increase in NRT1.1 protein levels in wild-type (Col-0) Arabidopsis plants were measured using microelectrode ion flux and green fluorescent protein (GFP)/ß-glucuronidase (GUS) staining, respectively. Both agar and hydroponic cultures showed that mutants lacking the NRT1.1 function in nrt1.1 and chl1-5 (chlorate resistant 1) exhibited lower Zn levels in the roots and shoots of Zn-stressed plants than the wild-type. A lack of NRT1.1 activity also alleviated Zn-induced photosynthetic damage and growth inhibition in plants. Further, we used a rotation system with synchronous or asynchronous uptakes of nitrate and Zn to demonstrate differences in Zn levels between the Col-0 and nrt1.1/chl1-5 mutants. Significantly lower difference in Zn levels were noted in the nitrate/Zn asynchronous treatment than in the nitrate/Zn synchronous treatment. From these results, it can be concluded that NRT1.1 modulates Zn accumulation in plants via a nitrate-dependent pathway.

Abscisic acid (ABA)-importing transporter 1 (AIT1) contributes to the inhibition of Cd accumulation via exogenous ABA application in Arabidopsis.

Soil cadmium (Cd) accumulation presents risks to crop safety and productivity. However, through an exogenous application of abscisic acid (ABA), its accumulation in plants can be reduced and its toxicity mitigated, thereby providing an alternative strategy to counteract Cd contamination of arable soil. In the present study, we demonstrated that exogenous ABA application alleviates Cd-induced growth inhibition and photosynthetic damage in wild-type (Col-0) Arabidopsis plants. However, these positive effects were weakened in the ABA-importing transporter (AIT1)-deficient mutant (ait1). Through further analysis, we found that upon ABA application, the decrease in Cd level significantly differed among ait1, Col-0, and the two AIT1-overexpressing transgenic plants (AIT1ox-1 and AIT1ox-2), suggesting that AIT1 mediates the Cd-reducing effects of ABA. ABA application also inhibited the expression of IRT1, ZIP1, ZIP4, and Nramp1 in Col-0 plants subjected to Cd stress. However, significant differences among the genotypes (ait1, Col-0 and AIT1ox) were only observed in terms of IRT1 expression. Overall, our findings suggest that the suppression of Cd accumulation and restoration of plant growth by exogenous ABA require the ABA-importing activity of AIT1 to inhibit IRT1 expression.

Microbial activity and community diversity in a variable charge soil as affected by cadmium exposure levels and time.

Effects of cadmium (Cd) on microbial biomass, activity and community diversity were assessed in a representative variable charge soil (Typic Aquult) using an incubation study. Cadmium was added as Cd(NO3)(2) to reach a concentration range of 0-16 mg Cd/kg soil. Soil extractable Cd generally increased with Cd loading rate, but decreased with incubation time. Soil microbial biomass was enhanced at low Cd levels (0.5-1 mg/kg), but was inhibited consistently with increasing Cd rate. The ratio of microbial biomass C/N varied with Cd treatment levels, decreasing at low Cd rate (<0.7 mg/kg available Cd), but increasing progressively with Cd loading. Soil respiration was restrained at low Cd loading (<1 mg/kg), and enhanced at higher Cd levels. Soil microbial metabolic quotient (MMQ) was generally greater at high Cd loading (1-16 mg/kg). However, the MMQ is also affected by other factors. Cd contamination reduces species diversity of soil microbial communities and their ability to metabolize different C substrates. Soils with higher levels of Cd contamination showed decreases in indicator phospholipids fatty acids (PLFAs) for Gram-negative bacteria and actinomycetes, while the indicator PLFAs for Gram-positive bacteria and fungi increased with increasing levels of Cd contamination.

Control of the pollution of antibiotic resistance genes in soils by quorum sensing inhibition.

To investigate whether pollution from antibiotic resistance genes (ARGs) could be affected by bacterial quorum sensing, the oxytetracycline (OTC)-containing manure was fertilized to establish the ARG-polluted soil environment. Under long-term OTC stress, substantial ARGs in the range from 10-4 to 10-3 RG/16S rRNA (resistance genes/16S rRNA) were detected in the antibiotics control (AC) group, in which OTC-containing manure was fertilized. Meanwhile, 10-6 RG/16S rRNA was detected in biological control (BC) group, in which non-OTC-containing manure was fertilized. Subsequently, two typical quorum sensing inhibitors, 4-nitropyridine N-oxide (4-NPO) and 3,4-dibromo-2H-furan-5-one (DBF), were used to treat the ARG-polluted soils. These two groups called 4-NPO treatments (NT) and DBF treatments (FT), respectively. There were no significant differences in bacterial growth and OTC degradation in NT and FT groups, compared to AC group. However, acyl-homoserine lactones such as C4-HSL, C6-HSL, and C8-HSL decreased significantly in both NT and FT groups, compared to AC group. Meanwhile, the abundance of most ARGs decreased dramatically. In FT group, the concentrations of tet(L) and tet(Q) were below the detection limits. It was demonstrated that quorum sensing inhibition could be an effective way to prevent and control the pollution of ARGs in soil.

Could humic acid relieve the biochemical toxicities and DNA damage caused by nickel and deltamethrin in earthworms (Eisenia foetida)?

The aim of the study was to determine whether humic acid (HA) prevented gene and biochemical toxic effects in earthworms (Eisenia foetida) exposed to nickel and deltamethrin (at 100 and 1 mg kg(-1), respectively) in soil. Cellular- and molecular-level toxic effects of nickel and deltamethrin in earthworms were evaluated by measuring damage to lipid membranes and DNA and the production of protein carbonyls over 42 days of exposure. Nickel and deltamethrin induced significant levels of oxidative stress in earthworms, increasing the production of peroxidation products (malondialdehyde and protein carbonyls) and increasing the comet assay tail DNA% (determined by single-cell gel electrophoresis). DNA damage was the most sensitive of the three indices because it gave a higher sample/control ratio than did the other indices. The presence of HA alleviated (in decreasing order of effectiveness) damage to DNA, proteins, and lipid membranes caused by nickel and deltamethrin. A low HA dose (0.5-1% HA in soil) prevented a great deal of lipid membrane damage, but the highest HA dose (3% HA in soil) prevented still more DNA damage. However, the malondialdehyde concentrations in earthworms were higher at the highest HA dose than at the lower HA doses. The amounts of protein carbonyls produced at different HA doses were not significantly different. The toxic effects to earthworms caused by increased oxidizable nickel concentrations could be relieved by adding HA.

Effect from low-level exposure of oxytetracycline on abundance of tetracycline resistance genes in arable soils.

To evaluate the effect from low-level exposure of antibiotics on the abundance of antibiotic resistance genes, unpolluted arable soils were treated with oxytetracycline (OTC)-containing manure, with OTC (0, 2, 20, or 70 µg kg(-1)) added every 2 weeks on 10 occasions. Six tetracycline resistance genes [TRGs-tet(A), tet(L), tet(M), tet(Q), tet(O), and tet(W)] and the 16S ribosomal RNA (rRNA) gene were monitored using real-time quantitative polymerase chain reaction. The relative abundance of tet(A), tet(L), tet(M), and tet(Q) genes in soil increased 10-1000 times after application of OTC-containing manure. Tet(A) abundance per unit of residual OTC on day 140 was 1.53-4.42 times higher than that on day 28, while tet(L) abundance was 1.04-1.74 times higher. Treatment with >40 µg kg(-1) OTC significantly increased abundance of tet(A) and tet(L), while tet(M) and tet(Q) abundance was positively correlated (R (2) = 0.965 and 0.932, p < 0.01) with residual OTC concentrations. There was a significant accumulation of TRGs associated with low-level OTC exposure in arable soils. Besides OTC residual, the effects from exposure time and application frequencies should also be considered to limit the increase in abundance of tet(A) and tet(L).

Bioaccessibility and health risk of heavy metals in ash from the incineration of different e-waste residues.

Ash from incinerated e-waste dismantling residues (EDR) may cause significant health risks to people through ingestion, inhalation, and dermal contact exposure pathways. Ashes of four classified e-waste types generated by an incineration plant in Zhejiang, China were collected. Total contents and the bioaccessibilities of Cd, Cu, Ni, Pb, and Zn in ashes were measured to provide crucial information to evaluate the health risks for incinerator workers and children living in vicinity. Compared to raw e-waste in mixture, ash was metal-enriched by category incinerated. However, the physiologically based extraction test (PBET) indicates the bioaccessibilities of Ni, Pb, and Zn were less than 50 %. Obviously, bioaccessibilities need to be considered in noncancer risk estimate. Total and PBET-extractable contents of metal, except for Pb, were significantly correlated with the pH of the ash. Noncancer risks of ash from different incinerator parts decreased in the order bag filter ash (BFA) > cyclone separator ash (CFA) > bottom ash (BA). The hazard quotient for exposure to ash were decreased as ingestion > dermal contact > inhalation. Pb in ingested ash dominated (>80 %) noncancer risks, and children had high chronic risks from Pb (hazard index >10). Carcinogenic risks from exposure to ash were under the acceptable level (<10(-6)) both for children and workers. Exposure to ash increased workers' cancer risks and children's noncancer risks. Given the risk estimate is complex including toxicity/bioaccessibility of metals, the ways of exposure, and many uncertainties, further researches are required before any definite decisions on mitigating health risks caused by exposure to EDR incinerated ash are made.

[Combined effects of copper and simulated acid rain on copper accumulation, growth, and antioxidant enzyme activities of Rumex acetosa].

A pot experiment was conducted to study the combined effects of Cu (0-1500 mg x kg(-1)) and simulated acid rain (pH 2.5-5.6) on the copper accumulation, growth, and antioxidant enzyme activities of Rumex acetosa. With the increasing concentration of soil Cu, the Cu accumulation in R. acetosa increased, being higher in root than in stem and leaf. The exposure to low pH acid rain promoted the Cu uptake by R. acetosa. With the increase of soil Cu concentration and/or of acid rain acidity, the biomass of R. acetosa decreased, leaf and root MDA contents increased and had good correlation with soil Cu concentration, and the SOD and POD activities in leaf and root displayed a decreasing trend after an initial increase. This study showed that R. acetosa had a strong adaptive ability to Cu and acid rain stress, exhibiting a high application potential in the remediation of Cu-contaminated soil in acid rain areas.

[Effects of dissolved organic matter derived from hyperaccumulator Sedum alfredii Hance rhizosphere on Zn adsorption and desorption in soil].

The study with pot experiment and simulation test showed that after planted hyperaccumulator Sedum alfredii Hance on mining soil, the water soluble Zn and NH4OAc extractable Zn in rhizosphere were decreased obviously, while the available Zn in non-rhizosphere had less change. The pH value of rhizosphere soil was decreased by 0. 3 units, whereas the organic matter and dissolved organic matter (DOM) contents were increased by 13.6% and 20.9%, respectively, compared with the soil without S. alfredii. The effects of DOM from S. alfredii rhizosphere on Zn absorption and desorption varied with the kinds of test soils. After the addition of rhizosphere DOM, the maximal absorption capacity (Xm) of mining soil, quaternary red clay soil, and fluavio-marine yellow loamy soil was reduced by 17.8%, 21.9% and 27.7%, respectively, whereas the addition of non-rhizosphere DOM had no effects on Zn absorption. The Zn desorption in the three soils, especially in fluavio-marine yellow loamy soil, was promoted by the addition of rhizosphere DOM. It was indicated that the DOM from S. alfredii rhizosphere could reduce the maximal absorption capacity and accelerate the desorption of adsorbed Zn, and thus, increase the Zn mobility and bioavailability.