Hexavalent chromium stress enhances the uptake of nitrate but reduces the uptake of ammonium and glycine in pak choi (Brassica chinensis L.)

Phytomelatonin maintained chromium toxicity induced oxidative burst in Brassica juncea L. through improving antioxidant system and gene expression

Chromium (Cr) contamination in soils reduces crop yields and poses a remarkable risk to human and plant system. The main objective of this study was to observe the protective mechanisms of exogenously applied melatonin (Mel- 0.05, 0.1, and 0.15 μM) in seedlings of Brassica juncea L. under Cr (0.2 mM) stress. This was accomplished by analysing the plant's morpho-physiological, biochemical, nuclear, membrane, and cellular characteristics, as well as electrolyte leakage. Superoxide, malondialdehyde, and hydrogen peroxide increased with Cr toxicity. Cr also increased electrolyte leakage. Seedlings under Cr stress had 86.4% more superoxide anion and 27.4% more hydrogen peroxide. Electrolyte leakage increased 35.7% owing to Cr toxicity. B. juncea L. cells with high radical levels had membrane and nuclear damage and decreased viability. Besides this, the activities of the antioxidative enzymes, as POD, APOX, SOD, GST, DHAR, GPOX and GR also elevated in the samples subjected to Cr toxicity. Conversely, the activity of catalase was downregulated due to Cr toxicity. In contrast, Mel reduced oxidative damage and conserved membrane integrity in B. juncea seedlings under Cr stress by suppressing ROS generation. Moreover, the activity of antioxidative enzymes that scavenge reactive oxygen species was substantially upregulated by the exogenous application of Mel. The highest concentration of Mel (Mel c- 0.15 μM) applied showed maximum ameliorative effect on the toxicity caused by Cr. It causes alleviation in the activity of SOD, CAT, POD, GPOX, APOX, DHAR, GST and GR by 51.32%, 114%, 26.44%, 48.91%, 87.51%, 149%, 42.30% and 40.24% respectively. Histochemical investigations showed that Mel increased cell survival and reduced ROS-induced membrane and nuclear damage. The findings showed that Mel treatment upregulated several genes, promoting plant development. Its supplementation decreased RBOH1 gene expression in seedling sunder stress. The results supported the hypothesis that Mel concentrations reduce Cr-induced oxidative burst in B. juncea.

LC-QTOF/MS-based non-targeted metabolomics to explore the toxic effects of di(2-ethylhexyl) phthalate (DEHP) on Brassica chinensis L

Di(2-ethylhexyl) phthalate (DEHP) is a widely used plasticizer known to pose health risks to humans upon exposure. Recognizing the toxic nature of DEHP, our study aimed to elucidate the response mechanisms in Brassica chinensis L. (Shanghai Qing) when subjected to varying concentrations of DEHP (2 mg kg-1, 20 mg kg-1, and 50 mg kg-1), particularly under tissue stress. The findings underscored the substantial impact of DEHP treatment on the growth of Brassica chinensis L., with increased DEHP concentration leading to a notable decrease in chlorophyll levels and alterations in the content of antioxidant enzyme activities, particularly superoxide dismutase (SOD) and peroxidase (POD). Moreover, elevated DEHP concentrations correlated with increased malondialdehyde (MDA) levels. Our analysis detected a total of 507 metabolites in Brassica chinensis L., with 331 in shoots and 176 in roots, following DEHP exposure. There was a significant difference in the number of metabolites in shoots and roots, with 79 and 64 identified, respectively (VIP > 1, p < 0.05). Metabolic pathway enrichment in Brassica chinensis L. shoots revealed significant perturbations in valine, leucine, and isoleucine biosynthesis and degradation, aminoacyl-tRNA, and glucosinolate biosynthesis. In the roots of Brassica chinensis L., varying DEHP levels exerted a substantial impact on the biosynthesis of zeatin, ubiquinone terpenoids, propane, piperidine, and pyridine alkaloids, as well as glutathione metabolic pathways. Notably, DEHP's influence was more pronounced in the roots than in the shoots, with higher DEHP concentrations affecting a greater number of metabolic pathways. This experimental study provides valuable insights into the molecular mechanisms underlying DEHP-induced stress in Brassica chinensis L., with potential implications for human health and food safety.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

Effect of different levels of EDTA on phytoextraction of heavy metal and growth of Brassica juncea L

Heavy metal pollution of soil is a major concern due to its non-biodegradable nature, bioaccumulation, and persistence in the environment. To explore the probable function of EDTA in ameliorating heavy metal toxicity and achieve the sustainable development goal (SDG), Brassica juncea L. seedlings were treated with different concentrations of EDTA (0, 1.0, 2.0, 3.0, and 4.0 mM Kg-1) in heavy metal-polluted soil. Plant samples were collected 60 days after sowing; photosynthetic pigments, H2O2, monoaldehyde (MDA), antioxidant enzymes, and ascorbic acid content, as well as plant biomass, were estimated in plants. Soil and plant samples were also examined for the concentrations of Cd, Cr, Pb, and Hg. Moreover, values of the phytoremediation factor were utilized to assess the accumulation capacity of heavy metals by B. juncea under EDTA treatments. In the absence of EDTA, B. juncea seedlings accrued heavy metals in their roots and shoots in a concentration-dependent manner. However, the highest biomass of plants (roots and shoots) was recorded with the application of 2 mM kg-1 EDTA. Moreover, high levels (above 3 mM kg-1) of EDTA concentration have reduced the biomass of plants (roots and shoots), photosynthetic area, and chlorophyll content. The effect of EDTA levels on photosynthetic pigments (chlorophyll a and b) revealed that with an increment in EDTA concentration, accumulation of heavy metals was also increased in the plant, subsequently decreasing the chlorophyll a and b concentration in the plant. TLF was found to be in the order Pb> Hg> Zn> and >Ni, while TF was found to be in the order Hg>Zn>Ni>Pb, and the best dose was 3 mM kg-1 EDTA for Hg and 4 mM kg-1 for Pb, Ni, and Zn. Furthermore, hyperaccumulation of heavy metals enhanced the generation of hydrogen peroxide (H2O2), superoxide anions (O2•-), and lipid peroxidation. It also interrupts mechanisms of the antioxidant defense system. Furthermore, heavy metal stress reduced plant growth, biomass, and chlorophyll (chl) content. These findings suggest that the exogenous addition of EDTA to the heavy metal-treated seedlings increases the bioavailability of heavy metals for phytoextraction and decreases heavy metal-induced oxidative injuries by restricting heavy metal uptake and components of their antioxidant defense systems.

Chromium toxicity and tolerance mechanisms in plants through cross-talk of secondary messengers: An overview of pathways and mechanisms

Environmental sources of chromium (Cr) such as solid waste, battery chemicals, industrial /waste, automotive exhaust emissions, mineral mining, fertilizers, and pesticides, have detrimental effects on plants. An excessive amount of Cr exposure can lead to toxic accumulations in human, animal, and plant tissues. In plants, diverse signaling molecules like hydrogen sulfide (H₂S) and nitric oxide (NO) play multiple roles during Cr stress. Consequently, the molecular mechanisms of Cr toxicity in plants, such as metal binding, modifying enzyme activity, and damaging cells are examined by several studies. The reactive oxygen species (ROS) that are formed when Cr reacts with lipids, membranes, DNA, proteins, and carbohydrates are all responsible for damage caused by Cr. ROS regulate plant growth, programmed cell death (PCD), cell cycle, pathogen defense, systemic communication, abiotic stress responses, and growth. Plants accumulate Cr mostly through the root system, with very little movement to the shoots. The characterization of stress-inducible proteins and metabolites involved in Cr tolerance and cross-talk messengers has been made possible due to recent advances in metabolomics, transcriptomics, and proteomics. This review discusses Cr absorption, translocation, subcellular distribution, and cross-talk between secondary messengers as mechanisms responsible for Cr toxicity and tolerance in plants. To mitigate this problem, soil-plant systems need to be monitored for the biogeochemical behavior of Cr and the identification of secondary messengers in plants.

Amelioration of Chromium-Induced Oxidative Stress by Combined Treatment of Selected Plant-Growth-Promoting Rhizobacteria and Earthworms via Modulating the Expression of Genes Related to Reactive Oxygen Species Metabolism in Brassica juncea

Chromium (Cr) toxicity leads to the enhanced production of reactive oxygen species (ROS), which are extremely toxic to the plant and must be minimized to protect the plant from oxidative stress. The potential of plant-growth-promoting rhizobacteria (PGPR) and earthworms in plant growth and development has been extensively studied. The present study was aimed at investigating the effect of two PGPR (Pseudomonas aeruginosa and Burkholderia gladioli) along with earthworms (Eisenia fetida) on the antioxidant defense system in Brassica juncea seedlings under Cr stress. The Cr toxicity reduced the fresh and dry weights of seedlings, enhanced the levels of superoxide anion (O₂•^-), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (EL), which lead to membrane as well as the nuclear damage and reduced cellular viability in B. juncea seedlings. The activities of the antioxidant enzymes, viz., superoxide dismutase (SOD), guaiacol peroxidase (POD), ascorbate peroxidase (APOX), glutathione peroxidase (GPOX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were increased; however, a reduction was observed in the activity of catalase (CAT) in the seedlings under Cr stress. Inoculation of the PGPR and the addition of earthworms enhanced the activities of all other antioxidant enzymes except GPOX, in which a reduction of the activity was observed. For total lipid- and water-soluble antioxidants and the non-enzymatic antioxidants, viz., ascorbic acid and glutathione, an enhance accumulation was observed upon the inoculation with PGPR and earthworms. The supplementation of PGPR with earthworms (combined treatment) reduced both the reactive oxygen species (ROS) and the MDA content by modulating the defense system of the plant. The histochemical studies also corroborated that the combined application of PGPR and earthworms reduced O₂•^-, H₂O₂, lipid peroxidation, and membrane and nuclear damage and improved cell viability. The expression of key antioxidant enzyme genes, viz., SOD, CAT, POD, APOX, GR, DHAR, and GST showed the upregulation of these genes at post-transcriptional level upon the combined treatment of the PGPR and earthworms, thereby corresponding to the improved plant biomass. However, a reduced expression of RBOH1 gene was noticed in seedlings supplemented under the effect of PGPR and earthworms grown under Cr stress. The results provided sufficient evidence regarding the role of PGPR and earthworms in the amelioration of Cr-induced oxidative stress in B. juncea.

Organic N compounds in plant nutrition: have methodologies based on stable isotopes provided unequivocal evidence of direct N uptake?

During the past two decades, interest has developed in regard to the possibility that plant roots can take up organic N compounds directly, a concept which challenges the conventional wisdom that soil inorganic N forms (NH4+ and NO3-) are the sole primary sources of N absorbed by plant roots. We reviewed the literature based on single or dual (¹⁵N, ¹³C) stable isotope labelling techniques to test the hypothesis of direct uptake. Both isotopically enriched and natural abundance approaches were reviewed. Of the methods examined, the dual enrichment technique, when combined with compound specific and position-specific stable isotope analysis, provided incontrovertible evidence for direct uptake of simple amino acids. We demonstrate that dual labelling lacks overall sensitivity due to the high C concentration in plant tissue relative to N, and the higher natural abundance of ¹³C cf. ¹⁵N, which limits the period of measurement due to isotope dilution, and hence an assessment of the long-term contribution of direct uptake to the N economy of plant communities.

Exogenous application of black cumin (Nigella sativa) seed extract improves maize growth under chromium (Cr) stress

Accumulation of non-essential heavy metals like chromium (Cr) is among major abiotic stresses, which adversely affect crop growth. Hexavalent chromium [Cr(VI)] is the most dangerous form negatively affecting the growth and productivity of crops. This study evaluated the role of black cumin extracts (BCE) in improving growth and productivity of maize genotypes under different concentrations of Cr(VI). Two maize genotypes ("Neelum" and "P1543") were grown under 0, 4, 8 and 12 mg Cr(VI) kg^-1 concentrations. The BCE was applied as foliar spray at three concentrations (0, 10 and 20%) at 25 and 45 days after sowing. Increasing Cr(VI) concentration significantly (p < 0.05) reduced seed germination, root and allometric traits, gas exchange attributes and relative water contents of tested genotypes. Hybrid maize genotype better tolerated tested Cr(VI) concentrations than synthetic genotype with lower Cr accumulation and better allometric and gas exchange traits. Exogenous application of 20% BCE proved effective in lowering the adverse effects of Cr(VI) toxicity on maize genotypes. It is concluded that 20% BCE could be used to improve maize performance through better allometric and gas exchange traits under different Cr(VI) concentrations. Nonetheless, actual mechanisms involved in improved Cr(VI)-tolerance of maize with BCE application must be explored. Novelty statement Black cumin has been widely used to reduce Cr toxicity in animals. However, the role of black cumin in reducing Cr toxicity in plants has never been studied. The present study was conducted to infer the role of different concentrations of black cumin extract in improving the growth of synthetic and hybrid maize genotypes under different levels of Cr stress. It is concluded that black cumin extract could be used to lower Cr toxicity in maize grown under Cr-contaminated soils.

Cellular pathologies and genotoxic effects arising secondary to heavy metal exposure: A review

Environmental pollution is significant and oftentimes hazardous in the areas, where mining, foundries and smelters and other metallurgical operations are located. Systematic research on the chronic effects of metals started during the past century; nevertheless, it is evident that even today, there are large gaps in knowledge regarding the assessment of the health effects caused by environmental and occupational exposures to these metals. Heavy metals induce the production of reactive oxygen species (ROS) causing oxidative stress, make several repair-inhibiting cellular changes and alter the DNA repair processes. They favour the ‘false’ repairing of double-strand breaks (DSBs), propagate DNA mutations and induce carcinogenesis. A detailed literature search was performed using the MedLine/PubMed database. Depending on the mechanism of action, arsenicals can act as genotoxins, non-genotoxic agents and carcinogens. Cadmium can bind to proteins, reduce DNA repair, activate protein degradation, up-regulate cytokines and proto-oncogenes (c-fos, c-jun and c-myc), induce the expression of metallothionein, haeme-oxygenases, glutathione transferases, heat-shock proteins, acute-phase reactants and DNA polymerase β at lower concentrations. Inorganic mercury damages oxidative phosphorylation and electron transport pathways at the ubiquinone–cytochrome b5 locus and thus induces ROS production. Abandoned mining areas generate environmentally persistent waste. These specific sites urgently require maximally efficient and cheap remediation. This bears the need for methodologies employing green and sustainable remediation. Phytoremediation is important in that it is a prevalent in situ remediation technique. Its advantages include the use of solar energy, cost-effectiveness, easy operation, reduction in secondary contaminants, the use of biomass for biofuel production and low-cost adsorbents.

Effects of surface-modified biochars and activated carbon on the transformation of soil inorganic nitrogen and growth of maize under chromium stress

Elevated chromium (Cr) level is challenging agricultural production and affecting soil biochemical process. This study evaluated the effect of amendments including surface-modified biochars (HBC: acid washing, Fe(III)-HBC: ferric iron loading, nZVI-HBC: nanoscale zero-valent iron loading) and activated carbon on hexavalent chromium (Cr(VI)) removal in soil and on N cycling enzyme activities, transformation of soil inorganic nitrogen, and growth of maize under Cr stress. The results showed that amendments increased Cr(VI) removal by 72.9%-96.34% at three levels of spiked Cr(VI) (low: 125 mg kg^-1, moderate: 250 mg kg^-1, high: 500 mg kg^-1). Under low Cr stress, amendments generally significantly decreased urease and nitrite reductase activities but increased nitrate reductase activity (p < 0.05). The NH₄⁺-N content had a significant positive correlation with urease activity (p < 0.01), while both NO₂^--N and NO₃^--N were absent correlations with N cycling enzyme studied. Amendments decreased NH₄⁺-N/NO₃^--N ratio under low Cr stress but increased it under moderate Cr stress, although the difference was not significant. Under high Cr stress, only Fe(III)-HBC significantly increased NH₄⁺-N/NO₃^--N ratio (p < 0.05). The decrease and increase of NH₄⁺-N/NO₃^--N ratios indicate the enhancement of nitrification and denitrification, respectively. The increase in Cr(VI) removal by amendments contributed to the increase in the migration of NO₃^--N from roots to shoots. Amendments (except for nZVI-HBC in soil under low Cr stress) increased maize height by 20%-59%. Under low Cr stress, however, nZVI-HBC significantly decreased maize height by 65% (p < 0.05), indicating the toxic effect of nZVI on maize growth overwhelmed low Cr stress.

Citric acid assisted phytoextraction of chromium by sunflower; morpho-physiological and biochemical alterations in plants

Soil and water contamination from heavy metals and metalloids is one of the most discussed and burning global issues due to its potential to cause the scarcity of healthy food and safe water. The scientific community is proposing a range of lab and field based physical, chemical and biological solutions to remedy metals and metalloids contaminated soils and water. The present study finds out a possibility of Chromium (Cr) extraction by sunflower from spiked soil under chelating role of citric acid (CA). The sunflower plants were grown under different concentrations of Cr (0, 5, 10 & 20mgkg^-1) and CA (0, 2.5 & 5mM). Growth, biomass, gas exchange, photosynthesis, electrolyte leakage (EL), reactive oxygen species (ROS; malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and the activities of antioxidant enzymes such as, superoxide dismutase (SOD), guaiacole values peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) were measured. The results depicted a clear decline in plant height, root length, leaf area, number of leaves and flowers per plant along with fresh and dry biomass of all parts of plant with increasing concentration of Cr in soil. Similar reduction was observed in chlorophyll a and b, total chlorophyll, carotenoids, soluble protein, gas exchange attributes and SPAD. The increasing concentration of Cr also enhanced the Cr uptake and accumulation in plant roots, stem and leaves along with the production of ROS and EL. The activities of antioxidant enzymes increased with increasing Cr concentration from 0 to 10mg, but decreased at 20mgkg^-1 soil. The CA application significantly alleviated Cr-induced inhibition of plant growth, biomass, photosynthesis, gas exchange, soluble proteins and SPAD value. Presence of CA also enhanced the activities of all antioxidant enzymes and reduced the production of ROS and EL. The chelating potential of CA increased the concentration and accumulation of Cr in plant roots, stem and leaves. It is concluded that the sunflower can be a potential candidate for the remediation of Cr under CA treatment, while the possibility may vary with genotype, Cr level and CA concentration.