Chromium(VI) Toxicity in Legume Plants: Modulation Effects of Rhizobial Symbiosis

Chromium stress induced oxidative burst in Vigna mungo (L.) Hepper: physio-molecular and antioxidative enzymes regulation in cellular homeostasis

Vigna mungo (L.) Hepper commonly known as blackgram is an important legume crop with good quality dietary proteins and vitamins. Low production of blackgram in the chromium rich soil of Odisha is a serious concern against its demand. Chromium (VI) was tested on V. mungo var. B3-8-8 at 100, 150, 200, 250 and 300 µM concentration on growth, anti-oxidative enzymes and chromium content at 15, 30 and 45 d of treatments. Seed germination and growth decreased with increase dose and duration. Cr uptake induced oxidative burst with significant increase of osmolytes was observed in cell at lower doses but failed to adjust homeostasis at higher dose. Increase of GPX and SOD and decrease of CAT was observed as dose dependent. Increased protein content was detected in < 200 µM Cr concentration whereas, significant decrease of protein was noted thereafter. Down regulation of proteins (29.2 kDa and 32.6 kDa) was observed at > 250 µM of Cr. Total Cr uptake was greater in root than in shoot which might be due to poor translocation of heavy metal or detoxification. Thus, blackgram was able to maintain homeostasis at lower concentrations of Cr by activating the cascade of enzymes following cellular detoxification mechanism.

SUPPLEMENTARY INFORMATION

The online version of this article contains supplementary material available at (10.1007/s12298-021-00941-3).

Biotechnological strategies for enhancing heavy metal tolerance in neglected and underutilized legume crops: A comprehensive review

Contamination of agricultural land and water by heavy metals due to rapid industrialization and urbanization including various natural processes have become one of the major constraints to crop growth and productivity. Several studies have reported that to counteract heavy metal stress, plants should be able to maneuver various physiological, biochemical and molecular processes to improve their growth and development under heavy metal stress. With the advent of modern biotechnological tools and techniques it is now possible to tailor legume and other plants overexpressing stress-induced genes, transcription factors, proteins, and metabolites that are directly involved in heavy metal stress tolerance. This review provides an in-depth overview of various biotechnological approaches and/or strategies that can be used for enhancing detoxification of the heavy metals by stimulating phytoremediation processes. Synthetic biology tools involved in the engineering of legume and other crop plants against heavy metal stress tolerance are also discussed herewith some pioneering examples where synthetic biology tools that have been used to modify plants for specific traits. Also, CRISPR based genetic engineering of plants, including their role in modulating the expression of several genes/ transcription factors in the improvement of abiotic stress tolerance and phytoremediation ability using knockdown and knockout strategies has also been critically discussed.

Menadione sodium bisulfite neutralizes chromium phytotoxic effects in okra by regulating cytosolutes, lipid peroxidation, antioxidant system and metal uptake

Chromium (Cr) is a major abiotic stress for plant species that significantly impacted plant development and impeded agricultural production. Menadione sodium bisulfite (MSB) has recently manifested a remarkable role in modulating plant defense responses. In the present experiment, Cr caused a significant decrease in growth, relative water contents, and chlorophyll in okra cultivars (Shabnam 786 and Arka Anamika). Cr produced an increase in proline, total soluble proteins, total free amino acids, phenolics, flavonoids, ascorbic acid, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and Cr accumulation. Besides, activities of antioxidant enzymes were also higher in Cr-stressed plants. MSB application (50, 100, 150, and 200 µM) profoundly impacted growth and important physio-biochemical characteristics in okra under Cr stress. Better growth in MSB treated plants was associated with lower oxidative damage and better oxidative defense system reflected in the form of higher antioxidant enzyme activities alongside the concentrations of non-enzymatic antioxidant compounds. In this background, cv. Shabnam-786 exhibited greater Cr tolerance over Arka Anamika. The degree of oxidative damage measured in the form of H₂O₂ and MDA was greater in cv. Arka Anamika. Lower MSB levels (50 and 100 µM) circumvented inhibitory Cr effects in okra, while higher doses proved lethal for plant growth and development.

Antagonistic effects of EDTA against biochemical toxicity induced by Cr(VI) in Hordeum vulgare L. seedlings

The present study aims at the amelioration of chromium Cr(VI) toxicity using ethylenediaminetetraacetic acid (EDTA), and to understand the interactive effects of Cr(VI) and EDTA with respect to seedling growth, lipid peroxidation as assessed from malondialdehyde, pigments and antioxidative enzymes in Hordeum vulgare L. Following multivariate statistical techniques were used to study binary interactions between Cr(VI) and EDTA: 2-way ANOVA, Tukey's multiple comparison test, multiple regression with interaction between Cr an EDTA, beta coefficients, path analysis and non-metric multidimensional scaling (NMDS). The present study revealed that the EDTA decreases lipid peroxidation induced by Cr(VI) and ameliorates the antioxidative defence system and pigment constitution of seedlings grown in Cr(VI) containing media. EDTA-Cr(VI) interaction decreased the Cr content in the seedlings which may be attributed to the chelating effect of EDTA. The root and shoot bioconcentration factors, the ratio of Cr content in the plant to that in the medium, were decreased by addition of EDTA to Cr(VI), indicating a decrease in the uptake of Cr by the seedlings from the medium. NMDS revealed that the ranking of the studied parameters is maintained by ordination on two axes. The study established that EDTA is antagonistic to Cr(VI) induced biochemical toxicity, and improves the antioxidative defence system, increases the chlorophyll content, and decreases Cr uptake in barley seedlings.

Determination of Macro- and Microelements in the Inflorescences of Banana Tree Using ICP OES: Evaluation of the Daily Recommendations of Intake for Humans

The inflorescence of Musa paradisiaca, known as “banana heart” is a structure that includes flowers and bracts of banana, commonly used as food source worldwide. The aims of this study were (1) to determine the mineral components of Musa paradisiaca and (2) to compare the obtained results with previously reported data of Recommendation Dietary Allowances (RDAs) and edible plant permissible limits set by FAO/WHO. The samples were digested using microwave-assisted equipment, while elemental contents were determined by inductively coupled plasma optical emission spectroscopy (ICP OES). Metal (Mg, Ca, Cr, Ni, Cu, Fe, and Zn) and nonmetal (S and P) contents were detected. According to RDA, the inflorescences could be excellent sources of Mg, P, Cr, Cu, Zn, and Fe for females, males, and pregnant women, all age 31–50 y, as well as children (4–8 y). Bracts are good source of Zn for male and pregnant women and good source of Fe for children. All the samples contained considerable amounts of Mg, Ca, P, Ni, Cu, Zn, and Fe, which were quite low to induce deleterious effects (UL). FAO/WHO limits for edible plants have not yet been established for S, P, Mg, and Ca, but Ni and Zn are below of those limit values. However, Cr and Cu concentrations are higher than the values established for edible plants and may pose a threat to human health. Farmers should be encouraged by government agencies, not only for sustainability of production but also to ensure the storage and trade of banana tree inflorescence.

The effect of wheat seedling density on photosynthesis may be associated with the phyllosphere microorganisms

Wheat seedlings are significantly impacted by the presence of bacteria. However, bacteria are unavoidably growing together with wheat. The study aimed to reveal wheat photosynthesis, phyllosphere bacterial community composition, and a shift in the bacterial community following different density treatments in a closed artificial ecosystem. Here, we report the relationship between photosynthesis and bacterial community in wheat seedlings for different planting densities. In this closed artificial ecosystem, a total of 30 phyla were detected, with 17 of them were simultaneously present in four treatments, under high light intensity and carbon dioxide growth environment. The key phyla detected include Firmicutes, Proteobacteria, and Bacteroidetes. We found that planting densities significantly impacted the photosynthetic characteristics of wheat and bacterial genetic biodiversity, but not on species composition of the bacterial community. Network analysis shows that the phyllosphere bacteria network structures were characterized by the clustering coefficient and modularity. Network for the 1000 plants/m² treatment group exhibits the highest levels of average clustering coefficient but lowest modularity and number of modules, among all plant densities tested. In addition, the network for the 1200 plants/m² treatment group exhibits the best characteristics in terms of net photosynthesis rate and intrinsic water use efficiency, higher complex phyllosphere community network structures, higher abundance of Corynebacterium, and more function of "Amino acid metabolism", which encourages the plants to grow better. The findings presented in this work elucidated the role of plant density in the growth of phyllosphere bacteria during wheat seedlings and provided theoretical support for reasonable wheat density cultivation in closed artificial ecosystems and wheat field production.

Hydrogen peroxide as a signalling molecule in plants and its crosstalk with other plant growth regulators under heavy metal stress

Hydrogen peroxide (H₂O₂) acts as a significant regulatory component interrelated with signal transduction in plants. The positive role of H₂O₂ in plants subjected to myriad of abiotic factors has led us to comprehend that it is not only a free radical, generated as a product of oxidative stress, but also helpful in the maintenance of cellular homeostasis in crop plants. Studies over the last two centuries has indicated that H₂O₂ is a key molecule which regulate photosynthesis, stomatal movement, pollen growth, fruit and flower development and leaf senescence. Exogenously-sourced H₂O₂ at nanomolar levels functions as a signalling molecule, facilitates seed germination, chlorophyll content, stomatal opening, and delays senescence, while at elevated levels, it triggers oxidative burst to organic molecules, which could lead to cell death. Furthermore, H₂O₂ is also known to interplay synergistically or antagonistically with other plant growth regulators such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid, nitric oxide and Ca²⁺ (as signalling molecules), and brassinosteroids (steroidal PGRs) under myriad of environmental stresses and thus, mediate plant growth and development and reactions to abiotic factors. The purpose of this review is to specify accessible knowledge on the role and dynamic mechanisms of H₂O₂ in mediating growth responses and plant resilience to HM stresses, and its crosstalk with other significant PGRs in controlling various processes. More recently, signal transduction by mitogen activated protein kinases and other transcription factors which attenuate HM stresses in plants have also been dissected.

Ameliorative effect of co-application of Bradyrhizobium japonicum EI09 and Se to mitigate chromium stress in Capsicum annum L

The present study was conducted to explore the potential of Bradyrhizobium japonicum EI09 (EI09) and selenium (Se) alone or in combination to mitigate hexavalent chromium (Cr⁶⁺) stress in Capsicum annum L. Chromium stressed plants exhibited significant reduction in biomass, chlorophyll content and gas exchange characteristics. The inoculated seedlings subjected to Cr⁶⁺stress showed improvement in growth, proline content, gas exchange attributes and total soluble proteins. Likewise, inoculated C. annum seedlings exhibited augmented activity of ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) under Cr⁶⁺ stress. The Cr⁶⁺ stress mitigation in inoculated seedlings was ascribed to reduction in malondialdehyde (MDA) content, hydrogen peroxide (H₂O₂) besides increase activity of flavonoids, proline, phenolic content along with modulation of antioxidative enzymes. The growth-enhancing attributes of bacteria such as indole acetic acid (IAA) content and 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity enhanced growth in Cr⁶⁺-stressed plants. Moreover, co-treatment of EI09 and 5 µM Se effectively mitigated Cr (VI) stress in C. annum plants. Current studies provide a novel insight into potential of B. japonicum EI09 and Se in reduction of Cr⁶⁺ toxicity in C. annum plants.

Organic chelates decrease phytotoxic effects and enhance chromium uptake by regulating chromium-speciation in castor bean (Ricinus communis L.)

There is limited information available on changes in the uptake of essential nutrients and secondary metabolites accumulation in castor bean under Cr toxicity. Besides, the role of organic chelates (EDTA and citric acid) mediated improvement in Cr uptake by castor bean is mostly unknown. Three independent experiments (sand, hydroponics, and soil) were executed to determine the Cr phytoextraction potential of Ricinus communis L. In the sand experiment, optimum doses of organic chelates (EDTA and citric acid) were selected. These optimum doses of chelates were used in the hydroponics and soil experiments. The results of hydroponics and soil experiments manifested a significant decrease in growth characteristics and leaf pigments in response to Cr stress applied as K2Cr2O7 (a source of Cr6+). The application of organic chelates (2.5 and 5 mM) showed a noticeable improvement in oxidative defense and secondary metabolites accumulation that might have decreased oxidative injury reflected as lower hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents. Moreover, chelates improved the uptake of essential nutrients (K+, Ca2+, Mg2+, Fe2+ and P) alongside significant enhancement in total Cr contents of plants. Our results advocated that chelates application resulted in greater endogenous levels of Cr3+ in plants compared with Cr6+ which is more toxic. In nutshell, organic chelates improved growth by regulating Cr species, ion homeostasis and secondary metabolites accumulation in Ricinus communis L.

Green copper nanoparticles from a native Klebsiella pneumoniae strain alleviated oxidative stress impairment of wheat plants by reducing the chromium bioavailability and increasing the growth

Chromium (Cr) concentration has been increasing substantially in the environment due to industrial and anthropogenic factors. Plants can absorb Cr and undergo unrestrained oxidation cascades, resulting in cell injury. The ameliorative role of biogenic copper nanoparticles to relieve wheat plants from Cr stress by supporting their growth is still unclear. The present work aims at the biosynthesis and characterization of copper nanoparticles (CuNPs) from a native Klebsiella pneumoniae strain, followed by assessment of wheat growth and physiological responses to CuNPs mixed in Cr-rich soil. The taxonomic rank of K. pneumoniae SN35 was established by the 16 S rRNA gene sequence analysis. The properties of biogenic CuNPs were elucidated by using UV-vis spectroscopy, FTIR, XRD, SEM, and TEM. It was found that 19.01-47.47 nm spherical shaped CuNPs were stabilized by different functional groups produced extracellularly by the strain SN35. The XRD data revealed the crystalline nature of CuNPs as a face-centered cubic structure. Different concentrations of CuNPs (0, 25, 50 and 100 mg kg^-1 of soil) were added into the soil mixed with 3.5 mg kg^-1 K₂Cr₂O₇ and the pots were placed in a growth chamber for 30 days. The results revealed that the CuNPs, at 25 and 50 mg kg^-1 of soil, augmented plant growth, biomass, and cellular antioxidants contents, whereas decreased the reactive oxygen species and Cr translocation from soil to roots and shoots as compared to control plants. Overall, the results revealed that the soil amendment of CuNPs could immobilize the Cr in the soil to prevent its translocation to the upper plant parts and support wheat growth by relieving cellular oxidative stress.

Chromium Bioaccumulation and Its Impacts on Plants: An Overview

Chromium (Cr) is an element naturally occurring in rocky soils and volcanic dust. It has been classified as a carcinogen agent according to the International Agency for Research on Cancer. Therefore, this metal needs an accurate understanding and thorough investigation in soil-plant systems. Due to its high solubility, Cr (VI) is regarded as a hazardous ion, which contaminates groundwater and can be transferred through the food chain. Cr also negatively impacts the growth of plants by impairing their essential metabolic processes. The toxic effects of Cr are correlated with the generation of reactive oxygen species (ROS), which cause oxidative stress in plants. The current review summarizes the understanding of Cr toxicity in plants via discussing the possible mechanisms involved in its uptake, translocation and sub-cellular distribution, along with its interference with the other plant metabolic processes such as chlorophyll biosynthesis, photosynthesis and plant defensive system.

β-Pinene moderates Cr(VI) phytotoxicity by quenching reactive oxygen species and altering antioxidant machinery in maize

We examined the possible role of monoterpene β-pinene in providing protection against Cr(VI) toxicity in maize (Zea mays). Treatment with β-pinene (10 μM) significantly alleviated Cr(VI) accumulation and recuperated Cr(VI) caused decline in root and coleoptile growth in maize. β-Pinene addition caused a decline in Cr(VI)-induced accumulation of superoxide anion, hydroxyl ion, hydrogen peroxide and confirmed by in-situ detection of ROS using histochemical localization. It suggested that the β-pinene quenches/neutralizes enhanced ROS generated under Cr(VI) exposure. β-Pinene also reduced Cr(VI)-induced electrolyte leakage, thereby suggesting its role in membrane stabilization. Further, β-pinene regulated the activity of scavenging enzymes, thereby suggesting a role in modulating Cr(VI)-induced oxidative damage. In conclusion, our results suggest that the addition of β-pinene has a protective role against Cr(VI) stress and provides resistance to maize against Cr(VI) toxicity.

Cytotoxic Assessment of Chromium and Arsenic Using Chromosomal Behavior of Root Meristem in Allium cepa L

A study was performed for phyto-genotoxic assay of chromium (Cr) and arsenic (As) through Allium cepa. Various concentrations (0, 1, 3, 6 and 12 mg L^-1) of Cr and As for 48 and 168 h time points exposed to A. cepa. The phytotoxic effects of metal(loid) were evident through inhibited root length and root protein. Metal(loid) toxicity also lead to genotoxic effects, which included depression of mitotic index and increased frequency of chromosomes aberrations like break, fragments, c-metaphase, multipolar arrangements etc. Genotoxic endpoint as progressive frequency of micronuclei in interphase of root meristem cells in treated plants was also observed. This genotoxic endpoint revealed carcinogenic nature of both aforementioned metal(loid). Along with inhibition in root length and protein content, depression in mitotic index as well as stimulation of various abnormality in mitotic cell division indicated that both metal(loid) are hazardous in nature and causing harmful effect on the environment.