Combined ability of chromium (Cr) tolerant plant growth promoting bacteria (PGPB) and salicylic acid (SA) in attenuation of chromium stress in maize plants

Deciphering the alleviation potential of nitric oxide, for low temperature and chromium stress via maintaining photosynthetic capacity, antioxidant defence, and redox homeostasis in rice (Oryza sativa)

Sodium nitroprusside (SNP) is a potent nitric oxide (NO) donor that enhances plant tolerance to various abiotic stresses. This research aims to assess the effect of SNP application on rice seedlings subjected to individual and combined exposure to two abiotic stresses viz., low-temperature (LT) and chromium (Cr). Exposure to LT, Cr, and LT+Cr caused severe oxidative damage by stimulating greater production and accumulation of reactive oxygen species (ROS) leading to lipid peroxidation and cell membrane instability. The combined LT+CR stress more intensly increased the cellular oxidative stress and excessive Cr uptake that in turn deteriorated the chlorophyll pigments and photosynthesis, as well as effected the level of tetrapyrrole biosynthesis in rice plants. The reduction in rice seedling growth was more obvious under LT+Cr treatment than their individual effects. The exogenous application of SNP diminished the toxic impact of LT and Cr stress. This was attributed to the positive role of SNP in regulating the endogenous NO levels, free amino acids (FAAs) contents, tetrapyrrole biosynthesis and antioxidants. Consequently, SNP-induced NO decreased photorespiration, ROS generation, lipid peroxidation, and electrolyte leakage. Moreover, exogenous SNP diminished the Cr uptake and accumulation by modulating the ionic homeostasis and strengthening the heavy metals detoxification mechanism, thus improving plant height, biomass and photosynthetic indexes. Essentially, SNP boosts plant tolerance to LT and Cr stress by regulating antioxidants, detoxification mechanism, and the plant's physio-biochemical. Hence, applying SNP is an effective method for boosting rice plant resilience and productivity in the face of escalating environmental stresses and pollutants.

Enhancing chromium resistance and bulb quality in onion (Allium cepa L.) through copper nanoparticles and possible health risk

Chromium (Cr) is a toxic metal in soil-plant system, hence causing possible health risks prominently in the areas with forgoing industrial activities. Copper nanoparticles (Cu NPs) have been reported as an excellent adsorbent for pollutants. Therefore, this study investigates how copper nanoparticles enhance onion growth while decreasing chromium uptake in onion plants. Additionally, it examines the potential health risks of consuming onion plants with elevated chromium levels. The results demonstrated that the addition of CuNPs at 15 mg kg-1 significantly improved the plant height (48%), leaf length (37%), fresh weight of root (61%), root dry weight (70%), fresh weight of bulb (52%), bulb dry weight (59%), leaves fresh weight (52%) and dry weight of leaves (59%), leaf area (72%), number of onion leaves per plant (60%), Chl. a (42%), chl. b (36%), carotenoids (40%), total chlorophyll (40%), chlorophyll contents SPAD value (56%), relative water contents (35%), membrane stability index (16%), total sugars (25%), crude protein (21%), ascorbic acid (19%) and ash contents (64%) at 10 mg kg-1 Cr. Whereas, maximum decline of Cr by 46% in roots, 68% in leaves and 92% in bulb was found with application of 15 mg kg-1 of Cu NPs in onion plants under 10 mg kg-1 Cr toxicity. The health risk assessment parameters of onion plants showed minimum values 0.0028 for average daily intake (ADI), 0.001911 for Non-cancer risk (NCR), and 0.001433 for cancer risk (CR) in plants treated with Cu NPs at 15 mg kg-1 concentration grown in soil spiked with 10 mg kg-1 chromium. It is concluded that Cu NPs at 15 mg kg-1 concentration improved growth of plants in control as well as Cr contaminated soil. Therefore, use of Cu NPs at 15 mg kg-1 concentration is recommended for improving growth of plants under normal and metal contaminated soils.

PGPR-Enabled bioremediation of pesticide and heavy metal-contaminated soil: A review of recent advances and emerging challenges

The excessive usage of agrochemicals, including pesticides, along with various reckless human actions, has ensued discriminating prevalence of pesticides and heavy metals (HMs) in crop plants and the environment. The enhanced exposure to these chemicals is a menace to living organisms. The pesticides may get bioaccumulated in the food chain, thereby leading to several deteriorative changes in the ecosystem health and a rise in the cases of some serious human ailments including cancer. Further, both HMs and pesticides cause some major metabolic disturbances in plants, which include oxidative burst, osmotic alterations and reduced levels of photosynthesis, leading to a decline in plant productivity. Moreover, the synergistic interaction between pesticides and HMs has a more serious impact on human and ecosystem health. Various attempts have been made to explore eco-friendly and environmentally sustainable methods of improving plant health under HMs and/or pesticide stress. Among these methods, the employment of PGPR can be a suitable and effective strategy for managing these contaminants and providing a long-term remedy. Although, the application of PGPR alone can alleviate HM-induced phytotoxicities; however, several recent reports advocate using PGPR with other micro- and macro-organisms, biochar, chelating agents, organic acids, plant growth regulators, etc., to further improve their stress ameliorative potential. Further, some PGPR are also capable of assisting in the degradation of pesticides or their sequestration, reducing their harmful effects on plants and the environment. This present review attempts to present the current status of our understanding of PGPR's potential in the remediation of pesticides and HMs-contaminated soil for the researchers working in the area.

Copper stress in rice: Perception, signaling, bioremediation and future prospects

Copper (Cu) is an indispensable micronutrient for plants, animals, and microorganisms and plays a vital role in different physiological processes. However, excessive Cu accumulation in agricultural soil, often through anthropogenic action, poses a potential risk to plant health and crop productivity. This review article provided a comprehensive overview of the available information regarding Cu dynamics in agricultural soils, major sources of Cu contamination, factors influencing its mobility and bioavailability, and mechanisms of Cu uptake and translocation in rice plants. This review examined the impact of Cu toxicity on the germination, growth, and photosynthesis of rice plants. It also highlighted molecular mechanisms underlying Cu stress signaling and the plant defense strategy, involving chelation, compartmentalization, and antioxidant responses. This review also identified significant areas that need further research, such as Cu uptake mechanism in rice, Cu signaling process, and the assessment of Cu-polluted paddy soil and rice toxicity under diverse environmental conditions. The development of rice varieties with reduced Cu accumulation through comprehensive breeding programs is also necessary. Regulatory measures, fungicide management, plant selection, soil and environmental investigation are recommended to prevent Cu buildup in agricultural lands to achieve sustainable agricultural goals.

Decontamination of Chlorpyrifos Residue in Soil by Using Mentha piperita (Lamiales: Lamiaceae) for Phytoremediation and Two Bacterial Strains

This study utilizes Mentha piperita (MI) for the first time to investigate the uptake and translocation of chlorpyrifos (CPF; 10 µg g-1) from soil, introducing a new approach to improve the efficacy of this technique, which includes using biosurfactants (Bacillus subtilis and Pseudomonas aeruginosa) at 107 CFU/mL to degrade CPF under greenhouse conditions. Moreover, antioxidant enzymes, including superoxide dismutase (SOD) and peroxidase (Prx), and oxidative stress due to hydrogen peroxide (H2O2) and malondialdehyde (MDA) in MI roots and leaves were evaluated under CPF stress. Our results demonstrated that amending soil with MI and B. subtilis followed by P. aeruginosa significantly reduced CPF levels in the soil (p > 0.05) and enhanced CPF concentrations in MI roots and leaves after 1, 3, 7, 10, and 14 days of the experiment. Furthermore, CPF showed its longest half-life (t1/2) in soil contaminated solely with CPF, lasting 15.36 days. Conversely, its shortest half-life occurred in soil contaminated with CPF and treated with MI along with B. subtilis, lasting 4.65 days. Soil contaminated with CPF and treated with MI and P. aeruginosa showed a half-life of 7.98 days. The half-life (t1/2) of CPF-contaminated soil with MI alone was 11.41 days. A batch equilibrium technique showed that B. subtilis is better than P. aeruginosa for eliminating CPF from soil in In vitro experiments. Notably, CPF-polluted soil treated with coadministration of MI and the tested bacteria improved the activities of SOD and Prx and reduced H2O2 and MDA compared with CPF-polluted soil treated with MI alone. Our findings demonstrated that using B. subtilis and P. aeruginosa as biosurfactants to augment phytoremediation represents a commendable strategy for enhancing the remediation of CPF contamination in affected sites while reducing the existence of harmful pesticide remnants in crop plants.

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

Salicylic Acid and Calcium Chloride Seed Priming: A Prominent Frontier in Inducing Mineral Nutrition Balance and Antioxidant System Capacity to Enhance the Tolerance of Barley Plants to Salinity

The current investigation aims to underline the impact of salicylic acid or calcium chloride seed pre-treatments on mineral status and oxidative stress markers, namely levels of electrolyte leakage (EL) and lipid peroxidation levels, measured as thiobarbituric reactive substances (TBARS), and the activity of some antioxidant enzymes in roots and leaves of plants in two barley species grown under various salt treatments. Overall, our results revealed that salinity inhibits essential nutrient absorption such as iron, calcium, magnesium and potassium and stimulates the absorption of sodium. Also, this environmental constraint induced oxidative stress in plants in comparison with the control conditions. This state of oxidative stress is reflected by an increase in TBARS content as well as the stimulation of EL values. In addition, salinity induced disturbances in the activity of antioxidant enzymes, which were mainly dependent on the applied salt concentration and the species. In addition, Hordeum marinum maintained high antioxidant enzyme activity and low levels of oxidative stress parameters, which reinforces its salt-tolerant character. Importantly, salicylic acid or calcium chloride seed priming alleviated the mineral imbalance and the oxidative damage induced by salinity. Moreover, seed priming improves iron, calcium magnesium and potassium content and limitsthe accumulation of sodium. Also, both treatments not only decrease TBARS levels and limit EL, but they also stimulate the antioxidant enzyme activities in the leaves and roots of the stressed plants as compared with stressed plants grown from non-primed seeds. Interestingly, the beneficial effects of the mentioned treatments were more notable on Hordeum vulgare species.

Nitric oxide and AMF-mediated regulation of soil enzymes activities, cysteine-H2S system and thiol metabolites in mitigating chromium (Cr (VI)) toxicity in pigeonpea genotypes

Cr (VI) hampers plant growth and yield by reducing essential nutrient uptake as it competes for phosphate and sulfate transporters. Nitric oxide (NO) and mycorrhization play important roles in mitigating Cr (VI) toxicity. Present study aimed to compare the potential of AMF (Arbuscular mycorrhizal fungi)-Rhizoglomus intraradices and NO (0.25 mM) in alleviating Cr (VI) stress (0, 10 and 20 mg/kg) in two differentially tolerant pigeonpea genotypes (Pusa 2001 and AL 201). Cr (VI) toxicity reduced growth, mycorrhizal colonization, nutrient uptake, and overall productivity by inducing reactive oxygen species (ROS) generation, with AL 201 more sensitive than Pusa 2001. NO and AM enhanced activities of soil enzymes, thereby increasing nutrients availability as well as their uptake, with AM more effective than NO. Both amendments reduced oxidative stress and restricted Cr (VI) uptake by increasing the activities of antioxidant and S- assimilatory enzymes, with Pusa 2001 more responsive than AL 201. NO was relatively more efficient in regulating cysteine-H2S system by increasing the activities of biosynthetic enzymes (ATP-sulfurylase (ATPS), O-acetylserine thiol lyase (OASTL), D-cysteine desulfhydrase (DCD) and L-cysteine desulfhydrase (LCD), while AM significantly increased glutathione reductase (GR), γ-glutamylcysteine synthetase (γ-ECS) enzymes activities and resultant glutathione (GSH), phytochelatins (PCs), and non-protein thiols (NP-SH) synthesis. Moreover, co-application of NO and AM proved to be highly beneficial in negating the toxic effects of Cr (VI) due to functional complementarity between them. Study suggested the combined use of NO and AM as a useful strategy in re-establishing pigeonpea plants growing in Cr (VI)-stressed environments.

ZnO nanoparticles mediated by Azadirachta indica as nano fertilizer: Improvement in physiological and biochemical indices of Zea mays grown in Cr-contaminated soil

The current investigation aimed at evaluating the impact of Azadirachta indica-mediated zinc oxide nanoparticles (Ai-ZnONPs) on the growth and biochemical characteristics of maize (sweet glutinous 3000) under exposure to 50 mg kg-1Ai-ZnONPs with Cr (VI) concentrations of 50 and 100 mg kg-1. The results indicate that plants exposed to Cr (VI) only experienced a decline in growth parameters. Conversely, the inclusion of Ai-ZnONPs caused a noteworthy increase in physiological traits. Specifically, shoot and root fresh weight increased by 28.02% and 16.51%, and 63.11% and 97.91%, respectively, when compared to Cr-50 and 100 treatments. Additionally, the SPAD chlorophyll of the shoot increased by 91.08% and 15.38% compared to Cr-50 and 100 treatments, respectively. Moreover, the antioxidant enzyme traits of plant shoot and root, such as superoxide dismutase (SOD 7.44% and 2.70%, and 4.45% and 3.53%), catalase (CAT 1.18% and 3.20%, and 5.03% and 5.78%), and peroxidase (POD 0.31% and 5.55%, and 4.72% and 3.61%), exhibited significant increases in Cr 50 and 100 treatments, respectively. The addition of Ai-ZnONPs to the soil also enhanced soil nutrient status and reduced Cr (VI) concentrations by 40.69% and 19.82% compared to Cr-50 and 100 treated soils. These findings suggest that Ai-ZnONPs can trigger the activation of biochemical pathways that enable biomass accumulation in meristematic cells. Further investigations are required to elucidate the mechanisms involved in growth promotion.

Progress in phytoremediation of chromium from the environment

As chromium (Cr) in ecosystems affects human health through food chain exposure, phytoremediation is an environmentally friendly and efficient way to reduce chromium pollution in the environment. Here, we review the mechanism of absorption, translocation, storage, detoxification, and regulation of Cr in plants. The Cr(VI) form is more soluble, mobile, and toxic than Cr(III), reflecting how various valence states of Cr affect environmental risk characteristics, physicochemical properties, toxicity, and plant uptake. Plant root's response to Cr exposure leads to reactive oxygen species (ROS) generation and apoptosis. Cell wall immobilization, vacuole compartmentation, interaction of defense proteins and organic ligand with Cr, and removal of reactive oxygen species by antioxidants continue plant life. In addition, the combined application of microorganisms, genetic engineering, and the addition of organic acids, nanoparticles, fertilization, soil amendments, and other metals could accelerate the phytoremediation process. This review provides efficient methods to investigate and understand the complex changes of Cr metabolism in plants. Preferably, fast-growing, abundantly available biomass species should be modified to mitigate Cr pollution in the environment as these green and efficient remediation technologies are necessary for the protection of soil and water ecology.

A Pseudomonas Plant Growth Promoting Rhizobacterium and Arbuscular Mycorrhiza differentially modulate the growth, photosynthetic performance, nutrients allocation, and stress response mechanisms triggered by a mild Zinc and Cadmium stress in tomato

This study aimed to assess the effectiveness of plant growth-promoting rhizobacteria (PGPR; Pseudomonas strain So_08) and arbuscular mycorrhizal fungi (AMF; Rhizoglomus irregulare BEG72 and Funneliformis mosseae BEG234) in mitigating the detrimental effects of cadmium (Cd) and zinc (Zn) stress in tomato plants. Plant biomass, root morphology, leaf relative water content, membrane stability, photosynthetic performance, chlorophyll content, and heavy metals (HMs) accumulation were determined. Furthermore, an ionomic profile was conducted to investigate whether microbial inoculants affected the uptake and allocation of macro- and micronutrients. Metabolomics with pathway analysis of both roots and leaves was performed to unravel the mechanisms underlying the differential responses to HMs stress. The findings revealed that the levels of HMs did not significantly affect plant growth parameters; however, they affected membrane stability, photosynthetic performance, nutrient allocation, and chlorophyll content. Cadmium was mainly accumulated in roots, whilst Zn exhibited accumulation in various plant organs. Our findings demonstrate the beneficial effects of PGPR and AMF in mitigating Cd and Zn stress in tomato plants. The microbial inoculations improved physiological parameters and induced differential accumulation of macro- and micronutrients, modulating nutrient uptake balance. These results provide insights into the mechanisms underlying the plant-microbe interactions and highlight the differential modulation of the biosynthetic pathways of secondary metabolites related to oxidative stress response, membrane lipids stability, and phytohormone crosstalk.

Heavy metal stress in plants: Ways to alleviate with exogenous substances

Accumulation and enrichment of excessive heavy metals due to industrialization and modernization not only devastate our ecosystem, but also pose a threat to the global vegetation, especially crops. To improve plant resilience against heavy metal stress (HMS), numerous exogenous substances (ESs) have been tried as the alleviating agents. After a careful and thorough review of over 150 recently published literature, 93 reported ESs and their corresponding effects on alleviating HMS, we propose that 7 underlying mechanisms of ESs be categorized in plants for: 1) improving the capacity of the antioxidant system, 2) inducing the synthesis of osmoregulatory substances, 3) enhancing the photochemical system, 4) detouring the accumulation and migration of heavy metals, 5) regulating the secretion of endogenous hormones, 6) modulating gene expressions, and 7) participating in microbe-involved regulations. Recent research advances strongly indicate that ESs have proven to be effective in mitigating a potential negative impact of HMS on crops and other plants, but not enough to ultimately solve the devastating problem associated with excessive heavy metals. Therefore, much more research should be focused and carried out to eliminate HMS for the sustainable agriculture and clean environmental through minimizing towards prohibiting heavy metals from entering our ecosystem, phytodetoxicating polluted landscapes, retrieving heavy metals from detoxicating plants or crop, breeding for more tolerant cultivars for both high yield and tolerance against HMS, and seeking synergetic effect of multiply ESs on HMS alleviation in our feature researches.

Elucidating growth and biochemical characteristics of rice seedlings under stress from chromium VI salt and nanoparticles

Plants are usually provoked by a variety of heavy metal (HM) stressors that have adverse effects on their growth and other biochemical characterizations. Among the HMs, chromium has been considered the most toxic for both plants and animals. The present study was conducted to compare the phytotoxic effects of increasing chromium (VI) salt and nanoparticles (NPs) concentrations on various growth indexes of rice (Oryza sativa L. var. swat 1) seedlings grown in a hydroponic system. The 7-day rice seedlings were exposed to Cr (VI) salt and NPs hydroponic suspensions which were adjusted to the concentration of 0, 50, 100, 150, 200 and 250 mg/L. Both the Cr (VI) salt and NPs with lower concentrations (up to 100mg/L) exerted minimum inhibitory effects on the growth performance of rice seedlings. However, a significant decrease in shoot and root length and their fresh and dry weight was recorded at higher doses of Cr (VI) salt (200 mg/L) and NPs (250 mg/L). The stress induced by Cr (VI) salt has drastically affected the roots, whereas, Cr (VI) NPs significantly affected the shoot tissues. Photosynthetic pigments decreased significantly in a dose-dependent manner, and the reduction was more pronounced in rice seedlings exposed to Cr (VI) NPs compared to Cr (VI) salt. Cr (VI) NPs enhanced the membrane permeability in shoots and roots as compared to that of Cr (VI) salt, which resulted in higher concentration of reactive oxygen species (ROS) and increased lipid peroxidation. The activities of antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) increased significantly in shoot/root tissue following exposure to higher doses of Cr (VI) salt (200 mg/L) and NPs stress (250 mg/L), while minor changes in CAT and APX activities were observed in root and shoot tissues after exposure to higher concentration of Cr (VI) NP. Furthermore, the increasing concentrations of Cr (VI) NPs increased the length of stomatal guard cells. Conclusively, Cr (VI) salt and NPs in higher concentrations have higher potential to damage the growth and induce oxidative stress in rice plants.

Responses of rhizosphere microbial community structure and metabolic function to heavy metal coinhibition

Metal mineral mining results in releases of large amounts of heavy metals into the environment, and it is necessary to better understand the response of rhizosphere microbial communities to simultaneous stress from multiple heavy metals (HMs), which directly impacts plant growth and human health. In this study, by adding different concentrations of cadmium (Cd) to a soil with high background concentrations of vanadium (V) and chromium (Cr), the growth of maize during the jointing stage was explored under limiting conditions. High-throughput sequencing was used to explore the response and survival strategies of rhizosphere soil microbial communities to complex HM stress. The results showed that complex HMs inhibited the growth of maize at the jointing stage, and the diversity and abundance of maize rhizosphere soil microorganisms were significantly different at different metal enrichment levels. In addition, according to the different stress levels, the maize rhizosphere attracted many tolerant colonizing bacteria, and cooccurrence network analysis showed that these bacteria interacted very closely. The effects of residual heavy metals on beneficial microorganisms (such as Xanthomonas, Sphingomonas, and lysozyme) were significantly stronger than those of bioavailable metals and soil physical and chemical properties. PICRUSt analysis revealed that the different forms of V and Cd had significantly greater effects on microbial metabolic pathways than all forms of Cr. Cr mainly affected the two major metabolic pathways: microbial cell growth and division and environmental information transmission. In addition, significant differences in rhizosphere microbial metabolism under different concentrations were found, and this can serve as a reference for subsequent metagenomic analysis. This study is helpful for exploring the threshold for the growth of crops in toxic HM soils in mining areas and achieving further biological remediation.

Allantoin alleviates chromium phytotoxic effects on wheat by regulating osmolyte accumulation, secondary metabolism, ROS homeostasis and nutrient acquisition

Allantoin is a nitrogen metabolite with significant potential to mediate plant defense responses under salinity. However, the impact of allantoin on ions homeostasis and ROS metabolism has yet to be established in plants under Cr toxicity. In the current study, chromium (Cr) notably diminished growth, photosynthetic pigments, and nutrient acquisition in two wheat cultivars (Galaxy-2013 and Anaj-2017). Plants subjected to Cr toxicity displayed excessive Cr accumulation. Chromium produced substantial oxidative stress reflected as higher levels of O₂^•, H₂O₂, MDA, methylglyoxal (MG) and lipoxygenase activity. Plants manifested marginally raised antioxidant enzyme activities due to Cr stress. Further, reduced glutathione (GSH) levels diminished with a concurrent rise in oxidized glutathione levels (GSSG). Plants exhibited a considerable abridge in GSH:GSSG due to Cr toxicity. Allantoin (200 and 300 mg L¹) subsided metal phytotoxic effects by strengthening the activities of antioxidant enzymes and levels of antioxidant compounds. Plants administered allantoin displayed a considerable rise in endogenous H₂S and nitric oxide (NO) levels that, in turn, lessened oxidative injury in Cr-stressed plants. Allantoin diminished membrane damage and improved nutrient acquisition under Cr stress. Allantoin markedly regulated the uptake and distribution of Cr in wheat plants, abridging the degree of metal phytotoxic effect.

Exogenous hydrogen sulfide alleviates chromium toxicity by modulating chromium, nutrients and reactive oxygen species accumulation, and antioxidant defence system in mungbean (Vigna radiata L.) seedlings

Chromium (Cr), a highly toxic redox-active metal cation in soil, seriously threatens global agriculture by affecting nutrient uptake and disturbing various physio-biochemical processes in plants, thereby reducing yields. Here, we examined the effects of different concentrations of Cr alone and in combination with hydrogen sulfide (H₂S) application on the growth and physio-biochemical performance of two mungbeans (Vigna radiata L.) varieties, viz. Pusa Vishal (PV; Cr tolerant) and Pusa Ratna (PR; Cr sensitive), growing in a pot in hydroponics. Plants were grown in the pot experiment to examine their growth, enzymatic and non-enzymatic antioxidant levels, electrolyte balance, and plasma membrane (PM) H⁺-ATPase activity. Furthermore, root anatomy and cell death were analysed 15 days after sowing both varieties in hydroponic systems. The Cr-induced accumulation of reactive oxygen species caused cell death and affected the root anatomy and growth of both varieties. However, the extent of alteration in anatomical features was less in PV than in PR. Exogenous application of H₂S promoted plant growth, thereby improving plant antioxidant activities and reducing cell death by suppressing Cr accumulation and translocation. Seedlings of both cultivars treated with H₂S exhibited enhanced photosynthesis, ion uptake, glutathione, and proline levels and reduced oxidative stress. Interestingly, H₂S restricted the translocation of Cr to aerial parts of plants by improving the nutrient profile and viability of root cells, thereby relieving plants from oxidative bursts by activating the antioxidant machinery through triggering the ascorbate-glutathione cycle. Overall, H₂S application improved the nutrient profile and ionic homeostasis of Cr-stressed mungbean plants. These results highlight the importance of H₂S application in protecting crops against Cr toxicity. Our findings can be utilised to develop management strategies to improve heavy metal tolerance among crops.

Combined application of allantoin and strain JIT1 synergistically or additively promotes the growth of rice under 2, 4-DCP stress by enhancing the phosphate solubility, improving soil enzyme activities and photosynthesis

Environmental pollution by 2, 4 dichlorophenol (2, 4-DCP) has become a widespread concern due to its detrimental influence on human and natural ecosystem. With the increasing accumulation of 2, 4-DCP in soil, it is of great significance to explore some appropriate approaches for enhancing plant tolerance to 2, 4-DCP stress. In the current study, a strain resistant to 2, 4-DCP was obtained from the tall fescue rhizosphere soil and named as Pseudomonas sp. JIT1. The strain JIT1 exhibited several remarkable plant growth-promoting traits, including the production of IAA, fixation of biological nitrogen and solubilization of phosphate. The inoculation of strain JIT1 significantly increased biomass, photosynthesis, antioxidant levels, chlorophyll contents and the osmotic substance contents in rice seedlings exposed to 2, 4-DCP. Meanwhile, inoculation of strain JIT1 also enhanced activities of soil alkaline phosphatase, urease, sucrase and cellulase. Moreover, under 2, 4-DCP stress, the content of allantoin in seedlings significantly increased and the pretreatment of exogenous allantoin noticeably ameliorated the negative effects caused by 2, 4-DCP stress in rice seedlings. Interesting, allantoin treatment also enhanced phosphate solubilization properties of strain JIT1. The chlorophyll contents, photosynthesis and osmotic substance further increased by combination use of strain JIT1 and allantoin, which improved the growth of seedlings, most likely to be attributed to the synergistic or additive effect between allantoin and strain JIT1. The results of this study highlight the important roles of combined use of strain JIT1 and allantoin for improving the tolerance of rice to 2, 4-DCP to stress.

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.

Effect of nanobiochar (nBC) on morpho-physio-biochemical responses of black cumin (Nigella sativa L.) in Cr-spiked soil

Chromium is a highly toxic heavy metal. High concentrations of Cr (III) can affect metabolic processes in plants, resulting in different morphological, physiological, and biochemical defects. Agricultural practices such as sewage irrigation, over-fertilization, and sewage sludge application contribute significantly to Cr contamination. It can reduce the growth of plants by affecting the activity of antioxidant enzymes. The materials in nano form play an important role in nano-remediation and heavy metals absorption due to their high surface area and micropores. This research was conducted to study the potential of foliar application of nanobiochar/nBC (100 mg/L^-1 and 150 mg/L^-1) for mitigation of Cr (III) stress (200 mg/kg and 300 mg/kg) in black cumin (Nigella sativa) plants. The results showed that increased Cr stress (300 mg/kg) decreased the plant growth parameters, chlorophyll content, total soluble sugars, and proteins. However, increased the level of hydrogen peroxide (H₂O₂) and malondialdehyde acetate (MDA) as a result of the activity of antioxidant enzymes (Catalase, Superoxide dismutase, peroxidase dismutase, and ascorbic peroxidase) increased in Nigella sativa seedlings. Foliar application of the nBC (100 mg/L^-1) increased plant growth parameters, chlorophyll content, and osmoprotectants, while decreasing the levels of oxidative stress markers (H₂O₂ and MDA). Furthermore, with the application of nBC, the antioxidant enzyme activity considerably improved. Improved antioxidant activity shows that nBC helped to decrease oxidative stress, which in return improved the growth of Nigella sativa seedlings. Overall, present study findings concluded that foliar application of nBC in Nigella sativa seedlings improved growth, chlorophyll, and antioxidant enzymes. The nBC treatment of 100 mg/L^-1 showed better results compared to 150 mg/L^-1 under chromium stress.

Study of the competition between Pi and Cr (VI) for the use of Pi-transporter at Vicia faba L. using molecular modeling

Recent studies have shown that Cr uses other element transporters such as phosphate transporters to enter cells. The aim of this work is to explore the interaction between dichromate and inorganic phosphate (Pi) in the plant of Vicia faba L. To study this interaction, we used three concentrations of Dipotassium hydrogen phosphate (K₂HPO₄) 10 mM (Pi10), 50 mM (Pi50) and 100 mM (Pi100) added alone or in combination with potassium dichromate (K₂Cr₂O₇) Cr + Pi10, Cr + Pi50 and Cr + Pi100. In order to investigate the impact of this interaction on morpho-physiological parameters, the biomass, chlorophyll content, proline level, H₂O₂ level, Catalase and Ascorbate peroxidase activity and Cr-bioaccumulation has been determined. For the molecular scale, the theoretical chemistry was used via molecular docking to explore the various interactions between dichromate Cr₂O₇^2-/HPO₄^2-/H₂O₄P^- and the phosphate-transporter. We have selected the eukaryotic phosphate transporter (PDB: 7SP5) as the module. The results showed that K₂Cr₂O₇ negatively affects morpho-physiological parameters and generates oxidative damage (+84% H₂O₂ than the control), which involved the production of antioxidant enzymes (+147% Catalase and +176% Ascorbate-peroxidase) and Proline (+108%). The addition of Pi improved the growth of Vicia faba L. and induces the partial restoration of the parameters affected by Cr (VI) to the normal levels. Also, it decreased oxidative damage and reduce Cr (VI) bioaccumulation in shoots and roots. Molecular docking has shown that the dichromate structure is more compatible and establishes more bonds with the Pi-transporter which generates a very stable complex compared to HPO₄^2-/H₂O₄P^-. Overall, these results confirmed that there is a strong relationship between dichromate uptake and the Pi-transporter.

Physiological and transcriptomic evidence of antioxidative system and ion transport in chromium detoxification in germinating seedlings of soybean

Chromium (Cr) toxicity impairs the productivity of crops and is a major threat to food security worldwide. However, the effect of Cr toxicity on seed germination and transcriptome of germinating seedlings of soybean crop has not been fully explored. In this study, two Cr-tolerant lines (J82, S125) and two Cr-sensitive ones (LD1, RL) were screened out of twenty-one soybean (Glycine max L.) genotypes based on seed germination rate, seed germinative energy, seed germination index, and growth of germinating seedlings under 50 mg L^-1 Cr treatment. We found that Cr stress inhibits the growth of soybean seed germinating seedlings due to the Cr-induced overaccumulation of reactive oxygen species (ROS). Significantly different levels of element contents, antioxidant enzyme activities, malondialdehyde content were observed in the four soybean genotypes with contrasting Cr tolerance. Further, a total of 13,777 differentially expressed genes (DEGs) were identified in transcriptomic sequencing and 1298 DEGs in six gene modules were found highly correlated with the physiological traits by weighted correlation network analysis (WGCNA) analysis. The DEGs encoding antioxidant enzymes, transcription factors, and ion transporters are proposed to confer Cr tolerance in soybean germinating seedlings by reducing the uptake and translocation of Cr, decreasing the level of ROS, and keeping the osmotic balance in soybean germinating seedings. In conclusion, our study provided a molecular regulation network on soybean Cr tolerance at seed germinating stage and identified candidate genes for molecular breeding of low Cr accumulation soybean cultivars.

Burkholderia cepacia CS8 improves phytoremediation potential of Calendula officinalis for tannery solid waste polluted soil

Microbes have shown potential for the bioremediation of tannery waste polluted soil. During our previous study, it was observed that heavy metal resistant Burkholderia cepacia CS8 augmented growth and phytoremediation capability of an ornamental plant. Objective of the present research work was to evaluate the capability of B. cepacia CS8 assisted Calendula officinalis plants for the phytoremediation of tannery solid waste (TSW) polluted soil. The TSW treatment significantly reduced growth attributes and photosynthetic pigments in C. officinalis. However, supplementation of B. cepacia CS8 which exhibited substantial tolerance to the TSW amended soil, augmented growth traits, carotenoid, proline, and antioxidant enzymes level in C. officinalis under toxic and nontoxic regimes. Inoculation of B. cepacia CS8 augmented plant growth (shoot length 13%, root length 11%), physiological attributes (chlorophyll a 14%, chlorophyll b 17%), antioxidant enzyme activities (peroxidase 24%, superoxide dismutase 31% and catalase 19%), improved proline 36%, phenol 32%, flavonoids 14% and declined malondialdehyde (MDA) content 15% and hydrogen peroxide (H2O2) level 12% in C. officinalis at TSW10 stress compared with relevant un-inoculated plants of TSW10 treatment. Moreover, B. cepacia CS8 application enhanced labile metals in soil and subsequent metal uptake, such as Cr 19%, Cd 22%, Ni 35%, Fe 18%, Cu 21%, Pb 34%, and Zn 30%, respectively in C. officinalis plants subjected to TSW10 stress than that of analogous un-inoculated treatment. Higher plant stress tolerance and improved phytoremediation potential through microbial inoculation will assist in the retrieval of agricultural land in addition to the renewal of native vegetation.

Effects of Cr Stress on Bacterial Community Structure and Composition in Rhizosphere Soil of Iris tectorum under Different Cultivation Modes

With the rapid development of industry, Cr has become one of the major heavy metal pollutants in soil, severely impacting soil microecology, among which rhizosphere microorganisms can improve the soil microenvironment to promote plant growth. However, how rhizosphere bacterial communities respond to Cr stress under different cultivation modes remains to be further studied. Therefore, in this study, a greenhouse pot experiment combined with 16S rRNA high-throughput sequencing technology was used to study the effects of Cr stress at 200 mg kg−1 on the bacterial community structure and diversity in the rhizosphere soil of Iris tectorum under different cultivation modes. The results showed that the rhizosphere bacterial community diversity index (Shannon and Simpson) and abundance index (Ace and Chao) increased significantly with wetland plant diversity under Cr stress. Moreover, the bacterial community changed by 20.1% due to the addition of Cr, further leading to a 15.9% decrease in the common species of the bacterial community, among which Proteobacteria, Actinobacteria, Chloroflexi, and Acidobacteriota accounted for more than 74.8% of the total sequence. However, with the increase in plant diversity, the abundance of rhizosphere-dominant bacteria and plant growth-promoting bacteria communities increased significantly. Meanwhile, the symbiotic network analysis found that under the two cultivation modes, the synergistic effect between the dominant bacteria was significantly enhanced, and the soil microenvironment was improved. In addition, through redundancy analysis, it was found that C, N, and P nutrients in uncontaminated soil were the main driving factors of bacterial community succession in the rhizosphere of I. tectorum, and Cr content in contaminated soil was the main driving factor of bacterial community succession in I. tectorum rhizosphere. In summary, the results of this study will provide a basis for the response of the rhizosphere bacterial community to Cr and the interaction between wetland plants and rhizosphere bacteria in the heavy metal restoration of wetland plants under different cultivation modes.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Moringa oleifera smoke induced positive changes in biochemical, metabolic, and antioxidant profile of rice seedling under cadmium stress

Cadmium as a heavy metal contaminates the agricultural soil and effect plant growth due to rapid increases in industrialization and anthropogenic activities. Smoke water of Moringa oleifera was used in the current study to alleviate the effect of cadmium on the physiological, biochemical, metabolic, and antioxidant profile of Basmati 385 and Shaheen Basmati seedling. Cadmium stress of 100, 200, and 400 µM were given to 28 days-old seedlings along with smoke water (1:1,000) for one week in hydroponic culture. As a result, Cd⁺² toxicity negatively affects the seedling length, fresh and dry weight, photosynthetic pigment, and electrolytes leakage, while the application of smoke water alleviated those effects. Furthermore, Cd⁺² content, cell injury, metabolic parameters (proline, total soluble sugar), and antioxidants (peroxidase, catalase) were increased with increasing Cd⁺² concentration while smoke water-treated seedlings showed reduction at high concentration. From present study, it can be concluded that smoke water had some regulatory compound which could reduce the Cd⁺² stress level in rice seedlings and improve plant growth.

Enhancing Mechanisms of the Plant Growth-Promoting Bacterial Strain Brevibacillus sp. SR-9 on Cadmium Enrichment in Sweet Sorghum by Metagenomic and Transcriptomic Analysis

To explore the mechanism by which the plant growth-promoting bacterium Brevibacillus sp. SR-9 improves sweet sorghum tolerance and enriches soil cadmium (Cd) under pot conditions, the effect of strain SR-9 inoculation on the microbial community of sorghum rhizosphere soil was analyzed by metagenomics. Gene expression in sweet sorghum roots was analyzed using transcriptomics. The results showed that strain SR-9 promoted the growth of sweet sorghum and improved the absorption and enrichment of Cd in the plants. Compared with the uninoculated treatment, the aboveground part and root dry weight in strain SR-9 inoculated with sorghum increased by 21.09% and 17.37%, respectively, and the accumulation of Cd increased by 135% and 53.41%, respectively. High-throughput sequencing showed that strain SR-9 inoculation altered the rhizosphere bacterial community, significantly increasing the relative abundance of Actinobacteria and Firmicutes. Metagenomic analysis showed that after inoculation with strain SR-9, the abundance of genes involved in amino acid transport metabolism, energy generation and conversion, and carbohydrate transport metabolism increased. KEGG functional classification showed that inoculation with strain SR-9 increased the abundance of genes involved in soil microbial metabolic pathways in the rhizosphere soil of sweet sorghum and the activity of soil bacteria. Transcriptome analysis identified 198 upregulated differentially expressed genes in sweet sorghum inoculated with strain SR-9, including those involved in genetic information processing, biological system, metabolism, environmental information processing, cellular process, and human disease. Most of the annotated differentially expressed genes were enriched in the metabolic category and were related to pathways such as signal transduction, carbohydrate metabolism, amino acid metabolism, and biosynthesis of other secondary metabolites. This study showed that plant growth-promoting bacteria can alter the rhizosphere bacterial community composition, increasing the activity of soil bacteria and upregulating gene expression in sweet sorghum roots. The findings enhance our understanding of the microbiological and botanical mechanisms by which plant growth-promoting bacterial inoculation improves the remediation of heavy metals by sorghum.

Chromium in plant growth and development: Toxicity, tolerance and hormesis

Research over the last three decades showed that chromium, particularly the oxyanion chromate Cr(VI) behaves as a toxic environmental pollutant that strongly damages plants due to oxidative stress, disruption of nutrient uptake, photosynthesis and metabolism, and ultimately, represses growth and development. However, mild Cr(VI) concentrations promote growth, induce adventitious root formation, reinforce the root cap, and produce twin roots from single root meristems under conditions that compromise cell viability, indicating its important role as a driver for root organogenesis. In recent years, considerable advance has been made towards deciphering the molecular mechanisms for root sensing of chromate, including the identification of regulatory proteins such as SOLITARY ROOT and MEDIATOR 18 that orchestrate the multilevel dynamics of the oxyanion. Cr(VI) decreases the expression of several glutamate receptors, whereas amino acids such as glutamate, cysteine and proline confer protection to plants from hexavalent chromium stress. The crosstalk between plant hormones, including auxin, ethylene, and jasmonic acid enables tissues to balance growth and defense under Cr(VI)-induced oxidative damage, which may be useful to better adapt crops to biotic and abiotic challenges. The highly contrasting responses of plants manifested at the transcriptional and translational levels depend on the concentration of chromate in the media, and fit well with the concept of hormesis, an adaptive mechanism that primes plants for resistance to environmental challenges, toxins or pollutants. Here, we review the contrasting facets of Cr(VI) in plants including the cellular, hormonal and molecular aspects that mechanistically separate its toxic effects from biostimulant outputs.

Brassinosteroid Supplementation Alleviates Chromium Toxicity in Soybean (Glycine max L.) via Reducing Its Translocation

Chromium (Cr) phytotoxicity severely inhibits plant growth and development which makes it a prerequisite to developing techniques that prevent Cr accumulation in food chains. However, little is explored related to the protective role of brassinosteroids (BRs) against Cr-induced stress in soybean plants. Herein, the morpho-physiological, biochemical, and molecular responses of soybean cultivars with/without foliar application of BRs under Cr toxicity were intensely investigated. Our outcomes deliberated that BRs application noticeably reduced Cr-induced phytotoxicity by lowering Cr uptake (37.7/43.63%), accumulation (63.92/81.73%), and translocation (26.23/38.14%) in XD-18/HD-19, plant tissues, respectively; besides, improved seed germination ratio, photosynthetic attributes, plant growth, and biomass, as well as prevented nutrient uptake inhibition under Cr stress, especially in HD-19 cultivar. Furthermore, BRs stimulated antioxidative defense systems, both enzymatic and non-enzymatic, the compartmentalization of ion chelation, diminished extra production of reactive oxygen species (ROS), and electrolyte leakage in response to Cr-induced toxicity, specifically in HD-19. In addition, BRs improved Cr stress tolerance in soybean seedlings by regulating the expression of stress-related genes involved in Cr accumulation, and translocation. Inclusively, by considering the above-mentioned biomarkers, foliar spray of BRs might be considered an effective inhibitor of Cr-induced damages in soybean cultivars, even in Cr polluted soil.

Nitric oxide, salicylic acid and oxidative stress: Is it a perfect equilateral triangle?

Nitric oxide (NO) is an endogenous free radical involved in the regulation of a wide array of physio-biochemical phenomena in plants. The biological activity of NO directly depend on its cellular concentration which usually changes under stress conditions, it participates in maintaining cellular redox equilibrium and regulating target checkpoints which control switches among development and stress. It is one of the key players in plant signalling and a plethora of evidence supports its crosstalk with other phytohormones. NO and salicylic acid (SA) cooperation is also of great physiological relevance, where NO modulates the immune response by regulating SA linked target proteins i.e., non-expressor of pathogenesis-related genes (NPR-1 and NPR-2) and Group D bZIP (basic leucine zipper domain transcription factor). Many experimental data suggest a functional cooperative role between NO and SA in mitigating the plant oxidative stress which suggests that these relationships could constitute a metabolic "equilateral triangle".

Salicylic Acid, a Multifaceted Hormone, Combats Abiotic Stresses in Plants

In recent decades, many new and exciting findings have paved the way to the better understanding of plant responses in various environmental changes. Some major areas are focused on role of phytohormone during abiotic stresses. Salicylic acid (SA) is one such plant hormone that has been implicated in processes not limited to plant growth, development, and responses to environmental stress. This review summarizes the various roles and functions of SA in mitigating abiotic stresses to plants, including heating, chilling, salinity, metal toxicity, drought, ultraviolet radiation, etc. Consistent with its critical roles in plant abiotic tolerance, this review identifies the gaps in the literature with regard to the complex signalling network between SA and reactive oxygen species, ABA, Ca²⁺, and nitric oxide. Furthermore, the molecular mechanisms underlying signalling networks that control development and stress responses in plants and underscore prospects for future research on SA concerning abiotic-stressed plants are also discussed.

Effects of Chromium Toxicity on Physiological Performance and Nutrient Uptake in Two Grapevine Cultivars (Vitis vinifera L.) Growing on Own Roots or Grafted onto Different Rootstocks

Chromium toxicity is considered within the most severe and dangerous nutritional disorders, and it can often be observed in crops grown in industrial areas. The present study aims to determine the effects of Cr(VI) toxicity on the growth, nutrition, and physiological performance of grapevines. In a pot hydroponic experiment, own-rooted Merlot and Cabernet Franc grapevine cultivars or cultivars grafted onto 1103P and 101-14 Mgt rootstocks were exposed to 120 μM Cr(VI). Leaf interveinal chlorosis appeared after forty-five days of treatment. Overall leaf chlorosis and brown root coloration after sixty days was reported. A significant effect on the majority of the measured parameters due to the Cr(VI) treatment was observed. Chromium stress increased the total Cr concentrations in all parts of the vines, i.e., leaves, shoots, roots, and trunks. When comparing between the studied plant sections, the roots presented the highest Cr concentrations, ranging from 396 to 868 mg kg−1 d. w., and then, in descending order, the Cr concentrations ranged from 41 to 102 mg kg−1 d. w. in the trunks, from 2.0 to 3.3 mg kg−1 d. w. in the leaves, and from 1.9 to 3.0 mg kg−1 d. w. in the shoots. Between the assessed rootstocks, 1103P was identified to be a better excluder of Cr concentration in the roots and other aerial parts of the vines. Additionally, chromium toxicity negatively affected the concentrations and compartmentalization of the most important nutrients. Leaf chlorophyll (Chl) concentration decreased down to approximately 53% after sixty days of Cr stress. Chromium toxicity significantly reduced the stem water potential (SWP), net CO2 assimilation rate (A), stomatal conductance (gs), and PSII maximum quantum yield in all the cases of grafted or own-rooted vines. At this stage, chromium stress increased the leaf total phenolic content from 46.14% in Merlot vines to 75.91% in Cabernet Franc vines.

Exploring plant growth-promoting rhizobacteria as stress alleviators: a methodological insight

Rhizospheric and root-endophyte bacteria can stimulate plant growth through diverse biochemical mechanisms and pathways, particularly under biotic and abiotic stresses. For this reason, biotechnological trends on plant growth-promoting rhizobacteria (PGPR) application as biofertilizers, bioremediators, and stress alleviators are gaining increasing interest as ecofriendly strategies for sustainable agriculture management and soil restoration. The first steps needed to implement these technologies are isolation, screening, and characterization of PGPR that can be potentially applied as bioinoculants to alleviate biotic and/or abiotic stresses. Therefore, a complete and accurate methodological study and laboratory techniques are required to warrant the correct achievement of these steps. This review compiles and details the fundamentals, methods, and procedures of key protocols used in isolation and characterization of PGPR for plant stress alleviation.

Identification and Characterization of Arthrobacter nicotinovorans JI39, a Novel Plant Growth-Promoting Rhizobacteria Strain From Panax ginseng

A bacterial strain JI39 that had plant growth-promoting traits was isolated from the rhizosphere soil of Panax ginseng. It had the ability to produce high indole-3-acetic acid (13.1 μg/ml), phosphate solubilization (164.2 μg/ml), potassium solubilization (16.1 μg/ml), and nitrogen fixation. The strain JI39 was identified to be Arthrobacter nicotinovorans based on morphological, physiological, and biochemical traits and through 16S rDNA sequence analysis. The optimal culture environment for strain growth was 1.0% NaCl, 30°C, pH 6.0, and without UV irradiation. The strain can produce cellulase and protease. The strain JI39 can significantly promote the growth of ginseng. After ginseng seeds were treated with 3 × 108 CFU/ml of JI39 bacterial suspension, the shoot's length was significantly increased by 64.61% after 15 days. Meanwhile, the fresh weight of 2-year-old ginseng roots was significantly increased by 24.70% with a treatment by the 108 CFU/ml bacterial suspension after 150 days in the field. The gene expression of phenylalanine ammonia-lyase (PAL), β-1.3 glucanase (β-1,3-GA), chitinase (CHI), superoxide dismutase (SOD), and peroxidase (POD) of ginseng was upregulated, and it also can improve the soil urease, phosphatase, invertase, and catalase activity. In conclusion, the bacterial strain JI39 could efficiently promote the growth of ginseng and has the potential to be a good microbial fertilizer for ginseng.

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.

Preventative effect of crop straw-derived biochar on plant growth in an arsenic polluted acidic ultisol

In different regions of the world, arsenic (As) contaminated soils poses a serious threat to plant growth and its physiological processes. Organic amendments are a cost-effective and environmentally friendly way to improve plant growth under stress conditions in contaminated soils. In As polluted acidic ultisol, a greenhouse trial was conducted to investigate the protective effects of peanut straw biochar (PSB) and canola straw biochar (CSB) on soybean mineral nutrition, antioxidant enzymes, and physiological growth parameters. The current study used eighteen treatments with different levels of As ((1) 0 mg kg^-1, (2) 30 mg kg^-1, (3) 60 mg kg^-1) and biochar (PSB and CSB) (0%, 1%, and 2%). The result suggests that biochar addition under As stress in highly weathered acidic ultisol soil increased soybean growth attributes and defense mechanisms. The PSB was more effective than the CSB in a dose-dependent manner. The application of 2% PSB in polluted soil resulted in significant increases in soybean height (58%), biomass production (root (44%) and shoot length (52%)), chlorophyll contents (92%), soybean functional leaves (62%), total soluble sugars (TSS) (71%) and base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺). However, biochar application decreased proline, MDA, H₂O₂, and O₂^- by 64%, 82%, 49%, and 45% respectively. Furthermore, biochar application increased (Phosphate) P and As uptake in soybean, with PSB application exhibiting a greater increase than CSB application. As a result, crop straw-derived biochar can reduce As-induced soybean plant damage and insert a protective effect in As-contaminated acidic ultisol soils.

Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands

Land that has little to no utility for agriculture or industry is considered marginal land. This kind of terrain is frequently found on the edge of deserts or other arid regions. The amount of land that can be used for agriculture continues to be constrained by increasing desertification, which is being caused by climate change and the deterioration of agriculturally marginal areas. Plants and associated microorganisms are used to remediate and enhance the soil quality of marginal land. They represent a low-cost and usually long-term solution for restoring soil fertility. Among various phytoremediation processes (viz., phytodegradation, phytoextraction, phytostabilization, phytovolatilization, phytofiltration, phytostimulation, and phytodesalination), the employment of a specific mechanism is determined by the state of the soil, the presence and concentration of contaminants, and the plant species involved. This review focuses on the key economically important plants used for phytoremediation, as well as the challenges to plant growth and phytoremediation capability with emphasis on the advantages and limits of plant growth in marginal land soil. Plant growth-promoting bacteria (PGPB) boost plant development and promote soil bioremediation by secreting a variety of metabolites and hormones, through nitrogen fixation, and by increasing other nutrients' bioavailability through mineral solubilization. This review also emphasizes the role of PGPB under different abiotic stresses, including heavy-metal-contaminated land, high salinity environments, and organic contaminants. In our opinion, the improved soil fertility of marginal lands using PGPB with economically significant plants (e.g., Miscanthus) in dual precession technology will result in the reclamation of general agriculture as well as the restoration of native vegetation.

Plant growth promoting rhizobacteria improve growth and yield related attributes of chili under low nitrogen availability

Nitrogen (N) is a macronutrient desired by crop plants in large quantities. However, hiking fertilizer prices need alternative N sources for reducing its requirements through appropriate management practices. Plant growth promoting rhizobacteria (PGPR) are well-known for their role in lowering N requirements of crop plants. This study assessed the impact of PGPR inoculation on growth, allometry and biochemical traits of chili under different N doses. Two PGPR, i.e., Azospirillum 'Er-20' (nitrogen fixing) and Agrobacterium 'Ca-18' (phosphorous solubilizing) were used for inoculation, while control treatment had no PGPR inoculation. Six N doses, i.e., 100, 80, 75, 70, 60 and 50% of the N required by chili were included in the study. Data relating to growth traits, biochemical attributes and yield related traits were recorded. Interaction among N doses and PGPR inoculation significantly altered all growth traits, biochemical attributes and yield related traits. The highest values of the recorded traits were observed for 100% N with and without PGPR inoculation and 75% N with PGPR inoculation. The lowest values of the recorded traits were noted for 50% N without PGPR inoculation. The PGPR inoculation improved the measured traits compared to the traits recorded noted in same N dose without PGPR inoculation. Results revealed that PGPR had the potential to lower 25% N requirement for chili. Therefore, it is recommended that PGPR must be used in chili cultivation to lower N requirements.

Salicylic acid alleviates chromium (VI) toxicity by restricting its uptake, improving photosynthesis and augmenting antioxidant defense in Solanum lycopersicum L

Contamination of agricultural soil by chromium (Cr) is a serious menace to environmental safety and global food security. Although potential of salicylic acid (SA) in mitigating heavy metal (HM) toxicity in plants is well recognized, detailed physiological mechanisms behind such beneficial effects under Cr-stress in tomato (Solanum lycopersicum L.) plant are far from being completely unravelled. The present study evaluated the efficacy of exogenously applied SA, in alleviating Cr-mediated alterations on photosynthesis and antioxidant defense in tomato exposed to three different concentrations of Cr(VI) [0, 50, and 100 mg Cr(VI) kg^-1 soil]. Exposure of tomato plants to Cr resulted in increased Cr-accumulation and oxidative damage, as signposted by high Cr concentration in root as well as shoot, augmented malondialdehyde (MDA) and superoxides levels, and inhibition in enzymes of ascorbate-glutathione (AsA-GSH) cycle. Furthermore, a significant (P ≤ 0.05) reduction in photosynthetic pigments and gas exchange parameters was also evident in Cr-stressed tomato plants. Findings of the present study showed that exogenous application of 0.5 mM SA not only promoted plant growth subjected to Cr, but also restored Cr-mediated disturbances in plant physiology. A significant (P ≤ 0.05) decrease in Cr acquisition and translocation as evidenced by improved growth and photosynthesis in SA-treated plants was observed. Additionally, exogenous SA application by virtue of its positive effect on efficient antioxidant system ameliorated the Cr-mediated oxidative stress in tomato plants as signposted by lower MDA and superoxide levels and improved AsA-GSH cycle. Overall, current study advocates the potential of exogenous SA application in amelioration of Cr-mediated physiological disturbances in tomato plant.

Improved chromium tolerance of Medicago sativa by plant growth-promoting rhizobacteria (PGPR)

Background

Soil pollution by heavy metals increases the bioavailability of metals like hexavalent chromium (Cr (VI)), subsequently limiting plant growth and reducing the efficiency of phytoremediation. Plant growth-promoting rhizobacteria (PGPR) have substantial potential to enhance plant growth as well as plant tolerance to metal stress. The aim of this research was to investigate Cr (VI) phytoremediation enhancement by PGPR.

Results

The results showed that the 27 rhizobacterial isolates studied were confirmed as Cr (VI)-resistant PGPR, by using classical biochemical tests (phosphate solubilization, nitrogen fixation, indole acetic acid, exopolysaccharides, hydrogen cyanide, siderophores, ammonia, cellulase, pectinase, and chitinase production) and showed variable levels of Cr (VI) resistance (300–600 mg/L). The best four selected Cr (VI)-resistant PGPR (NT15, NT19, NT20, and NT27) retained most of the PGP traits in the presence of 100–200 mg/L concentrations of Cr (VI). The inoculation of Medicago sativa with any of these four isolates improved the shoot and root dry weight. The NT27 isolate identified using 16S rDNA gene sequence analyses as a strain of Pseudomonas sp. was most effective in terms of plant growth promotion and stress level decrease. It increased shoot and root dry weights of M. sativa by 97.6 and 95.4%, respectively, in the presence of Cr (VI) when compared to non-inoculated control plants. It also greatly increased chlorophyll content and decreased the levels of stress markers, malondialdehyde, hydrogen peroxide, and proline. The results of the effect of Pseudomonas sp. on Cr content and bioaccumulation factor (BAF) of the shoots and roots of M. sativa plants showed the increase of plant biomass concomitantly with the increase of Cr root concentration in inoculated plants. This would lead to a higher potential of Cr (VI) phytostabilization.

Conclusions

This study demonstrates that the association M. sativa-Pseudomonas sp. may be an efficient biological system for the bioremediation of Cr (VI)-contaminated soils.

Salicylic acid pre-treatment modulates Pb2+-induced DNA damage vis-à-vis oxidative stress in Allium cepa roots

The current study investigated the putative role of salicylic acid (SA) in modulating Pb²⁺-induced DNA and oxidative damage in Allium cepa roots. Pb²⁺ exposure enhanced free radical generation and reduced DNA integrity and antioxidant machinery after 24 h; however, SA pre-treatment (for 24 h) ameliorated Pb²⁺ toxicity. Pb²⁺ exposure led to an increase in malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) accumulation and enhanced superoxide radical and hydroxyl radical levels. SA improved the efficiency of enzymatic antioxidants (ascorbate and guaiacol peroxidases [APX, GPX], superoxide dismutases [SOD], and catalases [CAT]) at 50-μM Pb²⁺ concentration. However, SA pre-treatment could not improve the efficiency of CAT and APX at 500 μM of Pb²⁺ treatment. Elevated levels of ascorbate and glutathione were observed in A. cepa roots pre-treated with SA and exposed to 50 μM Pb²⁺ treatment, except for oxidized glutathione. Nuclear membrane integrity test demonstrated the ameliorating effect of SA by reducing the number of dark blue-stained nuclei as compared to Pb²⁺ alone treatments. SA was successful in reducing DNA damage in cell exposed to higher concentration of Pb²⁺ (500 μM) as observed through comet assay. The study concludes that SA played a major role in enhancing defense mechanism and protecting against DNA damage by acclimatizing the plant to Pb²⁺-induced toxicity.

Potential use of Solanum lycopersicum and plant growth promoting rhizobacterial (PGPR) strains for the phytoremediation of endosulfan stressed soil

This study aimed to assess the role of Solanum lycopersicum and plant growth promoting rhizobacterial (PGPR) strains to remove endosulfan present in the soil. S. lycopersicum was grown in endosulfan amended soil (5, 10, 25, and 50 mg kg^-1) inoculated with PGPR strains for 40, 80, and 120 days. The influence of PGPR inoculation on endosulfan accumulation in plant tissues, endosulfan degradation in soil, and plant growth parameters were evaluated. The oxidative stress tolerance was assessed by determining the malondialdehyde formation in S. lycopersicum planted in endosulfan stressed soil inoculated with PGPR strains. The results showed that uptake of endosulfan followed root > shoot pathway in association with a reduction in endosulfan accumulation in inoculated plants as related to un-inoculated plants. Moreover, inoculation of PGPR strains showed a beneficial influence on the degradation of endosulfan, Bacillus sp. PRB101 showed maximum degradation (89% at 5 mg kg^-1 of soil) of endosulfan at 120 days after sowing. Furthermore, the content of malondialdehyde was lower in inoculated plants as related to un-inoculated plants. Inoculation of PGPR strains efficiently enhanced plant biomass. The findings showed the effectiveness of PGPR strains to increase the decontamination of endosulfan stressed soil and decline endosulfan concentration in the plant tissues.

Effects of antibiotics stress on growth variables, ultrastructure, and metabolite pattern of Brassica rapa ssp. chinensis

Antibiotics frequently contaminate agricultural fields and through plant uptake enter into the food chain. This study aimed to explore the effects of antibiotics; tetracycline (TC), oxytetracycline (OTC) and norfloxacin (NF) on the growth, cell ultrastructure, and metabolite pattern of Brassica rapa ssp. chinensis. Oxytetracycline accumulated more than other antibiotics followed by TC and NF. Plant growth, chlorophyll fluorescence, and antioxidant activities were negatively affected under all antibiotic treatments. Ultrastructural investigation of mesophyll of leaves performed by transmission electron microscopy indicated that antibiotic stress caused the changes in thylakoid orientation, number of plastoglobuli, and starch grains. Identification of functional groups through Fourier transform infrared analysis indicated that carboxyl group, carbonate and ammonium ions are involved in the adsorption of antibiotics. The metabolic profiling of B. rapa leaves demonstrated that all of the antibiotics treatments distorted phenylalanine, tyrosine and tryptophan biosynthesis, d-glutamine and d-glutamate metabolism, alanine, aspartate and glutamate metabolism, phenylalanine metabolism and TCA cycle. Metabolic alterations as a result of antibiotics stress provide insights of metabolites that affect the physiological changes attributed to antibiotic stress. These results will improve the understanding of antibiotic contamination effects on plants.

Foliar application of seed water extract of Nigella sativa improved maize growth in cadmium-contaminated soil

Cadmium (Cd) is a widespread heavy metal, which commonly exert negative impacts on agricultural soils and living organisms. Foliar application of seed water extract of black cumin (Nigella sativa L.) can mitigate the adverse impacts of Cd-toxicity in plants through its rich antioxidants. This study examined the role of seed water extracts of N. sativa (NSE) in mitigating the adverse impacts of Cd-toxicity on maize growth. Two maize genotypes (synthetic ‘Neelum’ and hybrid ‘P1543’) were grown under 0, 4, 8 and 12 mg Cd kg^-1 soil. The NSE was applied at three different concentrations (i.e., 0, 10 and 20%) as foliar spray at 25 and 45 days after sowing. All Cd concentrations had no effect on germination percentage of both genotypes. Increasing Cd concentration linearly decreased root and allometric attributes, gas exchange traits and relative water contents of hybrid genotype. However, gas exchange traits of synthetic genotype remained unaffected by Cd-toxicity. Overall, hybrid genotype showed better tolerance to Cd-toxicity than synthetic genotype with better germination and allometric attributes and less Cd accumulation. Foliar application of NSE lowered negative effects of Cd-toxicity on all studied traits, except relative water contents. In conclusion, foliar application of NSE seemed a viable option to improve maize growth in Cd-contaminated soil.

Menadione sodium bisulfite alleviated chromium effects on wheat by regulating oxidative defense, chromium speciation, and ion homeostasis

Menadione sodium bisulfite (MSB) is a crucial growth regulator mediating plant defense response. MSB-mediated regulation of defense mechanisms in wheat under chromium (Cr) toxicity has not been reported in the literature. Therefore, the present study was undertaken to appraise the efficacy of exogenous MSB on circumventing Cr phytotoxic effects on wheat. We also compared the effects of water-soluble MSB with that of water-insoluble menadiol diacetate (MD). The levels used in the present investigation for MSB and MD were 100 and 200 mg L-1. Wheat plants grown in soil contaminated with 25 mg kg-1 Cr in the form of K2Cr2O7 showed a notable reduction in growth, chlorophyll molecules, relative water contents, grain yield, total soluble sugars, phenolics, flavonoids, ascorbic acid, activities of antioxidant enzymes (SOD, POD, CAT), and uptake of essential nutrients (K, P, and Ca). Cr toxicity caused a noticeable accretion in total free amino acids, proline, malondialdehyde, H2O2, O2•-, relative membrane permeability, methylglyoxal contents, activities of enzymes (lipoxygenase, glutathione-S-transferase, and ascorbate peroxidase), nitric oxide and H2S contents, glutathione and oxidized glutathione contents, total Cr contents, and Cr6+ and Cr3+ accumulation. MSB application significantly reduced lipid peroxidation, ROS overproduction, methylglyoxal levels, total Cr contents, and maintained higher Cr3+:Cr6+ ratio in aerial parts. Besides, Cr-mediated inhibition in essential nutrient uptake was significantly circumvented by exogenous MSB. Consequently, MSB enhanced wheat growth by lessening oxidative damage, total Cr contents in aerial parts, and strengthening antioxidant enzyme activities. MD was not effective in mediating defense responses in wheat under Cr toxicity.

Effects of chromium stress on the rhizosphere microbial community composition of Cyperus alternifolius

Wetland plants are often used as the main body of soil, and the rhizosphere is a hot spot migration and transformation. Response mechanism to rhizosphere microorganisms on chromium(Cr) stressing could help improve the phytoremediation system. Cyperus alternifolius(CA) is selected as the research object by Cr-stress treatments and uncontaminated treatments with different cultivated pattern, included sole cultivated pattern(CAI), two-cultivated pattern (CAII), three-cultivated pattern (CAIII), and the un-planted blank samples (CK). 16s rRNA gene sequencing and metagenomic sequencing are performed to measure rhizosphere microbial community. And Five common enzymes in rhizosphere soils were observed: β-1,4-glucosidase (BG), β-N-acetylglucosaminidase (NAG), β-1,4-xylosidase (BX), cellobiohydrolase (CBH) and Leucine amino peptidase (LAP) in the rhizosphere. The results show that Gammaproteobacteria, Actinobacteria, Alphaproteobacteria, Gemmatimonadetes, Deltaproteobacteria are top five (63.97%) of the total sequence number. Wetland plants enriched a large amount of soil Cr in themselves, and the rhizosphere microorganisms don't show significant difference in community structure after affecting. 10.48% variation of microbial community is caused by Cr-stress. Acidovorax showed a great potential for chromium resistance. BX involvement in tolerance processes indirectly affects microbial communities (P < 0.01), there is a strong linear relationship between enzyme activity and the plants accumulating Cr and microbial community within 15.58% variant. The material accumulation and microbial quantity of CAIII are relatively low, but high biodiversity remains after affecting. These results provide references for in-depth understanding of rhizosphere microbial response to heavy metal pollution in wetland phytoremediation and interaction between wetland plants and rhizosphere microorganisms.

Effect of Pharmaceutical Effluents on Growth, Oxidative Defense, Secondary Metabolism, and Ion Homeostasis in Carrot

Pharmaceutical wastes are environmental micro pollutant and potential risk for the ecosystem. Therefore, the present study was planned to find out the effects of different pharmaceutical effluent (PE) regimes on growth, secondary metabolism, and oxidative defense in 2 carrot lines. The seeds of 2 carrot lines (DC-3 and T-29) were spread in plastic pots containing sandy loam soil. The design of experiment was completely randomized with 3 replicates per treatment. At vegetative stage, plants were irrigated with 5 different doses (control), 25%, 50%, 75% and 100%) of PE on every 3-day interval, while control plants were irrigated with canal water. The carrot roots were harvested after 25 days’ application of the treatments to determine various attributes. High concentration of PE caused a substantial decline in growth, beta carotenoids, anthocyanin, total soluble protein, free amino acids, total soluble sugar, phenolic and flavonoid contents and an increase in proline, levels of H₂O₂ and MDA, activities of antioxidant enzymes such as peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) in both lines. Moreover, PE caused significant reduction in the levels of essential nutrients (K⁺, Ca²⁺) and increased in Na⁺ content. However, T-29 line was found to be more PE tolerant because it had less H₂O₂, MDA and ascorbic acid contents. Thus, our findings showed that diluted PE (25%) could not be used for irrigation to increase the growth of plants in nutrients deprived environments without using bio filtration and biocarbon sorption technologies for treatments.

Study on the oxidative stress and transcriptional level in Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1 isolated from chromium-contaminated soil

The bioreduction of Cr(VI) and Hg(II) has become a hot topic in the field of heavy metals bioremediation. However, the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria is still not clear. In this work, a novel Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1, was isolated from chromium landfill at a chromate factory, which was used to investigate the mechanism of antioxidant stress during the Cr(VI) and Hg(II) reduction process. The results demonstrated that the removal of Cr(VI) and Hg(II) by A. indicus yy-1 from solution was through reduction rather than biosorption. The reduction rates of Cr(VI) and Hg(II) by resting cells reached 59.71% and 31.73% at 24 h with initial concentration of 10 mg L^-1, respectively. X-ray photoelectron spectroscopy (XPS) analysis further showed that Cr(III) and Hg(0) were mainly the Cr(VI)- and Hg(II)-reduced productions, respectively. Results of physiological assays showed Hg(II) was more toxic to A. indicus yy-1 than Cr(VI), and the activities of antioxidant enzymes (SOD and CAT) were significantly increased in A. indicus yy-1 for relieving the oxidative stress. The transcriptional level of genes related to Cr(VI) and Hg(II) reductases and antioxidant enzymes were up-regulated, indicating that the reductases have participated in the reduction of Cr(VI) and Hg(II), and SOD and CAT served as the vital antioxidant enzymes for defending the oxidative stress. This work provides a deep insight into the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria, which helps seek the highly resistant heavy metal reducing bacteria.

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.

Combined ability of salicylic acid and spermidine to mitigate the individual and interactive effects of drought and chromium stress in maize (Zea mays L.)

Application of the growth regulator salicylic acid (SA) and the polyamine spermidine (Spd) can be used to manage various plant abiotic stresses. We aimed to evaluate the sole and combined effects of SA and Spd on maize (Zea mays) under individual and combined drought and chromium (Cr) stress. Drought, Cr, and drought + Cr treatments caused oxidative stress by inducing higher production of reactive oxygen species (H₂O₂, O₂^-), enhanced malondialdehyde content and increased relative membrane permeability. Increased oxidative stress and higher Cr uptake in the host plant reduced the content of carotenoids, other photosynthetic pigments and protein, and changed carbohydrate metabolism. Combined drought + Cr stress was more damaging for the growth of maize plants than the individual stresses. Exogenous treatments of SA and Spd alleviated the adverse effects of drought and Cr toxicity, reflected by accumulations of osmolytes, antioxidants and endogenous polyamines. Single applications of Spd (0.1 mM) increased plant height, shoot fresh weight, leaf area, above-ground dry matter accumulation and polyamine content under drought, Cr, and drought + Cr stress conditions. However, the combined treatment SA + Spd (0.25 mM + 0.05 mM) was more effective in increasing protein and water contents, photosynthetic pigments, and carotenoids. The same treatment increased Cr tolerance in the maize plants by decreasing uptake of this heavy metal from root to shoot. The SA + Spd treatment also decreased oxidative stress by promoting antioxidant enzyme activities, and enhanced levels of proline, soluble sugars, and carbohydrate contents under individual and combined stress conditions. Results indicate that the combined half-dose application of SA + Spd may be utilized to boost the tolerance in maize under individual as well as combined drought and Cr stress conditions.