The Role of Salicylic Acid in Plants Exposed to Heavy Metals.

Salicylic acid (SA) is a very simple phenolic compound (a C7H6O3 compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

Phytoremediation of lead by a wild, non-edible Pb accumulator Coronopus didymus (L.) Brassicaceae.

Coronopus didymus was examined in terms of its ability to remediate Pb-contaminated soils. Pot experiments were conducted for 4 and 6 weeks to compare the growth, biomass, photosynthetic efficiency, lead (Pb) uptake, and accumulation by C. didymus plants. The plants grew well having no visible toxic symptoms and 100% survivability, exposed to different Pb-spiked soils 100, 350, 1500, and 2500 mg kg-1, supplied as lead nitrate. After 4 weeks, root and shoot concentrations reached 1652 and 502 mg Pb kg-1 DW, while after 6 weeks they increased up to 3091 and 527 mg Pb kg-1 DW, respectively, at highest Pb concentration. As compared to the 4 week experiments, the plant growth and biomass yield were higher after 6 weeks of Pb exposure. However, the chlorophyll content of leaves decreased but only a slight decline in photosynthetic efficiency was observed on exposure to Pb at both 4 and 6 weeks. The Pb accumulation was higher in roots than in the shoots. The bioconcentration factor of Pb was > 1 in all the plant samples, but the translocation factor was < 1. This suggested C. didymus as a good candidate for phytoremediation of Pb-contaminated soils and can be used for future remediation purposes.

Arsenic acquisition, toxicity and tolerance in plants - From physiology to remediation: A review.

Globally, environmental contamination by potentially noxious metalloids like arsenic is becoming a critical concern to the living organisms. Arsenic is a non-essential metalloid for plants and can be acclimatised in plants to toxic levels. Arsenic acquisition by plants poses serious health risks in human due to its entry in the food chain. High arsenic regimes disturb plant water relations, promote the generation of reactive oxygen species (ROS) and induce oxidative outburst in plants. This review evidences a conceivable tie-up among arsenic levels, speciation, its availability, uptake, acquisition, transport, phytotoxicity and arsenic detoxification in plants. The role of different antioxidant enzymes to confer plant tolerance towards the enhanced arsenic distress has also been summed up. Additionally, the mechanisms involved in the modulation of different genes coupled with arsenic tolerance have been thoroughly discussed. This review is intended to present an overview to rationalise the contemporary progressions on the recent advances in phytoremediation approaches to overcome ecosystem contamination by arsenic.

Copper bioavailability, uptake, toxicity and tolerance in plants: A comprehensive review.

Copper (Cu) is an essential element for humans and plants when present in lesser amount, while in excessive amounts it exerts detrimental effects. There subsists a narrow difference amid the indispensable, positive and detrimental concentration of Cu in living system, which substantially alters with Cu speciation, and form of living organisms. Consequently, it is vital to monitor its bioavailability, speciation, exposure levels and routes in the living organisms. The ingestion of Cu-laced food crops is the key source of this heavy metal toxicity in humans. Hence, it is necessary to appraise the biogeochemical behaviour of Cu in soil-plant system with esteem to their quantity and speciation. On the basis of existing research, this appraisal traces a probable connexion midst: Cu levels, sources, chemistry, speciation and bioavailability in the soil. Besides, the functions of protein transporters in soil-plant Cu transport, and the detrimental effect of Cu on morphological, physiological and nutrient uptake in plants has also been discussed in the current manuscript. Mechanisms related to detoxification strategies like antioxidative response and generation of glutathione and phytochelatins to combat Cu-induced toxicity in plants is discussed as well. We also delimits the Cu accretion in food crops and allied health perils from soils encompassing less or high Cu quantity. Finally, an overview of various techniques involved in the reclamation and restoration of Cu-contaminated soils has been provided.

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.

Insights into the tolerance and phytoremediation potential of Coronopus didymus L. (Sm) grown under zinc stress.

Zinc (Zn) is a vital micronutrient for plants, but its abundance can be calamitous. In this study, a screenhouse experiment was conducted over a 6-week period to assess the effect of soil enrichment with Zn regimes (100, 250 and 500 mg kg-1) on growth, Zn accumulation, photosynthetic pigment concentration, oxidative stress markers and activities of antioxidant enzymes in Coronopus didymus. Results revealed that Zn concentration in C. didymus roots and shoots reached up to 1848 mg kg-1 DW and 1845 mg kg-1 DW at 500 mg kg-1 Zn regime, respectively. The plant growth (root-shoot length and biomass) increased, while leaf pigment concentration and soluble protein content in C. didymus tissues decreased progressively with the increased Zn regimes in the soil. At 500 mg kg-1 Zn regime, hydrogen peroxide and malondialdehyde level increased ∼219% and 111% in roots, while ∼170% and 105% in shoots, with respect to the control. Likewise, superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase and glutathione reductase activities increased significantly with elevated Zn levels. Contrarily, compared to the control, CAT activity declined gradually and reached a minimum of ∼45% in roots and 12% in shoots under highest Zn regime. The results suggested that C. didymus displayed high Zn accumulation and emerged as a tolerant plant species towards Zn stress. Elevated Zn regimes provoked reactive oxygen species generation in C. didymus tissues which was effectively neutralised and scavenged by the antioxidant enzymes, thus marked its efficacy to be potentially employed in phytoremediation and reclamation of Zn-contaminated soils.

Pollution assessment of heavy metals in soils of India and ecological risk assessment: A state-of-the-art.

Soil is substantive component of biosphere, which is exposed to plethora of pollutants including heavy metals. These are added by natural as well as anthropogenic activities. Upsurge in heavy metal content affects all organisms by biomagnification. So, it becomes vital to create a database of heavy metals concentration in soil. This is relevant in countries where unsustainable intensive agriculture, industrial and urban development is in progress. The present review of the scientific literature from 1991 to 2018 on heavy metals in soils in India shows that Zn and Pb exceeded their limits for Indian natural soil guidelines (Zn 22.1 and Pb 13.1 µg/g), Canada (Zn 200 µg/g), Swedish (80 µg/g) and Poland (Zn 300 µg/g) soil guidelines. The mean values of As and Cu for all soil types except for roadside soils, exceeded the limits. The average value of Cd for all soil types exceeded their limit. The mean values obtained for soils of India are Fe (23774.84 µg/g), Mn (872.54 µg/g), Zn (359.94 µg/g), Cu (183.67 µg/g), Cr (161.42 µg/g), As (148.70 µg/g), Ni (112.41 µg/g), Pb (61.87 µg/g), Co (37.63 µg/g) and Cd (14.16 µg/g). Cluster analysis and factor analysis were employed to different soil types and showed multiple sources of these metals. The contamination factor (CF), enrichment factor (EF) and potential contamination index (Cp) showed that Cd and As are the main contaminants. The results of ecological risk index indicated that Cd is the main pollutant in the different soils of India.

Phytohormones Regulate Accumulation of Osmolytes Under Abiotic Stress.

Plants face a variety of abiotic stresses, which generate reactive oxygen species (ROS), and ultimately obstruct normal growth and development of plants. To prevent cellular damage caused by oxidative stress, plants accumulate certain compatible solutes known as osmolytes to safeguard the cellular machinery. The most common osmolytes that play crucial role in osmoregulation are proline, glycine-betaine, polyamines, and sugars. These compounds stabilize the osmotic differences between surroundings of cell and the cytosol. Besides, they also protect the plant cells from oxidative stress by inhibiting the production of harmful ROS like hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals. The accumulation of osmolytes is further modulated by phytohormones like abscisic acid, brassinosteroids, cytokinins, ethylene, jasmonates, and salicylic acid. It is thus important to understand the mechanisms regulating the phytohormone-mediated accumulation of osmolytes in plants during abiotic stresses. In this review, we have discussed the underlying mechanisms of phytohormone-regulated osmolyte accumulation along with their various functions in plants under stress conditions.

Global evaluation of heavy metal content in surface water bodies: A meta-analysis using heavy metal pollution indices and multivariate statistical analyses.

Water is polluted by increasing activities of population and the necessity to provide them with goods and services that use water as a vital resource. The contamination of water due to heavy metals (HMs) is a big concern for humankind; however, global studies related to this topic are scarce. Thus, the current review assesses the content of HMs in surface water bodies throughout the world from 1994 to 2019. To achieve this goal, multivariate analyses were applied in order to determine the possible sources of HMs. Among the analyzed HMs in a total of 147 publications, the average content of Cr, Mn, Co, Ni, As and Cd exceeded the permissible limits suggested by WHO and USEPA. The results of the heavy metal pollution index, evaluation index, the degree of contamination, water pollution and toxicity load showed that the examined water bodies are highly polluted by HMs. The results of median lethal toxicity index showed maximum toxicity in As, Co, Cr and Ni in the surface water bodies. Results of ingestion and dermal pathways for adults and children in the current analyzed review showed that As is the major contaminant. Moreover, Cr, Ni, As and Cd showed values that could be considered as a high risk for cancer generation via the ingestion pathway as compared to the dermal route. It is recommended that remediation techniques such as the introduction of aquatic phytoremediation plant species and adsorbents should be included in land management plans in order to reduce human risks.

Ethylenediamine disuccinic acid enhanced phytoextraction of nickel from contaminated soils using Coronopus didymus (L.) Sm.

In a screenhouse, the applicability of biodegradable chelant ethylenediamine disuccinic acid (EDDS) to enhance Ni-phytoextraction by Coronopus didymus was tested for the first time. This study assayed the hypothesis based upon the role of EDDS on physiological and biochemical alterations and ameliorating phytoextraction capacity of C. didymus under nickel (Ni) stress. Pot experiments were conducted for 6 weeks and C. didymus plants were cultivated in soil artificially contaminated with 30, 50, and 70 mg kg-1 Ni treatments. Soil was amended with EDDS (2 mmol kg-1). Plants were harvested, 1 week after EDDS application. At 70 mg kg-1 Ni level, EDDS application dramatically enhanced the root and shoot Ni concentration from 665 and 644 to 1339 and 1338 mg kg-1, respectively. Combination of Ni + EDDS induced alterations in biochemical parameters of plants. EDDS addition posed pessimistic effects on growth, biomass, photosynthetic activity and protein content of the plants. Besides, application of EDDS stimulated the generation of superoxide anion, H2O2 content and MDA level. However, EDDS assisted mount in antioxidant activities (superoxide dismutase, catalase and glutathione peroxidase) considerably neutralised the toxicity induced by reactive oxygen species in plant tissues. The results revealed EDDS efficacy to ameliorate the performance of antioxidant enzymes and improved Ni translocation in plant tissues, thus strongly marked its affinity to be used together with C. didymus for Ni-phytoextraction.