Mechanisms of arsenic tolerance and detoxification in plants and their application in transgenic technology: a critical appraisal

Didactical approaches and insights into environmental processes and cardiovascular hazards of arsenic contaminants

Arsenic, as a metalloid, has the ability to move and transform in different environmental media. Its widespread contamination has become a significant environmental problem and public concern. Arsenic can jeopardize multiple organs through various pathways, influenced by environmental bioprocesses. This article provides a comprehensive overview of current research on the cardiovascular hazards of arsenic. A bibliometric analysis revealed that there are 376 papers published in 145 journals, involving 40 countries, 631 institutions, and 2093 authors, all focused on arsenic-related concerns regarding cardiovascular health. China and the U.S. have emerged as the central hubs of collaborative relationships and have the highest number of publications. Hypertension and atherosclerosis are the most extensively studied topics, with redox imbalance, apoptosis, and methylation being the primary mechanistic clues. Cardiovascular damage caused by arsenic includes arrhythmia, cardiac remodeling, vascular leakage, and abnormal angiogenesis. However, the current understanding is still inadequate over cardiovascular impairments, underlying mechanisms, and precautionary methods of arsenic, thus calling an urgent need for further studies to bridge the gap between environmental processes and arsenic hazards.

Detoxifying the heavy metals: a multipronged study of tolerance strategies against heavy metals toxicity in plants

Heavy metal concentrations exceeding permissible limits threaten human life, plant life, and all other life forms. Different natural and anthropogenic activities emit toxic heavy metals in the soil, air, and water. Plants consume toxic heavy metals from their roots and foliar part inside the plant. Heavy metals may interfere with various aspects of the plants, such as biochemistry, bio-molecules, and physiological processes, which usually translate into morphological and anatomical changes. They use various strategies to deal with the toxic effects of heavy metal contamination. Some of these strategies include restricting heavy metals to the cell wall, vascular sequestration, and synthesis of various biochemical compounds, such as phyto-chelators and organic acids, to bind the free moving heavy metal ions so that the toxic effects are minimized. This review focuses on several aspects of genetics, molecular, and cell signaling levels, which integrate to produce a coordinated response to heavy metal toxicity and interpret the exact strategies behind the tolerance of heavy metals stress. It is suggested that various aspects of some model plant species must be thoroughly studied to comprehend the approaches of heavy metal tolerance to put that knowledge into practical use.

Recent advances in the bioremediation of arsenic-contaminated soils: a mini review

The carcinogenic metalloid arsenic (As), owing to its persistent behavior in elevated levels in soils, aggravates environmental and human health concerns. The current strategies used in the As decontamination involve several physical and chemical approaches. However, it involves high cost and even leads to secondary pollution. Therefore, it is quite imperative to explore methods that can eradicate As menace from the environment in an eco-friendly, efficient, and cost-competitive way. Searching for such viable alternatives leads to the option of bioremediation technology by utilizing various microorganisms, green plants, enzymes or even their integrated methods. This review is intended to give scientific and technical details about recent advances in the bioremediation strategies of As in soil. It takes into purview the extent, toxicological manifestations, pathways of As exposure and exemplifies the substantive need of bioremediation technologies such as phytoremediation and biosorption in a descriptive manner. Additionally, the paper looks into the wide potential of some plant growth promoting microorganisms (PGPMs) that improve plant growth on one hand and alleviate As toxicity on the other. Furthermore, it also makes a modest attempt to assimilate the use of nanoparticles, non-living biomass and transgenic crops which are the emerging alternative bioremediation technologies.

Arsenic-Induced Oxidative Stress and Antioxidant Defense in Plants

The non-essential metalloid arsenic (As) is widely distributed in soil and underground water of many countries. Arsenic contamination is a concern because it creates threat to food security in terms of crop productivity and food safety. Plants exposed to As show morpho-physiological, growth and developmental disorder which altogether result in loss of productivity. At physiological level, As-induced altered biochemistry in chloroplast, mitochondria, peroxisome, endoplasmic reticulum, cell wall, plasma membrane causes reactive oxygen species (ROS) overgeneration which damage cell through disintegrating the structure of lipids, proteins, and DNA. Therefore, plants tolerance to ROS-induced oxidative stress is a vital strategy for enhancing As tolerance in plants. Plants having enhanced antioxidant defense system show greater tolerance to As toxicity. Depending upon plant diversity (As hyperaccumulator/non-hyperaccumulator or As tolerant/susceptible) the mechanisms of As accumulation, absorption or toxicity response may differ. There can be various crop management practices such as exogenous application of nutrients, hormones, antioxidants, osmolytes, signaling molecules, different chelating agents, microbial inoculants, organic amendments etc. can be effective against As toxicity in plants. There is information gap in understanding the mechanism of As-induced response (damage or tolerance response) in plants. This review presents the mechanism of As uptake and accumulation in plants, physiological responses under As stress, As-induced ROS generation and antioxidant defense system response, various approaches for enhancing As tolerance in plants from the available literatures which will make understanding the to date knowledge, knowledge gap and future guideline to be worked out for the development of As tolerant plant cultivars.

miRNAs play critical roles in response to abiotic stress by modulating cross-talk of phytohormone signaling

One of the most interesting signaling molecules that regulates a wide array of adaptive stress responses in plants are the micro RNAs (miRNAs) that are a unique class of non-coding RNAs constituting novel mechanisms of post-transcriptional gene regulation. Recent studies revealed the role of miRNAs in several biotic and abiotic stresses by regulating various phytohormone signaling pathways as well as by targeting a number of transcription factors (TFs) and defense related genes. Phytohormones are signal molecules modulating the plant growth and developmental processes by regulating gene expression. Studies concerning miRNAs in abiotic stress response also show their vital roles in abiotic stress signaling. Current research indicates that miRNAs may act as possible candidates to create abiotic stress tolerant crop plants by genetic engineering. Yet, the detailed mechanism governing the dynamic expression networks of miRNAs in response to stress tolerance remains unclear. In this review, we provide recent updates on miRNA-mediated regulation of phytohormones combating various stress and its role in adaptive stress response in crop plants.

Effect of the irrigation method and genotype on the bioaccumulation of toxic and trace elements in rice

The total concentration of three toxic elements (As, Cd and Pb) and five oligoelements (Cu, Mn, Mo, Ni and Se) has been determined using an original and completely validated ICP-MS method. This was applied to rice grains from 26 different genotypes cultivated in the same soil and irrigated with the same water in three different ways: by the traditional continuous flooding (CF) and by two intermittent methods, the sprinkler irrigation (SP) and the periodical saturation of the soil (SA). The adoption of SP hugely minimizes the average amounts of almost all elements in kernels (-98% for As, -90% for Se and Mn, -60% for Mo, -50% for Cd and Pb), with the only exception of Ni, whose concentration increases the average amount found in the CF rice by 7.5 times. Also SA irrigation is able to reduce the amounts of As, Mo and Pb in kernels but it significantly increases the amounts of Mn, Ni and - mainly - Cd. Also the nature of the genotype determined a wide variability of data within each irrigation method. Genotypes belonging to Indica subspecies are the best bioaccumulators of elements in both CF and SP methods and, never, the worst bioaccumulators for any element/irrigation method combination. In the principal component analysis, PC1 can differentiate samples irrigated by SP by those irrigated by CF and SA, whereas PC2 provides differentiation of CF samples by SA samples. When looking at the loading plot Ni is negatively correlated to the majority of the other elements, except Cu and Cd having negative loadings on PC2. These results allow to envisage that a proper combination of the irrigation method and the nature of rice genotype might be a very valuable tool in order to successfully achieve specific objectives of food safety or the attainment of functional properties.

Artificial Floating Island with Vetiver for Treatment of Arsenic-Contaminated Water: A Real Scale Study in High-Andean Reservoir

Arsenic found in agriculture water reservoirs represents a threat to water security and safe agricultural products in developing countries. Small farms do not implement traditional water treatments due to the high cost; hence, a nature-based solution is an alternative to tackling this challenge. This paper investigated the potential of artificial floating island with Vetiver (AFIV) for the geogenic arsenic removal present in the reservoir of the Ilinizas páramo in Ecuador. We constructed two AFIV systems using PVC pipes in a reservoir batch type with a 3.6 m3 treatment capacity. Arsenic and iron were analyzed in duplicated every 30 days at the affluent and effluent through 120 days. The average remediation of arsenic was recorded as 97% in water and 84% in sediment, while the average remediation of iron was 87% in sediment. The survival rate of macrophytes was 92%; they accumulated arsenic in its roots that acted as a barrier against the translocation. The research demonstrated that the use of AFIV has the potential to rehabilitate reservoirs contaminated with arsenic under adverse climatic conditions such as the páramo ecosystem.

Effect of Light Intensity on the Mechanism of Inorganic Arsenic Accumulation and Patterns in the Red Macroalga, Sarcodia suiae

Arsenic is a noted dangerous metalloid found in many organisms, including humans, that accumulate via food consumption of aquatic products such as macroalgae, particularly where they are a major component of the human diet. The mechanism of accumulation of inorganic arsenic (iAs) as the most toxic form of arsenic (As) was investigated under three different light intensities (LI) (30, 55, and 80 μmol photons m^-2 s^-1) at varied arsenite (As (III)) concentrations (conc) (0, 125, 250, and 500 μg L^-1) using Sarcodia suiae, a red marine macroalga. The depigmentation of the algal fronds from deep red to slightly pinkish-orange and solid green has been confirmed as a form of developmental acclimation, and the direct toxic effects of conc and LI were manifested by the degree of severity of this symptom. Two-way ANOVA showed a significant difference in iAs accumulation which depended upon conc and LI. Stepwise regression analysis showed LI as the second most important variable after conc in all treatments. S. suiae did not appear to intracellularly transform As (III) to arsenate (As (V)); hence, As (III) oxidation seemed to be a preliminary biotransformation as reflected by the dominance and increased accumulation of toxic As (III) by the alga. These findings may render it a prospective environmentally friendly candidate for reducing toxic As hazard risk and as a biological component in the treatment of wastewater. Moreover, these results also suggest that a more concerted effort is required in developing protocols for public health concerns regarding food safety and quality regulations in seafood and products sourced from macroalgae including S. suiae.

Prospects of genetic engineering utilizing potential genes for regulating arsenic accumulation in plants

The rapid pace of industrial, agricultural and anthropogenic activities in the 20^th century has resulted in contamination of heavy metals across the globe. Arsenic (As) is a ubiquitous, naturally occurring toxic metalloid, contaminating the soil and water and affecting human health in several countries. Several physicochemical methods exist for the cleanup of As contamination but these are expensive and disastrous to microbes and soil. Plant based remediation approaches are low cost and environmentally safe. Hence, extensive biochemical, molecular and genetic experiments have been conducted to understand plants' responses to As stress and have led to the identification of potential genes. The available knowledge needs to be utilized to either reduce As accumulation in crop plants (rice) or to enhance As levels in shoots of hyperaccumulators (Pteris vittata). Gene manipulation using biotechnological tools can be an effective approach to exploit the potential genes (plasmamembrane and vacuolar transporters, glutathione and phytochelatin biosynthetic enzymes, etc.) playing pivotal roles in uptake, translocation, transformation, complexation, and compartmentalization of As in plants. The transgenic plants with increased tolerance to As and altered (increased/decreased) As accumulation have been developed. The need, however, exists to design plants with altered expression of two or more genes for harmonizing various events (like arsenate reduction, arsenite complexation, sequestration and translocation) so as to achieve desirable reduction (crop plants) or increase (phytoremediator plants) in As content. This review sheds light on transgenic approaches adopted to modulate As levels in plants and proposes future directions to achieve desirable results.

Arsenic tolerance and phytoremediation potential of Conocarpus erectus L. and Populus deltoides L

The present study was conducted to explore arsenic (As) tolerance and phytostabilization potential of the two tree species, buttonwood (Conocarpus erectus) and eastern cottonwood (Populus deltoides). Both plant species were grown in pots and were exposed to various soil As levels (control, 5, 10, 15, and 20 mg kg^-1). The plants were harvested after 9 months for the evaluation of growth parameters as well as root and shoot As concentrations. With increasing soil As levels, plant height stress tolerance index (PHSTI) was significantly decreased in both tree species, whereas root length stress tolerance index (RLSTI) and dry matter stress tolerance index (DMSTI) were not affected. Root and shoot As concentrations significantly increased in both tree species with increasing soil As levels. Translocation factor and bioconcentration factor were less than 1.0 for both plant species. This study revealed that both tree species are non-hyperaccumulators of As, but they could be used for phytostabilization of As-contaminated soils.

miRNA-based heavy metal homeostasis and plant growth

Plants have been naturally gifted with mechanisms to adjust under very high or low nutrient concentrations. Heavy metal toxicity is considered as a major growth and yield-limiting factor for plants. This stress includes essential as well as non-essential metals. MicroRNAs (miRNAs) are known for mediating post-transcriptional regulation by cleaving transcripts or translational inhibition. It is commonly agreed that an extensive understanding of plant miRNAs will significantly help in the induction of tolerance against environmental stresses. With the introduction of the latest technology like next generation sequencing (NGS), a growing figure of miRNAs has been productively recognized in several plants for their diverse roles. These miRNAs are well-known modulators of plant responses to heavy metal (HM) stress. Data regarding metal-responsive miRNAs point out the vital role of plant miRNAs in supplementing metal detoxification by means of transcription factors (TF) or gene regulation. Acting as systemic signals, miRNAs also synchronize different physiological processes for plant responses to metal toxicities. In contrast to practicing techniques, using miRNA is a greatly helpful, pragmatic, and feasible approach. The earlier findings point towards miRNAs as a prospective target to engineer heavy metal tolerance in plants. Therefore, there is a need to augment our knowledge about the orchestrated functions of miRNAs during HM stress. We reviewed the deterministic significance of plant miRNAs in heavy metal tolerance and their role in mediating plant responses to HM toxicities. This review also summarized the topical developments by identification and validation of different metal stress-responsive miRNAs.

miRNAs: Major modulators for crop growth and development under abiotic stresses

Cumulatively, biotic and abiotic stresses of various magnitudes can decrease the production of crops by 70%. miRNAs have emerged as a genetic tool with enormous potential that can be exploited to understand stress tolerance at the molecular level and eventually regulate stress in crops. Plant miRNA targets frequently fit into diverse families of TFs that control the expression of genes related to a certain trait. As key machinery in gene regulatory networks, it is agreed that a broad understanding of miRNAs will greatly increase our understanding of plant responses to environmental stresses. miRNA-led stress regulatory networks are being considered as novel tools for the development of abiotic stress tolerance in crops. At this time, we need to expand our knowledge about the modulatory role of miRNAs during environmental fluctuations. It has become exceedingly clear that with increased understanding of the role of miRNAs during stress, the techniques for using miRNA-mediated gene regulation to enhance plant stress tolerance will become more effective and reliable. In this review we present: (1) miRNAs as a potential avenue for the modulation of abiotic stresses, and (2) summarize the research progress regarding plant responses to stress. Current progress is explained through discussion of the identification and validation of several miRNAs that enhance crop tolerance of salinity, drought, etc., while missing links on different aspects of miRNAs related to abiotic stress tolerance are noted.

Arsenic hyperaccumulation induces metabolic reprogramming in Pityrogramma calomelanos to reduce oxidative stress

Arsenic (As) pollution is a major environmental concern due to its worldwide distribution and high toxicity to organisms. The fern Pityrogramma calomelanos is one of the few plant species known to be able to hyperaccumulate As, although the mechanisms involved are largely unknown. This study aimed to investigate the metabolic adjustments involved in the As-tolerance of P. calomelanos. For this purpose, ferns with five to seven fronds were exposed to a series of As concentrations. Young fronds were used for biochemical analysis and metabolite profiling using gas chromatography-mass spectrometry. As treatment increased the total concentration of proteins and soluble phenols, enhanced peroxidase activities, and promoted disturbances in nitrogen and carbon metabolism. The reduction of the glucose pool was one of the striking responses to As. Remarkable changes in amino acids levels were observed in As-treated plants, including those related to biosynthesis of glutathione and phenols, osmoregulation and two photorespiratory intermediates. In addition, increases in polyamines levels and antioxidant enzyme activities were observed. In summary, this study indicates that P. calomelanos tolerates high concentration of As due to its capacity to upregulate biosynthesis of amino acids and antioxidants, without greatly disturbing central carbon metabolism. At extremely high As concentrations, however, this protective mechanism fails to block reactive oxygen species production, leading to lipid peroxidation and leaf necrosis.

MicroRNAs As Potential Targets for Abiotic Stress Tolerance in Plants

The microRNAs (miRNAs) are small (20-24 nt) sized, non-coding, single stranded riboregulator RNAs abundant in higher organisms. Recent findings have established that plants assign miRNAs as critical post-transcriptional regulators of gene expression in sequence-specific manner to respond to numerous abiotic stresses they face during their growth cycle. These small RNAs regulate gene expression via translational inhibition. Usually, stress induced miRNAs downregulate their target mRNAs, whereas, their downregulation leads to accumulation and function of positive regulators. In the past decade, investigations were mainly aimed to identify plant miRNAs, responsive to individual or multiple environmental factors, profiling their expression patterns and recognizing their roles in stress responses and tolerance. Altered expressions of miRNAs implicated in plant growth and development have been reported in several plant species subjected to abiotic stress conditions such as drought, salinity, extreme temperatures, nutrient deprivation, and heavy metals. These findings indicate that miRNAs may hold the key as potential targets for genetic manipulations to engineer abiotic stress tolerance in crop plants. This review is aimed to provide recent updates on plant miRNAs, their biogenesis and functions, target prediction and identification, computational tools and databases available for plant miRNAs, and their roles in abiotic stress-responses and adaptive mechanisms in major crop plants. Besides, the recent case studies for overexpressing the selected miRNAs for miRNA-mediated enhanced abiotic stress tolerance of transgenic plants have been discussed.

Biochemical mechanisms of signaling: perspectives in plants under arsenic stress

Plants are the ultimate food source for humans, either directly or indirectly. Being sessile in nature, they are exposed to various biotic and abiotic stresses because of changing climate that adversely effects their growth and development. Contamination of heavy metals is one of the major abiotic stresses because of anthropogenic as well as natural factors which lead to increased toxicity and accumulation in plants. Arsenic is a naturally occurring metalloid toxin present in the earth crust. Due to its presence in terrestrial and aquatic environments, it effects the growth of plants. Plants can tolerate arsenic using several mechanisms like phytochelation, vacuole sequestration and activation of antioxidant defense systems. Several signaling mechanisms have evolved in plants that involve the use of proteins, calcium ions, hormones, reactive oxygen species and nitric oxide as signaling molecules to cope with arsenic toxicity. These mechanisms facilitate plants to survive under metal stress by activating their defense systems. The pathways by which these stress signals are perceived and responded is an unexplored area of research and there are lots of gaps still to be filled. A good understanding of these signaling pathways can help in raising the plants which can perform better in arsenic contaminated soil and water. In order to increase the survival of plants in contaminated areas there is a strong need to identify suitable gene targets that can be modified according to needs of the stakeholders using various biotechnological techniques. This review focuses on the signaling mechanisms of plants grown under arsenic stress and will give an insight of the different sensory systems in plants. Furthermore, it provides the knowledge about several pathways that can be exploited to develop plant cultivars which are resistant to arsenic stress or can reduce its uptake to minimize the risk of arsenic toxicity through food chain thus ensuring food security.

Analysis of arsenic induced physiological and biochemical responses in a medicinal plant, Withania somnifera

Withania somnifera has been an important herb in the Ayurvedic and indigenous medical systems for centuries in India. However, these grow as weeds mostly in the wastelands, which receive contaminated water from municipal and industrial sources. In the present investigation, plants of Withania somnifera were exposed to various concentrations of arsenate (AsV) and arsenite (AsIII) (0, 10, 25, 50, 100 μM) for 10 days and analysed for accumulation of arsenic (As) and physiological and biochemical changes. Plants showed more As accumulation upon exposure to AsIII (320 μg g(-1) DW in roots and 161 μg g(-1) DW in leaves) than to AsV (173 μg g(-1) DW in roots and 100 μg g(-1) DW in leaves) after 10 days of treatment. Consequently, AsIII exposure caused more toxicity to plants as compared to that AsV, as evaluated in terms of the level of photosynthetic pigments and oxidative stress parameters (superoxide, hydrogen peroxide and lipid peroxidation), particularly at higher concentrations and on longer durations. Plants could tolerate low concentrations (variable for AsIII and AsV) until longer durations (10 days) and high concentrations for shorter durations (1-5 days) through increase in antioxidant enzymes and by augmented synthesis of thiols. In conclusion, As tolerance potential of Withania plants on one hand advocates its prospective use for remediation under proper supervision and on the other demonstrates possible threat of As entry into humans due to medicinal uses.

Transcriptomics profiling of Indian mustard (Brassica juncea) under arsenate stress identifies key candidate genes and regulatory pathways

Arsenic (As) is a non-essential element, a groundwater pollutant, whose uptake by plants produces toxic effects. The use of As-contaminated groundwater for irrigation can affect the crop productivity. Realizing the importance of the Brassica juncea as a crop plant in terms of oil-yield, there is a need to unravel mechanistic details of response to As stress and identify key functional genes and pathways. In this research, we studied time-dependent (4-96 h) transcriptome changes in roots and shoots of B. juncea under arsenate [As(V)] stress using Agilent platform. Among the whole transcriptome profiled genes, a total of 1,285 genes showed significant change in expression pattern upon As(V) exposure. The differentially expressed genes were categorized to various signaling pathways including hormones (jasmonate, abscisic acid, auxin, and ethylene) and kinases. Significant effects were also noticed on genes related to sulfur, nitrogen, CHO, and lipid metabolisms along with photosynthesis. Biochemical assays were conducted using specific inhibitors of glutathione and jasmonate biosynthesis, and kinases. The inhibitor studies revealed interconnection among sulfur metabolism, jasmonate, and kinase signaling pathways. In addition, various transposons also constituted a part of the altered transcriptome. Lastly, we profiled a set of key functional up- and down-regulated genes using real-time RT-PCR, which could act as an early indicators of the As stress.

MicroRNA mediated regulation of metal toxicity in plants: present status and future perspectives

The human population is increasing at an alarming rate, whereas heavy metals (HMs) pollution is mounting serious environmental problem, which could lead to serious concern about the future sufficiency of global food production. Some HMs such as Mn, Cu, and Fe, at lower concentration serves as an essential vital component of plant cell as they are crucial in various enzyme catalyzed biochemical reactions. At higher concentration, a vast variety of HMs such as Mn, Cu, Cd, Fe, Hg, Al and As, impose toxic reaction in the plant system which greatly affect the crop yield. Recently, microRNAs (miRNAs) that are small class of non-coding riboregulator have emerged as central regulator of numerous abiotic stresses including HMs. Increasing reports indicate that plants have evolved specialized inbuilt mechanism viz. signal transduction, translocation and sequestration to counteract the toxic response of HMs. Combining computational and wet laboratory approaches have produced sufficient evidences concerning active involvement of miRNAs during HMs toxicity response by regulating various transcription factors and protein coding genes involved in plant growth and development. However, the direct role of miRNA in controlling various signaling molecules, transporters and chelating agents of HM metabolism is poorly understood. This review focuses on the latest progress made in the area of direct involvement of miRNAs in signaling, translocation and sequestration as well as recently added miRNAs in response to different HMs in plants.

Identification and profiling of arsenic stress-induced microRNAs in Brassica juncea

MicroRNAs (miRNAs) constitute a novel mechanism of gene regulation affecting plant development, growth, and stress response. To study the role of miRNAs in arsenic (As) stress, microarray profiling of miRNAs was performed in Brassica juncea using a custom Phalanx Plant OneArray containing 381 unique miRNA probes representing 618 miRNAs from 22 plant species. miRNA microarray hybridization of roots exposed to As for 1h and 4h revealed that a total of 69 miRNAs belonging to 18 plant miRNA families had significantly altered expression. The As-responsive miRNAs also exhibited a time- and organ-dependent change in their expression. Putative target prediction for the miRNAs suggested that they regulate various developmental processes (e.g. miR156, miR169, and miR172), sulphur uptake, transport, and assimilation (miR395, miR838, and miR854), and hormonal biosynthesis and/or function (e.g. miR319, miR167, miR164, and miR159). Notable changes were observed in the level of auxins [indole-3-acetic acid (IAA), indole-3- butyric acid, and naphthalene acetic acid], jasmonates [jasmonic acid (JA) and methyl jasmonate], and abscisic acid. The exogenous supply of JA and IAA improved growth of plants under As stress and altered expression of miR167, miR319, and miR854, suggesting interplay of hormones and miRNAs in the regulation of As response. In conclusion, the present work demonstrates the role of miRNAs and associated mechanisms in the plant's response towards As stress.