Evaluation of the protective role of exogenous growth regulators against Ni toxicity in woody shrub Daphne jasminea

The impact of nickel on plant growth and oxidative balance

This review summarizes the impact of nickel (Ni) in hydroponics on the growth, basic biochemical parameters and oxidative balance in angiosperms using data from 66 papers (and 181 treatments). Generally, changes in biomass, pigments (chlorophylls and carotenoids) and proteins were negative when comparing concentration (≤100 and >100 μM) and time (≤14 and >14 days). However, we could deduce a higher tolerance to Ni excess in dicots than in monocots. Growth and basic metabolites were often significantly positively correlated. In contrast to proteins, amino acids were positively affected by Ni, indicating proline accumulation and/or protein catabolism. The increase in hydrogen peroxide (H2O2) content was stimulated by time and Ni concentration, and it is higher in dicots and usually negatively correlated with basic metabolites. An increase in Ni concentration stimulates the increase of thiols, but a longer exposure has a neutral or negative effect. On the contrary, the amount of vitamin C (ascorbic acid) is positively influenced by the dose of Ni in roots and the duration of excess Ni in shoots, which points to dynamic changes of this antioxidant in individual organs. Soluble phenols were not as affected, but their importance appears especially in shoots during long-term exposure to Ni with a simultaneous increase in H2O2 content, confirming their antioxidative role. We emphasize that due to the significant quantitative variability in the published studies, we analyze the presented parameters as a percentage change.

In Vitro Culture Studies for the Mitigation of Heavy Metal Stress in Plants

Heavy metals are among the most common and dangerous contaminants; their action on plants, as well as the possibility for plants to effectively absorb and translocate them, have been studied for several years, mainly for exploitation in phytoremediation, an environmentally friendly and potentially effective technology proposed and studied for the recovery of contaminated soils and waters. In this work, the analysis has focused on the studies developed using in vitro techniques on the possibilities of mitigating, in plants, the stress due to the presence of heavy metals and/or improving their absorption. These objectives can be pursued with the use of different substances and organisms, which have been examined in detail. The following are therefore presented in this review: an analysis of the role of metals and metalloids; the use of several plant growth regulators, with their mechanisms of action in different physiological phases of the plant; the activity of bacteria and fungi; and the role of other effective compounds, such as ascorbic acid and glutathione.

The regulatory metabolic networks of the Brassica campestris L. hairy roots in response to cadmium stress revealed from proteome studies combined with a transcriptome analysis

Brassica campestris L., a cadmium (Cd) hyperaccumulating herbaceous plant, is considered as a promising candidate for the bioremediation of Cd pollution. However, the molecular mechanisms regulating these processes remain unclear. The present work, using proteome studies combined with a transcriptome analysis, was carried out to reveal the response mechanisms of the hairy roots of Brassica campestris L. under Cd stress. Significant tissue necrosis and cellular damage occurred, and Cd accumulation was observed in the cell walls and vacuoles of the hairy roots. Through quantitative proteomic profiling, a total of 1424 differentially expressed proteins (DEPs) were identified, and are known to be enriched in processes including phenylalanine metabolism, plant hormone signal transduction, cysteine and methionine metabolism, protein export, isoquinoline alkaloid biosynthesis and flavone biosynthesis. Further studies combined with a transcriptome analysis found that 118 differentially expressed genes (DEGs) and their corresponding proteins were simultaneously up- or downregulated. Further Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis of the 118 shared DEGs and DEPs indicated their involvement in calcium, ROS and hormone signaling-mediated response, including regulation of carbohydrate and energy metabolism, biosynthesis of GSH, PCs and phenylpropanoid compounds that play vital roles in the Cd tolerance of Brassica campestris L. Our findings contribute to a better understanding of the regulatory networks of Brassica campestris L. under Cd stress, as well as provide valuable information on candidate genes (e.g., BrPAL, BrTAT, Br4CL, BrCDPK, BrRBOH, BrCALM, BrABCG1/2, BrVIP, BrGCLC, BrilvE, BrGST12/13/25). These results are of particular importance to the subsequent development of promising transgenic plants that will hyperaccumulate heavy metals and efficient phytoremediation processes.

Multivariate characterization of biochemical and physiological attributes of quinoa (Chenopodium quinoa Willd.) genotypes exposed to nickel stress: implications for phytoremediation

Nickel (Ni) is an essential element for plants; however, excessive uptake of Ni causes phytotoxicity in plants. The phytotoxic effects of Ni on the growth of quinoa and the underlaying mechanisms for Ni tolerance and phytoremediation are unknown. Hence, the present study investigated Ni tolerance and accumulation potential of two quinoa genotypes (Puno and Vikinga). Both genotypes were exposed to Ni (0, 100, 200, 300, and 400 μM) in half-strength Hoagland nutrient solution for three weeks. Results revealed that shoot and root lengths, biomass, stomatal conductance, and chlorophyll contents were decreased with the increase of Ni concentration. Excessive uptake of Ni resulted in the limited uptake of K by root and its translocation to shoot. Ni caused oxidative stress in plants by overproduction of H2O2 leading to lipid peroxidation of cell membranes. Genotype Puno showed greater tolerance to Ni than Vikinga based on tolerance index, lower bioconcentration factor, and translocation factor. Greater tolerance of Puno was mainly attributed to improved physiological responses and amelioration of oxidative stress by induction of antioxidant enzymes such as peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX). It was revealed through multivariate analysis that Ni had strong negative correlations with growth and physiological attributes and positive associations with oxidative stress attributes. The study demonstrated genotypic variation in response to varying Ni concentrations and Puno performed better than Vikinga for phytostabilization of Ni-contaminated soils.

Transcriptome Analysis Reveals Changes in Whole Gene Expression, Biological Process, and Molecular Functions Induced by Nickel in Jack Pine (Pinus banksiana)

Understanding the genetic response of plants to nickel stress is a necessary step to improving the utility of plants in environmental remediation and restoration. The main objective of this study was to generate whole genome expression profiles of P. banksiana exposed to nickel ion toxicity compared to reference genotypes. Pinus banksiana seedlings were screened in a growth chamber setting using a high concentration of 1600 mg of nickel per 1 kg of soil. RNA was extracted and sequenced using the Illumina platform, followed by de novo transcriptome assembly. Overall, 25,552 transcripts were assigned gene ontology. The biological processes in water-treated samples were analyzed, and 55% of transcripts were distributed among five categories: DNA metabolic process (19.3%), response to stress (13.3%), response to chemical stimuli (8.7%), signal transduction (7.7%) and response to biotic stimulus (6.0%). For molecular function, the highest percentages of genes were involved in nucleotide binding (27.6%), nuclease activity (27.3%) and kinase activity (10.3%). Sixty-two percent of genes were associated with cellular compartments. Of these genes, 21.7% were found in the plasma membrane, 16.1% in the cytosol, 12.4% with the chloroplast and 11.9% in the extracellular region. Nickel ions induced changes in gene expression, resulting in the emergence of differentially regulated categories. Overall, there were significant changes in gene expression with a total 4128 genes upregulated and 3754 downregulated genes detected in nickel-treated genotypes compared to water-treated control plants. For biological processes, the highest percentage of upregulated genes in plants exposed to nickel were associated with the response to stress (15%), the response to chemicals (11,1%), carbohydrate metabolic processes (7.4%) and catabolic processes (7.4%). The largest proportions of downregulated genes were associated with the biosynthetic process (21%), carbohydrate metabolic process (14.3%), response to biotic stimulus (10.7%) and response to stress (10.7%). For molecular function, genes encoding for enzyme regulatory and hydrolase activities represented the highest proportion (61%) of upregulated gene. The majority of downregulated genes were involved in the biosynthetic processes. Overall, 58% of upregulated genes were located in the extracellular region and the nucleus, while 42% of downregulated genes were localized to the plasma membrane and 33% to the extracellular region. This study represents the first report of a transcriptome from a conifer species treated with nickel.

Nickel (Ni) phytotoxicity and detoxification mechanisms: A review

Scientists studying the environment, physiology, and biology have been particularly interested in nickel (Ni) because of its dual effects (essentiality and toxicity) on terrestrial biota. It has been reported in some studies that without an adequate supply of Ni, plants are unable to finish their life cycle. The safest Ni limit for plants is 1.5 μg g^-1, while the limit for soil is between 75 and 150 μg g^-1. Ni at lethal levels harms plants by interfering with a variety of physiological functions, including enzyme activity, root development, photosynthesis, and mineral uptake. This review focuses on the occurrence and phytotoxicity of Ni with respect to growth, physiological and biochemical aspects. It also delves into advanced Ni detoxification mechanisms such as cellular modifications, organic acids, and chelation of Ni by plant roots, and emphasizes the role of genes involved in Ni detoxification. The discussion has been carried out on the current state of using soil amendments and plant-microbe interactions to successfully remediate Ni from contaminated sites. This review has identified potential drawbacks and difficulties of various strategies for Ni remediation, discussed the importance of these findings for environmental authorities and decision-makers, and concluded by noting the sustainability concerns and future research needs regarding Ni remediation.

Effect of gibberellic acid on photosynthesis and oxidative stress response in maize under weak light conditions

Gibberellin (GA) is an important endogenous hormone involved in plant responses to abiotic stresses. Experiments were conducted at the Research and Education Center of Agronomy, Shenyang Agricultural University (Shenyang, China) in 2021.We used a pair of near-isogenic inbred maize lines comprising, SN98A (light-sensitive inbred line) and SN98B (light-insensitive inbred line) to study the effects of exogenous gibberellin A3 (GA₃) application on different light-sensitive inbred lines under weak light conditions. The concentration of GA₃ was selected as 20, 40 and 60 mg L^-1. After shade treatment, the photosynthetic physiological indexes of SN98A were always lower than SN98B, and the net photosynthetic rate of SN98A was 10.12% lower than SN98B on the 20th day after shade treatment. GA₃ treatments significantly reduced the barren stalk ratios in SN98A and improved its seed setting rates by increasing the net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), photosynthetic pigment contents, photochemical efficiency of photosystem II (PS II) (Fv/Fm), photochemical quenching coefficient (qP), effective quantum yield of PSII photochemistry (Φ_PSII), and antioxidant enzyme activities, where the most effective treatment was 60 mg L^-1GA₃. Compared with CK group, the seed setting rate increased by 33.87%. GA₃ treatment also regulated the metabolism of reactive oxygen species (ROS) and reduced the superoxide anion ( O 2 - ) production rate, H₂O₂ content, and malondialdehyde content. The superoxide anion ( O 2 - ) production rate, H₂O₂ content and malondialdehyde content of SN98A sprayed with 60 mg L^-1 GA₃ decreased by 17.32%,10.44% and 50.33% compared with CK group, respectively. Compared with the control, GA₃ treatment significantly (P < 0.05) increased the expression levels of APX and GR in SN98A, and APX, Fe-SOD, and GR in SN98B. Weak light stress decreased the expression of GA20ox2, which was related to gibberellin synthesis, and the endogenous gibberellin synthesis of SN98A. Weak light stress accelerated leaf senescence, and exogenous GA₃ application inhibited the ROS levels in the leaves and maintained normal physiological functions in the leaves. These results indicate that exogenous GA₃ enhances the adaptability of plants to low light stress by regulating photosynthesis, ROS metabolism and protection mechanisms, as well as the expression of key genes, which may be an economical and environmentally friendly method to solve the low light stress problem in maize production.

Brassinosteroids and metalloids: Regulation of plant biology

Metalloid contamination in the environment is one of the serious concerns posing threat to our ecosystems. Excess of metalloid concentrations (including antimony, arsenic, boron, selenium etc.) in soil results in their over accumulation in plant tissues, which ultimately causes phytotoxicity and their bio-magnification. So, it is very important to find some ecofriendly approaches to counter negative impacts of above mentioned metalloids on plant system. Brassinosteroids (BRs) belong to family of plant steroidal hormones, and are considered as one of the ecofriendly way to counter metalloid phytotoxicity. This phytohormone regulates the plant biology in presence of metalloids by modulating various key biological processes like cell signaling, primary and secondary metabolism, bio-molecule crosstalk and redox homeostasis. The present review explains the in-depth mechanisms of BR regulated plant responses in presence of metalloids, and provides some biotechnological aspects towards ecofriendly management of metalloid contamination.

Molecular Interaction and Evolution of Jasmonate Signaling With Transport and Detoxification of Heavy Metals and Metalloids in Plants

An increase in environmental pollution resulting from toxic heavy metals and metalloids [e.g., cadmium (Cd), arsenic (As), and lead (Pb)] causes serious health risks to humans and animals. Mitigation strategies need to be developed to reduce the accumulation of the toxic elements in plant-derived foods. Natural and genetically-engineered plants with hyper-tolerant and hyper-accumulating capacity of toxic minerals are valuable for phytoremediation. However, the molecular mechanisms of detoxification and accumulation in plants have only been demonstrated in very few plant species such as Arabidopsis and rice. Here, we review the physiological and molecular aspects of jasmonic acid and the jasmonate derivatives (JAs) in response to toxic heavy metals and metalloids. Jasmonates have been identified in, limiting the accumulation and enhancing the tolerance to the toxic elements, by coordinating the ion transport system, the activity of antioxidant enzymes, and the chelating capacity in plants. We also propose the potential involvement of Ca2+ signaling in the stress-induced production of jasmonates. Comparative transcriptomics analyses using the public datasets reveal the key gene families involved in the JA-responsive routes. Furthermore, we show that JAs may function as a fundamental phytohormone that protects plants from heavy metals and metalloids as demonstrated by the evolutionary conservation and diversity of these gene families in a large number of species of the major green plant lineages. Using ATP-Binding Cassette G (ABCG) transporter subfamily of six representative green plant species, we propose that JA transporters in Subgroup 4 of ABCGs may also have roles in heavy metal detoxification. Our paper may provide guidance toward the selection and development of suitable plant and crop species that are tolerant to toxic heavy metals and metalloids.

Priming Strategies for Benefiting Plant Performance under Toxic Trace Metal Exposure

Combating environmental stress related to the presence of toxic elements is one of the most important challenges in plant production. The majority of plant species suffer from developmental abnormalities caused by an exposure to toxic concentrations of metals and metalloids, mainly Al, As, Cd, Cu, Hg, Ni, Pb, and Zn. However, defense mechanisms are activated with diverse intensity and efficiency. Enhancement of defense potential can be achieved though exogenously applied treatments, resulting in a higher capability of surviving and developing under stress and become, at least temporarily, tolerant to stress factors. In this review, I present several already recognized as well as novel methods of the priming process called priming, resulting in the so-called “primed state” of the plant organism. Primed plants have a higher capability of surviving and developing under stress, and become, at least temporarily, tolerant to stress factors. In this review, several already recognized as well as novel methods of priming plants towards tolerance to metallic stress are discussed, with attention paid to similarities in priming mechanisms activated by the most versatile priming agents. This knowledge could contribute to the development of priming mixtures to counteract negative effects of multi-metallic and multi-abiotic stresses. Presentation of mechanisms is complemented with information on the genes regulated by priming towards metallic stress tolerance. Novel compounds and techniques that can be exploited in priming experiments are also summarized.

Photosynthetic apparatus efficiency, phenolic acid profiling and pattern of chosen phytohormones in pseudometallophyte Alyssum montanum

The present study investigated the response of non-metallicolous (NM) and metallicolous (M) Alyssum montanum shoots cultured in vitro on a medium supplemented simultaneously with heavy metals (HMs) to identify mechanisms involved in alleviating metal-induced damage. Plant status in respect to photosynthetic apparatus efficiency was determined and linked with changes in biochemical composition of shoots, namely phenolic acids' and stress-related phytohormones. Results showed the considerable inter-ecotype differences in (1) the photosynthetic pigments' amount, (2) the functioning of membrane electron transporters as well as (3) the linear and alternative electron transport pathways, whose lower values were reported in NM than in M HM-treated culture. Photosynthetic apparatus protection in M specimens was assured by the activation of cinnamic acid synthesis (by phenylalanine ammonia lyase) and its further transformations to benzoic acid derivatives with high ability to counteract oxidative stress, that was accompanied by the overexpression of jasmonic acid stimulating antioxidant machinery. In turn, detrimental HM effects on NM shoots could result from the diminution of most phenolics' accumulation, and only the content of coumarate (produced by bifunctional phenylalanine/tyrosine ammonia lyase) and rosmarinic acid increased. All these together with an enhanced concentration of abscisic acid might suggest that NM strategy to cope with HMs is based mostly on a restriction of metal movement with transpiration flow and their limited distribution in leaves. Summarizing, our findings for the first time point out the physiological and metabolic adaptation of pseudometallophyte A. montanum to adverse conditions.

Effect of enhancers on the phytoremediation of soils polluted by pyrene and Ni using Sudan grass (Sorghum sudanense (Piper) Stapf.)

Remediation of heavy metal and polycyclic aromatic hydrocarbon (PAH)-co-contaminated soils has drawn much more attention; phytoremediation is an often-used technique. Sudan grass (Sorghum sudanense (Piper) Stapf.) with developed root system and strong PAHs and heavy metal tolerance is a potential choice for phytoremediation. In this study, the application of tea saponin (TS) (1 g kg^-1 soil) and nitrilotriacetic acid (NTA) (1 g kg^-1 soil) was to improve the removal efficiency of Ni and pyrene. TS and NTA had no obvious effects on the growth and soluble proteins of Sudan grass. Ni concentration in root was higher than that in the shoot. The addition of TS and NTA increased the Ni concentration in the root by 25.98% in Ni-contaminated treatment. Pyrene was mainly accumulated in the shoot of Sudan grass. Pyrene concentration in shoot increased by 20.14% with TS-NTA in pyrene-contaminated treatment and increased by 31.97% in Ni-contaminated treatment. TS and NTA had significantly improved dissolved organic matter and soil microbial activity. Microbial activity increased by 16.75%, 18.07%, and 23.364% in pyrene-contaminated, Ni-contaminated, and pyrene and Ni-co-contaminated treatment, respectively. This study showed that phytoremediation of pyrene and Ni-co-contaminated soil by Sudan grass could be enhanced by the application of TS-NTA and the interaction between pyrene and Ni impacted the accumulation of Ni and pyrene in Sudan grass.

Salicylic acid enhances nickel stress tolerance by up-regulating antioxidant defense and glyoxalase systems in mustard plants

The present study identified inverse relationships between nickel (Ni) levels and growth, photosynthesis and physio-biochemical attributes, but increasing levels of Ni stress enhanced methylglyoxal, electrolyte leakage, hydrogen peroxide, and lipid peroxidation content. Exogenous application of salicylic acid (SA) (10^-5 M) ameliorated the ill-effects of Ni by restoring growth, photosynthesis and physio-biochemical attributes and increasing the activities of enzymes associated with antioxidant systems, especially the ascorbate-glutathione (AsA-GSH) cycle and glyoxalase system. In addition, SA application to Ni-stressed plants had an additive effect on the activities of the ascorbate and glutathione pools, and the AsA-GSH cycle enzymes (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase), superoxide dismutase, catalase, glutathione S-transferase, and osmolyte biosynthesis). This trend also follows in glyoxalase system viz. glyoxalase I and glyoxalase II enzymes. Nevertheless, exogenous SA supplementation restored mineral nutrient contents. Principal component analysis showed that growth, photosynthesis, and mineral nutrient parameters were positively correlated with each other and negatively correlated with antioxidant enzymes and oxidative stress biomarkers. Hence, SA is an alternative compound with potential application in the phytoremediation of Ni.