Reduced nodulation in alfalfa induced by arsenic correlates with altered expression of early nodulins

Literature Review on the Effects of Heavy Metal Stress and Alleviating Possibilities through Exogenously Applied Agents in Alfalfa (Medicago sativa L.)

Heavy metals (HMs) are among the most important toxic agents since they reach the soil through various routes and accumulate in the food chain. Therefore, HMs induce problems in soil integrity and in plant, animal, and human health. Alfalfa (Medicago sativa L.) is a significant crop worldwide, utilized in animal production. Furthermore, because of its nitrogen-absorbing ability via symbiotic strains of bacteria, it increases soil productivity. However, there are relatively few studies investigating the effects of HMs and their alleviation possibilities on alfalfa plants. Therefore, the goal of this review is to clarify the current state of research into HM-induced alterations in alfalfa and to determine the extent to which externally applied microorganisms and chemical compounds can mitigate the negative effects. The aim is to indicate areas of development towards further understanding of HM detoxification in alfalfa and to identify future research directions.

Zero valent iron nanoparticles and organic fertilizer assisted phytoremediation in a mining soil: Arsenic and mercury accumulation and effects on the antioxidative system of Medicago sativa L

Zero valent iron nanoparticles (nZVI) attract interest given their effectiveness in soil remediation. However, little attention has been given to their impacts on plants. Likewise, although fertilizers are commonly used to enhance phytoremediation, their effects on As mobilization, resulting in potential toxic effects, require further study. In this context, we examined the impact of As and Hg accumulation on the antioxidative system of Medicago sativa grown in a soil amended with organic fertilizer and/or nZVI. The experiment consisted of 60 pots. Plants were pre-grown and transferred to pots, which were withdrawn along time for monitoring purposes. As and Hg were monitored in the soil-plant system, and parameters related to oxidative stress, photosynthetic pigments, and non-protein thiol compounds (NPTs) were measured. In general, the application of nZVI immobilized As in soil and increased Hg accumulation in the plant, although it surprisingly decreased oxidative stress. Plants in nZVI-treated soil also showed an increase in NPT content in roots. In contrast, the application of the fertilizer mobilized As, thereby improving bioaccumulation factors. However, when combining fertilizer with nZVI, the As accumulation is mitigated. This observation reveals that simultaneous amendments are a promising approach for soil stabilization and the phytomanagement of As/Hg-polluted soils.

Leveraging arsenic resistant plant growth-promoting rhizobacteria for arsenic abatement in crops

Arsenic is a toxic metalloid categorized under class 1 carcinogen and is detrimental to both plants and animals. Agricultural land in several countries is contaminated with arsenic, resulting in its accumulation in food grains. Increasing global food demand has made it essential to explore neglected lands like arsenic-contaminated lands for crop production. This has posed a severe threat to both food safety and security. Exploration of arsenic-resistant plant growth-promoting rhizobacteria (PGPR) is an environment-friendly approach that holds promise for both plant growth promotion and arsenic amelioration in food grains. However, their real-time performance is dependent upon several biotic and abiotic factors. Therefore, a detailed analysis of associated mechanisms and constraints becomes inevitable to explore the full potential of available arsenic-resistant PGPR germplasm. Authors in this review have highlighted the role and constraints of arsenic-resistant PGPR in reducing the arsenic toxicity in food crops, besides providing the details of arsenic transport in food grains.

Exploring apoplast reorganization in the nodules of Lotus corniculatus L. growing on old Zn-Pb calamine wastes

Nodulation and symbiotic nitrogen fixation are important factors that determine legume growth. A pot experiment was carried out to determine the effects of Zn-Pb contamination on nodule apoplast (cell walls and intercellular spaces) of bird's foot trefoil (Lotus corniculatus L.) that spontaneously colonized old calamine wastes. The plants were grown in pots filled with sterile calamine substrate (M, metal treated) or expanded clay (NM, untreated) and inoculated with calamine-derived Lotus-nodulating Bradyrhizobium liaoningense. Apoplast reorganization in the nodules was examined using specific dyes for cellulose, pectin and lignin detection, and immuno-histochemical techniques based on monoclonal antibodies against xyloglucan (Lm25), pectins (Jim5 and Jim7), and structural proteins (arabinogalactan protein - Lm14 and extensin - Jim12). Microscopic analysis of metal-treated nodules revealed changes in the apoplast structure and composition of nodule cortex tissues and infected cells. Wall thickening was accompanied by intensified deposition of cellulose, xyloglucan, esterified pectin, arabinogalactan protein and extensin. The metal presence redirected also lignin and suberin deposition in the walls of the nodule cortex tissues. Our results showed reorganization of the apoplast of cortex tissues and infected cells of Lotus nodules under Zn-Pb presence. These changes in the apoplast structure and composition may have created actual barriers for the toxic ions. For this reason, they can be regarded as an element of legume defense strategy against metal stress that enables effective functioning of L. corniculatus-rhizobia symbiosis on Zn-Pb polluted calamine tailings.

Unraveling the role of plant growth-promoting rhizobacteria in the alleviation of arsenic phytotoxicity: A review

The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant's inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic-resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.

Assessing the Plant Growth Promoting and Arsenic Tolerance Potential of Bradyrhizobium japonicum CB1809

Accumulation of heavy metals in soil is of concern to the agricultural production sector, because of the potential threat to food quality and quantity. Inoculation with plant growth-promoting bacteria (PGPR) has previously been shown to alleviate heavy metal stress but the mechanisms are unclear. Potential mechanisms by which inoculation with Bradyrhizobium japonicum CB1809 affected the legume soybean (Glycine max cv. Zeus) and the non-legume sunflower (Helianthus annus cv. Hyoleic 41) were investigated in solution culture under 5 μM As stress. Adding As resulted in As tissue concentrations of up to 5 mg kg^-1 (shoots) and 250 mg kg^-1 (roots) in both species but did not reduce shoot or root biomass. Inoculation increased root biomass but only in the legume (soybean) and only with As. Inoculation resulted in large (up to 100%) increases in siderophore concentration but relatively small changes (±10-15%) in auxin concentration in the rhizosphere. However, the increase in siderophore concentration in the rhizosphere did not result in the expected increases in tissue N or Fe, especially in soybean, suggesting that their function was different. In conclusion, siderophores and auxins may be some of the mechanisms by which both soybean and sunflower maintained plant growth in As-contaminated media.

Fulvic acid increases forage legume growth inducing preferential up-regulation of nodulation and signalling-related genes

The use of potential biostimulants is of broad interest in plant science for improving yields. The application of a humic derivative called fulvic acid (FA) may improve forage crop production. FA is an uncharacterized mixture of chemicals and, although it has been reported to increase growth parameters in many species including legumes, its mode of action remains unclear. Previous studies of the action of FA have lacked appropriate controls, and few have included field trials. Here we report yield increases due to FA application in three European Medicago sativa cultivars, in studies which include the appropriate nutritional controls which hitherto have not been used. No significant growth stimulation was seen after FA treatment in grass species in this study at the treatment rate tested. Direct application to bacteria increased Rhizobium growth and, in M. sativa trials, root nodulation was stimulated. RNA transcriptional analysis of FA-treated plants revealed up-regulation of many important early nodulation signalling genes after only 3 d. Experiments in plate, glasshouse, and field environments showed yield increases, providing substantial evidence for the use of FA to benefit M. sativa forage production.

Structural Adaptation and Physiological Mechanisms in the Leaves of Anthyllis vulneraria L. from Metallicolous and Non-Metallicolous Populations

Calamine wastes highly contaminated with trace metals (TMs) are spontaneously inhabited by a legume plant Anthyllis vulneraria L. This study determined an adaptation strategy of metallicolous (M) A. vulneraria and compared it with that of the non-metallicolous (NM) ecotype. We hypothesized that TMs may lead to (i) leaf apoplast modifications and (ii) changes in the antioxidant machinery efficiency that facilitate plant growth under severe contamination. To verify our hypothesis, we implemented immunolabelling, transmission electron microscopy and biochemical measurements. NM leaves were larger and thicker compared to the M ecotype. Microscopic analysis of M leaves showed a lack of dysfunctions in mesophyll cells exposed to TMs. However, changes in apoplast composition and thickening of the mesophyll and epidermal cell walls in these plants were observed. Thick walls were abundant in xyloglucan, pectins, arabinan, arabinogalactan protein and extensin. The tested ecotypes differed also in their physiological responses. The metallicolous ecotype featured greater accumulation of photosynthetic pigments, enhanced activity of superoxide dismutase and increased content of specific phenol groups in comparison with the NM one. Despite this, radical scavenging activity at the level of 20% was similar in M and NM ecotypes, which may implicate effective reduction of oxidative stress in M plants. In summary, our results confirmed hypotheses and suggest that TMs induced cell wall modifications of leaves, which may play a role in metal stress avoidance in Anthyllis species. However, when TMs reach the protoplast, activation of antioxidant machinery may significantly strengthen the status of plants naturally growing in TM-polluted environment.

Impact of double inoculation with Bradyrhizobium japonicum E109 and Azospirillum brasilense Az39 on soybean plants grown under arsenic stress

Inoculation practice with plant growth-promoting bacteria (PGPB) has been proposed as a good biotechnological tool to enhance plant performance and alleviate heavy metal/metalloid stress. Soybean is often cultivated in soil with high arsenic (As) content or irrigated with As-contaminated groundwater, which causes deleterious effects on its growth and yield, even when it was inoculated with rhizobium. Thus, the effect of double inoculation with known PGPB strains, Bradyrhizobium japonicum E109 and Azospirillum brasilense Az39 was evaluated in plants grown in pots under controlled conditions and treated with As. First, the viability of these co-cultivated bacteria was assayed using a flow cytometry analysis using SYTO9 and propidium iodide (PI) dyes. This was performed in vitro to evaluate the bacterial population dynamic under 25 μM AsV and AsIII treatment. A synergistic effect was observed when bacteria were co-cultured, since mortality diminished, compared to each growing alone. Indole acetic acid (IAA) produced by A. brasilense Az39 would be one of the main components involved in B. japonicum E109 mortality reduction, mainly under AsIII treatment. Regarding in vivo assays, under As stress, plant growth improvement, nodule number and N content increase were observed in double inoculated plants. Furthermore, double inoculation strategy reduced As translocation to aerial parts thus improving As phytostabilization potential of soybean plants. These results suggest that double inoculation with B. japonicum E109 and A. brasilense Az39 could be a safe and advantageous practice to improve growth and yield of soybean exposed to As, accompanied by an important metalloid phytostabilization.

Metal resistant rhizobia and ultrastructure of Anthyllis vulneraria nodules from zinc and lead contaminated tailing in Poland

This present paper studies the response of Anthyllis vulneraria-Rhizobium symbiosis to heavy metal stress. The symbiotic rhizobium bacteria isolated from root nodules of A. vulneraria from zinc and lead wastes were examined in this project. Light microscopy (LM) and transmission electron microscopy (TEM) were used to analyze the nodule anatomy and ultrastructure and conduct a comparison with nonmetal-treated nodules. 16S ribosomal DNA sequence analysis of bacteria isolated from metal-treated nodules revealed the presence of Rhizobium metallidurans and Bradyrhizobium sp. In regard to heavy metal resistance/tolerance, a similar tolerance to Pb was shown by both strains, and a high tolerance to Zn and a lower tolerance to Cd and Cu by R. metallidurans, whereas a high tolerance to Cd and Cu and a lower tolerance to Zn by Bradyrhizobium were found. The nodules of Anthyllis from metal-polluted tailing sites were identified as the typical determinate type of nodules. Observed under TEM microscopy changes in nodules ultrastructure like: (1) wall thickening; (2) infection thread reduction; (3) vacuole shrinkage; (4) synthesis of phenolics in vacuoles; (5) various differentiation of bacteroids and (6) simultaneous symbiosis with arbuscular mycorrhiza fungi could be considered as a form of the A.vulneraria-Rhizobium symbiosis adaptation to metal stress.

Effect of combined microbes on plant tolerance to Zn-Pb contaminations

The presence and composition of soil microbial communities has been shown to have a large impact on plant-plant interactions and consequently plant diversity and composition. The goal of the present study was to evaluate impact of arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria, which constitutes an essential link between the soil and the plant's roots. A greenhouse pot experiment was conducted to evaluate the feasibility of using selected microbes to improve Hieracium pilosella and Medicago sativa growth on Zn-Pb-rich site. Results of studies revealed that biomass, the dry mass of shoots and roots, increased significantly when plants were inoculated with mycorrhizal fungi and nitrogen-fixing bacteria. The addition of Azospirillum sp. and Nostoc edaphicum without mycorrhiza suppressed plant growth. Single bacterial inoculation alone does not have a positive effect on M. sativa growth, while co-inoculation with AMF improved plant growth. Plant vitality (expressed by the performance index) was improved by the addition of microbes. However, our results indicated that even dry heat sterilization of the substratum created imbalanced relationships between soil-plant and plants and associated microorganisms. The studies indicated that AMF and N2-fixers can improve revegetation of heavy metal-rich industrial sites, if the selection of interacting symbionts is properly conducted.

Hydrogen sulfide alleviates toxic effects of arsenate in pea seedlings through up-regulation of the ascorbate-glutathione cycle: Possible involvement of nitric oxide

In plants, hydrogen sulfide (H2S) is an emerging novel signaling molecule that is involved in growth regulation and abiotic stress responses. However, little is known about its role in the regulation of arsenate (As(V)) toxicity. Therefore, hydroponic experiments were conducted to investigate whether sodium hydrosulfide (NaHS; a source of H2S) is involved in the regulation of As(V) toxicity in pea seedlings. Results showed that As(V) caused decreases in growth, photosynthesis (measured as chlorophyll fluorescence) and nitrogen content, which was accompanied by the accumulation of As. As(V) treatment also reduced the activities of cysteine desulfhydrase and nitrate reductase, and contents of H2S and nitric oxide (NO). However, addition of NaHS ameliorated As(V) toxicity in pea seedlings, which coincided with the increased contents of H2S and NO. The cysteine level was higher under As(V) treatment in comparison to all other treatments (As-free; NaHS; As(V)+NaHS). The content of reactive oxygen species (ROS) and damage to lipids, proteins and membranes increased by As(V) while NaHS alleviated these effects. Enzymes of the ascorbate-glutathione cycle (AsA-GSH cycle) showed inhibition of their activities following As(V) treatment while their activities were increased by application of NaHS. The redox status of ascorbate and glutathione was disturbed by As(V) as indicated by a steep decline in their reduced/oxidized ratios. However, simultaneous NaHS application restored the redox status of the ascorbate and glutathione pools. The results of this study demonstrated that H2S and NO might both be involved in reducing the accumulation of As and triggering up-regulation of the AsA-GSH cycle to counterbalance ROS-mediated damage to macromolecules. Furthermore, the results suggest a crucial role of H2S in plant priming, and in particular for pea seedlings in mitigating As(V) stress.

Improving legume nodulation and Cu rhizostabilization using a genetically modified rhizobia

The rhizobia-legume interaction has been proposed as an interesting and appropriate tool for rhizostabilization of soils contaminated with heavy metals. One of the main requirements to use this symbiosis is the availability of tolerant and symbiotically effective rhizobia. The aim of this work was to improve the symbiotic properties of the arsenic-resistant wild-type strain Ensifer medicae MA11 in Cu-contaminated substrates. The copAB genes from a Cu-resistant Pseudomonas fluorescens strain were expressed in E. medicae MA11 under the control of the nifH promoter. The resulting strain E. medicae MA11-copAB was able to alleviate the toxic effect of Cu in Medicago truncatula. At 300 µM Cu, root and shoot dry matter production, nitrogen content, number of nodules and photosynthetic rate were significantly reduced in plants inoculated with the wild-type strain. However, these parameters were not altered in plants inoculated with the genetically modified strain. Moreover, nodules elicited by this strain were able to accumulate twofold the Cu measured in nodules formed by the wild-type strain. In addition, the engineered E. medicae strain increased Cu accumulation in roots and decreased the content in shoots. Thus, E. medicae MA11-copAB increased the capacity of M. truncatula to rhizostabilize Cu, decreasing the translocation factor and avoiding metal entry into the food chain. The plasmid containing the nifH promoter-copAB construct could be a useful biotool for Cu rhizostabilization using legumes, since it can be transferred to different rhizobia microsymbionts of authoctonous legumes growing in Cu-contaminated soils.

Unraveling the effect of arsenic on the model Medicago-Ensifer interaction: a transcriptomic meta-analysis

The genetic regulation underlying the effect of arsenic (As(III)) on the model symbiosis Medicago-Ensifer was investigated using a combination of physiological (split-roots), microscopy and genetic (microarrays, qRT-PCR and composite plants) tools. Nodulation was very sensitive to As(III) (median inhibitory dose (ID50) = 20 μM). The effect on root elongation and on nodulation was local (nonsystemic). A battery of stress (salt, drought, heat shock, metals, etc.)-related genes were induced. Glutathione played a pivotal role in tolerance/detoxification, together with secondary metabolites ((iso)flavonoids and phenylpropanoids). However, antioxidant enzymes were not activated. Concerning the symbiotic interaction, molecular evidence suggesting that rhizobia alleviate As stress is for the first time provided. Chalcone synthase (which is involved in the first step of the legume-rhizobia cross-talk) was strongly enhanced, suggesting that the plants are biased to establish symbiotic interactions under As(III) stress. In contrast, 13 subsequent nodulation genes (involved in nodulation factors (Nod factors) perception, infection, thread initiation and progression, and nodule morphogenesis) were repressed. Overexpression of the ethylene responsive factor ERN in composite plants reduced root stress and partially restored nodulation, whereas overexpression of the early nodulin ENOD12 enhanced nodulation both in the presence and, particularly, in the absence of As, without affecting root elongation. Several transcription factors were identified, which could be additional targets for genetic engineering aiming to improve nodulation and/or alleviate root stress induced by this toxic.

Phosphorus improves arsenic phytoremediation by Anadenanthera peregrina by alleviating induced oxidative stress

Due to similarities in their chemical behaviors, studies examining interactions between arsenic (As)--in special arsenate--and phosphorus (P) are important for better understanding arsenate uptake, toxicity, and accumulation in plants. We evaluated the effects of phosphate addition on plant biomass and on arsenate and phosphate uptake by Anadenanthera peregrina, an important Brazilian savanna legume. Plants were grown for 35 days in substrates that received combinations of 0, 10, 50, and 100 mg kg(-1) arsenate and 0, 200, and 400 mg kg(-1) phosphate. The addition of P increased the arsenic-phytoremediation capacity of A. peregrina by increasing As accumulation, while also alleviating As-induced oxidative stress. Arsenate phytotoxicity in A. peregrina is due to lipid peroxidation, but not hydrogen peroxide accumulation. Added P also increased the activity of important reactive oxygen species-scavenging enzymes (catalase and ascorbate peroxidase) that help prevent lipid peroxidation in leaves. Our findings suggest that applying P represents a feasible strategy for more efficient As phytoremediation using A. peregrina.

Engineering copper hyperaccumulation in plants by expressing a prokaryotic copC gene

In this work, engineering Cu-hyperaccumulation in plants was approached. First, the copC gene from Pseudomonas sp. Az13, encoding a periplasmic Cu-binding protein, was expressed in Arabidopsis thaliana driven by the CaMV35S promoter (transgenic lines 35S-copC). 35S-copC lines showed up to 5-fold increased Cu accumulation in roots (up to 2000 μg Cu. g(-1)) and shoots (up to 400 μg Cu. g(-1)), compared to untransformed plants, over the limits established for Cu-hyperaccumulators. 35S lines showed enhanced Cu sensitivity. Second, copC was engineered under the control of the cab1 (chlorophyll a/b binding protein 1) promoter, in order to drive copC expression to the shoots (transgenic lines cab1-copC). cab1-copC lines showed increased Cu translocation factors (twice that of wild-type plants) and also displayed enhanced Cu sensitivity. Finally, subcellular targeting the CopC protein to plant vacuoles was addressed by expressing a modified copC gene containing specific vacuole sorting determinants (transgenic lines 35S-copC-V). Unexpectedly, increased Cu-accumulation was not achieved-neither in roots nor in shoots-when compared to 35S-copC lines. Conversely, 35S-copC-V lines did display greatly enhanced Cu-hypersensitivity. Our results demonstrate the feasibility of obtaining Cu-hyperaccumulators by engineering a prokaryotic Cu-binding protein, but they highlight the difficulty of altering the exquisite Cu homeostasis in plants.

Arsenic toxicity: the effects on plant metabolism

The two forms of inorganic arsenic, arsenate (AsV) and arsenite (AsIII), are easily taken up by the cells of the plant root. Once in the cell, AsV can be readily converted to AsIII, the more toxic of the two forms. AsV and AsIII both disrupt plant metabolism, but through distinct mechanisms. AsV is a chemical analog of phosphate that can disrupt at least some phosphate-dependent aspects of metabolism. AsV can be translocated across cellular membranes by phosphate transport proteins, leading to imbalances in phosphate supply. It can compete with phosphate during phosphorylation reactions, leading to the formation of AsV adducts that are often unstable and short-lived. As an example, the formation and rapid autohydrolysis of AsV-ADP sets in place a futile cycle that uncouples photophosphorylation and oxidative phosphorylation, decreasing the ability of cells to produce ATP and carry out normal metabolism. AsIII is a dithiol reactive compound that binds to and potentially inactivates enzymes containing closely spaced cysteine residues or dithiol co-factors. Arsenic exposure generally induces the production of reactive oxygen species that can lead to the production of antioxidant metabolites and numerous enzymes involved in antioxidant defense. Oxidative carbon metabolism, amino acid and protein relationships, and nitrogen and sulfur assimilation pathways are also impacted by As exposure. Readjustment of several metabolic pathways, such as glutathione production, has been shown to lead to increased arsenic tolerance in plants. Species- and cultivar-dependent variation in arsenic sensitivity and the remodeling of metabolite pools that occurs in response to As exposure gives hope that additional metabolic pathways associated with As tolerance will be identified.