Risk mitigation of genetically modified bacteria and plants designed for bioremediation

Advances in amelioration of air pollution using plants and associated microbes: An outlook on phytoremediation and other plant-based technologies

Globally, air pollution is an unfortunate aftermath of rapid industrialization and urbanization. Although the best strategy is to prevent air pollution, it is not always feasible. This makes it imperative to devise and implement techniques that can clean the air continuously. Plants and microbes have a natural potential to transform or degrade pollutants. Hence, strategies that use this potential of living biomass to remediate air pollution seem to be promising. The simplest future trend can be planting suitable plant-microbe species capable of removing air pollutants like SO2, CO2, CO, NOX and particulate matter (PM) along roadsides and inside the buildings. Established wastewater treatment strategies such as microbial fuel cells (MFC) and constructed wetlands (CW) can be suitably modified to ameliorate air pollution. Green architecture involving green walls and green roofs is facile and aesthetic, providing urban ecosystem services. Certain microbe-based bioreactors such as bioscrubbers and biofilters may be useful in small confined spaces. Several generative models have been developed to assist with planning and managing green spaces in urban locales. The physiological limitations of using living organisms can be circumvent by applying biotechnology and transgenics to improve their potential. This review provides a comprehensive update on not just the plants and associated microbes for the mitigation of air pollution, but also lists the technologies that are available and/or can be modified and used for air pollution control. The article also gives a detailed analysis of this topic in the form of strengths-weaknesses-opportunities-challenges (SWOC). The strategies mentioned in this review would help to attain corporate Environmental Social and Governance (ESG) and Sustainable Development Goals (SDGs), while reducing carbon footprint in the urban scenario. The review aims to emphasise that urbanization is possible while tackling air pollution using facile, green techniques involving plants and associated microbes.

eCell Technology for Cell-Free Protein Synthesis, Biosensing, and Remediation

The eCell technology is a recently introduced, specialized protein production platform with uses in a multitude of biotechnological applications. This chapter summarizes the use of eCell technology in four selected application areas. Firstly, for detecting heavy metal ions, specifically mercury, in an in vitro protein expression system. Results show improved sensitivity and lower limit of detection compared to comparable in vivo systems. Secondly, eCells are semipermeable, stable, and can be stored for extended periods of time, making them a portable and accessible technology for bioremediation of toxicants in extreme environments. Thirdly and fourthly, applications of eCell technology are shown to facilitate expression of correctly folded disulfide-rich proteins and incorporate chemically interesting derivatives of amino acids into proteins which are toxic to in vivo protein expression. Overall, eCell technology presents a cost-effective and efficient method for biosensing, bioremediation, and protein production.

Oral toxicity evaluation of genetically modified lactic acid bacteria in three generations of Sprague Dawley rats

Genetic modification has great advantages in improving performance of bacteria, but its oral safety has not been systematically evaluated. In this study, the toxicity including the reproductive toxicity of two genetically modified bacteria engineered using food-grade vectors on three generations of rats (F0, F1 and F2) were studied. Sprague Dawley rats were administrated by gavage with corresponding parent and genetically modified Lactobacillus plantarum and Lactobacillus delbrueckii at 2.5 × 10 ⁹ CFU every other day for 8 weeks. Results showed that the transgenic Lactobacillus had no significant toxicity to the body weight, food intake or blood biochemical parameters of three generations of rats. There was no significant effect on the reproductive parameters (gestational weight and pregnancy time) and related hormones (FSH, LH, estradiol, progesterone and testosterone) of the parent rats (F0 and F1). Consistently, their offsprings had no abnormal physical conditions including body weight, body length and anogenital distances. Moreover, their pups had normal body organ weight and reasonable abnormal sperm rate. Further analyses were conducted to evaluate SOD, MDA and GPX of the ovarium in the F1 and F2 female rats, which showed no malady phenotypes. In conclusion, transgenic Lactobacillus has no toxicity to the three generations of rats.

Recent Trends in Microbial Approaches for Soil Desalination

Soil salinization has become a major problem for agriculture worldwide, especially because this phenomenon is continuously expanding in different regions of the world. Salinity is a complex mechanism, and in the soil ecosystem, it affects both microorganisms and plants, some of which have developed efficient strategies to alleviate salt stress conditions. Currently, various methods can be used to reduce the negative effects of this problem. However, the use of biological methods, such as plant-growth-promoting bacteria (PGPB), phytoremediation, and amendment, seems to be very advantageous and promising as a remedy for sustainable and ecological agriculture. Other approaches aim to combine different techniques, as well as the utilization of genetic engineering methods. These techniques alone or combined can effectively contribute to the development of sustainable and eco-friendly agriculture.

Phytoremediation of multiple persistent pollutants co-contaminated soil by HhSSB transformed plant

The high toxicity of persistent pollutants limits the phytoremediation of pollutants-contaminated soil. In this study, heterologous expressing Halorhodospira halophila single-stranded DNA binding protein gene (HhSSB) improves tolerance to 2,4,6-trinitrotoluene (TNT), 2,4,6-trichlorophenol (2,4,6-TCP), and thiocyanate (SCN^-) in A. thaliana and tall fescue (Festuca arundinacea). The HhSSB transformed Arabidopsis, and tall fescue also exhibited enhanced phytoremediation of TNT, 2,4,6-TCP, and SCN^- separately contaminated soil and co-contaminated soil compared to control plants. TNT assay was selected to explore the mechanism of how HhSSB enhances the phytoremediation of persistent pollutants. Our result indicates that HhSSB enhances the phytoremediation of TNT by enhancing the transformation of TNT in Arabidopsis. Moreover, transcriptomics and comet analysis revealed that HhSSB improves TNT tolerance through three pathways: strengthening the defense system, enhancing the ROS scavenging system, and reducing DNA damage. These results presented here would be particularly useful for further studies in the remediation of soil contaminated by organic and inorganic pollutants.

Lead toxicity in plants: Impacts and remediation

Lead (Pb) is the second most toxic heavy metal after arsenic (As), which has no role in biological systems. Pb toxicity causes a range of damages to plants from germination to yield formation; however, its toxicity is both time and concentration dependent. Its exposure at higher rates disturbs the plant water and nutritional relations and causes oxidative damages to plants. Reduced rate of seed germination and plant growth under stress is mainly due to Pb interference with enzymatic activities, membrane damage and stomatal closure because of induction of absicic acid and negative correlation of Pb with potassium in plants. Pb induced structural changes in photosynthetic apparatus and reduced biosynthesis of chlorophyll pigments cause retardation of carbon metabolism. In this review, the noxious effects of Pb on germination, stand establishment, growth, water relations, nutrient uptake and assimilation, ultra-structural and oxidative damages, carbon metabolism and enzymatic activities in plants are reported. The Pb dynamics in soil rhizosphere and role of remediation strategies i.e. physical, chemical and biological to decontaminate the Pb polluted soils has also been described. Among them, biological strategies, including phytoremediation, microbe-assisted remediation and remediation by organic amendments, are cost effective and environmentally sound remedies for cleaning Pb contaminated soils. Use of organic manures and some agricultural practices have the potential to harvest better crops yield of good quality form Pb contaminated soils.

Microbe and plant assisted-remediation of organic xenobiotics and its enhancement by genetically modified organisms and recombinant technology: A review

Environmental problems such as the deterioration of groundwater quality, soil degradation and various threats to human, animal and ecosystem health are closely related to the presence of high concentrations of organic xenobiotics in the environment. Employing appropriate technologies to remediate contaminated soils is crucial due to the site-specificity of most remediation methods. The limitations of conventional remediation technologies include poor environmental compatibility, high cost of implementation and poor public acceptability. This raises the call to employ biological methods for remediation. Bioremediation and microbe-assisted bioremediation (phytoremediation) offer many ecological and cost-associated benefits. The overall efficiency and performance of bio- and phytoremediation approaches can be enhanced by genetically modified microbes and plants. Moreover, phytoremediation can also be stimulated by suitable plant-microbe partnerships, i.e. plant-endophytic or plant-rhizospheric associations. Synergistic interactions between recombinant bacteria and genetically modified plants can further enhance the restoration of environments impacted by organic pollutants. Nevertheless, releasing genetically modified microbes and plants into the environment does pose potential risks. These can be minimized by adopting environmental biotechnological techniques and guidelines provided by environmental protection agencies and other regulatory frameworks. The current contribution provides a comprehensive overview on enhanced bioremediation and phytoremediation approaches using transgenic plants and microbes. It also sheds light on the mitigation of associated environmental risks.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

Plasmids as Tools for Containment

Active containment systems are a major tool for reducing the uncertainty associated with the introduction of monocultures, genetically engineered or not, into target habitats for a large number of biotechnological applications (e.g., bioremediation, bioleaching, biopesticides, biofuels, biotransformations, live vaccines, etc.). While biological containment reduces the survival of the introduced organism outside the target habitat and/or upon completion of the projected task, gene containment strategies reduce the lateral spread of the key genetic determinants to indigenous microorganisms. In fundamental research, suicide circuits become relevant tools to address the role of gene transfer, mainly plasmid transfer, in evolution and how this transfer contributes to genome plasticity and to the rapid adaptation of microbial communities to environmental changes. Many lethal functions and regulatory circuits have been used and combined to design efficient containment systems. As many new genomes are being sequenced, novel lethal genes and regulatory elements are available, e.g., new toxin-antitoxin modules, and they could be used to increase further the current containment efficiencies and to expand containment to other organisms. Although the current containment systems can increase the predictability of genetically modified organisms in the environment, containment will never be absolute, due to the existence of mutations that lead to the appearance of surviving subpopulations. In this sense, orthogonal systems (xenobiology) appear to be the solution for setting a functional genetic firewall that will allow absolute containment of recombinant organisms.

Integrated phytobial remediation for sustainable management of arsenic in soil and water

Arsenic (As), cited as the most hazardous substance by the U.S. Agency for Toxic Substance and Disease Registry (ATSDR, 2005), is an ubiquitous metalloid which when ingested for prolonged periods cause extensive health effects leading to ultimate untimely death. Plants and microbes can help mitigate soil and groundwater As problem since they have evolved elaborate detoxification machineries against this toxic metalloid as a result of their coexistence with this since the origin of life on earth. Utilization of the phytoremediation and bioremediation potential of the plants and microbes, respectively, is now regarded as two innovative tools that encompass biology, geology, biotechnology and allied sciences with cutting edge applications for sustainable mitigation of As epidemic. Discovery of As hyperaccumulating plants that uptake and concentrate large amounts of this toxic metalloid in their shoots or roots offered new hope to As phytoremediation, solar power based nature's own green remediation. This review focuses on how phytoremediation and bioremediation can be merged together to form an integrated phytobial remediation which could synergistically achieve the goal of large scale removal of As from soil, sediment and groundwater and overcome the drawbacks of the either processes alone. The review also points to the feasibility of the introduction of transgenic plants and microbes that bring new hope for more efficient treatment of As. The review identifies one critical research gap on the importance of remediation of As contaminated groundwater not only for drinking purpose but also for irrigation purpose and stresses that more research should be conducted on the use of constructed wetland, one of the most suitable areas of application of phytobial remediation. Finally the review has narrowed down on different phytoinvestigation and phytodisposal methods, which constitute the most essential and the most difficult part of pilot scale and field scale applications of phytoremediation programs.

Genetically modified microorganism Spingomonas paucimobilis UT26 for simultaneously degradation of methyl-parathion and γ-hexachlorocyclohexane

Bioremediation of pesticide residues by bacteria is an efficient and environmentally friendly method to deal with environmental pollution. In this study, a genetically modified microorganism (GMM) named UT26XEGM was constructed by introducing a parathion hydrolase gene into an initially γ-hexachlorocyclohexane (γ-HCH) degrading bacterium Spingomonas paucimobilis UT26. In order to reduce its potential risk of gene escaping into the environment for the public concern on biosafety, a suicide system was also designed that did not interfere with the performance of the GMM until its physiological function was activated by specific signal. The system was designed with circuiting suicide cassettes consisting of killing genes gef and ecoRIR from Escherichia coli controlled by Pm promoter and the xylS gene. The cell viability and original degradation characteristics were not affected by the insertion of exogenous genes. The novel GMM was capable of degrading methyl-parathion and γ-HCH simultaneously. In laboratory scale testing, the recombinant bacteria were successfully applied to the bioremediation of mixed pesticide residues with the activity of self-destruction after 3-methylbenzoate induction.

Enhanced phytoremediation of toxic metals by inoculating endophytic Enterobacter sp. CBSB1 expressing bifunctional glutathione synthase

To engineer plant-bacteria symbionts for remediating complex sites contaminated with multiple metals, the bifunctional glutathione (GSH) synthase gene gcsgs was introduced into endophytic Enterobacter sp. CBSB1 to improve phytoremediation efficiency of host plant Brassica juncea. The GSH contents of shoots inoculated with CBSB1 is 0.4μMg(-1) fresh weight. However, the GSH concentration of shoots with engineered CBSB1-GCSGS increased to 0.7μMg(-1) fresh weight. The shoot length, fresh weight and dry weight of seedlings inoculated with CBSB1-GCSGS increased 67%, 123%, and 160%, compared with seedlings without inoculation, respectively. The Cd and Pb concentration in shoots with CBSB1-GCSGS increased 48% and 59% compared with seedlings without inoculation, respectively. The inoculation of CBSB1 and CBSB1-GCSGS could increase the Cd and Pb extraction amounts of seedlings significantly compared with those without inoculation (P<0.05), the seedlings inoculated with CBSB1-GCSGS showed the highest Cd and Pb extraction amounts. It was concluded that the gcsgs gene introduced into Enterobacter sp. CBSB1 upgraded the phytoremediation efficacy of B. juncea. So the engineered Enterobacter sp. CBSB1-GCSGS showed potentials in remediation sites contaminated with complex contaminants by inoculating into remediating plants.

Improved detection of microbial risk of releasing genetically modified bacteria in soil by using massive sequencing and antibiotic resistance selection

High-throughput 16S rRNA gene-targeted pyrosequencing was used with commonly used risk assessment techniques to evaluate the potential microbial risk in soil after inoculating genetically modified (GM) Corynebacterium glutamicum. To verify the risk, reference experiments were conducted in parallel using well-defined and frequently used GM Escherichia coli and wild-type strains. The viable cell count showed that the number of GM bacteria in the soil was reduced to below the detection limit within 10 days, while the molecular indicator for GM plasmids was detected throughout the experiment by using quantitative real-time polymerase chain reactions. Subsequent pyrosequencing showed an insignificant influence of the GM bacteria and/or their GM plasmids on the structure of the soil bacterial community this was similar to non-GM wild-type strains. However, pyrosequencing combined with kanamycin-resistant bacteria selection uncovered a potential risk of GM bacteria on the soil bacterial community and pathogens. The results of the improved methodology showed that the microbial risk attributable to GM C. glutamicum was relatively lower than that attributable to the reference GM E. coli.

Glyphosate degradation in glyphosate-resistant and -susceptible crops and weeds

High levels of aminomethylphosphonic acid (AMPA), the main glyphosate metabolite, have been found in glyphosate-treated, glyphosate-resistant (GR) soybean, apparently due to plant glyphosate oxidoreductase (GOX)-like activity. AMPA is mildly phytotoxic, and under some conditions the AMPA accumulating in GR soybean correlates with glyphosate-caused phytotoxicity. A bacterial GOX is used in GR canola, and an altered bacterial glyphosate N-acetyltransferase is planned for a new generation of GR crops. In some weed species, glyphosate degradation could contribute to natural resistance. Neither an isolated plant GOX enzyme nor a gene for it has yet been reported in plants. Gene mutation or amplification of plant genes for GOX-like enzyme activity or horizontal transfer of microbial genes from glyphosate-degrading enzymes could produce GR weeds. Yet, there is no evidence that metabolic degradation plays a significant role in evolved resistance to glyphosate. This is unexpected, considering the extreme selection pressure for evolution of glyphosate resistance in weeds and the difficulty in plants of evolving glyphosate resistance via other mechanisms.

Burkholderia fungorum DBT1: a promising bacterial strain for bioremediation of PAHs-contaminated soils

An extensive taxonomic analysis of the bacterial strain Burkholderia sp. DBT1, previously isolated from an oil refinery wastewater drainage, is discussed here. This strain is capable of transforming dibenzothiophene through the 'destructive' oxidative pathway referred to as the Kodama pathway. Burkholderia DBT1 has also been proved to use fluorene, naphthalene and phenanthrene as carbon and energy sources, although growth on the first two compounds requires a preinduction step. This evidence suggests that the strain DBT1 exerts a versatile metabolism towards polycyclic aromatic hydrocarbons other than condensed thiophenes. Phylogenetic characterization using a polyphasic approach was carried out to clarify the actual taxonomic position of this strain, potentially exploitable in bioremediation. In particular, investigations were focused on the possible exclusion of Burkholderia sp. DBT1 from the Burkholderia cepacia complex. Analysis of the sequences of 16S, recA and gyrB genes along with the DNA-DNA hybridization procedure indicated that the strain DBT1 belongs to the species Burkholderia fungorum, suggesting the proposal of the taxonomic denomination B. fungorum DBT1.

Phytoremediation of polychlorinated biphenyls: new trends and promises

Transgenic plants and associated bacteria constitute a new generation of genetically modified organisms for efficient and environment-friendly treatment of soil and water contaminated with polychlorinated biphenyls (PCBs). This review focuses on recent advances in phytoremediation for the treatment of PCBs, including the development of transgenic plants and associated bacteria. Phytoremediation, or the use of higher plants for rehabilitation of soil and groundwater, is a promising strategy for cost-effective treatment of sites contaminated by toxic compounds, including PCBs. Plants can help mitigate environmental pollution by PCBs through a range of mechanisms: besides uptake from soil (phytoextraction), plants are capable of enzymatic transformation of PCBs (phytotransformation); by releasing a variety of secondary metabolites, plants also enhance the microbial activity in the root zone, improving biodegradation of PCBs (rhizoremediation). However, because of their hydrophobicity and chemical stability, PCBs are only slowly taken up and degraded by plants and associated bacteria, resulting in incomplete treatment and potential release of toxic metabolites into the environment. Moreover, naturally occurring plant-associated bacteria may not possess the enzymatic machinery necessary for PCB degradation. To overcome these limitations, bacterial genes involved in the metabolism of PCBs, such as biphenyl dioxygenases, have been introduced into higher plants, following a strategy similar to the development of transgenic crops. Similarly, bacteria have been genetically modified that exhibit improved biodegradation capabilities and are able to maintain stable relationships with plants. Transgenic plants and associated bacteria bring hope for a broader and more efficient application of phytoremediation for the treatment of PCBs.

Low level of pollen-mediated gene flow from cultivated to wild grapevine: consequences for the evolution of the endangered subspecies Vitis vinifera L. subsp. silvestris

A parentage and a paternity-based approach were tested for estimation of pollen-mediated gene flow in wild grapevine (Vitis vinifera L. subsp. silvestris), a wind-pollinated species occurring in Mediterranean Europe and southwestern Asia. For this purpose, 305 seedlings collected in 2 years at 2 locations in France from 4 wild female individuals and 417 wild individuals prospected from France and Italy were analyzed using 20 highly polymorphic microsatellite loci. Their profiles were compared with a database consisting of 3203 accessions from the Institut National de la Recherche Agronomique Vassal collection including cultivars, rootstocks, interspecific hybrids, and other wild individuals. Paternity was assigned for 202 (66.2%) of the 305 seedlings, confirming the feasibility of the method. Most of the fertilizing pollen could be assigned to wild males growing nearby. Estimates of pollen immigration from the cultivated compartment (i.e., the totality of cultivars) ranged from 4.2% to 26% from nearby vineyards and from hidden pollinators such as cultivars and rootstocks that had escaped from farms. In an open landscape, the pollen flow was correlated to the distance between individuals, the main pollinator being the closest wild male (accounting for 51.4-86.2% of the pollen flow). In a closed landscape, more complex pollination occurred. Analysis of the parentage of the 417 wild individuals also revealed relationships between nearby wild individuals, but in the case of 12 individuals (3%), analysis revealed pollen immigration from vineyards, confirming the fitness of the hybrid seedlings. These pollen fluxes may have a significant effect on the evolution of wild populations: on the one hand, the low level of pollen-mediated gene flow from cultivated to wild grapevine could contribute to a risk of extinction of the wild compartment (i.e., the totality of the wild individuals). On the other hand, pollen dispersal within the wild populations may induce inbreeding depression of wild grapevines.

Advances in development of transgenic plants for remediation of xenobiotic pollutants

Phytoremediation-the use of plants for cleaning up of xenobiotic compounds-has received much attention in the last few years and development of transgenic plants tailored for remediation will further enhance their potential. Although plants have the inherent ability to detoxify some xenobiotic pollutants, they generally lack the catabolic pathway for complete degradation/mineralization of these compounds compared to microorganisms. Hence, transfer of genes involved in xenobiotic degradation from microbes/other eukaryotes to plants will further enhance their potential for remediation of these dangerous groups of compounds. Transgenic plants with enhanced potential for detoxification of xenobiotics such as trichloro ethylene, pentachlorophenol, trinitro toluene, glycerol trinitrate, atrazine, ethylene dibromide, metolachlor and hexahydro-1,3,5-trinitro-1,3,5-triazine are a few successful examples of utilization of transgenic technology. As more genes involved in xenobiotic metabolism in microorganisms/eukaryotes are discovered, it will lead to development of novel transgenic plants with improved potential for degradation of recalcitrant contaminants. Selection of suitable candidate plants, field testing and risk assessment are important considerations to be taken into account while developing transgenic plants for phytoremediation of this group of pollutants. Taking advantage of the advances in biotechnology and 'omic' technologies, development of novel transgenic plants for efficient phytoremediation of xenobiotic pollutants, field testing and commercialization will soon become a reality.