Effect of plant growth promoting bacterium; Pseudomonas putida UW4 inoculation on phytoremediation efficacy of monoculture and mixed culture of selected plant species for PAH and lead spiked soils

Plant growth promoting bacteria (PGPB) enhanced phytoremediation (PEP) is an attractive remedial strategy for the remediation of polycyclic aromatic hydrocarbon (PAH) and heavy metal (HM) contaminated sites. The effect of PGPB; Pseudomonas putida UW4 inoculation on the phytoremediation efficiency of Medicago sativa, Festuca arundinacea, Lolium perenne, and mixed plants (L. perenne and F. arundinacea) was assessed. This involved two contaminant treatments; “PAH” (phenanthrene; 300?mg·kg^?1, fluoranthene; 200?mg·kg^?1, and benzo[a]pyrene; 5?mg·kg^?1) and “PAH?+?HM” (‘PAH’ treatments +100?mg of Pb/kg). PGPB inoculation significantly enhanced root biomass yield of F. arundinacea in PAH treatment, and the mixed plant shoot biomass and L. perenne root biomass yields of the PAH?+?HM treatment. PGPB significantly enhanced dissipation of phenanthrene and fluoranthene for M. sativa-PAH?+?PGPB treatment and fluoranthene for F. arundinacea-PAH?+?HM?+?PGPB treatment. In others, PGPB inoculation either had no impact or inhibited PAH dissipation. PAH dissipation for the single and mixed plant treatments with PGPB inoculation were not different. The efficiency of PEP is dependent on different factors such as PGPB inoculum biomass, plant species, plant–microbe specificity and type of contaminants. Exploiting PEP technology would require proper understanding of plant tolerance and growth promoting mechanisms, and rhizosphere activities.     

Enhanced Cd extraction of oilseed rape (Brassica napus) by plant growth-promoting bacteria isolated from Cd hyperaccumulator Sedum alfredii Hance

Four plant growth-promoting bacteria (PGPB) were used as study materials, among them two heavy metal-tolerant rhizosphere strains SrN1 (Arthrobacter sp.) and SrN9 (Bacillus altitudinis) were isolated from rhizosphere soil, while two endophytic strains SaN1 (Bacillus megaterium) and SaMR12 (Sphingomonas) were identified from roots of the cadmium (Cd)/zinc (Zn) hyperaccumulator Sedum alfredii Hance. A pot experiment was carried out to investigate the effects of these PGPB on plant growth and Cd accumulation of oilseed rape (Brassica napus) plants grown on aged Cd-spiked soil. The results showed that the four PGPB significantly boosted oilseed rape shoot biomass production, improved soil and plant analyzer development (SPAD) value, enhanced Cd uptake of plant and Cd translocation to the leaves. By fluorescent in situ hybridization (FISH) and green fluorescent protein (GFP), we demonstrated the studied S. alfredii endophytic bacterium SaMR12 were able to colonize successfully in the B. napus roots. However, all four PGPB could increase seed Cd accumulation. Due to its potential to enhance Cd uptake by the plant and to restrict Cd accumulation in the seeds, SaMR12 was selected as the most promising microbial partner of B. napus when setting up a plant–microbe fortified remediation system.     

Characterization of bacterial communities associated with Brassica napus L. growing on a Zn-contaminated soil and their effects on root growth

The interaction between plant growth-promoting bacteria (PGPB) and plants can enhance biomass production and metal tolerance of the host plants. This work aimed at isolating and characterizing the cultivable bacterial community associated with Brassica napus growing on a Zn-contaminated site, for selecting cultivable PGPB that might enhance biomass production and metal tolerance of energy crops. The effects of some of these bacterial strains on root growth of B. napus exposed to increasing Zn and Cd concentrations were assessed. A total of 426 morphologically different bacterial strains were isolated from the soil, the rhizosphere, and the roots and stems of B. napus. The diversity of the isolated bacterial populations was similar in rhizosphere and roots, but lower in soil and stem compartments. Burkoholderia, Alcaligenes, Agrococcus, Polaromonas, Stenotrophomonas, Serratia, Microbacterium, and Caulobacter were found as root endophytes exclusively. The inoculation of seeds with Pseudomonas sp. strains 228 and 256, and Serratia sp. strain 246 facilitated the root development of B. napus at 1,000 µM Zn. Arthrobacter sp. strain 222, Serratia sp. strain 246, and Pseudomonas sp. 228 and 262 increased the root length at 300 µM Cd.     

Soil CO2 venting as one of the mechanisms for tolerance of Zn deficiency by rice in flooded soils

We sought to explain rice (Oryza sativa) genotype differences in tolerance of zinc (Zn) deficiency in flooded paddy soils and the counter‐intuitive observation, made in earlier field experiments, that Zn uptake per plant increases with increasing planting density. We grew tolerant and intolerant genotypes in a Zn‐deficient flooded soil at high and low planting densities and found (a) plant Zn concentrations and growth increased with planting density and more so in the tolerant genotype, whereas the concentrations of other nutrients decreased, indicating a specific effect on Zn uptake; (b) the effects of planting density and genotype on Zn uptake could only be explained if the plants induced changes in the soil to make Zn more soluble; and (c) the genotype and planting density effects were both associated with decreases in dissolved CO₂ in the rhizosphere soil solution and resulting increases in pH. We suggest that the increases in pH caused solubilization of soil Zn by dissolution of alkali‐soluble, Zn‐complexing organic ligands from soil organic matter. We conclude that differences in venting of soil CO₂ through root aerenchyma were responsible for the genotype and planting density effects.     

Effect of bacterial inoculation of strains of pseudomonas aeruginosa,alcaligenes feacalis and bacillus subtilis on germination,growth and heavy metal (cd,cr, and ni) uptake of brassica juncea

Bacterial inoculation may influence Brassica juncea growth and heavy metal (Ni, Cr, and Cd) accumulation. Three metal tolerant bacterial isolates (BCr3, BCd33, and BNi11) recovered from mine tailings, identified as Pseudomonas aeruginosa KP717554, Alcaligenes feacalis KP717561, and Bacillus subtilis KP717559 were used. The isolates exhibited multiple plant growth beneficial characteristics including the production of indole-3-acetic acid, hydrogen cyanide, ammonia, insoluble phosphate solubilization together with the potential to protect plants against fungal pathogens. Bacterial inoculation improved seeds germination of B. juncea plant in the presence of 0.1 mM Cr, Cd, and Ni, as compared to the control treatment. Compared with control treatment, soil inoculation with bacterial isolates significantly increased the amount of soluble heavy metals in soil by 51% (Cr), 50% (Cd), and 44% (Ni) respectively. Pot experiment of B. juncea grown in soil spiked with 100 mg kg^?1 of NiCl₂, 100 mg kg^?1 of CdCl₂, and 150 mg kg^?1 of K₂Cr₂O₇, revealed that inoculation with metal tolerant bacteria not only protected plants against the toxic effects of heavy metals, but also increased growth and metal accumulation of plants significantly. These findings suggest that such metal tolerant, plant growth promoting bacteria are valuable tools which could be used to develop bio-inoculants for enhancing the efficiency of phytoextraction.     

Root development and heavy metal phytoextraction efficiency: comparison of different plant species in the field

Heavy metal phytoextraction is a soil remediation technique which implies the optimal use of plants to remove contamination from soil. Plants must thus be tolerant to heavy metals, adapted to soil and climate characteristics and able to take up large amounts of heavy metals. Their roots must also fit the spatial distribution of pollution. Their different root systems allow plants to adapt to their environment and be more or less efficient in element uptake. To assess the impact of the root system on phytoextraction efficiency in the field, we have studied the uptake and root systems (root length and root size) of various high biomass plants (Brassica juncea, Nicotiana tabacum, Zea mays and Salix viminalis) and one hyperaccumulator (Thlaspi caerulescens) grown in a Zn, Cu and Cd contaminated soil and compared them with total heavy metal distribution in the soil. Changes from year to year have been studied for an annual (Zea mays) and a perennial plant (Salix viminalis) to assess the impact of the climate on root systems and the evolution of efficiency with time and growth. In spite of a small biomass, T. caerulescens was the most efficient plant for Cd and Zn removal because of very high concentrations in the shoots. The second most efficient were plants combining high metal concentrations and high biomass (willows for Cd and Zn and tobacco for Cu and Cd). A large cumulative root density/aboveground biomass ratio (L_A/B), together with a relative larger proportion of fine roots compared to other plants seemed to be additional favourable characteristics for increased heavy metal uptake by T. caerulescens. In general, for all plants correlations were found between L _A/B and heavy metal concentrations in shoots (r=0.758^***, r=0.594^***, r=0.798^*** (P<0.001) for Cd, Cu and Zn concentrations resp.). Differences between years were significant because of variations in climatic conditions for annual plants or because of growth for perennial plants. The plants exhibited also different root distributions along the soil profile: T. caerulescens had a shallow root system and was thus best suited for shallow contamination (0.2 m) whereas maize and willows were the most efficient in colonising the soil at depth and thus more applicable for deep contamination (0.7 m). In the field situation, no plant was able to fit the contamination properly due to heterogeneity in soil contamination. This points out to the importance and the difficulty of choosing plant species according to depth and heterogeneity of localisation of the pollution.     

Salt-tolerant and plant growth-promoting bacteria isolated from Zn/Cd contaminated soil: identification and effect on rice under saline conditions

The bacteria of PDMCd0501, PDMCd2007, and PDMZnCd2003 were isolated from a Zn/Cd contaminated soil. They were classified as salt-tolerant bacteria in this experiment. The bacteria had indole-3-acetic acids (IAA) production, nitrogen fixation, and phosphate solubilization, under 8% (w/v) NaCl condition. Biochemical test (API 20E) and 16S rDNA sequencing identified PDMCd2007 and PDMCd0501 as Serratia sp. and PDMZnCd2003 was Pseudomonas sp. The effect of Pseudomonas sp. PDMZnCd2003 on the germination and seedlings of Oryza sativa L.cv. RD6 was determined under a salinity of 0–16 dS/m. The salinity levels of 4–16 dS/m affected to decrease germination and seedlings of rice. Comparison between uninoculated and inoculated system, however, Pseudomonas sp. PDMZnCd2003 had a negative impact on the rice growth. This unexpected effect was a case that should be concerned and studied further before application as a plant growth-promoting bacteria (PGPB).     

POTENTIAL OF BRASSICA JUNCEA IN ORDER TO TREAT TEXTILE—EFFLUENT—CONTAMINATED SITES

Three plant species (Brassica juncea, Sorghum vulgare, and Phaseolus mungo) of different agronomic consequence were evaluated for the decolorization of the dyes from textile effluent. B. juncea, S. vulgare, and P. mungo showed textile effluent decolorization up to 79, 57, and 53%, respectively. A significant decrease in shoot and root height, but no significant injury, was observed in the case of P. mungo and S. vulgare. B. juncea (Indian mustard), the most tolerant and more effective metals accumulator than other tested agricultural plant species, showed enhanced growth with respect to the height of the shoot and root, 129 and 178%, respectively, when grown using original textile effluent. Textile effluent induced intracellular nicotinamide adenine dinucleotide reduced (NADH)–dichlorophenol indophenol reductase significantly in the case of S. vulgare and B. juncea with 209 and 194%, respectively. The extracellular riboflavin reductase activity was induced by 223% in the case of P. mungo as compared to control plants. Significant induction of intracellular laccase (266%) was observed in the case of B. juncea, indicating their crucial role for a potential metabolism and further degradation of the textile effluent. The metabolites were identified as napthalenesufamide (m/z 372) and 2-amino-4, 6-dichlorotriazine (m/z 167), when B. juncea was used to degrade a model dye, Reactive red 2.     

Pongamia pinnata inoculated with Bradyrhizobium liaoningense PZHK1 shows potential for phytoremediation of mine tailings

Mine tailings contain high concentrations of metal contaminants and only little nutrients, making the tailings barren for decades after the mining has been terminated. Effective phytoremediation of mine tailings calls for deep-rooted, metal accumulating, and soil fertility increasing plants with tolerance against harsh environmental conditions. We assessed the potential of the biofuel leguminous tree Pongamia pinnata inoculated with plant growth promoting rhizobia to remediate iron–vanadium–titanium oxide (V–Ti magnetite) mine tailing soil by pot experiment and in situ remediation test. A metal tolerant rhizobia strain PZHK1 was isolated from the tailing soil and identified as Bradyrhizobium liaoningense by phylogenetic analysis. Inoculation with PZHK1 increased the growth of P. pinnata both in V–Ti magnetite mine tailings and in Ni-contaminated soil. Furthermore, inoculation increased the metal accumulation capacity and superoxide dismutase activity of P. pinnata. The concentrations of Ni accumulated by inoculated plants were higher than the hyperaccumulator threshold. Inoculated P. pinnata accumulated high concentration of Fe, far exceeding the upper limit (1000 mg kg⁻¹) of Fe in plant tissue. In summary, P. pinnata–B. liaoningense PZHK1 symbiosis showed potential to be applied as an effective phytoremediation technology for mine tailings and to produce biofuel feedstock on the marginal land.

     

Induction of phytochelatins and antioxidant defence system in <Emphasis Type="Italic">Brassica juncea</Emphasis> and <Emphasis Type="Italic">Vigna radiata</Emphasis> in response to chromium treatments

Growth performance, chromium (Cr) accumulation potential and induction of antioxidative defence system and phytochelatins (PCs) were studied in hydroponically grown Brassica juncea (Indian mustard) and Vigna radiata (mungbean) at various levels of Cr treatments (0, 50, 100, 200 μM Cr). B. juncea accumulated twofolds and threefolds higher Cr in root and shoot, respectively than in V. radiata. Compared to B. juncea, V. radiata was found to be particularly sensitive to Cr as observed by the severity and development of Cr toxicity symptoms and decreased growth. Induction of PC and enzymes of antioxidant defence system were monitored as plant’s primary and secondary metal detoxifying responses, respectively. There was induction of PC and enzymes of antioxidant defence system in both the plants. PCs were induced significantly in roots and shoot of both the plants at all the levels of Cr treatments. Significantly higher activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) were observed in shoot of B. juncea than V. radiata at all the levels of Cr treatments. Induction of PCs along with antioxidant defence system in response to Cr stress suggests the cumulative role of PCs and antioxidants in conferring tolerance against accumulated Cr in B. juncea, and thereby signifies the suitability of this plant as one of the potential remediators of Cr.     

Can the use of natural biostimulants be a potential means of phytoremediating contaminated soils from goldmines in South Africa?

Biostimulants offer great potential in improving phytoremediation of contaminated soils. In the current greenhouse-based study, Brassica juncea seedlings grown on soils collected from Krugersdorp Goldmine and the adjourning areas (a Game Reserve and private farmland) were supplemented with different biostimulants (Kelpak® = KEL, vermicompost leachate = VCL, smoke-water = SW). Indole-3-butyric acid (IBA) was included in the study for comparative purposes because these biostimulants are known to enhance rooting. Prior to the pot trial, concentrations of elements in the three soil types were determined using Inductively Coupled Plasma-Optical Emission Spectroscopy. Plants were harvested after 105 days and the growth and concentrations of elements in the various plant organs were determined. TheB. juncea seedlings with and without biostimulants did not survive when growing in soil from the Krugersdorp Goldmine. The Game Reserve and private farmland soils supplemented with KEL produced the highest plant biomass and the lowest accumulation of metals in the organs of B. juncea. High concentrations (>13 000 mg kg^?1) of zinc and aluminium were quantified in the roots of IBA-supplemented soils from the Game Reserve. Generally, IBA and SW enhanced the phytoremediation of B. juncea due to elevated levels of elements that accumulated in their different organs.     

EFFECT OF METAL TOLERANT PLANT GROWTH PROMOTING BACTERIA ON GROWTH AND METAL ACCUMULATION IN ZEA MAYS PLANTS GROWN IN FLY ASH AMENDED SOIL

The present study was undertaken to examine the effect of the application of fly ash (FA) into Garden soil (GS), with and without inoculation of plant growth promoting bacteria (PGPB), on the growth and metal uptake by Zea mays plants. Three FA tolerant PGPB strains, Pseudomonas sp. PS5, PS14, and Bacillus sp. BC29 were isolated from FA contaminated soils and assessed for their plant growth promoting features on the Z. mays plants. All three strains were also examined for their ability to solubilize phosphate and to produce Indole Acetic Acid (IAA), siderophores, and hydrogencynide acid (HCN) production. Although inoculation of all strains significantly enhanced the growth of plants at both the concentration of FA but maximum growth was observed in plants inoculated with BC29 and PS14 at low level (25%) of FA concentration. The experimental results explored the plant growth promoting features of selected strains which not only enhanced growth and biomass of plants but also protected them from toxicity of FA.     

Isolation and expression analysis of partial sequences of heavy metal transporters from <Emphasis Type="Italic">Brassica juncea</Emphasis> by coupling high throughput cloning with a molecular fingerprinting technique

Heavy metal transporters play a key role in regulating metal accumulation and transport in plants. These are important candidate genes to study in metal tolerant and accumulator plants for their potential use in environmental clean up. We coupled a degenerate primer-based RT-PCR approach with a molecular fingerprinting technique based on amplified rDNA restriction analysis (ARDRA) to identify novel ESTs corresponding to heavy metal transporters from metal accumulator Brassica juncea. We utilized this technique to clone several family members of natural resistance-associated macrophage proteins (NRAMP) and yellow stripe-like proteins (YSL) in a high throughput manner to distinguish between closely related isoforms and/or allelic variants from the allopolyploid B. juncea. Partial clones of 23 Brassica juncea NRAMPs and 27 YSLs were obtained with similarity to known Arabidopsis thaliana and Noccaea (Thlaspi) caerulescens NRAMP and YSL genes. The cloned transporters showed Brassica-specific changes in domains, which can have important functional consequences. Semi-quantitative RT-PCR-based expression analysis of chosen members indicated that even closely related isoforms/allelic variants of BjNRAMP and BjYSL have distinct tissue-specific and metal-dependent expressions which might be essential for adaptive fitness and heavy metal tolerance. Consistent to this, BjYSL6.1 and BjYSL5.8 were found to show elevated expressions specifically in cadmium-treated shoots and lead-treated roots of B. juncea, respectively.     

The cation-efflux transporter BjCET2 mediates zinc and cadmium accumulation in <Emphasis Type="Italic">Brassica juncea</Emphasis> L. leaves

Brassica juncea L. is a Zn/Cd accumulator. To determine the physiological basis of its metal accumulation phenotype, the functional properties and role of the metal efflux transporter BjCET2 were investigated using transgenic technology. Heterologous expression of BjCET2 in the double mutant yeast strain Δzrc1Δcot1 enhanced the metal tolerance of the yeast strain and led to decrease in Zn or Cd accumulation. Detection of green fluorescence from green fluorescent protein (GFP) in the root tip of transgenic tobacco further revealed that BjCET2::GFP is localized at the plasma membrane. Semi-quantitative RT-PCR analysis showed that BjCET2 was most abundant in the root and was weakly expressed in the stem and leaves. The expression of BjCET2 was up-regulated by heavy metals. However, exposure to low temperature, salt and drought did not affect the expression of BjCET2. Overexpression of BjCET2 in transgenic B. juncea plants conferred heavy metal tolerance and increased Cd/Zn accumulation in the leaves. BjCET2-deficient B. juncea mediated by antisense RNA resulted in hypersensitivity to heavy metals and decreased Zn/Cd accumulation in the plants. These results suggest that the heavy metal efflux of BjCET2 plays important roles in the metal tolerance of B. juncea and in Zn/Cd accumulation in B. juncea.     

Assessment of Metal Uptake Capacity of Castor Bean and Mustard for Phytoremediation of Nickel from Contaminated Soil

The effect of increasing level of nickel (Ni) in soil was studied on biomass production, antioxidants, and Ni bioaccumulation and its translocation in castor bean (Ricinus communis) as well as Indian mustard (Brassica juncea) in similar agroclimatic conditions. The plants were exposed to 25, 50, 75, 100, and 150 mg Ni kg^?1 soil for up to 60 days. It was found that R. communis produced higher biomass during the same period at all the contamination levels than B. juncea, and reduction in fresh and dry weights due to the metal contamination in soil was significantly lower in R. communis than in B. juncea. Proline and malondialdehyde in the leaves increased with increase in Ni level in both the species, whereas soluble protein content was found decreased. A correlation between the protein and MDA contents in the leaves and Ni contamination levels revealed that higher r² values for protein and MDA were found in case of B. juncea, which indicates more toxic effects of the metal in this species. R. communis was found to have enhanced proline accumulation (higher correlation value, r²) at different Ni contamination levels. The bioaccumulation of Ni was higher in B. juncea on the basis of the per unit biomass; however, the total metal accumulation per plant was much higher in R. communis than in B. juncea during the same growing periods. The translocation of Ni from roots to shoots was higher in B. juncea at all Ni concentrations. R. communis appeared more tolerant and capable to clean more Ni from the contaminated soil in a given time and also in one crop cycle.     

The effect of plant growth-promoting rhizobacteria on the phytoextraction of Cd and Zn by Brassica napus L.

The test strains Bacteroidetes bacterium (Ba), Pseudomonas fluorescens (Pf) and Variovorax sp. (Va) were selected in advance for their in vitro capability for growth promotion of rapeseed in the presence of increased concentrations of Cd, Cu, Pb and Zn in the medium. In the pot experiment, the strains were used for single Ba, Pf, Va or combined Ba + Pf, Ba + Va, Pf + Va, and Ba + Pf + Va inoculation of B. napus growing in contaminated soil from alluvial deposits. The positive effect of bacterial strains on plant growth was observed in vitro, but was not confirmed in situ in the contaminated soil, where the tested strains inhibited biomass production, rather than stimulating it. However, single inoculation with Ba significantly increased the chlorophyll content and K⁺ concentration in the leaves. The inoculation of rapeseed with Ba and Va strains was indicated to be the most promising combination for phytoextraction of Cd and Zn from contaminated soil. Combined inoculation with Pf+Va and Pf + Ba+Va significantly decreased the concentration of heavy metals in the roots of rapeseed. We conclude that suitable combinations of PGPR can control the metal uptake of B. napus, selectively increasing either metal extraction or metal stabilization in the rhizosphere and offering promising applications in soil remediation.     

Effect of Salinity on Zinc uptake by Brassica juncea

Salinity is a major worldwide problem that affects agricultural soils and limits the reclamation of contaminated sites. Despite the large number of research papers published about salt tolerance in Brassica juncea L., there are very few accounts concerning the influence of salinity on the uptake of trace metals. In this study, B. juncea plants divided through soil sets comprising 0, 900 and 1800 mg Zn kg^?1, were treated with solutions containing 0, 60 and 120 mmol L^?1 of NaCl, with the purpose of observing the effect of salt on Zn uptake, and some physiological responses throughout the 90 days experiment. Increasing concentrations of NaCl and Zn produced a decline in the ecophysiological and biochemical properties of the plants, with observable synergistic effects on parameters like shoot dry weight, leaf area, or photochemical efficiency. Nevertheless, plants treated with 60 mmol L^?1 of NaCl accumulated striking harvestable amounts of Zn per plant that largely exceed those reported for Thlaspi caerulescens. It was concluded that salinity could play an important role on the uptake of Zn by B. juncea. The potential mechanisms behind these results are discussed, as well as the implications for phytoremediation of Zn on saline and non-saline soils.     

The Light Quanta Modulated Physiological Response of Brassica Juncea Seedlings Subjected to Ni(II) Stress

This work is a study of the inter‐relationship between parameters that principally affect the metal up‐take in the plant. The relationships between the concentration of metal in the growth medium, C_s, the concentration of metal absorbed by the plant, C_p, and the total biomass achieved, M, all of which are factors relevant to the efficiency of phytoremediation of the plant, have been investigated via the macro‐physiological response of Brassica juncea seedlings to Ni(II) stress. The factorial growth experiments treated the Ni(II) concentration in the agar gel and the diurnal light quanta (DLQ) as independently variable parameters. Observations included the evidence of light enhancement of Ni toxicity at the root as well as at the whole plant level, the shoot mass index as a possible indicator of shoot metal sequestration in B. juncea, the logarithmic variation of C_p with C_s and the power‐law dependence of M on C_p. The sum total of these observations indicates that for the metal accumulator B. juncea with regard to its capacity to accumulate Ni, the overall metabolic nature of the plant is important – neither rapid biomass increase nor a high metal concentration capability favor the removal of high metal mass from the medium, but rather the plant with the moderate photosynthetically driven biomass growth and moderate metal concentrations demonstrated the ability to remove the maximum mass of metal from the medium. The implications of these observations in the context of the perceived need in phytoremediation engineering to maximize C_p and M simultaneously in the same plant, are discussed.     

Molecular evidence of genetic modification of Sinorhizobium meliloti: enhanced PCB bioremediation

The genome of the nitrogen-fixing soil bacterium Sinorhizobium meliloti does not possess genes for bioremediation of aromatic pollutants. It has the well-known ability to interact specifically with the leguminous alfalfa plant, Medicago sativa. Our previous work has shown enhanced degradation of the nitroaromatic compound 2,4-dinitrotoluene (DNT) when a plasmid containing degradative genes was introduced in it. In this study we report molecular evidence of the transfer of a polychlorinated biphenyl (PCB)-biodegradative plasmid pE43 to S. meliloti strain USDA 1936. Several standard analytical tests and plant growth chamber studies were conducted to test the ability of S. meliloti to degrade 2′,3,4-PCB congener. Alfalfa plant alone was able to degrade 30% of PCBs compared with control. No enhanced dechlorination was noted when alfalfa plant was grown with wild-type S. meliloti, and when alfalfa plant was grown with the S. meliloti electrotransformants (genetically modified) dechlorination of PCBs was more than twice that when alfalfa plant was grown with wild-type S. meliloti. When alfalfa plant was grown with uncharacterized mixed culture (containing nodule formers), almost equally significant PCB degradation was observed. The significance of this work is that the naturally occurring nitrogen-fixing soil bacterium S. meliloti (genetically modified) has the ability to enhance fertility of soil in association with the leguminous alfalfa plant while simultaneously enhancing bioremediation of PCB-contaminated soils. Enhanced bioremediation of PCB and robust alfalfa plant growth was also noted when uncharacterized mixed cultures containing alfalfa plant nodule formers were used.