Mani Rajkumar

Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Technogenic activities (industrial-plastic, textiles, microelectronics, wood preservatives; mining-mine refuse, tailings, smelting; agrochemicals-chemical fertilizers, farm yard manure, pesticides; aerosols-pyrometallurgical and automobile exhausts; biosolids-sewage sludge, domestic waste; fly ash-coal combustion products) are the primary sources of heavy metal contamination and pollution in the environment in addition to geogenic sources. During the last two decades, bioremediation has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Exclusively derived processes by plants alone (phytoremediation) are time-consuming. Further, high levels of pollutants pose toxicity to the remediating plants. This situation could be ameliorated and accelerated by exploring the partnership of plant-microbe, which would improve the plant growth by facilitating the sequestration of toxic heavy metals. Plants can bioconcentrate (phytoextraction) as well as bioimmobilize or inactivate (phytostabilization) toxic heavy metals through in situ rhizospheric processes. The mobility and bioavailability of heavy metal in the soil, particularly at the rhizosphere where root uptake or exclusion takes place, are critical factors that affect phytoextraction and phytostabilization. Developing new methods for either enhancing (phytoextraction) or reducing the bioavailability of metal contaminants in the rhizosphere (phytostabilization) as well as improving plant establishment, growth, and health could significantly speed up the process of bioremediation techniques. In this review, we have highlighted the role of plant growth promoting rhizo- and/or endophytic bacteria in accelerating phytoremediation derived benefits in extensive tables and elaborate schematic sketches.

Potential of siderophore-producing bacteria for improving heavy metal phytoextraction

Phytoremediation holds promise for in situ treatment of heavy metal contaminated soils. Recently, the benefits of combining siderophore-producing bacteria (SPB) with plants for metal removal from contaminated soils have been demonstrated. Metal-resistant SPB play an important role in the successful survival and growth of plants in contaminated soils by alleviating the metal toxicity and supplying the plant with nutrients, particularly iron. Furthermore, bacterial siderophores are able to bind metals other than iron and thus enhance their bioavailability in the rhizosphere of plants. Overall, an increase in plant growth and metal uptake will further enhance the effectiveness of phytoremediation processes. Here, we highlight the diversity and ecology of metal resistant SPB and discuss their potential role in phytoremediation of heavy metals.

Endophytic bacteria and their potential to enhance heavy metal phytoextraction

Pollution of soils with heavy metals is becoming one of the most severe environmental and human health hazards. Due to its widespread contamination finding innovative ways to clean metal pollutant has become a priority in the remediation field. Phytoremediation, the use of plants for the restoration of environments contaminated with pollutants is a relatively new technology that is more benign than current engineering solutions to treat contaminated sites. Recently, the benefits of combining endophytic bacteria with plants for increased remediation of pollutants have been successfully tried for toxic metal removal from contaminated soils. Endophytic bacteria reside within plant hosts without causing disease symptoms. Further, the metal resistant endophytes are reported to be present in various hyperaccumulator plants growing on heavy metal contaminated soils and play an important role in successful survival and growth of plants. Moreover, the metal resistant endophytes are reported to promote plant growth by various mechanisms such as nitrogen fixation, solubilization of minerals, production of phytohormones, siderophores, utilization of 1-aminocyclopropane-1-carboxylic acid as a sole N source and transformation of nutrient elements. In this review we highlight the diversity and plant growth promoting features of metal resistant endophytic bacteria and discuss their potential in phytoextraction of heavy metals from contaminated soils.

Influence of metal resistant-plant growth-promoting bacteria on the growth of Ricinus communis in soil contaminated with heavy metals

The metal resistant-plant growth-promoting bacterial (PGPB) strains PsM6 and PjM15 isolated from a serpentine soil were characterized as Pseudomonas sp. and Pseudomonas jessenii, respectively, on the basis of their morphological, physiological, biochemical characteristics and 16S rDNA sequences. Assessment of plant growth-promoting parameters revealed the intrinsic ability of the strains for the utilization of 1-aminocyclopropane-1-carboxylic acid as the sole N source, solubilization of insoluble phosphate and production of indole-3-acetic acid (IAA). Further, a pot experiment was conducted to elucidate the effects of inoculating metal resistant PGPB on the plant growth and the uptake of Ni, Cu and Zn by Ricinus communis. Inoculation of Pseudomonas sp. PsM6 or P. jessenii PjM15 increased the shoot and root biomass of R. communis grown in non-contaminated and contaminated soil. However, the maximum biomass was observed in the plants inoculated with strain PjM15. This effect can be attributed to the solubilization of phosphate and production of IAA. Inoculation of Pseudomonas sp. PsM6 and PjM15 did not greatly alter the organ metal concentrations except Zn which concentration was higher in root, stem and leaf of inoculated plants. The results of metal extraction with PGPB strains showed that PsM6 was more efficient at solubilizing Zn than PjM15, and that PjM15 was better at solubilising Ni and Cu than PsM6. Owing to its wide action spectrum, the metal resistant PGPB could serve as an effective metal sequestering and growth-promoting bioinoculant for plants in metal-stressed soil. The present study has provided a new insight into the phytoremediation of metal-contaminated soil.

Inoculation of plant growth promoting bacterium Achromobacter xylosoxidans strain Ax10 for the improvement of copper phytoextraction by Brassica juncea

In this study, a copper-resistant plant growth promoting bacterial (PGPB) strain Ax10 was isolated from a Cu mine soil to assess its plant growth promotion and copper uptake in Brassica juncea. The strain Ax10 tolerated concentrations up to 600 mg CuL(-1) on a Luria-Bertani (LB) agar medium and utilized 1-aminocyclopropane-1-carboxylic acid (ACC) as a sole N source in DF salts minimal medium. The strain Ax10 was characterized as Achromobacter xylosoxidans based on its 16S rDNA sequence homology (99%). The bacterium A. xylosoxidans Ax10 has also exhibited the capability of producing indole acetic acid (IAA) (6.4 microg mL(-1)), and solubilizing inorganic phosphate (89.6 microg mL(-1)) in specific culture media. In pot experiments, inoculation of A. xylosoxidans Ax10 significantly increased the root length, shoot length, fresh weight and dry weight of B. juncea plants compared to the control. This effect can be attributed to the utilization of ACC, production of IAA and solubilization of phosphate. Furthermore, A. xylosoxidans Ax10 inoculation significantly improved Cu uptake by B. juncea. Owing to its wide action spectrum, the Cu-resistant A. xylosoxidans Ax10 could serve as an effective metal sequestering and growth promoting bioinoculant for plants in Cu-stressed soil. The present study has provided a new insight into the phytoremediation of Cu-contaminated soil.

Improvement of plant growth and nickel uptake by nickel resistant-plant-growth promoting bacteria

In this study, among a collection of Ni-resistant bacterial strains isolated from the rhizosphere of Alyssum serpyllifolium and Phleum phleoides grown on serpentine soil, five plant growth-promoting bacteria (PGPB) were selected based on their ability to utilize 1-aminocyclopropane-1-carboxylate (ACC) as the sole N source and promote seedling growth. All of the strains tested positive for indole-3-acetic acid (IAA) production and phosphate solubilization. In addition, four of the strains exhibited significant levels of siderophores production. Further, the efficiency of PGPB in enhancing Ni solubilization in soils was analyzed. Compared with control treatment, inoculation of PGPB strains significantly increased the concentrations of bioavailable Ni. Furthermore, a pot experiment was conducted to elucidate the effects of inoculating Ni-resistant PGPB on the plant growth and the uptake of Ni by Brassica juncea and B. oxyrrhina in soil contaminated with 450 mg kg(-1) Ni. Psychrobacter sp. SRA2 significantly increased the fresh (351%) and dry biomass (285%) of the B. juncea test plants (p<0.05), whereas Psychrobacter sp. SRA1 and Bacillus cereus SRA10 significantly increased the accumulation of Ni in the root and shoot tissues of B. juncea compared with non-inoculated controls. This result indicates that the strains SRA1 and SRA10 facilitated the release of Ni from the non-soluble phases in the soil, thus enhancing the availability of Ni to plants. A significant increase, greater than that of the control, was also noted for growth parameters of the B. oxyrrhina test plants when the seeds were treated with strain SRA2. This effect can be attributed to the utilization of ACC, solubilization of phosphate and production of IAA. The results of the study revealed that the inoculation of Ni mobilizing strains Psychrobacter sp. SRA1 and B. cereus SRA10 increases the efficiency of phytoextraction directly by enhancing the metal accumulation in plant tissues and the efficient PGPB, Psychrobacter sp. SRA2 increases indirectly by promoting the growth of B. juncea and B. oxyrrhina.

Inoculation of endophytic bacteria on host and non-host plants--effects on plant growth and Ni uptake.

Among a collection of Ni resistant endophytes isolated from the tissues of Alyssum serpyllifolium, four plant growth promoting endophytic bacteria (PGPE) were selected based on their ability to promote seedling growth in roll towel assay. Further, the PGPE screened showed the potential to produce plant growth promoting (PGP) substances and plant polymer hydrolyzing enzymes. These isolates were further screened for their PGP activity on A. serpyllifolium and Brassica juncea under Ni stress using a phytagar assay. None of the four isolates produced any disease symptoms in either plant. Further, strain A3R3 induced a maximum increase in biomass and Ni content of plants. Based on the PGP potential in phytagar assay, strain A3R3 was chosen for studying its PGP effect on A. serpyllifolium and B. juncea in Ni contaminated soil. Inoculation with A3R3 significantly increased the biomass (B. juncea) and Ni content (A. serpyllifolium) of plants grown in Ni contaminated soil. The strain also showed high level of colonization in tissue interior of both plants. By 16S rRNA gene sequencing analysis, A3R3 was identified as Pseudomonas sp. Successful colonization and subsequent PGP potentiality of Pseudomonas sp. A3R3 indicate that the inoculation with PGPE might have significant potential to improve heavy metal phytoremediation.

Isolation and characterization of Ni mobilizing PGPB from serpentine soils and their potential in promoting plant growth and Ni accumulation by Brassica spp.

The study was undertaken to assess the effects of Ni mobilizing bacteria on the plant growth and the uptake of Ni by Brassica juncea and Brassica oxyrrhina. Among a collection of Ni resistant bacterial strains isolated from the non-rhizosphere and rhizosphere soils of Alyssum serpyllifolium and Astragalus incanus at a serpentine site in Bragança, north-east of Portugal, nine strains were selected based on their ability to solubilize Ni in soil. Further assessment on plant growth-promoting parameters revealed the intrinsic ability of the Ni mobilizing strains to produce indole-3-acetic acid (IAA), siderophores, utilize 1-aminocyclopropane-1-carboxylic acid (ACC) as the sole N source and solubilize insoluble phosphate. All of the strains tested positive for IAA production and phosphate solubilization. In addition, all the strains, except SRS5 exhibited significant levels of siderophore production. Besides, five isolates showed positive for ACC deaminase activity. In pot experiments, inoculation of plants with Ni mobilizing strains increased the biomass of both B. juncea and B. oxyrrhina. Among the strains, Pseudomonas sp. SRI2, Psychrobacter sp. SRS8 and Bacillus sp. SN9 showed maximum increase in the biomass of the test plants. In addition, the strain SN9 significantly increased the Ni concentration in the root and shoot tissues of B. juncea and B. oxyrrhina. Further, a significantly positive correlation was observed between the bacterial Ni mobilization in soil and the total Ni uptake in both plant species. The findings, therefore, revealed that inoculation of Ni mobilizing plant growth-promoting bacterial strain SN9 increases the efficiency of phytoextraction directly by enhancing Ni accumulation in plant tissues and indirectly by promoting the shoot and root biomass of B. juncea and B. oxyrrhina.

Beneficial role of bacterial endophytes in heavy metal phytoremediation

Phytoremediation is an emerging technology that uses plants and their associated microbes to clean up pollutants from the soil, water and air. In recent years, phytoremediation assisted by bacterial endophytes has been highly recommended for cleaning up of metal polluted soils since endophytic bacteria can alleviate metal toxicity in plant through their own metal resistance system and facilitate plant growth under metal stress. Endophytic bacteria improve plant growth in metal polluted soils in two different ways: 1) directly by producing plant growth beneficial substances including solubilization/transformation of mineral nutrients (phosphate, nitrogen and potassium), production of phytohormones, siderophores and specific enzymes; and 2) indirectly through controlling plant pathogens or by inducing a systemic resistance of plants against pathogens. Besides, they also alter metal accumulation capacity in plants by excreting metal immobilizing extracellular polymeric substances, as well as metal mobilizing organic acids and biosurfactants. The present work aims to review the progress of recent research on the isolation, identification and diversity of metal resistant endophytic bacteria and illustrate various mechanisms responsible for plant growth promotion and heavy metal detoxification/phytoaccumulation/translocation in plants.

Climate change driven plant–metal–microbe interactions

Various biotic and abiotic stress factors affect the growth and productivity of crop plants. Particularly, the climatic and/or heavy metal stress influence various processes including growth, physiology, biochemistry, and yield of crops. Climatic changes particularly the elevated atmospheric CO₂ enhance the biomass production and metal accumulation in plants and help plants to support greater microbial populations and/or protect the microorganisms against the impacts of heavy metals. Besides, the indirect effects of climatic change (e.g., changes in the function and structure of plant roots and diversity and activity of rhizosphere microbes) would lead to altered metal bioavailability in soils and concomitantly affect plant growth. However, the effects of warming, drought or combined climatic stress on plant growth and metal accumulation vary substantially across physico-chemico-biological properties of the environment (e.g., soil pH, heavy metal type and its bio-available concentrations, microbial diversity, and interactive effects of climatic factors) and plant used. Overall, direct and/or indirect effects of climate change on heavy metal mobility in soils may further hinder the ability of plants to adapt and make them more susceptible to stress. Here, we review and discuss how the climatic parameters including atmospheric CO₂, temperature and drought influence the plant-metal interaction in polluted soils. Other aspects including the effects of climate change and heavy metals on plant-microbe interaction, heavy metal phytoremediation and safety of food and feed are also discussed. This review shows that predicting how plant-metal interaction responds to altering climatic change is critical to select suitable crop plants that would be able to produce more yields and tolerate multi-stress conditions without accumulating toxic heavy metals for future food security.