Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress

Our plain-English summary of Serpentine endophytic bacterium Pseudomonas azotoformans ASS1 accelerates phytoremediation of soil metals under drought stress, by Y. Ma, M. Rajkumar, A. Moreno, et al (Chemosphere, 2017).

The study

• Authors: Y. Ma, M. Rajkumar, A. Moreno, C. Zhang, H. Freitas
• Journal: Chemosphere
• Published: 2017;185:75-85
• Cited by: 51 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This study evaluates the potential of serpentine endophytic bacterium to foster phytoremediation efficiency of Trifolium arvense grown on multi-metal (Cu, Zn and Ni) contaminated soils under drought stress. A drought resistant endophytic bacterial strain ASS1 isolated from the leaves of Alyssum serpyllifolium grown in serpentine soils was identified as Pseudomonas azotoformans based on biochemical tests and partial 16S rRNA gene sequencing. P. azotoformans ASS1 possessed abiotic stress resistance (heavy metals, drought, salinity, antibiotics and extreme temperature) and plant growth promoting (PGP) properties (phosphate solubilization, nitrogen fixation, production of 1-aminocyclopropane-1-carboxylate deaminase, siderophore and ammonia). Inoculation of T. arvense with ASS1 considerably increased the plant biomass and leaf relative water content in both roll towel assay and pot experiments in the absence and presence of drought stress (DS). In the pot experiments, ASS1 greatly enhanced chlorophyll content, catalase, peroxidase, superoxide dismutase activities, and proline content (only in the absence of drought) in plant leaves, whereas they decreased the concentrations of malondialdehyde. Irrespective of water stress, ASS1 significantly improved accumulation, total removal, bio-concentration factor and biological accumulation coefficient of metals (Cu, Zn and Ni), while decreased translocation factors of Cu. The effective colonization and survival in the rhizosphere and tissue interior assured improved plant growth and successful metal phytoremediation under DS. These results demonstrate the potential of serpentine endophytic bacterium ASS1 for protecting plants against abiotic stresses and helping plants to thrive in semiarid ecosystems and accelerate phytoremediation process in metal polluted soils.

Copyright © 2017 Elsevier Ltd. All rights reserved.

In plain English

This laboratory and greenhouse study tested whether a bacterium helps a clover, Trifolium arvense, grow and take up metals under drought. The authors isolated strain ASS1, identified as Pseudomonas azotoformans, from leaves of Alyssum serpyllifolium growing in serpentine soils in Bragança, northeast Portugal. In laboratory tests the strain tolerated several metals, some antibiotics, salt, heat and drought-like conditions. Seeds soaked in the bacterium or in sterile water were grown in soil with or without added copper, nickel and zinc, and with or without drought. There were three replicates per treatment. Fresh weights of inoculated versus non-inoculated pot plants were 9.6 versus 4.8 mg (control), 7.9 versus 2.0 (drought), 6.7 versus 3.0 (metals) and 6.6 versus 1.3 (combined). Without drought, the text reports metal accumulation increases of 51, 63 and 26%. It names cadmium for the first figure, but the table lists copper. Under drought, the increases for copper, zinc and nickel were 27, 19 and 33%. The authors concluded the strain could serve as a biological agent to improve plant development and phytoremediation in drought environments. The article does not discuss asbestos.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory isolation and characterization of a bacterial strain, a roll towel seedling assay, and a greenhouse pot experiment. The pot experiment compared two bacterial treatments (no bacteria, strain ASS1) across four stress treatments (control, drought, multi-metals, drought plus multi-metals), with three replicates each.
• Who was studied: Bacterial strains were isolated from leaves of the nickel hyperaccumulator plant Alyssum serpyllifolium (subsp. malacitanum) growing in serpentine soils in Bragança, northeast Portugal. 18 resistant strains were found and one, ASS1, was chosen. The test plant was Trifolium arvense, grown in pots of soil from the Botanical Garden of the University of Coimbra, Portugal.
• Exposure: No asbestos or fibers were studied. The exposures were:
• Inoculation of seeds with Pseudomonas azotoformans ASS1.
• Soil artificially spiked to 400 mg copper, 350 mg nickel and 500 mg zinc per kg.
• Drought, imposed by keeping soil water at 50% of field capacity.
• Main results:
  • In the roll towel assay under 200 mg per litre PEG (simulated water stress), ASS1-inoculated seedlings had 93% higher fresh weight and 122% higher dry weight than non-inoculated seedlings.
  • In the pot experiment, non-inoculated plants had lower fresh weight than unstressed plants: 58% lower under drought, 38% lower under multi-metals and 73% lower under drought plus multi-metals.
  • The text reports that ASS1 increased plant fresh weight by 1, 3.0, 1.2 and 4.0-fold under control, drought, multi-metal and drought plus multi-metal conditions, respectively. These are increases over non-inoculated plants. Table 1 gives the pot-experiment fresh weights (mg), non-inoculated vs inoculated:
• Control: 4.8 vs 9.6.
• Drought: 2.0 vs 7.9.
• Multi-metals: 3.0 vs 6.7.
• Drought plus multi-metals: 1.3 vs 6.6.
  • ASS1 increased total chlorophyll (Chl a plus b) in leaves by 1.6, 2.3 and 1.7-fold under control, drought, and drought plus multi-metals, compared with non-inoculated plants.
  • ASS1 lowered leaf malondialdehyde (MDA, a marker of oxidative damage) by 60% under multi-metals and by 40% under drought plus multi-metals.
  • Without drought, ASS1 increased metal accumulation by 51, 63 and 26% compared with non-inoculated plants. The text labels these as Cd, Zn and Ni, but the study added copper, not cadmium, and Table 4 shows copper content rising from 99 to 149 mg per kg dry weight.
  • Under drought, ASS1 increased copper, zinc and nickel accumulation by 27, 19 and 33%, respectively, compared with non-inoculated plants.
  • Drought and metals reduced bacterial counts in the rhizosphere (the soil around the roots) but did not greatly affect colonization inside plant tissues.
• The authors' conclusion: The authors concluded that inoculation with ASS1 gave T. arvense greater phytoremediation potential under drought. They proposed that the strain could serve as an efficient biological agent to improve plant development and phytoremediation in drought environments.
• Limitations the authors state:
  • The authors state that the exact molecular mechanisms behind the bacterial increase in leaf relative water content still need to be explored.
  • The authors state that the mechanisms of microbial survival and functioning under combined extreme drought, metals and salinity require additional studies.
  • The authors note that comparing bacterial stress tolerance across studies is difficult because different culture media and incubation conditions are used.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Serpentine Group, Superoxide, Translocation.

Related studies

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• Characterization of metal-resistant plant-growth promoting Bacillus weihenstephanensis isolated from serpentine soil in Portugal — M. Rajkumar, Y. Ma, et al. (2008)
• Metal toxicity and biodiversity in serpentine soils: application of bioassay tests and microarthropod index — G. Visioli, C. Menta, et al. (2013)
• Framework for assessment and phytoremediation of asbestos-contaminated sites — C. Gonneau, K. Miller, et al. (2017)
• Occurrence of heavy metal-resistance in microflora from serpentine soil of Andaman — A. Pal, S. Dutta, et al. (2005)

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