Abstract
BackgroundSoil pollution by heavy metals increases the bioavailability of metals like hexavalent chromium (Cr (VI)), subsequently limiting plant growth and reducing the efficiency of phytoremediation. Plant growth-promoting rhizobacteria (PGPR) have substantial potential to enhance plant growth as well as plant tolerance to metal stress. The aim of this research was to investigate Cr (VI) phytoremediation enhancement by PGPR. ResultsThe results showed that the 27 rhizobacterial isolates studied were confirmed as Cr (VI)-resistant PGPR, by using classical biochemical tests (phosphate solubilization, nitrogen fixation, indole acetic acid, exopolysaccharides, hydrogen cyanide, siderophores, ammonia, cellulase, pectinase, and chitinase production) and showed variable levels of Cr (VI) resistance (300–600 mg/L). The best four selected Cr (VI)-resistant PGPR (NT15, NT19, NT20, and NT27) retained most of the PGP traits in the presence of 100–200 mg/L concentrations of Cr (VI). The inoculation of Medicago sativa with any of these four isolates improved the shoot and root dry weight. The NT27 isolate identified using 16S rDNA gene sequence analyses as a strain of Pseudomonas sp. was most effective in terms of plant growth promotion and stress level decrease. It increased shoot and root dry weights of M. sativa by 97.6 and 95.4%, respectively, in the presence of Cr (VI) when compared to non-inoculated control plants. It also greatly increased chlorophyll content and decreased the levels of stress markers, malondialdehyde, hydrogen peroxide, and proline. The results of the effect of Pseudomonas sp. on Cr content and bioaccumulation factor (BAF) of the shoots and roots of M. sativa plants showed the increase of plant biomass concomitantly with the increase of Cr root concentration in inoculated plants. This would lead to a higher potential of Cr (VI) phytostabilization. ConclusionsThis study demonstrates that the association M. sativa-Pseudomonas sp. may be an efficient biological system for the bioremediation of Cr (VI)-contaminated soils.
Highlights
Soil pollution by heavy metals increases the bioavailability of metals like hexavalent chromium (Cr (VI)), subsequently limiting plant growth and reducing the efficiency of phytoremediation
Our results showed that the 27 bacterial isolates studied showed various PGP properties (P solubilization, N2 fixation, Indole acetic acid (IAA), extracellular polymeric substances (EPS), Hydrogen cyanide (HCN), siderophores, NH3, cellulase, pectinase, and chitinase production) and variable levels of Cr (VI) resistance (300–600 mg/L)
We can conclude from our results that the inoculation of M. sativa species by Plant growth-promoting rhizobacteria (PGPR) overcoming the negative effects of Cr (VI) stress and increased the plant growth rate and the content of chlorophyll
Summary
Soil pollution by heavy metals increases the bioavailability of metals like hexavalent chromium (Cr (VI)), subsequently limiting plant growth and reducing the efficiency of phytoremediation. The intensive urbanization and civilization of society are responsible for the prominent increase of rapid industrial development and the spread of metals in soils. The excessive accumulation of Cr (VI) in the soil causes enormous problems for plant growth and crop productivity [6]. The interference of Cr (VI) with nutrient uptake, such as phosphorus, within the intracellular membrane structures and photosynthesis, increases plant phytotoxicity. This is due to lipid peroxidation through reactive oxygen species (ROS) and modification of antioxidant activities [9, 10]. Higher accumulation of Cr (VI) in plant tissues can affect the chlorophyll content, transpiration process, transport of electrons, CO2 fixation, photophosphorylation, photosynthetic enzyme activity, and stomatal conductance, which leads to a significant reduction of the photosynthetic rate [12,13,14]
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