Inoculation With Indigenous Rhizosphere Microbes Enhances Aboveground Accumulation of Lead in Salix integra Thunb. by Improving Transport Coefficients

Abstract

The application of plant–microbial remediation of heavy metals is restricted by the difficulty of exogenous microbes to form large populations and maintain their long-term remediation efficiency. We therefore investigated the effects of inoculation with indigenous heavy-metal-tolerant rhizosphere microbes on phytoremediation of lead (Pb) by Salix integra. We measured plant physiological indexes and soil Pb bioavailability and conducted widespread targeted metabolome analysis of strains to better understand the mechanisms of enhance Pb accumulation. Growth of Salix integra was improved by both single and co-inoculation treatments with Bacillus sp. and Aspergillus niger, increasing by 14% in co-inoculated plants. Transfer coefficients for Pb, indicating mobility from soil via roots into branches or leaves, were higher following microbial inoculation, showing a more than 100% increase in the co-inoculation treatment over untreated plants. However, Pb accumulation was only enhanced by single inoculation treatments with either Bacillus sp. or Aspergillus niger, being 10% greater in plants inoculated with Bacillus sp. compared with uninoculated controls. Inoculation mainly promoted accumulation of Pb in aboveground plant parts. Superoxide dismutase and catalase enzyme activities as well as the proline content of inoculated plants were enhanced by most treatments. However, soil urease and catalase activities were lower in inoculated plants than controls. Proportions of acid-soluble Pb were 0.34 and 0.41% higher in rhizosphere and bulk soil, respectively, of plants inoculated with Bacillus sp. than in that of uninoculated plants. We identified 410 metabolites from the microbial inoculations, of which more than 50% contributed to heavy metal bioavailability; organic acids, amino acids, and carbohydrates formed the three major metabolite categories. These results suggest that both indigenous Bacillus sp. and Aspergillus niger could be used to assist phytoremediation by enhancing antioxidant defenses of Salix integra and altering Pb bioavailability. We speculate that microbial strains colonized the soil and plants at the same time, with variations in their metabolite profiles reflecting different living conditions. We also need to consider interactions between inocula and the whole microbial community when applying microbial inoculation to promote phytoremediation.

Heavy Metal Stress Alleviation Through Omics Analysis of Soil and Plant Microbiome

“…This study also identified 410 metabolites, which mainly constituted organic acids, amino acids and carbohydrates, in response to microbial inoculation. Further, around half of the identified metabolites were associated with HM bioavailability (Niu et al, 2021), thereby corroborating the role of microbes in increasing the phytoremediation efficiency. In another recent report, metabolomics and proteomics approach was used to study the mechanism of enhanced tolerance of wheat to Cd and Pd stress on inoculation with Enterobacter bugandensis TJ6 strain, a HM-immobilizing bacterium (Han et al, 2021).…”

“…Inoculation with Pb-resistant Bacillus sp. and Aspergillus niger, showed enhanced proline levels as well as increased superoxide dismutase and catalase (antioxidants) activity (Niu et al, 2021). This study also identified 410 metabolites, which mainly constituted organic acids, amino acids and carbohydrates, in response to microbial inoculation.…”

“…Metabolomic studies of plants with or without microbial inoculation have been used to decipher the mechanism of HM tolerance. For instance, the Pb accumulation ability of Salix integra upon inoculation with some indigenous rhizospheric microbes was investigated by targeted metabolomics approach (Niu et al, 2021). Inoculation with Pb-resistant Bacillus sp.…”

“…However, its large-scale application is limited by its lower efficiency. This issue could be addressed by inoculation of plants with microbes, in an approach known as plant-microbial remediation (Niu et al, 2021). For instance, the legume-Rhizobia interaction has been exploited to increase the phytoremediation ability of plants.…”

Bamboo biochar greater enhanced Cd/Zn accumulation in Salix psammophila under non-flooded soil compared with flooded

“…Soil microorganisms, such as Bacillus sp. and Aspergillus niger were confirmed to enhance antioxidant defenses of Salix and promote HMs bioavailability and improve transport coefficients finally (Niu et al 2021). As a HMs remediation tree species, Salix has high tolerance and accumulation capacity for HMs.…”