Recruitment of beneficial microbes and differential metabolites promoted Pb transport in Salix integra Thunb. after inoculation with Bacillus megaterium B6

This study seeks to elucidate the mechanisms by which Bacillus megaterium B6 (strain B6) enhances lead (Pb) transport from roots to branches in Salix integra by approximately one time.

Variations in rhizosphere/endosphere microbes, along with changes in metabolites profiles in rhizosphere soil and roots, were assessed using amplicon sequencing and LC–MS, respectively, after inoculation with strain B6 in a controlled pot experiment.

Inoculation with strain B6 significantly enhanced species richness and diversities of microbes in both the rhizosphere and endosphere after inoculation with strain B6. The relative abundance of Bacillus, particularly within endosphere microbes, increased markedly. Beneficial microbes, essential for soil remediation, became the dominant species, especially within endosphere communities. Differential metabolites, for example up-regulated amino acids, chelated Pb, reducing its toxicity. These metabolites significantly enriched in "ABC transporters" pathway, promoting Pb transport directly in the root. In the soil, these metabolites either increased Pb bioavailability directly or indirectly by regulating soil properties. Additionally, differential metabolites were significantly correlated with dominant microbes, especially in root, and they played a crucial role in enhancing Pb transport than in Pb bioavailability.

After colonization, strain B6 recruited beneficial microbes for soil remediation, altered metabolites expression in plant roots and rhizosphere soil, and consequently enhanced Pb transfer, either directly or indirectly. Furthermore, endosphere microbes played a more significant role in facilitating Pb transport. These findings provide new insights into microbe-assisted phytoremediation in heavy metal-contaminated soil.