Inoculation of multi-metal-resistant Bacillus sp. to a hyperaccumulator plant Sedum alfredii for facilitating phytoextraction of heavy metals from contaminated soil

Co-contamination of soil by multiple heavy metals is a significant global challenge. An effective strategy to address this issue involves using hyperaccumulators such as Sedum alfredii (S. alfredii). The efficiency of phytoremediation can be improved by supplementing with plant growth-promoting bacteria (PGPB). However, bacteria resources of PGPB resistant to multi-heavy metal contamination are still lacking. This study focused nine different strains of Bacillus and screened for resistance to heavy metals including cadmium (Cd), zinc (Zn), copper (Cu), and lead (Pb). A superior strain, Bacillus subtilis PY79 (B. subtilis), showed tolerance for all tested metals. Inoculation with B. subtilis in the rhizosphere of S. alfredii increased the accumulation of Cd, Zn, Cu, and Pb by 88.02%, 58.99%, 90.22%, and 54.97% in the plant shoots after 30 days respectively. B. subtilis application lowered the pH of the rhizosphere soil, thereby increasing the bioavailability of nutrients and heavy metals. Furthermore, B. subtilis helped S. alfredii recruit PGPB and heavy metal-resistant bacteria such as Edaphobacter, Niastella, and Chitinophaga, enhancing the growth and phytoremediation efficiency. Moreover, inoculation with B. subtilis not only upregulated genes of the ABC, HMA, ZIP, and MTP families involved in the translocation and detoxification of heavy metals but also increased the secretion of antioxidants within the cells. These findings indicate that B. subtilis enhances the tolerance, uptake, and translocation of heavy metals in S. alfredii, offering valuable insights for the phytoremediation of multi-metal-contaminated soils.