Synergistic effects of heavy-metal-tolerant plant-growth-promoting bacteria on tall fescue growth and the phytoremediation of multi-metal-contaminated soils.

Heavy metal-contaminated soils pose serious threats to ecosystems, increasing the demand for sustainable and cost-effective remediation strategies. Although phytoremediation is an environmentally friendly approach, its efficiency is often limited under multi-metal stress, and the interactions among multiple plant growth-promoting bacteria (PGPB) remain insufficiently understood. In this study, we evaluated the effects of single and combined inoculation with three heavy metal-tolerant PGPB-Acinetobacter sp. ME1, Kosakonia sp. W18 and Burkholderia sp. ZF6-on the phytoremediation performance of tall fescue grown in soil co-contaminated with Cr, Zn, Cd and Pb. Dual inoculation with ME1 and W18 (D1 treatment) achieved over 30% removal efficiency for all four metals. Indole-3-acetic acid and dehydrogenase activities in the D1 and ME1 + ZF6 (D2) treatments were 1.1-1.3-fold higher than those in single-strain inoculations. The translocation factor in the D1 treatment was 1.7-fold higher than that of the uninoculated control, indicating enhanced metal uptake and translocation. Plant growth was also promoted, with root biomass exceeding 40 g. Microbial network and functional gene analyses revealed strengthened rhizosphere-endosphere connectivity and increased abundance of Cd/Zn tolerance genes under the D2 treatment. In contrast, triple-strain inoculation did not result in further improvements in metal removal, microbial functional indicators or plant growth compared with the dual-strain treatments. These results suggest that functional complementarity, rather than the number of inoculated strains, governs remediation efficiency. Overall, this study demonstrates that strategically designed multi-PGPB inoculation can enhance soil-plant-microbe interactions and improve phytoremediation performance in soils contaminated with multiple heavy metals.