Improvement of the Cu and Cd phytostabilization efficiency of perennial ryegrass through the inoculation of three metal-resistant PGPR strains

Highlights: • Metal toxicity on the isolates and grass seedlings depends on the type and concentration of metals. • The metal removal rate was positively correlated with bacterial cell numbers in the medium. • IAA production of the isolates was significantly enhanced at a low Cu²⁺ level (5 mg L⁻¹). • PGPR inoculation improved ryegrass growth and oxidative stress management. • Metal immobilization was enhanced and less metal was transferred to the above-ground parts. To explore a novel strategy for the remediation of soils polluted with Cu and Cd, three strains of plant-growth-promoting rhizobacteria (PGPRs) isolated from contaminated mines and two grass species (perennial ryegrass and tall fescue) were selected in this study. The performance of PGPR strains in metal adsorption, maintaining promotion traits under stress, and ameliorating phytostabilization potential was evaluated. Cd²⁺ exerted a stronger deleterious effect on microbial growth than Cu²⁺, but the opposite occurred for grass seedlings. Adsorption experiment showed that the growing PGPR strains were able to immobilize maximum 79.49% Cu and 81.35% Cd owing to biosorption or bioaccumulation. The strains exhibited the ability to secrete indole-3-acetic acid (IAA) and dissolve phosphorus in the absence and presence of metals, and IAA production was even enhanced in the presence of low Cu²⁺ (5 mg L⁻¹). However, the siderophore-producing ability of the isolates was strongly suppressed under Cu and Cd exposure. Ryegrass was further selected for pot experiments owing to its higher germination rate and tolerance under Cu and Cd stress than fescue. Pot-experiment results revealed that PGPR addition significantly increased the shoot and root biomasses of ryegrass by 11.49%–44.50% and 43.53%–90.29% in soil co-contaminated with 800 mg Cu kg⁻¹ and 30 mg Cd kg⁻¹, respectively. Metal uptake and translocation in inoculated ryegrass significantly decreased owing to the reduced diethylenetriamine pentaacetic acid-extractable metal content and increased residual metal-fraction percentage mediated by PGPR. Interestingly, stress mitigation was observed in these inoculated plants; in particular, their malondialdehyde content and superoxide dismutase activity were even significantly lower than those of ryegrass under normal conditions. Therefore, PGPR could be a promising option to enhance the phytostabilization efficiency of Cu and Cd in heavily polluted soils.