Two plant growth-promoting bacteria assist rice in promoting growth and reducing cadmium through different mechanisms

Cadmium (Cd) pollution threatens crop safety and human health. Plant growth-promoting bacteria (PGPB) can help plants maintain yield and cope with Cd toxicity under harmful conditions. However, the mechanisms of co-inoculation with two PGPB strains on rice under Cd stress were not well studied. This study performed flask-shaking, hydroponic and pot experiments using Pseudomonas sp. and Acinetobacter sp. It showed that inoculation reduced Cd content in rice and improved plant growth and grain yield, with co-inoculation having a greater effect than mono-inoculation. The two PGPB strains produced similar plant phenotypes through different mechanisms. Pseudomonas achieved these effects by upregulating phenylpropanoid and glutathione pathways, while Acinetobacter did so by upregulating calcium-binding proteins and WRKY transcription factors. The mechanism of co-inoculation resembled that of the strain with stronger adaptability in their symbiotic system. Additionally, the expression levels of key genes (e.g., OsPOX, OsGST), activities of antioxidant enzymes (e.g., POX, APX), and contents of related metabolites (e.g., GSH, MDA) revealed that the antioxidant capacity of rice increased after inoculation. This study confirmed that co-inoculation with two PGPB strains produced better results through the integration of their different Cd-tolerance mechanisms, which lays a foundation for future microbe-assisted rice yield improvement and low-Cd rice production.