Integrated impact of plant growth-promoting rhizobacteria and earthworms (Eisenia fetida) on cadmium detoxification, microbial activity, health risk assessment, and protein-related biochemical responses in rice (Oryza sativa L.)

The escalating threat of cadmium (Cd) contamination in agricultural soils poses serious challenges to food security and necessitates sustainable mitigation strategies. This study evaluated the combined effects of plant growth-promoting rhizobacteria (PGPR) and earthworms (Eisenia fetida) on Cd accumulation and associated physiological responses in rice (Oryza sativa L.). A controlled pot experiment was conducted under greenhouse conditions using plants exposed to Cd stress (100 mg kg^-1 soil), representing a severe contamination scenario used to evaluate mitigation responses under high Cd stress, with the application of Serratia marcescens, Pseudomonas fluorescens, and earthworms, individually and in combination. The results indicated that combined inoculation with PGPR and earthworms enhanced plant growth, improved photosynthetic efficiency, stimulated antioxidant defenses, increased osmolyte accumulation, and modulated cell wall-associated biochemical components, contributing to reduced Cd accumulation in O. sativa tissues. Furthermore, improvements in nutrient uptake and regulation of stress-related biochemical processes were observed, supporting enhanced tolerance under Cd exposure. Health risk indices (hazard quotient and hazard index) were markedly reduced, reflecting lower Cd transfer to edible tissues. Protein fraction and ribosomal protein analyses indicated increased total soluble protein content, enhanced protease activity, and greater ribosomal protein yield under biological treatments. Overall, the combined application of PGPR and earthworm reduced Cd accumulation and supported physiological resilience under controlled greenhouse conditions however, field validation and long-term assessment are required before broader agronomic application can be inferred.