Mechanisms of cadmium accumulation and translocation in rice affected by microbial agents under hydroponic conditions

Cadmium (Cd) accumulation in rice grains threatens global food security, yet the functional components of microbial agents that regulate plant internal Cd translocation remain unclear. Here, we compared the effects of live-cell treatment and secretion treatment on Cd accumulation and translocation in hydroponic rice. Rice was treated with Pseudomonas straminea, Allorhizobium oryzae, or their synthetic consortium, and integrated analyses of tissue Cd, root exudate metabolomics, rhizospheric microbial community, and structural equation modeling were performed. Live-cell treatment induced only modest and not statistically significant biomass changes but reduced shoot Cd concentrations by 50.1-55.6% and Cd translocation factor by 46.5-53.7%, while root Cd remained unchanged. Secretion treatment matched or slightly exceeded the live-cell-induced shoot Cd reduction and reproduced 60.6-91.8% of the corresponding TF-reduction effect, lowering shoot Cd by 52.9-58.5% and TF by up to 42.7%. Metabolomic and structural equation modeling analyses revealed a conserved low-translocation root exudate profile. Reduced Cd translocation was more closely associated with the coordinated suppression of pro-fatty acyl and pro-steroid modules than with the overall enhancement of anti-translocation metabolites. These findings support the conclusion that microbial extracellular products play a major role in regulating Cd translocation in rice, providing a mechanistic basis for developing secretion-based strategies to mitigate Cd accumulation in rice.