Manganese enrichment in rice seedlings shifts cadmium accumulation from inhibition to promotion during growth via transcriptomic mechanisms

Manganese (Mn)-rich seedling cultivation may reduce cadmium (Cd) uptake in rice; however, its stage-dependent effects and underlying mechanisms remain unclear. Mn-rich seedlings were cultivated in a hydroponic system. Biomass, Cd and Mn accumulation, and transcriptomic profiles were subsequently assessed at different growth stages. The results demonstrated that Mn enrichment increased Mn accumulation in the seedlings by 18.9-76.0-fold. On day 26 after transplantation, Cd concentrations in roots and shoots were reduced by 79.9% and 89.2%, respectively, in Mn-rich seedlings. However, this effect reversed at later stages, resulting in a 1.4- fold increase in Cd concentration in brown rice grains. Cd fluxes in the phloem and xylem decreased by 37.3%-49.4% on day 26, but increased by 1.10-2.74- fold on day 120 after the seedlings were enriched with Mn. Transcriptomic analysis was performed to elucidate the molecular mechanisms underlying the temporal dynamics of cadmium flux in rice tissues. Early-stage upregulation of genes such as Osj_27264, HSP17.4, and DREB1B was associated with enhanced Cd immobilization in roots, whereas the subsequent downregulation of key transcription factors including WRKY24, WRKY28, BHLH148, and TIFY11E in roots and nodes was associated with increased Cd uptake and translocation. Thus, the effect of Mn enrichment in rice seedlings on Cd uptake clearly indicates a time-dependent pattern. These findings suggest that elevated Mn levels should be maintained during the late reproductive stage to maximize Cd reduction in rice grains.