Multilevel mechanisms of potassium-mediated cadmium detoxification in Brassica napus: Insights from physiological, biochemical, and transcriptomic analyses

Brassica napus, a major oilseed crop in southern China, faces dual challenges of cadmium (Cd(II)) contamination and potassium (K) deficiency in soils. However, the mechanisms by which K alleviates Cd(II) toxicity in B. napus remain poorly understood. This study, therefore, aimed to elucidate the physiological, biochemical, and molecular mechanisms underlying K-mediated Cd(II) detoxification in B. napus using a hydroponic experimental system. The results demonstrated that elevated K application significantly alleviated Cd(II)-induced toxicity. A major finding was the substantial recovery of photosynthetic activity, with K supplementation increasing the photosynthetic rate by 40.7 % under Cd(II) stress, which corresponded with a significant reduction in biomass loss and decreased concentrations of malondialdehyde and H₂O₂. Mechanistically, this alleviation was linked to a fortified antioxidant defense system, evidenced by enhanced activities of catalase (CAT) and peroxidase (POD) by 54.2 % and 15.7 %, respectively. Transcriptomic analysis provided deeper insights, revealing that K-mediated tolerance involves enhanced Cd(II) sequestration. K supplementation significantly up-regulated genes encoding tonoplast-localized transporters (CAX and HMA) responsible for vacuolar Cd(II) import, while simultaneously down-regulating the vacuolar Cd(II) efflux gene NRAMP4. This was complemented by an increase in ion-bound pectin and cellulose in the cell wall, further restricting Cd(II) mobility. Furthermore, K appeared to modulate key signaling pathways, as the expression of PYL genes, encoding abscisic acid (ABA) receptors, was significantly induced. Collectively, our findings demonstrate that K alleviates Cd(II) toxicity in B. napus through a multi-pronged strategy encompassing the enhancement of antioxidant capacity and the promotion of Cd(II) sequestration in both vacuoles and cell walls, potentially regulated via an ABA-dependent pathway. This research provides a theoretical basis for optimizing K fertilization to improve phytoremediation efficiency in Cd-contaminated agricultural soils.