Silicon nanoparticles mitigate cadmium toxicity in rice by modulating root development and exudate dynamics

Cadmium (Cd) contamination severely hampers rice ( Oryza sativa L.) growth by disrupting root development, nutrient uptake, and exudation processes, posing a major risk to food security. This study investigated the role of silicon nanoparticle (SiNP) on root architecture and root exudation dynamics in rice grown hydroponically under two levels of Cd (10 and 20 μM). Cadmium markedly impaired root development, as evidenced by significant reductions in root length (8.94% and 17.93%), surface area (15% and 31.42%), diameter (15.78% and 28.94%), volume (8.15% and 25.54%), biomass (31.19% and 40.36%), and water uptake efficiency (11.94% and 28.05%), while increasing root branching. Co-application of SiNP mitigated these effects, enhancing root biomass (28.00% and 32.30%) and restoring water uptake (14.31% and 11.91%) relative to Cd-only treatments. Cadmium stress inhibited antioxidant enzymes (APX, CAT, SOD, POD) and increased oxidative damage (MDA, electrolyte leakage). SiNP co-application substantially mitigated Cd-induced oxidative stress in rice roots by restoring APX 17.8 and 39.4%, CAT 37.0 and 70%, SOD by 7.25 and 15.61% and POD by 14.59 and 38.00%, respectively, with both levels of Cd stress, while reduced MDA and electrolyte leakage by 27.7% and 29.6%, respectively, with the highest levels of Cd stress. Cadmium stress also significantly altered the profile of root exudates, decreasing oxalic acid (Cd 10 : 22.03%; Cd 20 : 32.60%) and citric acid (Cd 10 : 38.23%; Cd 20 : 49.67%) while increasing malic acid production (Cd 10 : 11.48%; Cd 20 : 17.84%). SiNP supplementation reversed these shifts by elevating oxalic (Cd 20 : 17.43%) and citric (Cd 20 : 24.29%) acids, suppressing malic acid release (Cd 10 : 21.39%; Cd 20 : 26.83%), and overall enhancing total organic acid secretion, thereby contributing to rhizosphere acidification. Over time, SiNP significantly lowered Cd bioavailability in the nutrient solution, likely via Si–Cd complexation and organic acid-mediated chelation. At the cellular scale, SiNP supplementation reduced Cd accumulation in both the cell wall and symplast, indicating restricted Cd uptake and transport. Moreover, although Cd stress substantially increased total organic carbon (TOC) (Cd 10 : 32.28% and Cd 20 : 58.13%) and total nitrogen (TN) (Cd 10 : 4.25% and Cd 20 : 27.71%) in rhizosphere exudates, SiNP co-treatment moderated these elevations and concurrently preserved cellular ultrastructure. Therefore, SiNP mitigated Cd toxicity by improving root architecture, modulating exudation, and reducing Cd bioavailability and uptake. These findings demonstrate the potential of SiNPs as a sustainable nanotechnology-based strategy for reducing Cd accumulation in rice and safeguarding food quality. • SiNP restored root growth traits and water uptake efficiency under Cd stress. • SiNP modulated organic acid secretion, enhancing rhizosphere acidification and Cd immobilization. • Cd accumulation in root cellular compartments was markedly reduced with SiNP treatment. • Protective effects of SiNP on root ultrastructure were confirmed via TEM analysis.