Auxin-Mediated Aerenchyma Formation Drives Selenium-Induced Rhizosphere Iron Barrier Strengthening to Restrict Toxic Element Uptake by Rice

Selenium (Se) application can reduce toxic element accumulation in rice, yet the mechanistic basis linking Se to rhizosphere oxygenation and Fe barrier formation remains unresolved. In a 103 day greenhouse pot experiment, sodium selenite was applied to multicontaminated paddy soil either as a soil amendment (1 mg kg-1) or foliar spray (40 mg L-1), with an untreated control. Soil Se application reduced grain concentrations of cadmium by 54%, arsenic by 34%, lead by 41%, chromium by 21%, nickel by 42%, and cobalt by 39%; foliar application achieved reductions of 12%-41%. To resolve the underlying mechanisms, we integrated planar optode and DGT-LA-ICP-MS imaging with physiological and molecular analyses. Selenium stimulated auxin accumulation by 2.74-fold and upregulated auxin biosynthesis and signaling genes (OsYUCCA1, OsTAA1, OsARF19), expanding constitutive aerenchyma from 34% to 65% of cortical area and sustaining radial oxygen loss. The resulting increase in rhizosphere oxygenation decreased labile Fe and Mn fluxes by 54%-89%, consistent with Fe/Mn oxide precipitation, and increased root surface Fe plaque by 5.64-fold, collectively restricting toxic element mobility through adsorption and coprecipitation. These findings establish an auxin-mediated aerenchyma-radial oxygen loss-iron barrier cascade as the mechanistic basis for Se-induced restriction of toxic element uptake by rice.