24-epibrassinolide mitigates cadmium toxicity in rice plants by activating the plant detoxification system and regulating genes expression involved in Cd/Fe uptake and translocation

Brassinosteroids (BRs) have received much research attention for their ability to mediate response to biotic and abiotic stress, with potential to counter the cadmium (Cd) toxicity in plants. Decreases in plant height and biomass in Cd-treated seedlings were alleviated by brassinosteroid analog 24-epibrassinolide (24-EBL) application, which has a negative effect on root growth. The Cd concentration significantly decreased in the roots and shoots under 24-EBL application, and high 24-EBL (0.1 µM) treatment was optimal for alleviating the deleterious effect of Cd stress. Fe and Mn accumulation in the 24-EBL-applied conditions resulted in an increase and decrease in rice varieties under Cd exposure, respectively. Additionally, 24-EBL improved the activity of antioxidant enzymes to reduce oxidative stress, decreased the nicotianamine (NA) content in shoots, and increased root NA accumulation. In molecular regulation, 24-EBL could directly alter the expression of Cd uptake/transport-related genes in the roots, including up-regulation of OsNRAMP1 and down-regulation of OsNRAMP5 and OsHMA2. The expression of genes involved in Fe homeostasis, including OsIRT1, OsNAS3, OsNAAT1, OsYSL2, OsYSL15, and OsTOM1, was up-regulated following 24-EBL application under Cd stress due to Fe accumulation and Cd-Fe antagonism. In addition, 24-EBL decreased the expression of OsHMA2, OsYSL2, and OsYSL15 in leaves, suggesting that BRs might inhibit Cd and Fe transport from roots to shoots. These novel findings highlight the critical role of BRs in mediating response of rice varieties to Cd exposure through their direct regulatory effects on Cd translocation and indirect effects on Fe metabolism.