Nitric oxide mitigates selenium phytotoxicity in barley: Inhibiting selenium absorption, altering selenium distribution, and strengthening the antioxidant system.

Selenium (Se) contamination may pose significant challenges to agricultural sustainability and food security in Se-rich environments. The potential of nitric oxide (NO) in improving Se tolerance in barley was investigated in this study. Selenium stress greatly increased endogenous NO content in barley, while exogenous NO donor (sodium nitroprusside, SNP) limited Se uptake by downregulating the expression of HvPT gene. Additionally, NO accelerated Se metabolism by upregulating the expression of HvSIR, HvCS, HvCγS, and HvCβL genes, thereby raising the proportion of organic Se in barley to 86.6 %. NO altered Se distribution by facilitating the translocation from nascent tissues to less active tissues, and shifted Se distribution from the soluble fraction to the cell wall at the subcellular level. SNP significantly upregulated antioxidant-related genes (Cu/Zn SOD, APX, GSH-Px), and elevated the content of non-enzymatic antioxidants (GSH, AsA). These protective effects were abolished by NO scavenger (cPTIO), confirming the specific role of NO. Additionally, NO alleviated Se-induced growth inhibition by enhancing root vigor, respiration rate, and proline accumulation. Overall, this study highlights the role of NO in modulating Se metabolism and antioxidant defenses in barley and provides novel insights for optimizing Se-enriched crops.