Alterations in HO-1 expression, heme oxygenase activity and endogenous NO homeostasis modulate antioxidant responses of Brassica nigra against nano silver toxicity

Silver nanoparticles (AgNPs) are one of the most widely-used nanomaterials, which are toxic and can cause physiological disorders in plants. The aim of the present study was to investigate whether a possible signaling link between heme oxygenase (HO) and nitric oxide (NO) is implicated in alleviating the toxicity of AgNPs as well as AgNO3. The results showed that exposure to 400 mg L−1 of AgNPs or AgNO3 reduced the chlorophyll content and the growth parameters in Brassica nigra. Data on Ag accumulation as well as the evaluation of lipid peroxidation and the H2O2 content in roots and shoots revealed that AgNP exerted more toxicity than AgNO3. Applying AgNP and AgNO3, respectively, increased HO transcripts by 87.5 and 37.3% and elevated the endogenous NO content 51.8 and 28.5%. The application of both hematin (as an inducer of HO) and sodium nitroprusside (SNP, as a NO donor) reversed the chlorosis and improved plant growth under AgNP and Ag+ ions stresses. Hematin decreased Ag accumulation in plants, indicating that this compound triggered an avoidance mechanism. Hematin and SNP enhanced the activities of antioxidant enzymes and proline accumulation, in parallel to increasing HO transcripts and NO release levels in the roots. ZnPPIX, as the inhibitor of HO, and cPTIO, as the specific NO scavenger, differentially blocked these effects. These findings revealed for the first time that HO might confer an increased tolerance to AgNP by activating the antioxidant systems, which was partially mediated by NO signal.