Ethylene renders silver nanoparticles stress tolerance in rice seedlings by regulating endogenous nitric oxide accumulation.

Developments in the field of nanotechnology over past years have increased the prevalence of silver nanoparticles (AgNPs) in the environment resulting in increased exposure of plants to AgNPs. Recently, various studies have reported the effect of AgNPs on plant growth at different concentrations. However, identifying the mechanisms and signalling molecules involved in plant responses against AgNPs stress is crucial to find an effective way to deal with the phytotoxic impacts of AgNPs on plant growth and development. Therefore, this study was envisaged to investigate the participation of ethylene in mediating the activation of AgNPs stress tolerance in rice (Oryza sativa L.) through a switch that regulates endogenous NO accumulation. Treatment of AgNPs alone hampered the growth of rice seedlings due to severe oxidative stress as a result of decline in sulfur assimilation, glutathione biosynthesis and alteration in the redox status of glutathione. These results are also accompanied by the higher endogenous NO level. But, addition of ethephon (a donor of ethylene) reversed the AgNPs-induced effects. Though silicon nanoparticles (SiNPs) alone stimulated rice growth but, interestingly when provided with AgNPs enhanced their toxicity through the routes as exhibited by AgNPs alone. However, addition of ethephon interestingly reversed the negative effects of SiNPs under AgNPs stress. These results suggest that ethylene might act as a switch to regulate the level of endogenous NO that in turn could be associated with AgNPs stress tolerance in rice. Furthermore, the results also indicated that addition of L-NAME (an inhibitor of endogenous NO synthesis) also reversed toxic effects of SiNPs together with AgNPs, further suggesting that low level of endogenous NO was associated with AgNPs stress tolerance. Overall, the results indicate that low level of endogenous NO triggers AgNPs stress tolerance while its high level leads to AgNPs toxicity by regulating sulfur assimilation, glutathione biosynthesis, redox status of glutathione and oxidative stress markers. The results revealed that ethylene might act as a switch in regulating AgNPs stress in rice seedlings by controlling endogenous NO accumulation.