Brassinosteroids Alleviate Ethylene-Induced Copper Oxide Nanoparticle Toxicity and Ultrastructural and Stomatal Damage in Rice Seedlings

Nanoparticle contamination has been associated with adverse impacts on crop productivity. Thus, effective approaches are necessary to ameliorate NP-induced phytotoxicity. The present study aimed to investigate the efficacy of brassinosteroids and ethylene in regulating CuO NPs toxicity in rice seedlings. Therefore, we comprehensively evaluated the crosstalk of 24-Epibrassinolide and ethylene in regulating CuO NP-induced phytotoxicity at the physiological, cellular ultrastructural, and biochemical levels. The results of the study illustrated that exposure to CuO NPs at 450 mg/L displayed a significant decline in growth attributes and induced toxic effects in rice seedlings. Furthermore, the exogenous application of ethylene biosynthesis precursor 1-aminocyclopropane-1-carboxylic acid (ACC) at 20 &micro:M with 450 mg/L of CuO NPs significantly enhanced the reactive oxygen species (ROS) accumulation that led to the stimulation of ultrastructural and stomatal damage and reduced antioxidant enzyme activities (CAT and APX) in rice tissues. On the contrary, it was noticed that 24-Epibrassinolide (BR) at 0.01 &micro:M improved plant biomass and growth, restored cellular ultrastructure, and enhanced antioxidant enzyme activities (CAT and APX) under exposure to 450 mg/L of CuO NPs. In addition, brassinosteroids reduced ROS accumulation and the toxic effects of 450 mg/L of CuO NPs on guard cells and the stomatal aperture of rice seedlings. Interestingly, when 0.01 &micro:M of brassinosteroids, 20 &micro:M of ACC, and 450 mg/L of CuO NPs were applied together, BRs and ethylene showed antagonistic crosstalk under CuO NP stress via partially reducing the ethylene-induced CuO NP toxicity on plant growth, cellular ultrastructure, stomatal aperture, and guard cell and antioxidant enzyme activities (CAT and APX) in rice seedlings. BR supplementation with ACC and CuO NPs notably diminished ACC-induced CuO NPs&rsquo: toxic effects on all of the mentioned attributes in rice seedlings. This study uncovered the interesting crosstalk of two main phytohormones under CuO NPs stress, providing basic knowledge to improve crop yield and productivity in CuO NPs-contaminated areas.