Nanoparticles（ZnO,CeO₂）Alleviates Cobalt Induced Physiological,Biochemical,and Ultrastructural Changes In Maize

Agriculture is directly linked to human life,providing food for survival and health.It is threatened by a number of challenges,such as climate change,resource depletion,and abiotic stresses,including heavy metals（HMs）,salinity,drought,etc.Heavy metals among them have been found to severely impair plant growth and development,posing a serious risk to food security and human health.Cobalt（Co）is one of the toxic and non-essential HM and has become a major pollutant affecting global food production and quality.Hence,innovative strategies are needed to discover the mechanisms of Co-induced toxicity in crops.Various strategies have been employed to palliate the phytotoxic effects of these stressors from the soil–plant system,but no method is efficient to reduce Co toxicity.Nanotechnology is an emerging field in crop sciences for its potential in improving crop production and mitigating various stresses.Nanotechnological approaches have emerged as a promising tool for increasing crop productivity and promoting sustainable agriculture.More importantly,the seed nano-priming approach has shown potential against abiotic stress factors and has improved crop productivity.Engineered nanoparticles（ENPs）such as zinc oxide nanoparticles（ZnO NPs）and cerium oxide nanoparticles（Ce O2 NPs）offer promising strategies for the improvement of novel agrochemical formulations of fertilizers.In this study,we subjected maize seedlings to investigate the toxic effects of Co on maize crops and unravel the underlying mechanism.Our results demonstrated that Co stress negatively affected plant growth and development by causing oxidative damage,nutrient deficiencies,and impairment of the photosynthetic apparatus.The increasing concentration of Co led to the overproduction of ROS both in root and shoot that principally caused oxidative damage as monitored by high MDA contents,cell death and ultrastructural damage via transmission electron microscopy.The results were further confirmed with ROS staining fluorescence via laser confocal microscopy.In response to stress,we observed an increasing trend in plant antioxidant defense activities with increasing Co concentration but it suddenly dropped at the highest used concentration of Co.More importantly,these results suggest that Co stress disrupts redox homeostasis which leads to the overproduction of ROS and causes oxidative damage.Our findings may pave potential strategies to develop resilient plant systems in order to increase crop production in Co-contaminated areas.Furthermore,we used nanoparticles as seed priming agent to mitigate Co induced toxicity and improve plant growth under stress conditions.The application NPs via seed priming is an innovative and cost-effective approach that improves seed germination and subsequent plant growth by activating plant physiological processes and providing tolerance against various stresses.Our results demonstrated that ZnO NPs significantly improved the plant growth,biomass,and photosynthetic machinery in maize under Co stress.The NPs priming reduced ROS and MDA accumulations in maize shoots.More importantly,ZnO NPs alleviated the toxic effects of Co by decreasing its uptake and conferred stability to plant ultra-cellular structures and photosynthetic apparatus.Furthermore,a higher accumulation of nutrient content and antioxidant enzymes were found in NPs-primed seedlings.Collectively,we provide the first evidence to demonstrate the alleviation of Co toxicity via ZnO NPs seed priming in maize,thus,illustrating the potential role of ZnO NPs to be applied as a stress mitigation agent for the crops grown in Co contaminated areas to enhance crop growth and yield.Furthermore,we checked the combined application of nanoparticles（Ce O2）together with hormones（brassinosteroids）.Cerium oxide nanoparticles have recently emerged as potent ROS scavengers and a potential agent to mitigate abiotic stress by acting as nanozyme.It is widely used in protecting plants from abiotic-induced stress and promoting plant growth and development.Similarly,brassinosteroids（BRs）is a new phytohormonal steroid and plant growth promotor that have the potential to reduce various environmental stresses and promote plant growth and development.Our results revealed various Co induced detrimental effects on maize such as inhibition of growth and photosynthesis-related attributes.The application of CeO₂ NPs and/or BRs reduced the oxidative damage by reducing ROS level,MDA content and enhanced plant antioxidant enzymatic activities.Similarly,the application of CeO₂ NPs and/or BRs restored the ultrastructural damage and improved light-harvesting pigments and other photosynthesis and growth-related attributes.More importantly,the improvement in growth and related attributes was more pronounced with the combine application of CeO₂ NPs and BRs under Co stress.Collectively,our observations depict that the application of CeO₂ NPs and BRs could be a beneficial practice in order to uplift plant tolerance against Co stress in crops grown in Co contaminated areas.