Multiple Pathways Revealing the CeO<sub>2</sub> Nanoparticle-Biostimulant-Based “Stress Training” Strategy for Enhanced <i>Medicago sativa</i> L. Antiviral Capability
Zhipeng Guo, Tingting Zhang, Zhao Chen, Junpeng Niu, Md Rasel, Enze Luo, Yuetong Li, Nan Xu, Mahmood Ul Hassan, Hafiz Abdul Kareem, Quanzhen Wang, Xuebing Yan, Yuxia Guo
Abstract
“Nanophytovirology” is a potential management approach to combat plant viral diseases. Herein, the impact of foliar application of CeO 2 nanoparticles (CeO 2 -NPs) on the growth, plant cell ultrastructure, and physiology of alfalfa mosaic virus (AMV)-infected alfalfa seedlings was explored at different concentrations and application strategies. In a solar greenhouse, 20 mL of CeO 2 -NP suspensions of 50, 100, 200, and 500 mg/L was sprayed onto the plant surface every other day for 9 days. As a result, the total volume applied per pot was 100 mL. It showed that the beneficial effects of CeO 2 -NPs on infected alfalfa seedlings were dependent on the concentration and application period. Compared to the infected control, foliar application with 100 mg/L CeO 2 -NPs before AMV infection (CeO 2 -NPs-BVI) showed the greatest virus suppression efficacy; significantly reduced the disease indices by 58.87%; improved growth, yield, and nutritional quality; and significantly increased plant height, plant dry biomass, crude protein, and crude fat contents and relative feeding value by 15.17, 37.12, 18.77, 71.19, and 10.09%, respectively. Moreover, CeO 2 -NPs-BVI maintained the chloroplast quantity and structure of leaf cells and significantly enhanced the chlorophyll content and photosynthesis rate of alfalfa leaves by 36.14 and 40.13%, respectively, as compared to infected control. Mechanistically, cell ultrastructure, physiology, and transcriptomic analyses revealed that (1) CeO 2 -NPs effectively mitigated AMV-induced chloroplast structure damage, subsequently enhancing photosynthesis and carbon fixation in photosynthetic organisms, providing sufficient energy and antiviral activities for maintaining plant growth and development; (2) CeO 2 -NPs reduced the AMV’s ability to bind to host receptors and evade host immune recognition, significantly activating and boosting plant systemic immunity by down-regulating ABA and ETH levels and upregulating SA, IAA, and BR levels; (3) CeO 2 -NPs activated the plant antioxidative systems to eliminate excess reactive oxygen species. These findings provide significant insight into the potential of CeO 2 -NPs as highly efficient antiviral agent.