Exogenous L-Citrulline Increases Tolerance to Tomato Brown Rugose Fruit Virus in Tomato Plants

Tomato brown rugose fruit virus (ToBRFV) is one of the most aggressive viral pathogens affecting commercial tomato cultivars, causing severe economic losses worldwide. Since plant viral diseases cannot be effectively controlled with pesticides, alternative strategies aimed at reducing disease severity and enhancing plant tolerance are urgently needed but scarcely explored. In this study, L-citrulline, an amino acid known to improve plant tolerance to abiotic stress, was evaluated for its potential to increase tomato tolerance to ToBRFV infection. Healthy tomato seedlings were grown in a greenhouse and treated weekly with foliar applications of citrulline for six weeks. One week after the first treatment, half of the plants were mechanically inoculated with ToBRFV. Disease progression was evaluated by assessing disease severity and virus accumulation in plants. Tomato morphological and physiological traits, including plant height, biomass, leaf area, color and number, and chlorophyll content, were also assessed. Citrulline treatment significantly reduced the disease severity index by 64.5% and improved growth and physiological performance in ToBRFV-infected plants, despite having no significant effect on viral accumulation. Treated infected plants also exhibited partial recovery from typical ToBRFV leaf symptoms. These findings provide the first evidence that L-citrulline application can enhance tolerance to ToBRFV in tomato plants by reducing disease severity and ameliorating plant performance. This study highlights the potential of L-citruline as a sustainable strategy for management of ToBRFV infection in tomato plants.