Exogenous Brassinosteroids Enhance Host Resistance to Rhizoctonia solani Through Reactive Oxygen Species Homeostasis and Photosynthesis Improvement in Oryza sativa

Brassinosteroids (BRs) are essential in regulating plant growth and development and response to stress. However, there are few reports on the mechanism of BRs regulating rice resistance to necrotrophic fungus. In this study, rice seedlings were pretreated with BR hormone and its synthetic inhibitor brassinazole and inoculated with Rhizoctonia solani to analyze the reactive oxygen species (ROS), photosynthetic indices, and expression of phytohormone signal components and defense-related genes in rice leaves during pathogen infection. BR treatment significantly decreased the lesion area, increased the activity of antioxidant enzymes and the content of antioxidant substances, and decreased the contents of superoxide anions, hydrogen peroxide, and malondialdehyde. At the same time, BR treatment enhanced the photosynthetic pigment content and Fv/Fm (maximum photochemical quantum yield of PSII) value of rice seedlings. In addition, BR treatment can cause high expression of endogenous BR synthesis and decomposition genes and signal transduction genes, cooperate with salicylic acid, and antagonize jasmonic acid signal gene expression. The structural equation analysis of tested indices uncovered first that a high BR level stimulated the BR signal transduction pathway to regulate photosynthesis and ROS homeostasis through ROS signaling, thereby enhancing the resistance of rice seedlings to R. solani. This study provides theoretical guidance for the application of BR analog chemical regulators.