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

Brassinosteroid (BR) is essential in regulating plant growth and development and response to stress. However, there are few reports on the mechanism of BR regulating rice resistance to necrotrophic fungus. In this study, rice seedlings were pretreated with BR hormone and its synthetic inhibitor Brassinazole (BRZ) and inoculated with Rhizoctonia solani to analyze the reactive oxygen species (ROS), photosynthetic indices, and expression of phytohormones signal components and defense-related genes in rice leaves during pathogen infection. BR treatment significantly decreased the lesion area, significantly increased the activity of antioxidant enzymes and the content of antioxidant substances, and significantly decreased the contents of O 2 -, H 2 O 2 , and MDA. At the same time, BR treatment enhanced photosynthetic pigment content and Fv/Fm 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 SA, and antagonize JA signal gene expression. The structural equation analysis of tested indices uncovered firstly that high BR level stimulated the BR signal transduction pathway to regulate photosynthesis and ROS homeostasis through ROS signal, thereby enhancing the resistance of rice seedlings to R. solani. This study provides theoretical guidance for the application of BR analogues chemical regulators.