Abstract
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Rice (Oryza sativa L.) production is constrained by fungal diseases such as brown spot caused by Bipolaris oryzae. Enhancing the endogenous defense capacity of rice plants represents a promising strategy for developing more sustainable approaches to disease management. This study investigated the effects of brassinosteroid treatment on disease resistance, plant growth, defense related gene responses, and total phenolic accumulation in rice challenged with B. oryzae under greenhouse conditions. Rice plants were inoculated with a conidial suspension of B. oryzae at 3 × 10⁵ conidia mL⁻¹ and subjected to four treatments: untreated control, NPK fertilizer, brassinosteroid, and conventional fungicide. Disease incidence and severity, plant growth parameters, and selected yield related traits were evaluated following pathogen inoculation. In addition, the expression of the defense-related genes PR2 and PR9, together with total phenolic content, was assessed to investigate the biochemical and molecular responses associated with disease resistance. Significant differences among treatments were observed for disease development, growth parameters, PR2 and PR9 expression, and total phenolic accumulation (p < 0.05). Compared with the untreated inoculated control, brassinosteroid treated plants showed a 48.5% reduction in disease severity, accompanied by increases of 34.7% and 60.9% in PR2 and PR9 expression, respectively. Total phenolic content increased by 52.73% following brassinosteroid application, suggesting enhanced activation of phenolic based defense mechanisms. The fungicide treatment reduced disease severity by 67.25%, whereas NPK primarily promoted vegetative growth. The coordinated increase in PR2 and PR9 expression and total phenolic accumulation in brassinosteroid treated plants was associated with improved resistance to B. oryzae. Overall, these findings provide evidence that brassinosteroids can enhance rice defense through complementary molecular and biochemical pathways and highlight their potential as biological regulators for strengthening plant immunity. Their integration into integrated disease management strategies could contribute to reducing dependence on conventional fungicides while maintaining rice health and productivity.
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