Microstructural and metabolic variations induced by Bipolaris oryzae inciting brown spot disease of rice

Brown spot disease, caused by Bipolaris oryzae, is a dominant lethal rice disease that causes qualitative and quantitative crop damage. The current study sought to identify various histological and metabolic changes that occur during brown spot development in susceptible rice plants. We present a conceptual framework that shows B. oryzae suppresses the production of immune-related metabolites in a susceptible cultivar, PR 124 using a comparative metabolomics approach. Un-inoculated rice leaves have an epidermis followed by cortex parenchyma with large intercellular spaces, and no fungal hyphae or distortion were found. Following pathogen inoculation, fungus hyphae grow intercellularly in photosynthetic areas and intracellularly in the bundle sheath, resulting in the microcracks on the surface of the rice leaf. Cellular depositions could have produced the clogging, which disrupted water channels and induced distortion of vascular bundles, ultimately leading to cellular collapse and the withering of rice plants under field conditions. Silica on an infected leaf surface suggests a more robust defence response, thus providing some degree of endurance at the later stages of infection. A significant decline in the total chlorophyll and lignin content was observed in the inoculated leaves compared to the un-inoculated ones. Higher relative injury was recorded post-inoculation. Early oxidative responses like malondialdehyde, proline and hydrogen peroxide accumulation occurred in the flag leaves at various intervals after inoculation. Reduced salicylic acid, phenol and lignin content post-inoculation could be attributed to lowered phenylalanine ammonia lyase activity. Significant declines in the activities of catalase, peroxidase, chitinase and glucanase suggest that immune suppression by this biotrophic pathogen impacts specialised plant metabolism. Thus, these findings form the basis for additional studies focussed on the characterisation of metabolic components involved in pathogen perception during the early stages of intracellular signal transduction.