Genome Sequencing and Comparative Transcriptomic Analysis of Rice Brown Spot Pathogen Bipolaris oryzae Adaptation to Osmotic Stress

Rice brown spot disease, caused by <i>Bipolaris oryzae</i>, is a significant fungal disease that poses a major threat to global rice production. Despite its widespread impact, genomic studies of <i>B. oryzae</i> remain limited, particularly those involving high-quality genomic data. In this study, we performed whole-genome sequencing of the <i>B. oryzae</i> strain RBD1, which was isolated from the demonstration field for upland rice cultivation in Haozhiba Village, Lancang County, Pu'er City, Yunnan Province, China, using a combination of second-generation Illumina sequencing and third-generation Single-Molecule Real-Time (SMRT) sequencing. The assembled genome was 37.5 Mb in size with a G + C content of 49.39%, containing 42 contigs with a contig N50 of 2.0 Mb. Genomic analysis identified genes related to carbon, nitrogen, and lipid metabolism, highlighting the strain's metabolic flexibility under diverse environmental conditions and host interactions. Additionally, we identified pathogenicity-related genes involved in MAPK signaling, G protein signaling, and oxidative stress responses. Under 1.2 M sorbitol-induced osmotic stress, we observed significant differences in growth responses between RBD1 and the rice blast fungus <i>Magnaporthe oryzae</i> H7. Transcriptomic analysis using Illumina sequencing revealed that RBD1 responds to osmotic stress by enhancing carbohydrate metabolism, fatty acid degradation, and amino acid synthesis, while H7 primarily relies on protein synthesis to enhance growth tolerance. This study provides a valuable foundation for understanding the pathogenic mechanisms of rice brown spot and future disease control strategies.