Characterization of virulence mechanisms of Burkholderia glumae and B. gladioli causing bacterial panicle blight of rice in Bangladesh

Bacterial panicle blight (BPB) of rice, caused by B. glumae and B. gladioli , threatens global rice yields and has been reported in Bangladesh. This study aimed to isolate and characterize BPB-associated Burkholderia strains, investigate their virulence mechanisms, and provide a basis for potential disease management strategies. Three hundred BPB-infected samples from 20 Bangladesh districts were analyzed using S-PG medium and gyrB PCR amplification, identifying 46 putative B. gladioli isolates and 5 putative B. glumae isolates. Twenty of these isolates were chosen for in-depth characterization. The virulence of these isolates was assessed using rice panicle inoculation and onion bulb assays, alongside phenotypic tests for toxoflavin production, hydrolytic enzyme activities, motility, and type III secretion. Comparative sequence analyses of BDBgla132A and BD_21g characterized targeted virulence-associated gene repertoires and amino acid identities. The expression of selected virulence genes was quantified by RT-qPCR in vitro to relate transcriptional profiles to phenotypic virulence traits. Pathogenicity tests identified the representative isolate BD_21g ( B. glumae ) as the most virulent, followed by BDBgla132A ( B. gladioli ). Disease severity on rice strongly correlated with onion bulb assays, validating them as a rapid virulence-screening tool. Phenotypic characterization of the 20 isolates revealed substantial variation in toxoflavin production, lipase activity, polygalacturonase activity, motility, and type III secretion system activity. Comparative sequence analysis of virulence-associated genes between BDBgla132A and BD_21g revealed high protein sequence identity, particularly within toxoflavin biosynthesis ( toxB , 98.11%) and transport ( toxH , 97.48%) genes. Conversely, genes encoding lipase ( lipA / lipB ), polygalacturonase ( pehA / pehB ), and motility displayed moderate to high identity. Both species retained virulence-related genes that are homologous to those of B. cepacia , but the examined isolates displayed differential virulence strategies. RT-qPCR demonstrated significantly higher expression of toxoflavin- and lipase-encoding genes in the characterized isolate BD_21g compared with BDBgla132A, which strongly correlates with BD_21g’s elevated enzymatic activities. Conversely, BD_21g showed lower expression of pectinolytic and flagellar genes than BDBgla132A, consistent with the greater pectinolytic activity and motility observed in BDBgla132A. These findings demonstrate that the representative isolates BD_21g ( B. glumae ) and BDBgla132A ( B. gladioli ) differ in virulence-associated gene expression and pathogenicity during rice infection, providing insights into potential BPB management approaches in Bangladesh.