Comparative Phenotypic and Genomic Analysis of Virulence-Associated Factors of <em>Burkholderia glumae</em> and <em>B. gladioli</em> Causing Bacterial Panicle Blight in Rice in Bangladesh

Bacterial panicle blight (BPB) of rice, a disease caused by Burkholderia glumae and B. gladioli, threatens global rice yields and has recently emerged in Bangladesh. Three hundred BPB-infected samples from 20 Bangladesh districts were analyzed using S-PG medium and gyrB PCR amplification, identifying 46 B. gladioli and 5 B. glumae potential isolates. Twenty of these isolates were chosen for in-depth characterization. Pathogenicity tests identified BD_21g (B. glumae) as the most virulent strain, followed by BDBgla132A (B. gladioli). Disease severity on rice strongly correlated with onion bulb assays, validating the assay 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. Comparative genomic analysis of virulence-associated genes between BDBgla132A and BD_21g showed high protein sequence identity, particularly in toxoflavin biosynthesis and transport genes, while genes encoding lipase (lipA/lipB), polygalacturonase (pehA/pehB), and those involved in motility, displayed moderate to high identity. Both strains retained virulence-related genes that are homologous to those of B. cepacia but displayed differential virulence strategies. Retrotranscribed-qPCR revealed significantly higher expression of toxoflavin and lipase-encoding genes in BD_21g compared with BDBgla132A, consistent with its elevated enzymatic activities. Conversely, BD_21g showed reduced expression of pectinolytic and flagellar genes over BDBgla132A, consistent with the enhanced pectinolytic activity and motility observed in BDBgla132A. These findings reveal that BD_21g (B. glumae) and BDBgla132A (B. gladioli) rely on different virulence strategies to infect rice, providing critical insights for developing targeted BPB management approaches in Bangladesh.