Physiological, biochemical and molecular characteristics associated with virulence of Burkholderia glumae: the major causative agent of bacterial panicle blight of rice

Burkholderia glumae is the bacterial pathogen causing bacterial panicle blight disease of rice (BPB). The pathogen, whose growth and pathogenicity is favored by high temperatures, has become a serious threat to rice production around the world possibly due to the current global climate changes. In this study, previously reported avirulent B. glumae strains were characterized in respect to the production of known virulence factors, toxoflavin, lipase and motility, and potential virulence factors, including polygalacturnase and type III secretion system (hypersensitive response). Considerable phenotypic variation was observed among the strains tested. Virulence of the B. glumae strains was closely related to their ability to produce various virulence factors. Interestingly, all the confirmed avirulent strains were defective in multiple virulence factors and most of them lost their ability to produce acyl-homoserine lactone (AHL) quorum-sensing signals implying that mutation in global regulatory system(s) for the virulence factors may be the major cause of the occurrence of avirulent B. glumae strains in nature. We developed a high throughput method for virulence tests of B. glumae by using onion bulb scales instead of rice panicles. The results indicated that the onion assay system can be a convenient alternative way for initial and preliminary virulence testing of B. glumae. In addition, some B. glumae strains produce melanin-like brown pigment. In order to identify the biosynthetic pathway and regulatory genes of this mechanism and role in virulence, the B. glumae 411gr-6, brown pigment producing strain genome was randomly mutagenized with a mini-Tn5 derivative, mini-Tn5gus. From this mutagenesis, several novel regulatory elements for B. glumae virulence factors were identified, including putative sensor histidine kinase, Clp protease, histone H1 like protein, prephenate dehydrogenase, and a putative sigma54 dependent response regulator. The melanin-like brown pigment of B. glumae may be involved in the hypervirulence of the pathogen providing a survival advantage under adverse climatic condition over the melanin non-producing strains