A molecular genetic study on the Tofl/TofR quorum-sensing system of Burkholderia glumae: the major pathogen that causes bacterial panicle blight of rice

Burkholderia glumae is the major causal agent of an economically important rice disease, bacterial panicle blight (BPB). The known virulence factors of B. glumae share the TofI/TofR quorum sensing system as their regulator. tofI and tofR genes encode the N-acyl homoserine lactone (AHL) synthase for the B. glumae quorum sensing signals, N-octanoyl homoserine lactone (C8-HSL) and N-hexanoyl homoserine lactone (C6-HSL), and the receptor for AHL, respectively. To better understand the relationship between quorum sensing and known virulence factors (toxoflavin, flagella and lipase), as well as, putative virulence factors (i.e. extracellular polysaccharide), mutagenetic and phenotypic analyses were applied to this study. A technical breakthrough is the creation of a novel deletion mutation system-pBBSacB vector, which can effectively delete target genes from the genome and gives more reliable results. Quorum sensing gene deletion mutants were successfully created by using pBBSacB with a sucrose-sensitive counter selective marker, SacB. The parental strain 336gr-1 and its mutants have undergone a series of phenotypic observations and quantification tests for virulence changes. Toxoflavin and swarming motility were confirmed as the major virulence factors in 336gr-1, whereas lipase and EPS were not determined as critical for causing symptoms. The results confirmed the importance of quorum sensing system in expressing virulence, but also indicated that other regulators may be implicated in pathogenicity. Additionally, orf1, which is located between tofI and tofR, was postulated as a functional regulatory component