Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Expression of type III secretion system (T3SS) genes, which are important for the virulence of phytopathogenic bacteria, is induced in the plant apoplastic environment or artificially amended growth conditions. Wild‐type <jats:italic>Burkholderia glumae</jats:italic> BGR1, which causes rice panicle blight, induced a hypersensitive response (HR) in tobacco plants, whereas the T3SS genes were not significantly expressed in the commonly used <jats:italic>hrp</jats:italic> induction medium. T3SS gene expression in <jats:italic>B. glumae</jats:italic> was dependent on HrpB, a well known T3SS gene transcriptional regulator. Here, we report a stepwise mechanism of T3SS gene regulation by the GluR response regulator and Lon protease in addition to HrpB‐mediated control of T3SS genes in <jats:italic>B. glumae</jats:italic>. The <jats:italic>gluR</jats:italic> mutant showed no HR in tobacco plants and exhibited attenuated virulence in rice plants. GluR directly activated <jats:italic>hrpB</jats:italic> expression, indicating that <jats:italic>hrpB</jats:italic> belongs to the GluR regulon. The <jats:italic>lon</jats:italic> mutation allowed high expression of the T3SS genes in nutrient‐rich media. Lon directly activated <jats:italic>gluR</jats:italic> expression but repressed <jats:italic>hrpB</jats:italic> expression, indicating that Lon acts as a regulator rather than a protease. However, the <jats:italic>lon</jats:italic> mutant failed to induce an HR and virulence, suggesting that Lon not only acts as a negative regulator, but also has an essential, yet to be determined role for T3SS. Our results demonstrate the involvement of the two‐component system response regulator GluR and Lon in T3SS gene regulation, providing new insight into the complex interplay mechanisms of regulators involved in T3SS gene expression in bacteria–plant interactions.</jats:p>