Bacterial blight of soybean: regulation of a pathogen gene determining host cultivar specificity.

Soybean cultivars resistant to Pseudomonas syringae pathovar glycinea (Psg), the causal agent of bacterial blight, exhibit a hypersensitive (necrosis) reaction (HR) to infection. Psg strains carrying the avrB gene elicit the HR in soybean cultivars carrying the resistance gene Rpg1. Psg expressing avrB at a high level and capable of eliciting the HR in the absence of de novo bacterial RNA synthesis have been obtained in in vitro culture. Nutritional signals and regions within the Psg hrp gene cluster, an approximately 20-kilobase genomic region also necessary for pathogenicity, control avrB transcription.