Distinct Virulence Mechanisms of Burkholderia gladioli in Onion Foliar and Bulb Scale Tissues

Slippery skin of onion caused by Burkholderia gladioli pv. alliicola (Bga) is a common bacterial disease reported from onion growing regions around the world. Despite the increasing attention in recent years, our understanding of the virulence mechanisms of this pathogen remains limited. In this study, we characterized the predicted genetic determinants of virulence in Bga strain 20GA0385 using reverse genetics approach. Using the closely related rice pathogen, B. glumae as a reference, comparative genomics analysis was performed to identify Bga candidate virulence factors and regulators. Marked and unmarked deletion mutants were generated using allelic exchange and the mutants were functionally validated using in vitro and in vivo assays. The role of mutants in pathogenic phenotypes was analyzed using onion foliar/seedling necrosis assays, the Red Scale Necrosis (RSN) assay and in planta bacterial population counts. The phytotoxin toxoflavin was a major contributor to foliar necrosis and bacterial populations whereas the type II and type III secretion system (T2SS/T3SS) were dispensable for foliar symptoms. In onion scale tissue, the T2SS single mutant gspC and its double and triple mutant derivatives all contributed to scale lesion area. Neither the lipocyclopeptide icosalide, toxoflavin, nor T3SS were required for scale symptoms. Our results suggest the quorum sensing tofIMR system in Bga regulates, toxoflavin, T2SS, and T3SS, contributing to onion symptom production. We show different virulence factors contribute to onion tissue-specific virulence patterns in Bga and that decreases in scale symptoms often do not result in decreased Bga populations in onion tissue.