Evidence for Burkholderia gladioli pv. alliicola Extracellular Detoxification of Thiosulfinates

Onion tissues produce antimicrobial thiosulfinates after tissue damage and cellular decompartmentalization. Burkholderia gladioli pv. alliicola (Bga), a common onion pathogen, encodes a thiosulfinate tolerance gene (TTG) cluster that protects the bacterium during thiosulfinate exposure. Previous work showed that the TTG cluster contributes to foliar infection but has little effect on infection of onion bulb tissue. To further examine Bga-thiosulfinate interactions in foliar and bulb tissues, we used a thiosulfinate-responsive PaltR-Lux reporter strain to determine when and where Bga encounters thiosulfinates.

In leaves, Bga-induced necrosis was associated with de-repression of the P _altR -Lux reporter and coincided with a contribution of the TTG cluster to bacterial population size, indicating thiosulfinate exposure during foliar infection. In contrast, TTG mutants and wild-type (WT) strains showed similar growth in scales, and P _altR -Lux signal declined as scale necrosis progressed, suggesting limited thiosulfinate exposure during bulb colonization. However, when necrosis was induced by the non-native toxin pantaphos, P _altR -Lux was de-repressed and recovery of the TTG mutant was reduced. These results indicate that Bga encounters thiosulfinates during foliar infection but largely avoids exposure during bulb infection.

Preconditioning the TTG mutant in onion scale tissue did not alter its thiosulfinate sensitivity in vitro, arguing against an infection-associated thiosulfinate exclusion mechanism. In contrast, partial rescue of the TTG mutant by the WT strain in zone-of-inhibition co-plating assays suggests extracellular thiosulfinate detoxification. Together, these findings indicate that Bga detoxifies thiosulfinates released during bulb necrosis, limiting thiosulfinate exposure during onion bulb infection. The molecular basis for detoxification and tissue specificity remain unresolved.