Plant-microbe co-evolution: allicin resistance in a Pseudomonas fluorescens strain ( Pf AR-1) isolated from garlic

The antibiotic defense substance allicin (diallylthiosulfinate) is produced by garlic ( L.) after tissue damage, giving garlic its characteristic odor. Allicin is a redox-toxin that oxidizes thiols in glutathione and cellular proteins. A highly allicin-resistant strain (AR-1) was isolated from garlic, and genomic clones were shotgun electroporated into an allicin-susceptible strain (4612). Recipients showing allicin-resistance had all inherited a group of genes from one of three similar genomic islands (GI), that had been identified in an analysis of the AR-1 genome. A core fragment of 8-10 congruent genes with redox-related functions, present in each GI, was shown to confer allicin-specific resistance to , and even to an unrelated strain. Transposon mutagenesis and overexpression analyses revealed the contribution of individual candidate genes to allicin-resistance. Moreover, AR-1 was unusual in having 3 () genes, two copies in two of the GIs, but outside of the core group, and one copy in the AR-1 genome. Glr activity was approximately 2-fold higher in AR-1 than in related susceptible 0-1, with only a single gene. Moreover, an Δ mutant showed increased susceptibility to allicin, which was complemented by AR-1 . Taken together, our data support a multi-component resistance mechanism against allicin, achieved through horizontal gene transfer during coevolution, and allowing exploitation of the garlic ecological niche. GI regions syntenic with AR-1 GIs are present in other plant-associated bacterial species, perhaps suggesting a wider role in adaptation to plants .