Plant-microbe co-evolution: allicin resistance in aPseudomonas fluorescensstrain (PfAR-1) isolated from garlic

Abstract

The antibiotic defense substance allicin (diallylthiosulfinate) is produced by garlic (Allium sativumL.) after tissue damage, giving garlic its characteristic odor. Allicin is a redox-toxin that oxidizes thiols in glutathione and cellular proteins. A highly allicin-resistantPseudomonas fluorescensstrain (PfAR-1) was isolated from garlic, and genomic clones were shotgun electroporated into an allicin-susceptibleP. syringaestrain (Ps4612). Recipients showing allicin-resistance had all inherited a group of genes from one of three similar genomic islands (GI), that had been identified in anin silicoanalysis of thePfAR-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 toP. syringae, and even to an unrelatedE. colistrain. Transposon mutagenesis and overexpression analyses revealed the contribution of individual candidate genes to allicin-resistance. Moreover,PfAR-1 was unusual in having 3glutathione reductase(glr) genes, two copies in two of the GIs, but outside of the core group, and one copy in thePfAR-1 genome. Glr activity was approximately 2-fold higher inPfAR-1 than in related susceptiblePf0-1, with only a singleglrgene. Moreover, anE. coliΔglrmutant showed increased susceptibility to allicin, which was complemented byPfAR-1glr1. 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 withPfAR-1 GIs are present in other plant-associated bacterial species, perhaps suggesting a wider role in adaptation to plantsper se.