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Abstract
Pantoea ananatis has emerged as one of the most economically impactful diseases of onion in the U.S. state of Georgia, with severe yield loses of up to 90% under permissive conditions. Unlike most characterized bacterial plant pathogens, P. ananatis lacks both the virulence-associated type III and type II secretion systems. In the absence of these typical virulence factors, P. ananatis induces necrotic symptoms and extensive cell death in onion tissue with a small molecule phosphonate toxin, pantaphos. Onion produces volatile antibacterial thiosulfinates upon cellular damage. However, the roles of endogenous thiosulfinate production in host-bacterial pathogen interactions have not been described. We conducted whole genome sequencing and pathogenicity testing on a panel of ten P. ananatis strains isolated from the Georgia farmscape. Our comparative genomic analysis indicated that six of the ten strains caused foliar necrosis and shared four plasmid-borne gene clusters, termed Onion Virulence Regions (OVR A-D) that were hypothesized to contribute to strain virulence. Experiments probing the function of the OVR gene clusters indicated that the first portion of the OVR A cluster was genetically required for P. ananatis to colonize necrotized onion tissue and its capacity to tolerate the thiosulfinate ‘‘allicin’’ based on the presence of an eleven-gene, plasmid-borne virulence cluster of sulfur redox genes. We named this eleven gene cluster the allicin tolerance cluster, alt. This novel virulence factor implicates endogenous onion thiosulfinates produced during cellular damage as major mediators of interactions with this bacterial pathogen. We also investigated the distribution of P. ananatis virulence genes by screening three unique collections of various Pantoea strains for the presence of absence of identified pathogenicity factors. We found an enrichment of alt positive strains among Pantoea strains isolated from diseased onion compared to those isolated from other sources. Future disease control strategies may rely on genetically manipulating the onion host to increase defensive thiosulfinates, which may translate to increased resistance to this bacterium that threatens Georgia’s Vidalia Onions.