Effect of plant systemic resistance elicited by biological and chemical inducers on the colonization of the lettuce and basil leaf apoplast by Salmonella enterica.

Mitigation strategies to prevent microbial contamination of crops are lacking. We tested the hypothesis that induction of plant systemic resistance by biological (ISR) and chemical (SAR) elicitors reduces endophytic colonization of leaves by Salmonella enterica serovars Senftenberg and Typhimurium. S. Senftenberg had greater endophytic fitness than S. Typhimurium in basil and lettuce. The apoplastic population sizes of serovars Senftenberg and Typhimurium in basil and lettuce, respectively, were significantly reduced approximately 10- to 100-fold by root treatment with microbial inducers of systemic resistance compared with the H2O treatment. Rhodotorula glutinis effected the lowest population increase of S. Typhimurium in lettuce (120-fold) and S. Senftenberg in basil leaves (60-fold) compared with the H2O treatment over 10 days post-inoculation. Trichoderma harzianum and Pichia guilliermondii did not have any significant effect on S. Senftenberg in the basil apoplast. The chemical elicitors acidobenzolar-S-methyl and DL-β-amino-butyric acid inhibited S. Typhimurium multiplication in the lettuce apoplast 10- and 2-fold, respectively, compared with H2O-treated plants. All ISR and SAR inducers applied to lettuce roots in this study increased leaf expression of the defense gene PR1, as did Salmonella apoplastic colonization in H2O-treated lettuce plants. Remarkably, both acidobenzolar-S-methyl- and R. glutinis-upregulation of PR1 was repressed by the presence of Salmonella in the leaves. However, enhanced PR1 expression was sustained longer and at greater levels upon elicitor treatment than by Salmonella induction alone. These results serve as proof of concept that priming of plant immunity may provide an intrinsic hurdle against the endophytic establishment of enteric pathogens in leafy vegetables. Importance Fruit and vegetables consumed raw have become an important vehicle of foodborne illness despite a continuous effort to improve their microbial safety. Salmonella enterica has caused numerous recalls and outbreaks of infection associated with contaminated leafy vegetables. Evidence is increasing that enteric pathogens can reach the leaf apoplast where they confront plant innate immunity. Plants may be triggered for induction of their defense signaling pathways by exposure to chemical or microbial elicitors. This priming for recognition of microbes by plant defense pathways has been used to inhibit plant pathogens and limit disease. Given that current mitigation strategies are insufficient in preventing microbial contamination of produce and associated outbreaks, we investigated the effect of plant induced resistance on S. enterica colonization of the lettuce and basil leaf apoplast in order to gain a proof of concept for the use of such an intrinsic approach to inhibit human pathogens in leafy vegetables.