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

Authors: CHALUPOWICZ, LAURA - Volcani Center (ARO); MANULIS-SASSON, SHULAMIT - Volcani Center (ARO); BARASH, ISAAC - Tel Aviv University; ELAD, YIGAL - Volcani Center (ARO); RAV, DAVID - Tel Aviv University; Brandl, Maria

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2021
Publication Date: 11/24/2021
Citation: Chalupowicz, L., Manulis-Sasson, S., Barash, I., Elad, Y., Rav David, D., Brandl, M. 2021. Effect of plant systemic resistance elicited by biological and chemical inducers on the colonization of the lettuce and basil leaf apoplast by Salmonella enterica. Applied and Environmental Microbiology. 87(24):e01151-21. https://doi.org/10.1128/AEM.01151-21.
DOI: https://doi.org/10.1128/AEM.01151-21

Interpretive Summary: Mitigation strategies to prevent microbial contamination of crops are lacking. Evidence is increasing that enteric bacterial pathogens can reach the internal tissue of leaves where they confront plant innate immunity. We tested the hypothesis that systemic resistance to microbial colonists that is triggered by treatment of roots with chemical compounds or microorganisms can reduce internal colonization of leafy vegetables by Salmonella enterica. We used two model systems, namely basil colonization by S. enterica serovar Senftenberg and lettuce colonization by S. enterica serovar Typhimurium. Root treatment with microbial elicitors of the induced systemic resistance caused a 10- to 100-fold reduction in the population sizes of both S. Senftenberg and S. Typhimurium in basil and lettuce leaves, respectively, compared with the control (water) treatment. Treatment with Rhodotorula glutinis strain Y13 effected the lowest survival of S. enterica in both plant species. Furthermore, root treatment with acidobenzolar-S-methyl (Bion 50 WG) and DL-ß-amino-butyric acid (BABA) significantly inhibited internal multiplication of S. Typhimurium in lettuce leaves 10- and 2-fold compared with water-treated plants. All elicitors of induced resistance applied to lettuce roots in this study increased leaf transcription of PR1, a marker gene for the salicylic acid defense pathway, as did also colonization by Salmonella in water-treated lettuce plants. However, enhanced PR1 transcription was sustained longer upon inducer treatment than by Salmonella-triggered induction alone. These results serve as proof of concept that priming of plant immunity, whether prophylactically or via future plant technologies, may act as an intrinsic hurdle against the internal establishment of enteric pathogens in leafy vegetables.

Technical Abstract: Mitigation strategies to prevent microbial contamination of crops are lacking. Evidence is increasing that enteric bacterial pathogens can reach the leaf apoplast where they confront plant innate immunity. We tested the hypothesis that induction of systemic resistance by chemical and biological elicitors reduces endophytic colonization of leafy vegetables by Salmonella enterica serovars Senftenberg and Typhimurium. While S. Senftenberg appeared to have greater endophytic fitness than S. Typhimurium and other serovars, the population sizes of both S. Senftenberg and S. Typhimurium in the basil and lettuce apoplast, respectively, were similarly and significantly reduced approximately 10- to 100-fold by root treatment with biological elicitors of the induced systemic resistance (ISR) compared with the H2O treatment. Among the latter elicitors, Rhodotorula glutinis strain Y13 effected the lowest survival of S. enterica in the leaves over 10 days post-inoculation, while Bacillus subtilis strain Mel 16 also caused significant suppression of the pathogen. Furthermore, the SAR elicitors acidobenzolar-S-methyl (Bion 50 WG) and DL-ß-amino-butyric acid (BABA) significantly inhibited multiplication of S. Typhimurium in the lettuce apoplast 10- and 2-fold compared with H2O-treated plants. All ISR and SAR elicitors applied to lettuce roots in this study increased leaf expression of the defense gene PR1, a marker of the salicylic acid signaling pathway, as did also apoplastic colonization by Salmonella in H2O-treated lettuce plants. Elicitor-triggered upregulation of PR1 could not by itself explain trends in the inhibitory effect of Bion and ISR-inducing colonists and remarkably, was repressed by the concomitant presence of Salmonella in the leaves, indicating crosstalk by defense signaling pathways. However, enhanced PR1 expression was sustained longer upon elicitor treatment than by Salmonella-triggered induction alone. These results serve as proof of concept that priming of plant immunity, whether prophylactically or via future plant technologies, may act as an intrinsic hurdle against the endophytic establishment of enteric pathogens in leafy vegetables.