Salmonella Newport Interacts with Plant-derived Reactive Oxygen and Nitrogen Species on Tomato Fruit and Leaves

Introduction: Plants recognize pathogens via pathogen-associated molecular patterns (PAMPs), mounting basal defense responses that are modulated by reactive nitrogen species (RNS) and reactive oxygen species (ROS). Plants are capable of recognizing enteric bacterial PAMPs, however the impact that the ensuing responses have on enteric bacterial colonization of plants is not known.

Purpose: The purpose of this project was to determine whether Salmonella enterica elicits generation of RNS and ROS in tomato seedlings and fruit, and in turn assess the effect of plant-derived nitric oxide (NO) and hydrogen peroxide (H₂O₂) on S. enterica populations.

Methods: Fruit of tomato cultivars (cv.) ‘Nyagous’, ‘Money maker’, and ‘Heinz’ and seedlings of ‘Heinz’ were treated with the NO scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or water (control), followed by Salmonella Newport or Salmonella Typhimurium inoculation, incubation, and retrieval for assessment of growth. To evaluate generation of ROS, 3 week old ‘Heinz’ leaves were syringe infiltrated with Salmonella Newport or water (control), and H₂O₂ production was measured through 3,3’-Diaminobenzidine (DAB) staining.

Results: Fruit of cv. ‘Heinz’ (P=0.034), ‘Nyagous’ (P=0.001) and ‘Money Maker’ (P=0.021) supported significantly higher Salmonella Newport populations on NO-scavenged tissue relative to control (n=5/treatment).  ‘Heinz’ fruit showed suppression of growth earlier (24 hours post-inoculation (hpi)) compared to other cultivars (48 hpi). Significantly larger populations of Salmonella Newport (P=0.008), but not the rpoS partial mutant Salmonella Typhimurium LT2, were measured on NO-scavenged leaves of sterile ‘Heinz’ seedlings.  ImageJ analysis of DAB staining revealed that Salmonella Newport infiltrated leaves produced significantly more H₂O₂ than the water control (P<0.001).

Significance: Cultivar-specific elicitation of NO and H₂O₂ in response to S. enterica was observed, with NO impacting Salmonella Newport populations on tomato fruit and leaves. This study suggests that S. enterica colonization triggers a basal immune response in tomato that restricts its growth in the tomato phyllosphere.