Plant-microbe Interactions Associated with Noroviruses and Leafy Greens

Introduction: Norovirus is the leading cause of acute gastroenteritis, including illness linked to contaminated produce. Methods by which norovirus comes into contact with and persists within the phyllosphere and rhizosphere of leafy greens is a complex issue that remains unsolved. The fields of food safety and plant science are merging to address plant-norovirus interactions to develop control strategies to reduce the number of illnesses associated with norovirus and leafy greens.

Purpose: The purpose of this study was to determine if norovirus elicits an immune response in plants. We also sought to determine if plant signaling hormones affect the persistence of internalized norovirus. 

Methods: Whole Arabidopsis thaliana plants or Romaine lettuce leaves were pooled (n = 4) and frozen in liquid nitrogen prior to RNA extraction and semi-quantitative PCR. Gene expression was quantified using markers for plant immune response pathways. Salicylic acid (SA) persistence studies were performed at 22°C for 20 min. SA was neutralized with FBS and total viral RNA extraction was performed prior to qPCR.

Results: In Arabidopsis thaliana, murine norovirus (MNV) was able to elicit an immune response through both the SA and jasmonic acid pathways shown through increased (> 2-fold) expression of PR-1, PDF1.2 and VSP2a at 6 hpi. Human norovirus was able to induce a 4-fold increase in the expression of the NPR-1 gene in lettuce 12 hpi indicating stimulation of the SA pathway. MNV and Tulane virus (TV) were shown to survive for 20 min in 500 mM (pH 2.4) SA indicating the ability of internalized norovirus to survive in plants (P = 0.80) although TV was able to persist at significantly higher levels in SA compared to MNV (P = 0.029).

Significance: This is the first study to show the biological recognition of human norovirus by plants as well as the ability of internalized virus to survive within plants during an immune response.