The Plant Rhizosphere-Phyllosphere Connection: Analysis of root-associated Pseudomonas sp. -induced changes in Fruit Surface Phytochemical Profiles of Heirloom and Modern Tomato cultivars and effect on Salmonella enterica association

Abstract

Despite more rigorous food safety regulation for fruit and vegetable production, foodborne illnesses and massive food recalls have resulted from the enteropathogen Salmonella enterica on Solanum lycopersicum (tomato) fruits. While some phytochemical profiles have been studied on modern cultivars in regards to minimizing Salmonella-plant association, little research has been done on heirloom varieties. The interaction between the plant rhizosphere and phyllosphere remains understudied and the inoculation of plants with plant-growth promoting rhizobacteria (PGPR) can modify phyllosphere components of plants including fruit properties. No research exists on whether PGPR colonization of tomato plant roots can alter the fruit’s phytochemical profile to affect Salmonella association. Through several chemical and microbial analyses, I investigated the Salmonella association with fruit of modern and heirloom cultivars, as well as the impact of PGPR inoculation on this system. In Chapter 3, I assessed fruit of modern and heirloom varieties, and explored possible associations between Salmonella growth in fruit surface washes and total sugar quantifications, citric acid and (for heirloom varieties) the fatty acid profiles. Microbial counts differed from variety to variety and heirloom varieties demonstrated higher levels of sugars and citric acid compared to modern varieties. Total sugar quantifications and citric acid were correlated with microbial counts in heirloom varieties, but not in modern varieties. Chapter 4 focuses on the effects of inoculating the heirloom varieties with a PGPR in the genus Pseudomonas sp.on the fruit surface phytochemical profile and changes in microbial association. Differing trends between the PGPR inoculated and non-inoculated groups were seen where citric acids were correlated with Salmonella association for the non-PGPR group, but not for the PGPR inoculated group. For both the PGPR inoculated and non-inoculated groups, we measured a high degree of variation between the cultivars for the phytochemical profiles, and for both groups hexadecanoic acid was negatively correlated with microbial counts. This study furthers the understanding of the relationship between tomato fruit phytochemistry and Salmonella association, as well as the effects of PGPR on modifying tomato fruit phytochemistry. These findings can help manage and improve the pre- and post-harvest food safety of tomato fruit.