Bacteria encode numerous antagonistic traits, including narrow-spectrum proteinaceous toxins, termed bacteriocins. Although bacteriocins are ubiquitous across phylogenetically diverse bacteria, in most cases, we lack sufficient understanding of their specific adaptive benefit. In particular, the patterns and processes underpinning the evolution of bacteriocins, as well as the environmental signals that stimulate their expression, are largely unknown. The goal of this study is to try to understand the evolutionary relationships and environmental conditions that contribute to regulation of bacteriocins in Pseudomonas syringae. P. syringae has served as a model plant epiphyte and pathogen for several decades, providing both a deep understanding of its ecology and evolution, as well as a developed molecular toolbox that can be used to assess environmental gene regulation. To address the question of evolution, this study will first use bioinformatic techniques to map out regions of bacteriocins in known P. syringae and possible evolutionary relationships. To address the question of regulation, the model phage-like bacteriocin from P. syringae, B728a, will be utilized to measure bacteriocin expression in combination with a fluorescent protein transcriptional reporter in various agriculturally important phyllosphere conditions. This study will combine knowledge of evolution and the environment to have a more comprehensive grasp of the role bacteriocins play to address questions regarding agriculturally important interactions in the phyllosphere.