This poster was presented at the 2025 IS-MPMI, Cologne Submitted Abstract Plasmids provide traits that can enable bacteria to adapt to unique and changing environments. Sphingomonas, an often-beneficial bacterial genus distributed across plants as well as other diverse environments, often have one or more native plasmids of varied sizes, and we identify many plasmids in our culture-collection through a long read sequencing approach. However, little is known about how these plasmids contribute to bacterial fitness or their potential roles in plant colonization. In this study, we used derivative strain of Sphingomonas melonis FR1, a known beneficial plant colonizer that contains 3 native plasmids, as a model to explore plasmid dynamics under stress conditions. By quantifying the copy number of the plasmids under heat stress, we observed a drop in the copy number of plasmid p1 (6.5 kb), with up to 75% of the surviving colonies losing p1 altogether. Additionally, FR1△p1 showed a faster growth rate compared to wild-type FR1 on standard growth media. We hypothesized that plant association may be the reason FR1 retains this plasmid in its natural environment, rather than losing it to enhance growth. We therefore explored whether association with a plant could enhance plasmid retention under heat stress. Our ongoing work aims to expand on these results to understand how plants may impact the native plasmids in Sphingomonas and how these plasmids contribute not only to bacterial survival but also ultimately to the ability of such beneficial bacteria to improve plant growth.