The plant–microbe partnership ryegrass – Burkholderia thailandensis prevails over biosurfactants addition to improve hydrocarbon rhizodegradation

Petroleum hydrocarbon soil contamination poses major environmental and health risks due to the persistence and toxicity of these pollutants. Rhizoremediation relies on synergistic interactions between plants and microorganisms and represents a sustainable and cost-effective strategy. However, its efficiency is often limited by the low bioavailability of hydrocarbons in soil. Previous research demonstrated that ryegrass ( Lolium multiflorum L.) tolerates low concentrations of biosurfactants such as rhamnolipids, whereas higher concentrations increase hydrocarbon phytotoxicity. In parallel, Burkholderia thailandensis has been identified as a promising hydrocarbon-degrading and biosurfactant-producing bacterium. Building on these findings, this study investigated whether petroleum hydrocarbon rhizodegradation could be improved by combining rygrass with B. thailandensis and rhamnolipids under controlled hydroponic conditions. The hydroponic system provided a simplified and well-controlled environment to isolate key biological mechanisms prior to soil application. After 120 days (2 g L -1 hydrocarbons; 1 × CMC water rhamnolipids), the ryegrass + B. thailandensis treatment achieved the highest dissipation rate (70%), particularly for C 10 –C 30 hydrocarbons. Biosurfactant production occurred even without rhamnolipid addition but did not enhance rhizodegradation. This indicates that endogenous biosurfactants production may already be sufficient in contaminated environments. Overall, these results question the relevance of biosurfactant supplementation, as it may increase phytotoxicity without improving rhizodegradation. They also highlight the ryegrass– B. thailandensis combination as a promising approach for the remediation hydrocarbon-contaminated environments.