Ethylene Perception and Response in Azospirillum brasilense Sp7

The plant hormone ethylene has been extensively studied for its role in many plant developmental processes. This has led to the discovery of five ethylene receptors that are all involved in a complex signaling pathway that overlaps with several other phytohormone pathways, as well as stress responses. The role that ethylene plays in stress responses makes it a central signaling pathway in plant-microbe interactions. This interface has been well studied from the plant perspective, but there is limited data on how bacteria respond directly to ethylene. This work shows that a plant growth-promoting bacteria, Azospirillum brasilense, has a functional ethylene receptor capable of binding ethylene and inducing a response, making it the first plant-associated bacteria to have a receptor identified. The application of ethylene to A. brasilense alters attachment behavior in both biofilm formation assays as well as root colonization of both Arabidopsis thaliana and tomato. These changes in attachment behavior are likely mediated by a large transcriptomic response to ethylene that leads to an upregulation of central carbon metabolism and a downregulation of genes involved in nitrogen fixation. In agreement with the changes in transcripts, ethylene treated cells also have a distinct metabolic profile when compared to untreated cells, and this change can be detected as early as 8 hours after exposure to ethylene. Together, this data show that ethylene perception by A. brasilense alters the nitrogen metabolism of the cell in a way that inhibits colonization of the root and upregulates carbon metabolism and storage.