Quorum sensing is a term used to describe cell-to-cell communication that allows cell density-dependent gene expression. 

We are working to understand how chemical signaling in the root microbiome contributes to the biology of the Eastern Cottonwood tree, Populus deltoides. Poplar is a promising biofuel plant because it grows very fast and can thrive in marginal soils. Working in collaboration with E. Peter Greenberg and scientists at Oak Ridge National Laboratory, we have found that acyl-homoserine lactone (AHL)-type quorum sensing (QS) genes are prevalent in the genomes of Proteobacteria isolated from Poplarroots. Quorum sensing is a cell-to-cell signaling system, which allows bacteria to control transcription in a cell density-dependent manner. The QS regulatory genes include both AHL signal synthases (luxI-type genes) and signal receptors (luxR-type genes). Often the AHL synthase and its co-evolved receptor genes are linked together on the chromosome, but many times the luxR homolog is unlinked to a luxI gene. These ‘extra' luxR genes are termed orphans and are abundant in the genomes of bacteria isolated from Poplar. We and others have found that evolution has co-opted orphan LuxR proteins for use in plant-bacterial interkingdom signaling instead of quorum sensing. We are focusing on an orphan LuxR protein termed PipR from a Pseudomonas species that responds to a compound present in Poplar leaf extracts. Current work is to identify the plant compound that binds to PipR to control bacterial gene expression. We will test the hypothesis that plant-bacterial signaling mediated by orphan LuxR protein is important for the ability of bacteria to colonize plants.