N-acyl homoserine lactone (AHL)-mediated quorum sensing by bacteria regulates traits that are involved in symbiotic, pathogenic and surface-associated relationships between microbial populations and their plant hosts. Recent advances demonstrate deviations from the classic LuxR/LuxI paradigm, which was first developed in Vibrio. For example, LuxR homologs can repress as well as activate gene expression, and non-AHL signals and signal mimics can affect the expression of genes that are controlled by quorum sensing. Many bacteria utilize multiple quorum-sensing systems, and these may be modulated via post-transcriptional and other global regulatory mechanisms. Microbes inhabiting plant surfaces also produce and respond to a diverse mixture of AHL signals. The production of AHL mimics by plants and the identification of AHL degradative pathways suggest that bacteria and plants utilize this method of bacterial communication as a key control point for influencing the outcome of their interactions.