Plant Responses to Bacterial Quorum Sensing

Quorum sensing (QS) is a population density-dependent regulatory mechanism, utilizing freely diffusible chemical signal molecules, which controls a wide range of phenotypes in many different bacteria. QS plays an essential role in the pathogenesis of many bacterial pathogens of plants. Both symbiotic and pathogenic bacteria rely on intercellular communication to coordinate the collective behavior towards the host plant. Mutually, the secretion of particular mimics by the plant might actively alter the synthesis of Acyl Homoserine Lactones (AHL) by bacteria. The genes encoding AHL synthesis are often positively regulated by the receptor for the AHL, leading to self-amplification of AHL synthesis by bacteria. Thus, the interaction of a plant mimic compound with an AHL receptor that regulates AHL synthesis could substantially affect rates of AHL synthesis, positively or negatively, and thereby affect host responses to the AHL. AHLs also induced the tissue-specific transcriptional activation of an auxin-inducible gene. Homoserine lactone is one of the products of enzymatic degradation of AHLs by soil bacteria. Increased transpiration, induced by homoserine lactone, should result in an increased flow of water and nutrients from the bulk soil to the rhizosphere, potentially benefiting both the plant and the bacteria. The consequences of this response may be improved root growth, altered plant architecture, induction of root hair formation, and even the formation of adventitious roots. Several plant-associated bacteria, known to induce systemic resistance lose this ability if blocked in AHL synthesis. This increased resistance is associated with a stronger activation of mitogen-activated protein kinases, followed by a higher expression of the defense-related transcription factors. In addition, some plants have high activities of lactonases, while others lack them and thus the AHLs can be transported from the root to the shoot. In both pathogenic and beneficial plant-associated bacteria, a large number of AHL-based QS systems are shown to affect processes such as swarming, biofilm formation, conjugal plasmid transfer, stress survival, and synthesis of colonization and virulence factors such as surfactants, exopolysaccharides, antibiotics, and extracellular enzymes.