Quorum sensing N-acyl-homoserine lactone signal molecules of plant beneficial Gram-negative rhizobacteria support plant growth and resistance to pathogens.

Quorum sensing (QS) is a well-known, cell density dependent regulatory process in bacterial communities. It is mediated by auto-induced synthesis of diffusible signal molecules once a critical cell density is reached. In Gram-negative bacteria, N-acyl-homoserine lactones (AHLs) are QS-signal molecules involved in, for e.g. biofilm formation, activation of virulence genes of pathogens, and induction of degradative enzyme activities. However, QS was not much characterized as being important for regulating beneficial interactions of plant growth promoting rhizobacteria (PGPR). In this article, we examine the involvement of QS-AHLs in the improvement of plant development and health. AHLs of Gluconacetobacter diazotrophicus were shown to improve growth of rice under water stress conditions. The AHLs of Rhizobium radiobacter, an endofungal bacterium of the PGP-fungus Serendipita indica, stimulated endophytic root colonization, improved growth and yields, and induced priming for pathogen resistance in wheat. Furthermore, the addition of AHL-compounds to plant seeds resulted in priming for better growth yields in winter wheat and enhanced resistance towards leaf rust in barley. Therefore, AHL signaling compounds and QS-AHL-producing rhizobacteria should be considered as promising candidates to be implemented in sustainable systems for consistent stimulation of plant growth and health