Role of Quorum Sensing Signals of Rhizobacteria for Plant Growth Promotion

Abstract

Signaling events between rhizosphere microbes and plants substantially contribute to establish different qualities of microbe-plant interactions from beneficial cooperativity to pathogenicity. In addition to the pathogen-associated molecular patterns (PAMPs), like exo- and lipopolysaccharides or flagellins, which are effectively recognized by the plants’ innate immune system, various secondary metabolites, such as antibiotics or the so-called autoinducers involved in the quorum sensing response of bacteria, are additional modulators of plants’ perception of associated microbes. In Gram-negative bacteria, N-acyl homoserine lactones (AHLs) are the major quorum sensing autoinducing molecules, which have a central role in the differentiation of specific phenotypes of sessile cells, living in root-attached microcolonies or biofilm consortia. AHLs turned out to have profound effects on plant development and/or defense priming and development of systemic resistance against pathogens. AHLs have different structural modifications (e.g., short or long hydrocarbon chain residues). While the hydrophilic ones can be taken up by plants, the lipophilic stay in the roots. Different modes of plant growth promotion by these AHL types in various plants are summarized in this chapter. We hypothesize, that in the absence of pathogenic patterns, AHLs support a beneficial to symbiotic interaction with plants. In cases when plant pathogens use AHLs for virulence development, AHLs reinforce plant’s defense. Alternatively, AHL degradation activities of certain rhizosphere bacteria can be used to suppress the pathogenic attack. To foster beneficial interactions of rhizotrophs with plants, consortia of bacteria using the same autoinducers could be developed.