Unraveling the secrets of rhizobacteria signaling in rhizosphere

Signaling among rhizobacteria and other soil microorganisms is an important mechanism to ensure a successful symbiotic relationship with their phytobionts. Quorum-sensing involves signaling molecules that provide essential networks for communication in the rhizosphere. These signaling molecules relay inter-and-intra-species information that coordinates and controls behavior in mixed communities, and the expression of these signaling molecules changes in response to the chemical cues. The diverse signaling molecules released in the rhizospheric zone affect the structural and physical heterogeneity of the soil and the quantity and identity of rhizobacteria. In general, rhizospheric signaling mechanisms can be categorized into three major types (i) plant to microorganisms signaling through low molecular weight molecules secreted by plants; (ii) interspecies and intraspecies microbial signaling, chiefly through quorum-sensing molecules; and (iii) microorganisms to plant signaling by microbially produced compounds. This review presents knowledge on the signaling molecules of the rhizosphere based on the above three mechanisms. The chemical nature of root exudates and their roles in attracting metabolically active rhizobacteria; the chemical properties of autoinducers secreted by rhizobacteria and their functions in intra- and inter-species interactions, including biofilm formation in the rhizosphere; and influence of quorum sensing on the root architecture, plant defense and biotic and abiotic stress responses, and gene expression are examined. The review provides a thorough understanding of rhizobacteria signaling and will help to develop novel strategies for agriculture, such as the novel use of plant growth-promoting rhizobacteria to enhance crops growth and quorum quenching technique to fight against plant pathogens.