Control of the Cooperation Between Plant Growth‐Promoting Rhizobacteria and Crops by Rhizosphere Signals

Abstract

In the rhizosphere, an extensive communication takes place between plants and root-associated microorganisms, and involving an astonishing diversity of compounds released by plants (as root exudates) and rhizobacteria. These organic compounds may play a role as nutrients, toxins, and/or signals, depending on the microbial partner as well as compound concentration. Therefore, they can be a major driver determining the assembly and functioning of rhizosphere communities. Until now, chemical signaling has been mostly studied during interaction between plants and pathogens, but recent work evidenced the involvement of several chemical signals in controlling the associative symbiosis between plant growth-promoting rhizobacteria (PGPR) and roots. In this review, recent findings about the identification of signals involved in the interaction of plants with PGPR, and of PGPR with each others, are presented. First, the focus is put on primary and secondary metabolites produced by plants, which may control the expression of PGPR genes, particularly those involved in plant-beneficial properties. Second, the role of signals released by PGPR, such as phytohormones, volatile organic compounds (VOC), N-acyl-homoserine lactones (AHL), or antimicrobial compounds, on modulation of plant growth and health, is considered. Third, signal molecules involved in cross-talk between PGPR are examined. Finally, the spatial distribution of PGPR populations on roots is emphasized as a key factor influencing PGPR's relationships with other rhizosphere-inhabiting partners.