Intracellular and extracellular PGPR: commonalities and distinctions in the plant–bacterium signaling processes

Plant growth promoting bacteria (PGPR) associations range in degree of bacterial proximity to the root and intimacy of association. In general, these can be separated into extracellular PGPR (ePGPR), existing in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex, and intracellular PGPR (iPGPR), which exist inside root cells, generally in specialized nodular structures. The latter includes rhizobia and Frankia species, both of which fix nitrogen in symbiosis with higher plants. There has been considerable development in understanding signaling mechanisms of rhizobia (iPGPR) during the establishment of the rhizobia–legume symbiosis, and this may serve as a model of knowledge regarding cross-talk and plant growth promoting mechanisms. We provide a detailed review of this process, including plant-to-bacteria signal molecules, followed by bacterial perception and consequent production of bacteria-to-plant signals. A history of PGPR discovery is also provided, indicating progress in understanding each of the PGPR groups. Recent advances in understanding plant growth responses to microbial signals are reviewed, along with the research areas that require attention. Based on new understandings of signaling mechanisms in the iPGPR (rhizobia) and recent findings with ePGPR we are able to speculate regarding general patterns of signaling in the ePGPR.