Molecular mechanisms modulating beneficial plant root-microbe interactions: What’s common?

In the current climate change context, there is a need to develop more sustainable agri-food strategies. As an alternative to the intensive use of chemically-synthesized fertilizers and pesticides that pollute water and impact biodiversity, there is a growing interest in using beneficial microbes as biostimulants and/or bioprotection agents. However, their implementation in agriculture remains a challenge due to highly variable outcomes and benefits. Furthermore, there are major knowledge gaps about the molecular mechanisms regulating the different plant-microbe interactions. In the present review, we summarize current knowledge on the molecular mechanisms controlling the different beneficial plant root-microbe interactions, namely arbuscular mycorrhiza, the rhizobium-legume symbiosis, ectomycorrhiza, as well as fungal and bacterial endophytic associations. This includes the signalling pathways required for microbes to be recognized as beneficial, metabolic pathways that provide nutritional benefits to the plant, and the regulatory pathways modulating the extent of the symbiosis establishment depending on soil nutrient availability and plant needs. The aim is to highlight what are the main common mechanisms, as well as the knowledge gaps, in order to promote their use, either individually or in consortia, within the framework of sustainable agriculture that is less dependent on chemicals and more protective of biodiversity and water resources.