Plant-microbe-soil interactions in the rhizosphere: an evolutionary perspective

Soils are the product of the activities ofplants, which supply organic matter and play a pivotalrole in weathering rocks and minerals. Many plantspecies have a distinct ecological amplitude thatshows restriction to specific soil types. In thenumerous interactions between plants and soil, microorganismsalso play a key role. Here we review theexisting literature on interactions between plants,microorganisms and soils, and include considerationsof evolutionary time scales, where possible. Some ofthese interactions involve intricate systems of communication,which in the case of symbioses such asthe arbuscular mycorrhizal symbiosis are severalhundreds of millions years old; others involve therelease of exudates from roots, and other products ofrhizodeposition that are used as substrates for soilmicroorganisms. The possible reasons for the survivalvalue of this loss of carbon over tens or hundreds ofmillions of years of evolution of higher plants arediscussed, taking a cost-benefit approach. Coevolutionof plants and rhizosphere microorganismsis discussed, in the light of known ecologicalinteractions between various partners in terrestrialecosystems. Finally, the role of higher plants, especiallydeep-rooted plants and associated microorganismsin the weathering of rocks and minerals,ultimately contributing to pedogenesis, is addressed.We show that rhizosphere processes in the long runare central to biogeochemical cycles, soil formationand Earth history. Major anticipated discoveries willenhance our basic understanding and allow applicationsof new knowledge to deal with nutrient deficiencies,pests and diseases, and the challenges of increasingglobal food production and agroecosystem productivityin an environmentally responsible manner.