Plant Symbionts Are Engineers of the Plant-Associated Microbiome

Plants interact throughout their lives with environmental microorganisms. These interactions determine plant development, nutrition, and fitness in a dynamic and stressful environment, forming the basis for the holobiont concept in which plants and plant-associated microbes are not considered as independent entities but as a single evolutionary unit. A primary open question concerns whether holobiont structure is shaped by its microbial members or solely by the plant. Current knowledge of plant–microbe interactions argues that the establishment of symbiosis directly and indirectly conditions the plant-associated microbiome. We propose to define the impact of the symbiont on the plant microbiome as the ‘symbiosis cascade effect’, in which the symbionts and their plant host jointly shape the plant microbiome. Plants are associated with an enormous diversity of microorganisms, some of which are symbiotic.Symbiont establishment is accompanied by structural and physiological changes in the host plant, including qualitative and quantitative changes in root exudates.Studies on plants impaired in their ability to enter symbiosis, or after controlled inoculation with symbionts, demonstrate that symbionts play an important role in the taxonomic and functional structuring of the phytomicrobiome.Plant symbionts drive the composition of the phytomicrobiome; hence, plant symbionts are ecological engineers of the holobiont.