Not a peripheral issue: secretion in plant–microbe interactions

The sessile nature of flowering plants and their capacity to thrive in soil habitats likely resulted in specific adaptation mechanisms enabling co-existence with a vast diversity of air-borne and soil-borne microorganisms. Only a small fraction of these microbial encounters result in pathogenic or symbiotic interactions, whilst the majority appear to give rise to commensalistic or mutualistic associations. Considering the abundance of the latter associations the question arises whether plants evolved, besides the plant immune system, other dedicated mechanisms to communicate with and to host microbial communities in the phyllosphere and rhizosphere. We hypothesize that the constitutive and microbe-induced secretion of specialized plant-derived biomolecules creates a critical interface for all types of plant–microbe associations. Thus, the plant secretory machinery might serve an important role in establishing an extended phenotype with microbial life.