Dark septate endophytes: mutualism from by-products?

Plant roots are abundantly colonized by dark septate endophytic (DSE) fungi in virtually all ecosystems. DSE fungi are functionally heterogeneous and their relationships with plants range from antagonistic to mutualistic. Here, we consider the role of by-product benefits in DSE and other root–fungal symbioses. We compared host investments against symbiont-derived benefits for the host plant and categorized these benefits as by-products or benefits requiring reciprocal investment from the host. By-product benefits may provide the variability required for the evolution of invested mutualisms between the host and symbiont. We suggest that DSE could be considered as ‘a by-product mutualist transitional phase’ in the evolution of cooperative mycorrhizal symbionts from saprotrophic fungi. Plant roots are abundantly colonized by DSE fungi in virtually all ecosystems, but the biology and ecological roles of these ubiquitous fungi are poorly understood.Development of genomics and other molecular methods has made research of root-associated fungal communities (including DSE symbiosis) timely.Understanding DSE symbiosis requires holistic evolutionary ecological approach. We consider by-product mutualism (sensu Connor Biol Rev. 1995;70:427–457) as a framework and present a schematic hypothesis of evolution of plant‐symbiont interactions.By-product mutualisms may be an important step between free-living saprotrophs and mutualistic mycorrhizal symbioses.