Endophytes with mycorrhizal potentials: biological and ecological implications

Generally, the root mycobiome is dominated by endophytic and mycorrhizal fungi with mutualistic potential to enhance plant fitness. In some cases, however, the distinction between the two biotrophic guilds is challenged by the ability of several endophytic fungi to colonize roots and transfer nutrients to the plants. With more research on harnessing plant-endophyte combinations using a gnotobiotic system, more endophytes endowed with mycorrhizal-like traits have been identified. They often benefit nonmycorrhizal plants by employing a set of responses to nutrient deficiency similar to those from mycorrhizal plants, orchestrate the development of ectomycorrhizal-like structures under controlled conditions, and share genetic traits with true mycorrhizal fungi, such as a lower content of plant cell wall-degrading enzymes and gene networks reminiscent of mutualistic interactions. Based on these characteristics, we propose the term 'mycorrhizal-like endophytes' to describe these fungi, which likely represent a transitional state along the endophyte-mycorrhizal mutualistic continuum.