Revisiting mycorrhizal dogmas: Are mycorrhizas really functioning as they are widely believed to do?

Abstract

Since the discovery of mycorrhizas, dogmas have been developed regarding their function, ecology, and distribution. We explore if these dogmas are valid, and if there are research biases toward regions inhabited by most researchers and away from regions inhabited by most plant species. We found, first, that the mycorrhizal status is known in less than 1% of plant species, with few having proven mycorrhizal benefits for growth and development. Second, the mycorrhizal status of a plant species varies within families, even genera. Third, mycorrhizas are important for phosphorus acquisition only within a narrow phosphorus range. Fourth, mycorrhizal plants are not uniform in their response to environment; taxonomy, phylogeny and continental history strongly drive mycorrhizal diversity. For example, the center of diversification of Ericaceae and Orchidaceae is in the neotropics, rather than, as recently claimed, in cold and high-latitude climates. Understanding the importance of mycorrhizas at the family-wide and biome-wide level is therefore conflicted by a research bias toward research-intensive regions outside the phylogenetic hotspots of diversification. The current precepts on mycorrhizas provide important starting points for hypotheses to robustly test when and where mycorrhizas play a role in determining ecological trajectory. The time has come to move forward and revisit past assumptions and update the dogmas, rather than assume functional significance in terms of nutritional benefits from studies on extraneous regions and species. We conclude that non-nutritional benefits deserve far greater attention.