A historical perspective on mycorrhizal mutualism emphasizing arbuscular mycorrhizas and their emerging challenges

Arbuscular mycorrhiza, one of the oldest interactions on earth (similar to 450 million years old) and a first-class partner for plants to colonize emerged land, is considered one of the most pervasive ecological relationships on the globe. Despite how important and old this interaction is, its discovery was very recent compared to the long story of land plant evolution. The story of the arbuscular mycorrhiza cannot be addressed apart from the history, controversies, and speculations about mycorrhiza in its broad sense. The chronicle of mycorrhizal research is marked by multiple key milestones such as the initial description of a ``persistent epiderm and pellicular wall structure'' by Hartig; the introduction of the ``Symbiotismus'' and ``Mycorrhiza'' concepts by Frank; the description of diverse root-fungal morphologies; the first description of arbuscules by Gallaud; Mosse's pivotal statement of the beneficial nature of the arbuscular mycorrhizal symbiosis; the impact of molecular tools on the taxonomy of mycorrhizal fungi as well as the development of in vitro root organ cultures for producing axenic arbuscular mycorrhizal fungi (AMF). An appreciation of the story - full of twists and turns - of the arbuscular mycorrhiza, going from the roots of mycorrhiza history, along with the discovery of different mycorrhiza types such as ectomycorrhiza, can improve research to help face our days' challenge of developing sustainable agriculture that integrates the arbuscular mycorrhiza and its ecosystem services.