Unity in diversity: structural and functional insights into the ancient partnerships between plants and fungi

Mycorrhizal symbiosis is an ancient and widespread mutualism between plants and fungi that facilitated plant terrestrialisation > 500 million years ago, with key roles in ecosystem functioning at multiple scales. Central to the symbiosis is the bidirectional exchange of plant‐fixed carbon for fungal‐acquired nutrients. Within this unifying role of mycorrhizas, considerable diversity in structure and function reflects the diversity of the partners involved. Early diverging plants form mutualisms not only with arbuscular mycorrhizal Glomeromycotina fungi, but also with poorly characterised Mucoromycotina, which may also colonise the roots of ‘higher’ plants as fine root endophytes. Functional diversity in these symbioses depends on both fungal and plant life histories and is influenced by the environment. Recent studies have highlighted the roles of lipids/fatty acids in plant‐to‐fungus carbon transport and potential contributions of Glomeromycotina fungi to plant nitrogen nutrition. Together with emerging appreciation of mycorrhizal networks as multi‐species resource‐sharing systems, these insights are broadening our views on mycorrhizas and their roles in nutrient cycling. It is crucial that the diverse array of biotic and abiotic factors that together shape the dynamics of carbon‐for‐nutrient exchange between plants and fungi are integrated, in addition to embracing the unfolding and potentially key role of Mucoromycotina fungi in these processes.