Strullu-Derrien, C. | bioRxiv Science

bioRxiv2025

An ancient plant symbiotic fungus with distinct features identified through advanced fluorescence and Raman imaging

Mycorrhizal associations between fungi and plants are a fundamental aspect of terrestrial ecosystems. The occurrence of mycorrhizae is now well established in c. 85% of extant plants, yet the geological record of these associations is sparse. Fossil evidence from early Paleozoic terrestrial environments provides rare insights into these ancient symbioses, but imaging micro-scale fossil fungi in surrounding plant tissues is challenging. We employed brightfield microscopy, confocal laser scanning microscopy (CLSM), fluorescence lifetime imaging microscopy (FLIM), and Raman spectroscopy to investigate a newly identified endomycorrhizal fungus within a 407-million-year-old plant from the Windyfield Chert. An exquisite preservation and our advanced imaging approach enabled the description of the fungal structures to unprecedented detail. The fungus, Rugosomyces lavoisieriae gen. nov., & sp. nov., exhibits structural features resembling modern Glomeromycotina arbuscular mycorrhizal fungi. Combining CLSM with FLIM provides extra information by separating features based upon fluorescence lifetime. Raman spectroscopy indicates that fungal arbuscules and plant water-conducting cells have undergone geological alterations, resulting in a similar chemical composition. These findings enhance our understanding of ancient plant-fungal symbioses and demonstrate the potential of advanced imaging techniques in paleobotanical research. Significance statementMost living plants form mycorrhizal associations with fungi, but how this ancient evolutionary relationship developed is poorly understood. Exceptional fossil materials preserve direct evidence of endomycorrhizae, but imaging micro-scale fossil fungi inside plant cells is challenging. Here we use confocal microscopy combined with fluorescence lifetime imaging and Raman spectroscopy to study a new endomycorrhizal fungus inside a 400-million-year-old plant. We document structures resembling modern Glomeromycotina. We show that the residual organic matter of the fungus and the plant cell walls has a very short fluorescence lifetime, which provides a new means of imaging these fossils. Raman spectroscopy indicates that both arbuscules and water-conducting cells were geologically altered by their diagenetic history and that they now have a similar chemical composition.

plant biology↗

bioRxiv2022

Structure and specialization of mycorrhizal networks in phylogenetically diverse tropical communities

BackgroundThe root mycobiome plays a fundamental role in plant nutrition and protection against biotic and abiotic stresses. In temperate forests or meadows dominated by angiosperms, the numerous fungi involved in root symbioses are often shared between neighboring plants, thus forming complex plant-fungus interaction networks of weak specialization. Whether this weak specialization also holds in rich tropical communities with more phylogenetically diverse sets of plant lineages remains unknown. We collected roots of 30 plant species in semi-natural tropical communities including angiosperms, ferns, and lycophytes, in three different habitat types on La Reunion island: a recent lava flow, a wet thicket, and an ericoid shrubland. We identified root-inhabiting fungi by sequencing both the 18S rRNA and the ITS2 variable regions. We assessed the diversity of mycorrhizal fungal taxa according to plant species and lineages, as well as the structure and specialization of the resulting plant-fungus networks. ResultsThe 18S and ITS2 datasets are highly complementary at revealing the root mycobiota. According to 18S, Glomeromycotina colonize all plant groups in all habitats forming the least specialized interactions, resulting in nested network structures, while Mucoromycotina (Endogonales) are more abundant in the wetland and show higher specialization and modularity compared to the former. According to ITS2, mycorrhizal fungi of Ericaceae and Orchidaceae, namely Helotiales, Sebacinales, and Cantharellales, also colonize the roots of most plant lineages, confirming that they are frequent endophytes. While Helotiales and Sebacinales present intermediate levels of specialization, Cantharellales are more specialized and more sporadic in their interactions with plants, resulting in highly modular networks. ConclusionsThis study of the root mycobiome in tropical environments reinforces the idea that mycorrhizal fungal taxa are locally shared between co-occurring plants, including phylogenetically distant plants (e.g. lycophytes and angiosperms), where they may form functional mycorrhizae or establish endophytic colonization. Yet, we demonstrate that, irrespectively of the environmental variations, the level of specialization significantly varies according to the fungal lineages, probably reflecting the different evolutionary origins of these plant-fungus symbioses. Frequent fungal sharing between plants questions the roles of the different fungi in community functioning and highlights the importance of considering networks of interactions rather than isolated hosts.

ecology↗