Observing invisible bridges: Non-invasive imaging of arbuscular mycorrhizal fungal structures in soil pore space

Arbuscular Mycorrhizal Fungi (AMF) are plant symbionts that colonize the root cortex, but also extend their extraradical hyphal networks deep into the soil. These networks increase root-soil contact, modify soil structure and facilitate water- and nutrient transport towards the root. The fine, almost “invisible” bridges formed by these networks may gain relevance when soil becomes dry and water and nutrient resources scarce. Their pore-bridging function may connect the roots to soil patches containing water and nutrient resources, potentially preventing root shrinkage while maintaining transport.

Only recently, a high-resolution non-invasive imaging tool became available that now allows us to study the fine, delicate AMF structures in pore space in situ. Here we are presenting a workflow based on synchrotron-based X-ray computed microtomography  imaging. We have developed setups to cultivate AMF at different levels of biotic complexity and subsequently image and analyze AMF hyphosphere and rhizosphere structures quantitatively and non-invasively. This approach has been successfully applied to two AMF species in contrasting soil textures, namely sand and loam. We present the 3D results of key architectural and morphological traits of AMF spores, hyphae and intraradical structures. These include structure counts, total hyphal length, branching frequency, volume, and surface area. Moreover, this study measured a set of novel parameters: (i) the AMF-soil and AMF-root interface areas, and (ii) the AMF pore space occupancy. These data can be linked to hyphal length densities measured destructively, as well as to the plant-scale data such as shoot biomass, C-, N- and P-contents in the leaves, and stomatal conductance.