Abstract
Interactions between plants and the soil’s microbial & fungal flora are crucial for the health of soil ecosystems and food production. Microbe-plant interactions are difficult to investigate in situ due to their intertwined relationship involving morphology and metabolism. Here, we describe an approach to overcome this challenge by elucidating morphology and the metabolic profile of Medicago truncatula root nodules using Magnetic Resonance (MR) Microscopy, at the highest magnetic field strength (22.3 T) currently available for imaging. A home-built solenoid RF coil with an inner diameter of 1.5 mm was used to study individual root nodules. A 3D imaging sequence with an isotropic resolution of (7 μm)3 was able to resolve individual cells, and distinguish between cells infected with rhizobia and uninfected cells. Furthermore, we studied the metabolic profile of cells in different sections of the root nodule using localised MR spectroscopy and showed that several metabolites, including betaine, asparagine/aspartate and choline, have different concentrations across nodule zones. The metabolite spatial distribution was visualised using chemical shift imaging. Finally, we describe the technical challenges and outlook towards future in vivo MR microscopy of nodules and the plant root system.
Highlights
Plants have solved the problem of biological nitrogen fixation through commensal processes, involving bacterial infection of plant roots[4]
Root nodule morphology resolved by Magnetic Resonance (MR) microscopy in cellular detail
The rhizobial infection zone, young cells infected by rhizobia bacteria could be seen as a region of alternating high- and low-intensity patches
Summary
Plants have solved the problem of biological nitrogen fixation through commensal processes, involving bacterial infection of plant roots[4]. More detailed understanding of key metabolites in plant-nodule metabolism and importantly, their localisation within root nodule tissues could, shed light on the mechanism by which Symbiotic Nitrogen Fixation (SNF) confers advantages to host plants. Gas Chromatography and Liquid Chromatography in combination with Mass Spectrometry (GC/LC-MS) has provided rich metabolic information but required destructive extraction procedures[14]. These in vitro methods may not faithfully reflect the native structural and molecular information. We applied state-of-the-art magnetic resonance microscopy (MRM) in conjunction with localised spectroscopy at ultra-high magnetic field (22.3 T), using a home-built solenoid RF coil in order to image root nodules of Medicago truncatula. We describe some of the technical challenges that were encountered and an outlook towards future in vivo imaging of nodules and plant root systems
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