A thin section of a mycorrhizal root sample was imaged for correlation with transmission electron micrographs. The sample was prepared from the grass Panicum hallii infected with the arbuscular mycorrhizal fungus Rhizophagus irregularis (formerly Glomus intraradices), grown as described in Hestrin et al. (2022). P. hallii seeds were germinated on Petri plates, transferred into cones filled with doubleautoclaved sand, inoculated with ~500 R. irregularis spores, and grown for eight weeks before being transplanted into cones filled with a 50:50 (v:v) mixture of double-autoclaved sand and a fine sandy loam soil collected from a pasture in Caddo County, OK (35.072417/-98.303667). The sand:soil mixture began at 15% moisture and declined to 5% over the course of the 3-month greenhouse experiment. The plants were grown with a 16 hr photoperiod and average daytime and nighttime temperatures of 27 °C and 24 °C, respectively. At harvest, roots were washed, fixed in 50% ethanol, and stored at 4 °C. To prepare samples for imaging, 1-2 mm root segments were excised and transferred to a fresh fixative buffer (2% Tween, 3% glutaraldehyde, 3% paraformaldehyde in 0.05M sodium cacodylate buffer, pH 7.2 (EMS, Hatfield, PA, USA)) and microwaved (Pelco BioWave; Ted Pella, Redding, CA, USA) while under vacuum at room temperature (2×; 1 min 150 W (watts) -hold 1 min -1 min 150 W -hold 1 min -1 min 150 W; 27 mbar (20 mm Hg vacuum)). Samples were then held under vacuum for 1 hr and stored overnight at 4 °C. Sample were rinsed (3×; 10 min) in 0.05M sodium cacodylate buffer (pH 7.2) and then immersed in a solution of 1% osmium tetroxide with 1.6% potassium ferricyanide in 0.05M sodium cacodylate buffer and microwaved (2×; 1 min 150 W -hold 1 min; 27 mbar at room temperature). Samples were rinsed in a solution of 0.05M sodium cacodylate buffer, pH 7.2 (3×; 10 min at room temperature) and then subjected to an ascending acetone gradient (10 min; 35%, 50%, 70%, 80%, 90%, 100%) followed by pure acetone (3×; 10 min at room temperature). Samples were progressively infiltrated while rocking with Epon resin and acetone mixtures (EMS, Hatfield, PA, USA). For the final 3 exchanges of 100% Epon resin, samples were microwaved at room temperature (1×; 1 min 250 W -hold 1 min; 27 mbar) and then rocked for 2 hr at each step. Finally, samples were polymerized at 60 C for 48 hours. Thin sections were cut using a Leica UC6 (Leica, Wetzlar, Germany). A 90 nm thin section was collected onto a formvar-coated copper-rhodium backed slot grid. The grid was post-stained for TEM imaging with 2% uranyl acetate followed by Reynold's lead citrate, for 5 min each. The section was imaged with a Tecnai 12 120-keV TEM (FEI, Hillsboro, OR, USA). Image data were recorded using a Gatan Rio16 CMOS camera with GWS software (Gatan Inc., Pleasanton, CA, USA). TEM sections were then sputter coated with ~5 nm of gold.