Barley control and mutant seeds were sterilized and germinated as described previously.75 Germinated seedlings were transferred to 9 × 9 cm pots containing autoclaved 1:1 silica sand:vermiculite mixture inoculated with 700 mg *R. irregularis* spore inoculum (10,000 spores·g-1 moist diatomaceous earth powder [50% water]) (Symplanta, Darmstadt, Germany). Approximately 500 mg of the inoculum was evenly mixed into the lower two-third substrate layer, further 200 mg were evenly sprinkled into a hole in the upper third of the substrate layer, into which the seedlings were transplanted. The seedlings were grown in the greenhouse and watered weekly with 30 mL of deionized water and tap water in a 1:1 ratio. Roots were harvested at 28 dpi and stored in 50% EtOH at 4 °C until staining.
