Materials and Methods

Plant materials and growth conditions
The mycorrhizal tubers of G. elata were developed from the symbiotic culture with Armillaria strain 2, according to a standard protocol (Yeh, Liao, Huang, Miyajima & Lee 2017). In brief, timber blocks of Liquidambar formosana were inoculated with PDA cubes containing Armillaria. After one month of culture, the protocorm-like bodies (PLBs) derived from G. elata callus were placed on the colonized timber blocks and grown until they formed juvenile tubers. During 18 months of symbiotic cultivation,
G. elata tubers were harvested at seven stages of development for analysis (Fig. S1). Mature tubers were collected from about 1.5 years of symbiotic cultivation. Tubers were frozen at -80 oC before analysis.
Arabidopsis thaliana and transgenic Arabidopsis plants used in this study were all Columbia ecotype (Col-0). For all experiments, seeds were surface-sterilized (30% CLOROX bleach and 0.1% Triton X100, v/v) and then stratified at 4 oC for three days.
Seeds were germinated and grown on one-half-strength Murashige and Skoog basal salt mixture agar media (½MS) or potting soils in a growth chamber (22 °C and 16/8 h light/dark regime, with ~80 μmol m-2 s-1 illumination) for most experiments (except where indicated). For root growth analysis, seeds were germinated on half-strength MS medium supplemented with 0%, 1%, 2%, 5% or 7% sucrose, and grown for 7 d before imaging. For leaf area measurement, seeds were sown and grown on half-strength MS medium containing 2 µg ml-1 hygromycin for 7 d under short-day conditions (10/14 h light/dark regime). Seedlings of similar sizes were then transferred to soil and grown for another 3 wks. All leaves were imaged by a scanner. Primary root length and leaf area were quantified using ImageJ.