*2.1. Detecting Mycorrhizal Fungus Colonization in A. roxburghii*

Pelotons (hyphae coils) formation is the important characteristic of orchid mycorrhizal association [38]. In the study, *Ceratobasidium* sp. AR2 would soon grow over the whole substrate for about two weeks. At this time, both morphological observation and toluidine blue staining, which showed no hyphae in the control group (Figure 1A) and that the intracellular pelotons existed in the treatment (Figure 1B), indicated that a symbiotic relationship had been established between *A. roxburghii* and mycorrhizal fungus *Ceratobasidium* sp. AR2.

**Figure 1.** The semi-thin sections of root of *A. roxburghii* after two weeks of symbiotic cultivation. (**A**) the control, scale bar = 50 mm; (**B**) the treatment; arrows represent pelotons, scale bar = 50 mm.
