S. indica is compatible with many orchid species A) D. catenatum seeds and their symbiotic protocorms or plantlets after inoculation with S. indica. Scale bars: 250 μm in left three images, 1 mm in right two images. B) S. indica (Si) promotes seed germination and protocorm development in six different orchid species: D. catenatum, D. wangliangii, D. nobile, D. chrysotoxum, C. mannii, and C. aloifolium. The seeds germinated in the presence of S. indica formed protocorms by about 20 days after inoculation with S. indica, with most protocorms developing into plantlets by 90 days, whereas no protocorms were formed from aseptically cultured seeds (controls, −). Data are mean ± SE, n ≥ 3 (10 plates on average for each value). Different letters next to error bars indicate statistically significant differences at P ≤ 0.05 (ANOVA with Tukey HSD post hoc test). Images are shown in Supplemental Figure S1. C) S. indica colonizes protocorm cells as an orchid mycorrhizal fungus. Sections of resin-embedded protocorms show the apical, uninfected region (a) and basal, mycorrhizal region (b) of orchid protocorms, with details of colonized cells at the bottom. Circles indicate lysed intracellular fungal pelotons (old pelotons), and living pelotons in the enlarged images. Scale bars, 100 μm.