Mycorrhizosphere Bacteria, sp. HPDA25, Promotes the Growth of and Its Parasitic Host .

is an entirely heterotrophic plant, the growth of which is completely reliant on , an orchid mycorrhizal fungus. To avoid damaging ecosystems, cultivation is shifting from woodland to farmland. However, whether the microbial community structure remains stable during this conversation is unknown. Here, we cultivated in woodland or farmland and found that woodland-cultivated produced a greater yield and larger tuber size. The relative abundance of was 22.84- and 122.25-fold higher in woodland- and farmland-cultivated soil samples, respectively, than that in uncultivated soil samples. To investigate how impacts the growth of and establishes symbiosis with , three spp. strains (HPDA25, SBD3, and SBD11) were isolated from mycorrhizosphere soil samples. It was found that these strains, especially HPDA25, promoted the growth of . Ultra-performance liquid chromatography coupled to a triple quadrupole mass spectrometry analysis detected the indole-3-acetic acid with 16.24 ng/ml in HPDA25 fermentation solution. Co-culturing with the strain HPDA25 or exogenous indole-3-acetic acid increased the branching and fresh weight of rhizomorphs and the growth rate and extracellular laccase activity of , compared with cultured alone. The results of RNA-seq and quantitative real-time polymerase chain reaction analysis showed that co-culturing with HPDA25 increased the expression level of the genes including hydrophobin, SUR7/PalI family, and pectin methylesterase, whereas decreased the expression levels of glycolysis-related genes. Furthermore, co-culturing with the strain HPDA25, promotes the growth of and enhances the tuber size of . These results provide new insights into an orchid mycorrhizal symbiosis and the cultivation of .