AUTHOR=Liu Tianrui , Hua Zhongyi , Han Pengjie , Zhao Yuyang , Zhou Junhui , Jin Yan , Li Xiaolin , Huang Luqi , Yuan Yuan 

TITLE=Mycorrhizosphere Bacteria, Rahnella sp. HPDA25, Promotes the Growth of Armillaria gallica and Its Parasitic Host Gastrodia elata

JOURNAL=Frontiers in Microbiology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.842893

DOI=10.3389/fmicb.2022.842893

ISSN=1664-302X

ABSTRACT=<p><italic>Gastrodia elata</italic> is an entirely heterotrophic plant, the growth of which is completely reliant on <italic>Armillaria gallica</italic>, an orchid mycorrhizal fungus. To avoid damaging ecosystems, <italic>G. elata</italic> cultivation is shifting from woodland to farmland. However, whether the microbial community structure remains stable during this conversation is unknown. Here, we cultivated <italic>G. elata</italic> in woodland or farmland and found that woodland-cultivated <italic>G. elata</italic> produced a greater yield and larger tuber size. The relative abundance of <italic>Rahnella</italic> 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 <italic>Rahnella</italic> impacts the growth of <italic>G. elata</italic> and establishes symbiosis with <italic>Armillaria gallica</italic>, three <italic>Rahnella</italic> spp. strains (HPDA25, SBD3, and SBD11) were isolated from mycorrhizosphere soil samples. It was found that these strains, especially HPDA25, promoted the growth of <italic>A. gallica</italic>. 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 <italic>A. gallica</italic>, compared with <italic>A. gallica</italic> cultured alone. The results of RNA-seq and quantitative real-time polymerase chain reaction analysis showed that co-culturing <italic>A. gallica</italic> 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, <italic>A. gallica</italic> promotes the growth of <italic>G. elata</italic> and enhances the tuber size of <italic>G. elata</italic>. These results provide new insights into an orchid mycorrhizal symbiosis and the cultivation of <italic>G. elata</italic>.</p>