AUTHOR=Zhang Bao , Weston Leslie A. , Li Mingjie , Zhu Xiaocheng , Weston Paul A. , Feng Fajie , Zhang Bingyong , Zhang Liuji , Gu Li , Zhang Zhongyi 

TITLE=Rehmannia glutinosa Replant Issues: Root Exudate-Rhizobiome Interactions Clearly Influence Replant Success

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01413

DOI=10.3389/fmicb.2020.01413

ISSN=1664-302X

ABSTRACT=<p>Production of medicinal tubers of <italic>Rehmannia glutinosa</italic> is severely hindered by replanting issues. However, a mechanistic understanding of the plant-soil factors associated with replant problems is currently limited. Thus, we aimed to identify the <italic>R. glutinosa</italic> root exudates, evaluate their potential phytotoxicity and profile the interactions between the plant and its associated rhizobiome. Stereomicroscopy and liquid chromatography coupled to a quadrupole/time of flight mass spectrometer were used to monitor and identify secreted metabolites, respectively. Seedling bioassays were used to evaluate the phytotoxicity of <italic>R. glutinosa</italic> root exudates. Two complimentary experiments were performed to investigate allelochemical fate in rhizosphere soil and profile the associated microbiota. Root specific microbes were further isolated from <italic>R. glutinosa</italic> rhizosphere. Impacts of isolated strains were evaluated by co-cultivation on plate and on seedlings in tissue culture, with a focus on their pathogenicity. Interactions between key <italic>R. glutinosa</italic> root exudates and isolated rhizobiomes were investigated to understand the potential for plant-soil feedbacks. Quantification and phytotoxic analysis of metabolites released from <italic>R. glutinosa</italic> indicated catalpol was the most abundant and bioactive metabolite in root exudates. Subsequent microbial profiling in soil containing accumulated and ecologically significant levels of catalpol identified several taxa (e.g., <italic>Agromyces</italic>, <italic>Lysobacter</italic>, <italic>Pseudomonas</italic>, <italic>Fusarium</italic>) that were specifically shifted. Isolation of <italic>R. glutinosa</italic> rhizobiomes obtained several root specific strains. A significant antagonistic effect between strain Rh7 (<italic>Pseudomonas aeruginosa</italic>) and two pathogenic strains Rf1 (<italic>Fusarium oxysporum</italic>) and Rf2 (<italic>Fusarium solani</italic>) was observed. Notably, the growth of strain Rh7 and catalpol concentration showed a hormesis-like effect. Field investigation further indicated catalpol was increasingly accumulated in the rhizosphere of replanted <italic>R. glutinosa</italic>, suggesting that interactions of biocontrol agents and pathogens are likely regulated by the presence of bioactive root exudates and in turn impact the rhizo-ecological process. In summary, this research successfully monitored the release of <italic>R. glutinosa</italic> root exudates, identified several abundant bioactive <italic>R. glutinosa</italic> secreted metabolites, profiled associated root specific microbes, and investigated the plant-soil feedbacks potentially regulated by catalpol and associated rhizobiomes. Our findings provide new perspectives toward an enhanced understanding <italic>R. glutinosa</italic> replant problems.</p>