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

Abstract

Production of medicinal tubers of <i>Rehmannia glutinosa</i> 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 <i>R. glutinosa</i> 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 <i>R. glutinosa</i> 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 <i>R. glutinosa</i> 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 <i>R. glutinosa</i> root exudates and isolated rhizobiomes were investigated to understand the potential for plant-soil feedbacks. Quantification and phytotoxic analysis of metabolites released from <i>R. glutinosa</i> 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., <i>Agromyces</i>, <i>Lysobacter</i>, <i>Pseudomonas</i>, <i>Fusarium</i>) that were specifically shifted. Isolation of <i>R. glutinosa</i> rhizobiomes obtained several root specific strains. A significant antagonistic effect between strain Rh7 (<i>Pseudomonas aeruginosa</i>) and two pathogenic strains Rf1 (<i>Fusarium oxysporum</i>) and Rf2 (<i>Fusarium solani</i>) 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 <i>R. glutinosa</i>, 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 <i>R. glutinosa</i> root exudates, identified several abundant bioactive <i>R. glutinosa</i> 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 <i>R. glutinosa</i> replant problems.