Abstract
Production of medicinal tubers of Rehmannia glutinosa 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 R. glutinosa 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 R. glutinosa 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 R. glutinosa 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 R. glutinosa root exudates and isolated rhizobiomes were investigated to understand the potential for plant-soil feedbacks. Quantification and phytotoxic analysis of metabolites released from R. glutinosa 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., Agromyces, Lysobacter, Pseudomonas, Fusarium) that were specifically shifted. Isolation of R. glutinosa rhizobiomes obtained several root specific strains. A significant antagonistic effect between strain Rh7 (Pseudomonas aeruginosa) and two pathogenic strains Rf1 (Fusarium oxysporum) and Rf2 (Fusarium solani) 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 R. glutinosa, 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 R. glutinosa root exudates, identified several abundant bioactive R. glutinosa 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 R. glutinosa replant problems.
Highlights
The bioactive chemical constituents in herbal plants are typically specific to their native geographical areas (Su et al, 2005; Liu et al, 2015), and such geo-authentic regionality has been recommended as an important marker for establishment of improved quality standards for traditional Chinese medicines (Zhang B. et al, 2010; Yang et al, 2017)
The plants cultivated in various soil treatments including the newly planted soil (NP), the replanted soil for 1 year (RP1) and the replanted soil for 2 years (RP2) were collected to investigate survival rate and root dry weight of R. glutinosa under continuously monocultured conditions
By direct comparison with the soil cultivated without R. glutinosa, we found that eleven metabolites specific to R. glutinosa root exudates and rhizosphere soil were present
Summary
The bioactive chemical constituents in herbal plants are typically specific to their native geographical areas (Su et al, 2005; Liu et al, 2015), and such geo-authentic regionality has been recommended as an important marker for establishment of improved quality standards for traditional Chinese medicines (Zhang B. et al, 2010; Yang et al, 2017). Replant problems, (the phenomena associated with the inhibition of germination, growth and yield of a plant when the same species or genus has been repeatedly cultivated in the same location) (Scotto la Massese, 1970; Bouhot, 1983; Yim et al, 2013), have severely limited the production of R. glutinosa (Zhang et al, 2013). Due to the lack of arable land in Jiaozuo City, R. glutinosa is frequently replanted in the same field at intervals of less than 8 years, or cultivated beyond its geo-authentic region to meet market demand, resulting in a high incidence of disease, lower tuber yield, and poor product quality. Given the serious replanting issues associated with production, R. glutinosa has been regarded as a model species for the study of replanting problems in perennial herbal plants (Zhang and Lin, 2009)
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