Asarum heterotropoides extracts showed strong antibacterial activity against selected phytopathogenic bacteria. Bioguided isolation was conducted to obtain 11 phenanthrene derivatives (1–11), 4 phenylpropanoids (12–15), a flavonoid (16), and a steroid (17), including a new phenanthrene derivative (1). In vitro bioassay results showed that phenanthrene derivatives are the main active components of A. heterotropoides extracts. The new compound aristoloxazine C (1) was found to exhibit outstanding antibacterial activity against Ralstonia solanacearum, Xanthomonas oryzae, Erwinia carolovora, Pseudomonas syringae, and Xanthomonas axonopodis, with MIC values of 0.05, 2.5, 2.5, 5, and 6.25 μg/mL, respectively. These values were significantly higher than that of the positive control, streptomycin sulfate. Aristoloxazine C (1) also demonstrated an excellent control effect on tobacco bacterial wilt. Physiological and biochemical experiments combined with electron microscopy showed that the antibacterial activity of aristoloxazine C (1) was primarily related to the destruction of the bacterial cell wall structure. Thus, aristoloxazine C (1) may have the potential to be used as a template for the development of new bactericides or as a probe for the discovery of new antimicrobial targets.
Four new 8-O-4′ neolignans, characterized at methoxy or ethoxy groups substituted at C-7, namely, (±) lasiandranins A–D ( 1 – 4 ), and two known analogs (±) pinnatifidanin BV ( 5 ) and (±) pinnatifidanin BVI ( 6 ) were isolated from the whole plants of Clematis lasiandra Maxim. The structures of 1 – 6 were determined by spectroscopic methods including 1D, 2D NMR, ECD, and HRESIMS analysis. Compounds 1 and 5 were determined as erythro configuration, while 2 – 4 and 6 were determined as threo configuration based on the chemical shift difference of H-9a and H-9b in CD 3 OD. The 8-O-4′ neolignans were found from the genus Clematis for the first time. Compounds 1 – 6 were evaluated for their antibacterial activity against three plant pathogenic bacteria Pseudomonas syringae pv. actinidiae , Ralstonia solanacearum, and Erwinia carotovora by agar and broth dilution methods. Compounds 1 – 6 showed potent antibacterial activity against R. solanacearum with MIC values of 25–50 μg/mL and relatively lower activity against P. syringae pv. actinidiae with MIC values of 50–100 μg/mL, while they were inactive to E. carotovora .
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