Abstract
Ligularia cymbulifera is one of the predominant species in the Hengduan Mountains, China, and has led to a decrease in the amount of forage grass in this area. However, little is known about the mechanism behind its predominance. In this study, two novel eremophilane sesquiterpenes, ligulacymirin A and B (1 and 2), together with seven other known terpenoids (3–9), were isolated from the roots of L. cymbulifera. The structures of 1 and 2 were determined by spectroscopic methods and single-crystal X-ray diffraction. Each compound showed phytotoxic activities against Arabidopsis thaliana, and each was detected and identified in rhizosphere soil by UHPLC-MS. Compound 3 was the most potent phytotoxin, showing remarkable inhibition against both seedling growth (EC50 = 30.33 ± 0.94 μg/mL) and seed germination (EC50 = 155.13 ± 0.52 μg/mL), with an average content in rhizosphere soil of 3.44 μg/g. These results indicate that terpenoids in L. cymbulifera roots might be released as phytotoxins in rhizosphere soil to interfere with neighboring plants.
Highlights
Plants have developed complex eco-physiological strategies that allow them to outcompete neighboring plants
The results indicated that phytotoxic terpenoids in L. cymbulifera might be released into rhizosphere soil, and might provide this species with a competitive advantage by interfering with the germination and root elongation of neighboring plants
Upon careful comparison of the nuclear magnetic resonance (NMR) data of 1 with furanoeremophilan-10β-ol (3) (Table 1), a typical eremophilane sesquiterpene isolated from the same plant, the three methyl group signals at δH 0.73 (d, J = 6.5 Hz, H3-15), 0.79 (s, H3-14), and 1.30 (s, H3-13), which are characteristic of eremophilane sesquiterpenes, were observed
Summary
Plants have developed complex eco-physiological strategies that allow them to outcompete neighboring plants. Many phytotoxic secondary metabolites are produced by plant roots, and their major mechanisms of release into the rhizosphere soil are root exudation and decomposition of plant root residue (Bertin et al, 2003). To shed light on these phytotoxins, it is important to detect and quantify them in rhizosphere soil (Macias et al, 2014). These phytotoxic secondary metabolites could offer interesting templates for potential agricultural applications, for example, as eco-friendly natural herbicides (Macías et al, 2008). Eremophilane sesquiterpenes have been shown to be an important class of secondary metabolites responsible for phytotoxic activities (Andolfi et al, 2013; Masi et al, 2014; Miranda et al, 2015; Wang et al, 2015). Various skeletons of eremophilane sesquiterpenes have been identified as major secondary metabolites in the genus Ligularia (Yang et al, 2011; Kuroda et al, 2012; Saito, 2012; Tori, 2016), and some have been reported to display phytotoxicity (Cantrell et al, 2007)
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