Phytotoxic phenylpropanoids isolated from Ophryosporus charua (Griseb.) Hieron.
Graphical abstract
Bioguided isolation of the EtOH extract from Ophryosporus charua, against the germination of Raphanus sativus seeds, yielded three phenylpropanoids, charuol A (1), charuepoxide (2) and charuol B (3). These compounds inhibited the germination and seedling development of different mono- and dicotyledonous species with different effectiveness.
Introduction
Plants produce many different secondary metabolites that are involved in interaction with other organisms. Some interact with other plants, interfering with their germination and growth. The active principles for this phytotoxic effect can be allelochemicals and it is widely accepted that those that are highly efficient can be used as natural herbicides (Morimoto et al., 2009).
In a search for plants with high phytotoxic activity, 101 native plant species of Central Argentina (Palacios et al., 2007, Palacios et al., 2010) were screened. Among the plants assayed, Ophryosporus charua (Griseb.) Hieron. (Astereceae) proved to be one of the most promising species, showing high inhibition of germination and of seedling growth, especially of dicotyledonous Raphanus sativus (Palacios et al., 2010). O. charua is a medicinal plant used as an antisyphilitic in traditional medicine (Toursarkissian, 1980). The ethanolic extract of this plant species was also assayed against insects (del Corral et al., 2014, Diaz Napal et al., 2015), bacteria (Joray et al., 2011), fungi (Carpinella et al., 2010b) and some enzymes (Carpinella et al., 2010a, Chiari et al., 2010) with poor results. The O. charua extract was, however, a good inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD) activity, an enzyme involved in the tyrosine degradation pathway in mammals as well as in the production of plastoquinone and tocopherol in plants (Chiari et al., 2015). Flavones, ent-labdanes, ent-halimanes (Favier et al., 1997), benzofurans, prenylated chromanones and prenylated acetophenone (de Lampasona et al., 1997) were isolated from this species but, as currently known, no bioactivity was reported for these compounds.
As part of a search for natural phytotoxic compounds (del Corral et al., 2012, Palacios et al., 2010), the ethanolic extract of O. charua was chosen for further investigation based on the previous screening results. Reported here are the bioguided isolation of three novel phenylpropanoids, 1, 2 and 3, as the phytotoxic principles of 0. charua aerial tissues.
Section snippets
Results and discussion
The ethanol extract from leaves of O. charua (yield 13%) was subjected to liquid-liquid partition to obtain diethyl ether and aqueous subfractions. Comparison of its germination inhibitory activity on R. sativus indicated that the major activity of the original extract appeared in the diethyl ether fraction (yield 86%, percent of germination inhibition (IG%) = 93). This bioactive fraction was subjected to flash silica gel column chromatography, and the greatest activity was observed in a
General experimental procedures
IR spectra were obtained using KBr disks on a Shimadzu spectrophotometer. 1H- and 13C-NMR spectra, including COSY, HMBC, HSQC and ROESY experiments, were recorded in CDCl3 with a Bruker AVANCE II 400 spectrometer (Bruker, Germany) operated at 400 MHz for 1H and at 100 MHz for the 13C nucleus. Chemical shifts (parts per million) are relative to internal tetramethylsilane (TMS) used as a reference (δ = 0.00). UV spectra were recorded on a Hewlett Packard 8452 A diode array spectrophotometer. CD
Acknowledgments
This work was supported by a grant from FonCyT- PICT 2013-1004 and from the Catholic University of Cordoba 2015-2016. M.M.M.B. gratefully acknowledges receipt of a fellowship from CONICET. The authors thank Joss Heywood for revising the English language.
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S.M. Palacios, R.V. Vico and M.C. Carpinella are members of the National Research Council of Argentina (CONICET).