Sesquiterpenyl epoxy-cyclohexenoids (SECs) show impressive biological activities. However, the key pathways for SECs still remain unambiguous. Unexpectedly, 11 new SECs and derivatives with diverse oxidation patterns were isolated after the deletion of gene 274. A high accumulation of toluquinol and its new glycosides in mutant Δ276 and further isolation of the most crucial precursors farnesyl hydroquinone, farnesyl quinone, and three new derivatives from mutant Δ278 confirm that farnesylation at toluquinol is the key step for SECs.

•Chemical profile of the latex of Euphorbia peplus was investigated.•13 terpenoids including two previously undescribed diterpenoids were identified.•The diterpenoids exhibited potential antifeedant activity.•The acyclic triterpenoid peplusol showed antifungal activity.•Latex terpenoids of Euphorbia peplus function as constitutive defense metabolites.Plant latex is an endogenous fluid secreted from highly specialized laticifer cells and has been suggested to act as a plant defense system. The chemical profile of the latex of Euphorbia peplus was investigated. A total of 13 terpenoids including two previously unknown diterpenoids, (2S*,3S*,4R*,5R*,6R*,8R*,l1R*,13S*,14S*,15R*, 16R*)-5,8,15-triacetoxy-3-benzoyloxy-11,16-dihydroxy-9-oxopepluane and (2R*,3R*, 4S*,5R*,7S*,8S*,9S*,l3S*,14S*,15R*)-2,5,8,9,14-pentaacetoxy-3-benzoyloxy-15-hydroxy-7-isobutyroyloxyjatropha-6(17),11E-diene), ten known diterpenoids, and a known acyclic triterpene alcohol peplusol, were identified, using HPLC and UPLC-MS/MS analyses and through comparison with the authentic compounds isolated from the whole plant. The diterpenoids exhibited significant antifeedant activity against a generalist plant-feeding insect, the cotton bollworm (Helicoverpa armigera), with EC50 values ranging from 0.36 to 4.60 μg/cm2. In particular, (2R*,3R*,4S*,5R*,7S*,8S*,9S*,l3S*,14S*,15R*)-2,5,9,14-tetraacetoxy-3-benzoyloxy-8,15-dihydroxy-7-isobutyroyloxyjatropha-6(17),11E-diene and (2R*,3R*, 4S*,5R*,7S*,8S*,9S*,l3S*,14S*,15R*)-2,5,14-triacetoxy-3-benzoyloxy-8,15-dihydroxy-7-isobutyroyloxy-9-nicotinoyloxyjatropha-6(17),11E-diene had EC50 values of 0.36 and 0.43 μg/cm2, respectively, which were approximately 7-fold more potent than commercial neem oil (EC50 = 2.62 μg/cm2). In addition, the major peplusol showed obvious antifungal activity against three strains of agricultural phytopathogenic fungi, Rhizoctonia solani, Colletotrichum litchi and Fusarium oxysporum f. sp. niveum. The results indicated that terpenoids in the latex of E. peplus are rich and highly diversified, and might function as constitutive defense metabolites against insect herbivores and pathogens for the plant.Chemical profile of the latex of Euphorbia peplus was investigated. Thirteen structurally diversified terpenoids including two previously undescribed diterpenoids were identified. Antifeedant and antifungal activities of these terpenoids were observed, indicating that they might function as constitutive defense metabolites against insect herbivores and pathogens for the plant.Download high-res image (201KB)Download full-size image

Glandular trichomes of plants produce a wide variety of secondary metabolites which are considered as major defensive chemicals. The capitate glandular trichomes of Oenothera glazioviana (Onagraceae) were collected with laser microdissection and analyzed by gas chromatography–mass spectrometry. The volatile compound 4-hydroxy-4-methylpentan-2-one (1) was identified. We found that compound 1 displays antimicrobial, insecticidal, and phytotoxic activities. These results suggest that compound 1 might function as a defensive compound in the capitate glandular trichomes of O. glazioviana against pathogens, insect herbivores, and presumably competitive plants as well.

Three new drimane sesquiterpenoids (1–3) together with the known 2α-hydroxyisodrimeninol (4), and a new isochromone derivative (5), were obtained from the solid cultures of fungal strain Pestalotiopsis sp. M-23, an endophytic fungus isolated from the leaves of Leucosceptrum canum (Labiatae). Their structures were determined by comprehensive 1D and 2D NMR, and MS analyses. The metabolites were evaluated for their antibacterial activities, and compound 3 showed weak inhibitory activity against Bacillus subtilis.

The morphology and chemical profile of the capitate glandular trichomes (CGTs) of Paragutzlaffia henryi (Acanthaceae) were investigated. Four new labdane diterpenoids named paraguhenryisins A–D (1–4), together with the known physacoztomatin (5), were localized to the CGTs using laser microdissection coupled with cryogenic 1H NMR and HPLC analyses and were traced and isolated from the CGT extract of inflorescences. Their structures were determined by spectroscopic methods and single-crystal X-ray diffraction. Bioassays indicated significant inhibitory effect for these diterpenoids on Arabidopsis thaliana seed germination and seedling root elongation. The most potent inhibitor, paraguhenryisin C (3), was interestingly detected in both the rhizosphere soil and water rinsed inflorescences extract of P. henryi but not the roots, with average contents in the rhizosphere soil relevant to its phytotoxic EC50 values. These results suggested that phytotoxic labdane diterpenoids in the CGTs might be released into the environment as a defensive measure for P. henryi against other competitive plants.

Seven new leucosceptroid degradation products possessing a C20, C21, or C25 framework, norleucosceptroids D–H (1–5), leucosceptroids P (6), and Q (7), have been isolated from Leucosceptrum canum. Their structures were determined by comprehensive NMR, MS, and single-crystal X-ray diffraction analyses. Discovery of these key intermediates, together with the biomimetic oxidation of a model system, supports the hypothesis that two biosynthetic pathways are operative. Antifeedant activity was observed for compounds 1–3.

A phytochemical investigation of the toxic tropical plant Dichapetalum gelonioides led to the isolation and identification of 14 new dichapetalins (1–14) and the known dichapetalins A (15) and K (16). The structures of the new compounds were determined by analyses of their NMR, MS, electronic circular dichroism, and X-ray diffraction data. The esterification at C-25 by 4-hydroxyphenylpropanoic acid and the hydroxylation at C-2′ are unique in this unusual class of natural products. In addition to the known cytotoxicity, an array of biological activities, including antifeedant, nematicidal, antifungal, and NO and AChE inhibitory activities, were observed for this class of compounds. These findings suggested that dichapetalin hybrid triterpenoids as a class have broad biologically active cellular functions including defense against insect herbivores and pathogens.

Eight new aphanamixoid-type aphanamixoids (C–J, 1–8) and six new prieurianin-type limonoids, aphanamixoids K–P (9–14), along with 10 known terpenoids were isolated from Aphanamixis polystachya, and their structures were established by spectroscopic data analysis. Among the new limonoids, 13 compounds exhibited antifeedant activity against the generalist Helicoverpa armigera, a plant-feeding insect, at various concentration levels. In particular, compounds 1, 4, and 5 showed potent activities with EC50 values of 0.017, 0.008, and 0.012 μmol/cm2, respectively. On the basis of a preliminary structure–activity relationship analysis, some potential active sites in the aphanamixoid-type limonoid molecules are proposed.

The perennial herbaceous plant Euphorbia jolkinii (Euphorbiaceae) is a noxious weed widely distributed in the grasslands of northwestern Yunnan and has greatly threatened the local biodiversity. Phytochemical investigation on the fresh roots of E. jolkinii afforded six new diterpenoids 1, 2, 4–6, and 8, together with fifteen known diterpenoids. Their structures were elucidated on the basis of 1D and 2D NMR and other spectroscopic methods. Casbane, lathyrane, abietane, and ent-kaurane diterpenoids were reported from this plant for the first time. Selected compounds were evaluated for their antifeedant and anti-RSV (respiratory syncytial virus) activities. Compound 2 and ingenol (3) exhibited moderate antifeedant activity against a generalist insect herbivore, Spodoptera exigua, with EC50 values of 17.88 and 17.71 μg/cm2 respectively. Compound 19 showed significant anti-RSV activity, with 50 % inhibition (IC50) value of 10.0 μM and selective index of 8.0. Compounds 1 and 2 were less active against RSV virus, both with IC50 value of 25 μM, and with selective indices of 1.0 and 3.2 respectively. These findings provided new evidence for the biological functions and utilization of the diversified diterpenoid metabolites in the roots of this rich but harmful plant.

A new class of unique sesterterpenoids, colquhounoids A–C (1–3), were identified from the peltate glandular trichomes of Colquhounia coccinea var. mollis (Lamiaceae) through precise laser-microdissection coupled with UPLC/MS/MS and spectroscopic analyses and X-ray diffraction. Very interestingly, their structural features and defensive function are closely related to leucosceptroid-class sesterterpenoids harbored by the glandular trichomes of another Lamiaceae taxon, Leucosceptrum canum, even though this is morphologically distinct and taxonomically distant.

Leucosceptroid O (1), an unusual sesterterpenoid possessing a spiro α,β-unsaturated γ-lactone moiety, was discovered from the flowers of a large woody Labiatae, Leucosceptrum canum. Its structure including absolute stereochemistry was determined by comprehensive NMR, MS, and single-crystal X-ray diffraction (with copper radiation) analyses. The obvious antifeedant activity of 1 against the generalist plant-feeding insect Helicoverpa armigera suggested that it to be a new defensive sesterterpenoid of Leucosceptrum canum.

Ten sesterterpenoids, leucosceptroids E–N (1–10), possessing an α,β-unsaturated γ-lactone moiety, were isolated from the leaves and flowers of a woody Labiatae, Leucosceptrum canum. Their structures including relative stereochemistry were determined by comprehensive 1D and 2D NMR, MS, and in the case of 1 and 10, by single crystal X-ray diffraction analyses. This class of unique plant terpenoids was designated as leucosceptrane sesterterpenoids (=leucosceptroids). The potent antifeedant activity of the most abundant compound, leucosceptroid G (3), and a representative compound, leucosceptroid N (10), against the generalist plant-feeding insect Helicoverpa armigera suggested that they might also be defense compounds of L. canum against insects.Graphical abstractDefense sesterterpenoids: A class of leucosceptrane sesterterpenoids 1–10, possessing an α,β-unsaturated γ-lactone moiety, has been isolated from the leaves and flowers of Leucosceptrum canum and found to show antifeedant activity against plant feeding insects.Highlights► A class of sesterterpenoid lactones has been isolated from Leucosceptrum canum. ► These compounds were identified by spectroscopic and single-crystal X-ray analyses. ► Representative compounds showed potent antifeedant activity against Helicoverpa armigera. ► These compounds might serve as defense chemicals of L. canum against insects.

The major pigment responsible for the dark brown nectar of the “bird’s Coca cola tree”, Leucosceptrum canum (Labiatae), was isolated and identified as a unique symmetric proline-quinone conjugate, 2,5-di-(N-(−)-prolyl)-para-benzoquinone (DPBQ). Behavioral experiments with both isolated and synthetic authentic samples indicated that DPBQ functions mainly as a color attractant to bird pollinators.

Three novel C20 terpenoids, norleucosceptroids A–C (1–3), were isolated from the leaves and flowers of Leucosceptrum canum (Labiatae) and were identified by comprehensive spectroscopic analysis and, in the case of 1, single-crystal X-ray diffraction. Structurally, compounds 1–3 should be categorized as pentanor-sesterterpenoids rather than diterpenoids. Moderate antifeedant activity of 1–3 against the generalist plant-feeding insect Helicoverpa armigera was detected, suggesting that they might also be involved in the plant defense against insect herbivores.

A phytochemical study of the invasive Eupatorium adenophorum indicated that the plant was rich in a phenolic compound o-coumaric acid (or 2-hydroxycoumaric acid). Biological investigations with the model plant Arabidopsis thaliana and crop plants showed that o-coumaric acid strongly inhibited seed germination, plant growth and root elongation, reduced the photosynthesis in old leaves, and induced the root cell death and the expression of genes related to senescence, oxidative stress, and systemic acquired resistance. The phytotoxic effects of o-coumaric acid exhibit selectivity between under- and above-ground parts of test plants and between E. adenophorum and other plants. These results indicate that o-coumaric acid is a potent toxin that might play an important role in the competition of E. adenophorum with its neighboring plants during its invasion and establishment.

The wild soybean (Glycine soja Sieb. et Zucc) has been reported to be relatively resistant to insect and pathogenic pests. However, the responsible secondary metabolites in the aerial part of this important plant are largely unknown. From the aerial part of G. soja, 13 compounds were isolated and identified, including seven isoflavonoids (1–7), a cyclitol (8), two sterol derivatives (9 and 10), and three triterpenoids (11–13). Compound 7 is a new isoflavonoid, and compounds 9 and 10 are reported as natural products for the first time. The growth inhibitory activity of 1, 3, 4, and 8 against the larvae of Spodoptera litura was investigated. The most abundant isoflavonoid in the aerial part of G. soja, daidzein (1), which could not be metabolized by S. litura, was found to inhibit the insect larvae growth significantly in 3 days after feeding diets containing the compound. Compounds 3, 4, and 8, which could be partially or completely metabolized, were inactive. Our results suggested that the isoflavonoid daidzein (1) might function as a constitutive defense component in G. soja against insect pests.

Pieris formosa is a poisonous plant to livestock and is used as an insecticide in rural areas of China. Two novel polyesterified 3,4-seco-grayanane diterpenoids, pierisoids A and B (1 and 2), were isolated from its flowers and were identified by spectroscopic analysis and X-ray diffraction. Both compounds showed obvious antifeedant activity against cotton bollworm, indicating their toxic properties, suggesting a defensive role of polyesterified 3,4-seco-grayanane diterpenoids for P. formosa against herbivores.

The invasive plant Eupatorium adenophorum Spreng. (or Ageratina adenophora (Spreng.) King and Robinson) (Compositae) has caused great economic loss in China, especially the southwestern region, and is gravely threatening the native biodiversity. The aerial part of this plant was phytochemically investigated for its allelochemicals. Eleven terpenes (2 monoterpenes and 9 sesquiterpenes) were isolated and identified, which include a new monoterpene, (−)-(1R*,2S,*4R*,5S*)-3,3-dimethyl-5-hydroxybicyclo[2,2,1]hept-2-ylmethanol (1), two new cadinane sesquiterpenes, (−)-(5S*,6S*,7S*,9R*,10S*)-7-hydroxy-5,7-epidioxycadinan-3-ene-2-one (2) and (+)-(5S*,6R*,9R*,10S*)-5,6-dihydroxycadinan-3-ene-2,7-dione (3), and eight known terpene compounds (4, 6−12). The new structures were established by spectroscopic studies such as 1D- and 2D-NMR and MS analyses. Meanwhile, the potential allelopathic effects of these compounds on the Arabidopsis seeds germination were tested. Compounds 3 and 7 retarded the Arabidopsis seeds germination at 0.5 mM and 1.0 mM concentrations, respectively, while other compounds showed no obvious inhibitory effects.