Potential benefits of iridoid glycoside sequestration in Longitarsus melanocephalus (Coleoptera,Chrysomelidae)

Whenever potentially noxious plant compounds are taken up and recycled by herbivorous insects, a protective function of these sequestered compounds is assumed. The flea beetle Longitarsus melanocephalus sequesters iridoid glycosides from its host plant up to a concentration of 2% DW, yet so far it remained unknown whether the insects gain protection from natural enemies by sequestering plant compounds at these comparatively low concentrations. Here we tested whether iridoid glycosides might deter or inhibit various soil and litter-dwelling potential enemies and pathogens. In choice experiments presenting L. melanocephalus pupae together with Tribolium castaneum pupae, the predator Lithobius forficatus was deterred by the iridoid glycoside containing pupae, while Forficula auricularia as well as the nematode Heterorhabditis bacteriophora were not deterred. L. forficatus also avoided artificial baits doted with 2% iridoid glycosides while F. auricularia showed no aversion to iridoid glycosides at these concentrations and H. bacteriophora did not suffer any toxic effect. Of the pathogens tested, the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae were not inhibited in their growth by iridoid glycosides ranging up to 2%. However, an inhibitory effect could be observed against the entomopathogenic bacterium Bacillus thuringiensis, even at the relatively small concentrations that are common in L. melanocephalus. The antibacterial effect might thus be another important selective value of iridoid glycoside sequestration in this species.     

Iridoid glucosides in Griselinia, Aralidium and Toricellia

From three of five investigated species of Griselinia a new iridoid glucoside, griselinoside, was isolated. It was found to be present also in foliage of Aralidium pinnatifidum and Toricellia angulata, accompanied in the former by aralidioside another novel iridoid glucoside. The structures and absolute configurations of the two iridoids were elucidated by NMR spectroscopy and chemical conversions. From G. littoralis and T. angulata the glucosides magnolioside and syringoside respectively were isolated. ¹³C NMR spectra are given for thirteen iridoid derivatives.     

Various types of tryptamine-iridoid alkaloids from Palicourea acuminata (=Psychotria acuminata,Rubiaceae)

The novel tryptamine-iridoid alkaloid lagamboside was isolated from the methanolic leaf extracts of Palicourea acuminata (Benth.) Borhidi (=Psychotria acuminata Benth.) collected in Costa Rica. Its structure was determined by extensive NMR experiments and is characterized by an unusual N-glycosylation and an iridoid moiety closely related to the less common vallesiachotamine. The co-occurrence with the rare bahienoside, containing two iridoid moieties, is of chemotaxonomic interest.     

Biosynthesis of the iridoid glucoside,lamalbid, in Lamium barbatum

The biosynthesis of the iridoid glucoside lamalbid in Lamium barbatum, a plant species in the Lamiaceae, was investigated by administrating ¹³C-labeled intermediates of MVA and MEP pathways, respectively. The results demonstrated that [3,4,5-¹³C₃]1-deoxy-d-xylulose 5-phosphate could be incorporated into lamalbid, whereas the incorporation of [2-¹³C₁]mevalonolactone was not observed. Based on the ¹³C labeling pattern of lamalbid and the incorporation data, we deduce that the iridoid glucoside in L. barbatum is biosynthesized through the MEP pathway, whereas the classic MVA pathway is not utilized.     

Impact of the dual defence system of Plantago lanceolata (Plantaginaceae) on performance,nutrient utilisation and feeding choice behaviour of Amata mogadorensis larvae (Lepidoptera,Erebidae)

Iridoid glycosides are plant defence compounds with potentially detrimental effects on non-adapted herbivores. Some plant species possess β-glucosidases that hydrolyse iridoid glycosides and thereby release protein-denaturing aglycones. To test the hypothesis that iridoid glycosides and plant β-glucosidases form a dual defence system, we used Plantago lanceolata and a polyphagous caterpillar species. To analyse the impact of leaf-age dependent differences in iridoid glycoside concentrations and β-glucosidase activities on insect performance, old or young leaves were freeze-dried and incorporated into artificial diets or were provided freshly to the larvae. We determined larval consumption rates and the amounts of assimilated nitrogen. Furthermore, we quantified β-glucosidase activities in artificial diets and fresh leaves and the amount of iridoid glycosides that larvae feeding on fresh leaves ingested and excreted. Compared to fresh leaves, caterpillars grew faster on artificial diets, on which larval weight gain correlated positively to the absorbed amount of nitrogen. When feeding fresh young leaves, larvae even lost weight and excreted only minute proportions of the ingested iridoid glycosides intact with the faeces, indicating that the hydrolysis of these compounds might have interfered with nitrogen assimilation and impaired larval growth. To disentangle physiological effects from deterrent effects of iridoid glycosides, we performed dual choice feeding assays. Young leaves, their methanolic extracts and pure catalpol reduced larval feeding in comparison to the respective controls, while aucubin had no effect on larval consumption. We conclude that the dual defence system of P. lanceolata consisting of iridoid glycosides and β-glucosidases interferes with the nutrient utilisation via the hydrolysis of iridoid glycosides and also mediates larval feeding behaviour in a concentration- and substance-specific manner.     

Chlorine-containing iridoid and iridoid glucoside, and other glucosides from leaves of Myoporum bontioides

Sixteen compounds were isolated from the MeOH extract of leaves of Myoporum bontioides. The five compounds hitherto unknown, were elucidated to be a chlorine-containing iridoid, named myopochlorin, and an iridoid glucoside, an acylated iridoid glucoside, a linear acetogenin glucoside, and an acyclic monoterpene glucoside, named myobontiosides A-D, respectively, by means of spectroscopic analyses.     

Iridoid glycosides in the butterfly Euphydryas cynthia (lepidoptera,nymphalidae)

Larvae and pupae of the alpine butterfly Euphydryas cynthia contain three iridoid glucosides: two of them, aucubin and catalpol, are very common in the Plantaginaceae, but the third, 6-O-glucopyranosylaucubin, has hitherto only been isolated from Odontites verna and Verbascum sinuatum (Scrophulariaceae). The amounts of the three iridoids found in the insects were 0.53, 0.31 and 1.48 % dry wt, respectively. Preliminary feeding experiments with an insectivorous bird indicated some unpalatability in the insect, which probably stems from the iridoid content.     

8-Epiloganin,an iridoid glucoside from Odontites verna

Odontites verna subsp. serotina contains besides odontoside, aucubin, mussaenoside, shanzhiside methyl ester and catalpol, a new iridoid glucoside, 8-epiloganin.     

Iridoid glucosides from Melampyrum

Melampyrum arvense and M. cristatum contain, besides aucubin, 8-epiloganin and melampyroside, a new natural iridoid glucoside: gardoside methyl ester. In addition, M. arvense contains mussaenoside and M. cristatum mussaenosidic acid, another novel iridoid glucoside.     

Influence of iridoid glycoside containing host plants on midgut β-glucosidase activity in a polyphagous caterpillar, Spilosoma virginica Fabricius (Arctiidae)

Iridoid glycosides are secondary plant compounds that have deterrent, growth reducing or even toxic effects on non-adapted herbivorous insects. To investigate the effects of iridoid glycoside containing plants on the digestive metabolism of a generalist herbivore, larvae of Spilosoma virginica (Lepidoptera: Arctiidae) were reared on three plant species that differ in their secondary plant chemistry: Taraxacum officinale (no iridoid glycosides), Plantago major (low iridoid glycoside content), and P. lanceolata (high iridoid glycoside content). Midguts of fifth instar larvae were assayed for the activity and kinetic properties of β-glucosidase using different substrates. Compared to the larvae on T. officinale, the β-glucosidase activity of larvae feeding on P. lanceolata was significantly lower measured with 4-nitrophenyl-β-d-glucopyranoside. Using the iridoid glycoside aucubin as a substrate, we did not find differences in the β-glucosidase activity of the larvae reared on the three plants. Heat inactivation experiments revealed the existence of a heat-labile and a more heat-stable β-glucosidase with similar Michaelis constants for 4-nitrophenyl-β-d-glucopyranoside. We discuss possible mechanisms leading to the observed decrease of β-glucosidase activity for larvae reared on P. lanceolata and its relevance for generalist herbivores in adapting to iridoid glycoside containing plant species and their use as potential host plants.     

5,7-Bisdeoxycynanchoside,an iridoid glucoside from Macfadyena cynanchoides

A new iridoid glucoside from Macfadyena cynanchoides leaves has been identified by spectral (¹H and ¹³C NMR) and chemical procedures as 5,7     

Three iridoid glycosides from Viburnum furcatum

Three new bitter iridoid glycosides having an 8,10,11-oxygen substituted iridoid skeleton with an isovaleryl moiety at C-1, have been isolated from the ether and ethyl acetate soluble fractions of the leaves of Viburnum furcatum. Two of them had a glucose moiety at C-11 of the iridoid skeleton and a p-coumaroyl group linked to C-6 of the sugar, and they were found to be geometrical isomers about the double bond of the p-coumaroyl2 moiety. The third one was characterized as a alloside of the same aglycone.     

Iridoids from the flowers of Gardenia jasminoides Ellis and their chemotaxonomic significance

Phytochemical investigation of the flowers of Gardenia jasminoides Ellis (Rubiaceae Gardenia) resulted in the isolation and identification of four iridoid aglycones (1–4) and eleven iridoid glycosides (5–15). This is the first report of the occurrence of these compounds in the genus Gardenia: garjasmine (1), dunnisin (2), α-gardiol (3), β-gardiol (4), diffusoside A (6), diffusoside B (7), genameside C (13), and deacetylasperulosidic acid (14). The chemotaxonomic significance and biosynthetic pathways of these iridoid aglycones (1–4) and iridoid glycosides (5–15) are summarized. Iridoid aglycones and iridoid glycosides are considered as important chemotaxonomic markers in Rubiaceae and this work indicates that the iridoid aglycones (1–4) have a limited distribution in the Rubiaceae.     

Iridoid and other constituents of Canthium subcordatum

The main constituent of the stem bark of Canthium subcordatum was found to be shanzhiside methyl ester, an iridoid recently described for two Mussaenda species; it is accompanied by the new iridoid shanzhisin methyl ester gentiobioside. No alkaloids were detected.     

Phylogeny-Aware Chemoinformatic Analysis of Chemical Diversity in Lamiaceae Enables Iridoid Pathway Assembly and Discovery of Aucubin Synthase

Countless reports describe the isolation and structural characterization of natural products, yet this information remains disconnected and underutilized. Using a cheminformatics approach, we leverage the reported observations of iridoid glucosides with the known phylogeny of a large iridoid producing plant family (Lamiaceae) to generate a set of biosynthetic pathways that best explain the extant iridoid chemical diversity. We developed a pathway reconstruction algorithm that connects iridoid reports via reactions and prunes this solution space by considering phylogenetic relationships between genera. We formulate a model that emulates the evolution of iridoid glucosides to create a synthetic data set, used to select the parameters that would best reconstruct the pathways, and apply them to the iridoid data set to generate pathway hypotheses. These computationally generated pathways were then used as the basis by which to select and screen biosynthetic enzyme candidates. Our model was successfully applied to discover a cytochrome P450 enzyme from Callicarpa americana that catalyzes the oxidation of bartsioside to aucubin, predicted by our model despite neither molecule having been observed in the genus. We also demonstrate aucubin synthase activity in orthologues of Vitex agnus-castus, and the outgroup Paulownia tomentosa, further strengthening the hypothesis, enabled by our model, that the reaction was present in the ancestral biosynthetic pathway. This is the first systematic hypothesis on the epi-iridoid glucosides biosynthesis in 25 years and sets the stage for streamlined work on the iridoid pathway. This work highlights how curation and computational analysis of widely available structural data can facilitate hypothesis-based gene discovery.     

Iridoid allosides from Viburnum opulus

Arbutin and four novel iridoid glycoside esters, named opulus iridoids I–IV, have been isolated from foliage of Viburnum opulus (Caprifoliaceae). Each opulus iridoid constitutes an inseparable mixture of two compounds, differing by containing either 2-methyl- or 3-methylbutyric acid in ester linkage at the 1-OH-group in an iridoid glycoside. In all glycosides 2′,3′-di-O-acetyl-β-D-allopyranose is linked through a glycosidic bond to C-11 in the iridoid aglycone. The opulus iridoids differ by the degree of acetylation of the aglycone and by the attachment, in III and IV, of a β-D-xylopyranosyl group at C-4 of the allose moiety. The structures have been elucidated by ¹H and ¹³C-NMR spectroscopy and by cleavage of the glycosidic linkage with boron trifluoride etherate in acetic anhydride, yielding the acetates of the cyclized aglycone and of the appropriate mono- or disaccharide. This is the second report of an iridoid attached to a sugar other than glucose and the second time allose has been encountered in higher plants. The systematic position of Viburnum is briefly discussed.     

Biosynthesis and therapeutic implications of iridoid glycosides from <Emphasis Type="Italic">Picrorhiza</Emphasis> genus: the road ahead

Picrorhiza genus is emerging as an important paradigm for herbal drug formulations due to its versatile iridoid glycosides exhibition and robustness in the treatment of diverse infections including hepatic amoebiasis, cancer, malaria, ulcerative colitis and cerebral ischemia reperfusion injury. Owing to the superiority of these bioactivities, iridoid glycosides from Picrorhiza have become a hot research area over the years. A metabolic pathway for the formation of iridoid glycosides has been proposed. However, some enzymes and genes of this route are still unidentified and demand the enumeration of facilitating pathways contributing to the biosynthesis of iridoid glycosides. This review summarizes the current knowledge of all naturally occurring iridoid glycosides from Picrorhiza, their biosynthesis and pharmacological capabilities which could provide the insight into metabolic regulation and the basis for the development of new drugs.     

LC–ESI-MS analysis of iridoid glucosides in Lamium species

An LC–ESI-MS analysis was performed to determine the iridoid composition of Lamium album, Lamium amplexicaule, Lamium garganicum, Lamium maculatum, and Lamium purpureum. Nine iridoids known to occur in these species, lamalbid, sesamoside, lamiol, 5-deoxylamiol, shanzhiside methyl ester, caryoptoside, penstemoside, lamioside, and barlerin, could be detected. Confident identification of the individual compounds was achieved by a combination of HPLC retention times, both positive and negative ionization modes in MS and the presence of cluster and fragment ions. Iridoid glucosides new to four of the studied species were reported here for the first time. In most cases, the observed iridoid profiles were consistent and only small intraspecific variations were detected. Some chemosystematic implications from the observed iridoid composition were discussed.     

5-deoxystansioside,an iridoid glucoside from Tecoma stans

In addition to plantarenaloside and stansioside, a new iridoid glucoside with a formyl group at C-4 has been isolated from Tecoma stans. The new glucoside was shown to be 5-deoxystansioside by ¹³C NMR and ¹H NMR spectroscopy.     

Structure and configuration of unedide,an iridoid glucoside from Arbutus unedo

Unedide, a novel iridoid glucoside isolated from Arbutus unedo (Ericaceae), has been established to be 6,7-dihydro-6β-hydroxymonotropein by detailed analysis of ¹H and ¹³ C NMR spectral data.