6-O-beta-glucopyranosylaucubin, a new iridoid from Odontites verna

Iridoid Glycosides Isolated from Bellardia trixago Identified as Inhibitors of Orobanche cumana Radicle Growth

Orobanche cumana is an obligate holoparasitic plant with noxious effects in sunflower crops. Bellardia trixago is a facultative hemiparasitic plant that infects ruderal plants without noxious significance in agriculture and is known to produce a wide spectrum of bioactive metabolites. The objective of this study was to evaluate the allelopathic effects of B. trixago on the growth of O. cumana seedlings. Three different extracts using solvents of increasing polarity (n-hexane, dichloromethane and ethyl acetate) were prepared from the flowers, aerial green organs and roots of two populations, a white-flowered and a yellow-flowered population of B. trixago, both collected in southern Spain. Each extract was studied using allelopathic screenings on O. cumana which resulted in the identification of allelopathic activity of the ethyl acetate extracts against Orobanche radicles. Five iridoid glycosides were isolated together with benzoic acid from the ethyl acetate extract of aerial green organs by bio-guided purification. These compounds were identified as bartsioside, melampyroside, mussaenoside, gardoside methyl ester and aucubin. Among them, melampyroside was found to be the most abundant constituent in the extract (44.3% w/w), as well as the most phytotoxic iridoid on O. cumana radicle, showing a 72.6% inhibition of radicle growth. This activity of melampyroside was significantly high when compared with the inhibitory activity of benzoic acid (25.9%), a phenolic acid with known allelopathic activity against weeds. The ecotoxicological profile of melampyroside was evaluated using organisms representing different trophic levels of the aquatic and terrestrial ecosystems, namely producers (green freshwater algae Raphidocelis subcapitata and macrophyte Lepidium sativum), consumers (water flea Daphnia magna and nematode Caenorhabditis elegans) and decomposers (bacterium Aliivibrio fischeri). The ecotoxicity of melampyroside differed significantly depending on the test organism showing the highest toxicity to daphnia, nematodes and bacteria, and a lower toxicity to algae and macrophytes. The findings of the present study may provide useful information for the generation of green alternatives to synthetic herbicides for the control of O. cumana.

Chemical Traits of Hemiparasitism in Odontites luteus

The study of the monoterpene glycosides content of Odontites luteus has shown the presence of a total of fifteen iridoid glucosides. The presence of compounds 1 - 5 and 7 - 10 is perfectly on-line with both the biogenetic pathway for iridoids biosynthesis in Lamiales and the current botanical classification of the species. On the other side, the presence of compounds like agnuside (6), adoxosidic acid (11), monotropein (12), 6,7-dihydromonotropein (13), methyl oleoside (14) and methyl glucooleoside (15) is of high interest because, first of all, they have never been reported before in Lamiales. In second instance, the majority of the last compounds are formally derived from a different biogenetic pathway which involves deoxyloganic acid/loganin and led to the formation of decarboxylated iridoid showing the 8β-configuration. Furthermore, a second abnormality was found during our study and this regards compounds 14 and 15 which are seco-iriodids and thus not typical for this family. The presence of these unusual compounds, biogenetically not related to species belonging to Lamiales, is a clear evidence of the metabolites transfer from the hosts. In fact, the collection area is also populated by species belonging to Oleaceae and Ericaceae which could be the possible hosts since the biosynthesis of seco-iridoids and or iridoids related to deoxyloganic acid/loganin pathway, with the 8β-configuration, is well documented in these species.