Chemical and Biological Activity of Triterpene Saponins from Medicago Species

Naturally occurring saponins are a large group of triterpene and steroid glycosides characterized by several biological and pharmacological properties. The Medicago genus represents a valuable source of saponins which have been extensively investigated. This review summarizes the chemical features of saponins from Medicago species and their biological activity, with particular attention to their antimicrobial, insecticidal, allelopathic and cytotoxic effects. Influence of saponins on animal metabolism is also reported.

Soyasapogenol B and trans-22-dehydrocam- pesterol from common vetch (Vicia sativa L.) root exudates stimulate broomrape seed germination

BACKGROUND

Orobanche and Phelipanche species (the broomrapes) are root parasitic plants, some of which represent serious weed problems causing severe yield losses on important crops. Control strategies have largely focused on agronomic practices, resistant crop varieties and herbicides, albeit with marginal success. An alternative control method is the induction of suicidal seed germination with natural substances isolated from root exudates of host and non-host plants.

RESULTS

Soyasapogenol B [olean-12-ene-3,22,24-triol(3β,4β,22β)] and trans-22-dehydrocampesterol [(ergosta-5,22-dien-3-ol, (3β,22E,24S)] were isolated from Vicia sativa root exudates. They were identified by comparing their spectroscopic and optical properties with those reported in the literature. Soyasapogenol B was very specific, stimulating the germination of O. minor seeds only, whereas trans-22-dehydrocampesterol stimulated P. aegyptiaca, O. crenata, O. foetida and O. minor.

CONCLUSION

Soyasapogenol B and trans-22-deydrocampesterol were isolated for the first time from Vicia sativa root exudates, and their biological activity as stimulants of Orobanche or Phelipanche sp. seed germination was reported.

Variation in saponin content during the growing season of spotted medic [Medicago arabica (L.) Huds.]

BACKGROUND

Spotted medic [Medicago arabica (L.) Huds.] is a minor forage species containing saponins which are reported to be biologically active. This study assessed the concentration and composition pattern of spotted medic saponins during the growing season and at senescence. The pattern of saponins was based on identification and quantification of their constituent sapogenins. At senescence, individual saponin concentrations of aerial and subterranean plant organs were also determined.

RESULTS

Leaf total saponin content did not vary during the growing season and decreased remarkably at senescence. Seven sapogenins were identified and quantified during the season, bayogenin and hederagenin being the most abundant ones throughout. Total saponin content varied among plant organs at senescence, with the highest concentration in roots. A variable number of saponins from one (in seeds) to 19 (in leaves) were quantified. A clear relationship between leaf concentrations of sapogenins and those of their derivative saponins was revealed by correlation analysis.

CONCLUSION

The species displayed a sapogenins/saponins pattern markedly different from those of other perennial or annual Medicago species. Saponins of queretaroic acid and 2β-hydroxy queretaroic acid had no precedent in the Leguminosae. The high concentration of biologically active hederagenin suggested further assessment of possible effects on feeding animals.

Triterpene saponins from aerial parts of Medicago arabica L

Eight major triterpene saponins have been isolated from the aerial parts of Medicago arabica and their structures elucidated by FAB-MS and NMR analysis. Three of them are new compounds and are identified as 3-O-(alpha-L-arabinopyranoside) bayogenin, 3-O-(alpha-L-arabinopyranosyl), 28-O-(beta-D-glucopyranoside) bayogenin, and 3-O-[alpha-L-arabinopyranosyl(1-->2)-beta-D-glucuronopyranosyl], 28-O-beta-D-glucopyranoside 2-beta-hydroxyoleanolic acid. Two saponins, identified as 3-O-(alpha-L-arabinopyranoside) hederagenin and 3-O-(alpha-L-arabinopyranosyl), 28-O-(beta-D-glucopyranoside) hederagenin are known compounds but not previously reported as saponin constituents of Medicago species, while three other saponins, being mono- and bidesmosides of hederagenin, have been previously isolated from roots of M. sativa.

Adaptations to herbivory in alfalfa (Medicago sativa)

Alfalfa (Medicago sativa L.) possesses numerous adaptations to herbivores. As with other wild plants, general chemical defences (notably medicagenic acid in alfalfa) and general anti-insect structures (notably trichomes) provide some protection against a wide spectrum of polyphagous insects. Several morphological adaptations protect alfalfa against specialized insect feeders, especially those that consume the seeds. Alfalfa has been disruptively selected by man to possess morphological adaptations and tolerance for two contrasting classes of herbivory: continuous grazing by livestock and intermittent harvesting for hay. Domestication of alfalfa has lowered the effectiveness of several natural adaptations against insects. Occasionally, however, artificial selection for resistance against certain insect species has, often unintentionally, resulted in selection for structural features discouraging these insects. Preadaptations of wild alfalfa for the horse facilitated this plant's domestication as a specialized horse feed, despite the fact that horses are poorly adapted in the wild to such a nutritious forage. Under conditions of domestication, however, alfalfa appears to offer horses several invaluable dietary advantages, and is much more suited to the horse than to man's other grazing herbivores. Anti-herbivorous hemolytic saponins are present in high concentrations in certain alfalfa lineages and cultivars. Horses in southern Europe and Asia have been fed low-saponin alfalfa for millenia, whereas high-saponin alfalfa has been utilized in northern Europe and much of the New World only since about the 16th century. While these compounds are poisonous for most herbivores, they could be beneficial for the horse. A very high degree of herbivorous coadaptation exists between alfalfa and its specialized pollinator, the alfalfa leafcutter bee (Megachile rotundata), and there is evidence that this relationship has coevolved. As the world's most efficient protein crop, alfalfa is preeminently adapted to mankind's future need for increased direct consumption of plant proteins. Keywords: adaptation, alfalfa, fodder, forage, herbivory, Medicago sativa.