Botany and Distribution of Astragalus
The genus Astragalus, the largest in the Fabaceae (Leguminosae), comprises ca. 2000 species. In Europe 133 species are found. The plants are cultivated and also selected from wild stock. Cultivated varieties are mostly selected from Astragalus membranaceus for medicinal use and are mainly cultivated in China.
Astragalus plants are annual or perennial herbs or small shrubs up to 150-200 cm. Their leaves are imparipinnate or paripinnate, sometimes terminating in a spine; leaflets entire. Flowers in racemes or axillary clusters, sessile or pedicellate. Calyx infundibuliform, tubular or campanulate, sometimes inflated in fruit, with distinct, equal or unequal teeth; keel not mucronate at apex (very rarely adaxially mucronate); stamens 10, diadelphous (very rarely 5, monadelphous); stigma and style glabrous. Legume is usually dehiscent, very varied in shape and texture, glabrous or hairy, unilocular to bilocular. Seeds are one to many.
History and Demand on the World Market
Astragalus was well known to Theophrastus and Dioscurides and is a plant of the Bible.
The medical use of Astragalus species dates back over 1000 years. The gum tragacanth (GT) obtained by incision from the stem of various species of Asiatic Astragalus has been used as an emulsifier and stabilizer in pharmaceutical products since 1820, and has been included in the US Pharmacopeia since 1820, and the European and other National Pharmacopeia since their inception. Because GT may originate legitimately from any Asiatic Astragalus species (Table Traditional sources of tragacanth), each of which gives a gum differing in chemical composition from other Astragalus species, commercial GT is an extremely variable commodity.
GT is the most efficient natural oil-water emulsifier known. Despite its high cost, ranging from £15 to 48 per kilogram in 1986-1987 according to quality/ grade (based on color and viscosity), GT remains cost-effective in formulations because very low levels (generally 0.02 to 0.8%) are effective. In terms of good manufacturing practice, its high cost makes GT self-limiting in use.
Annual production (Iran and Turkey) of GT has not exceeded 50001, and at least 50% of world production has always been used in pharmaceutical products. Over the past years, world production of GT has decreased to around 1000 t.
Table Traditional sources of tragacanth
|Astragalus sp.||Geographical distribution|
|1.||A. gummifera||Anatolia and Syria|
|2.||A. kurdicus||Northern Iraq and Syria|
|3.||A. brachycalyx||Western and SW Iran|
|4.||A. eriostylus||SW Iran|
|5.||A. pycnocladus||Kerman area|
|6.||A. verus||Western Iran|
|7.||A. leioclados||Western and central Iran|
|8.||A. adscendens||Southwestern Iran|
|9.||A. strobiliferus||Eastern Iran|
|10.||A. heratensis||Khorasan to Afghanistan|
Radix Astragali (A. membranaceus, A. mongholicus) is a very old and well-known drug in traditional Chinese medicine. Astragalus species are used in traditional medicine in Bulgaria, Russia, and other European and Asiatic countries. Some of them (extracts or isolated compounds) are clinically tested as well.
In recent years, advances in research on the Astragalus species have been made, due to their pharmacological constituents.
Conclusion and Prospects
Astragalus belongs to the group of plants used in medicine, essentially as immunostimulant or as a substitute for Panax ginseng. Especially saponins, polysaccharides, and flavonoids have been considered active principles for Astragalus pharmacological properties. A variety of experimental approaches have been used to increase the efficiency of the production of pharmacologically important products by tissue and cell culture techniques. Optimization of cultural conditions including chemical and physical factors was one of the common approaches. Selection of high product-yielding cell lines, using induction of hairy roots by Agrobacterium rhizogenes has also been applied.
Studies on saponin composition in cultured tissue of Astragalus gave a basic understanding with regard to biosynthesis. The saponin and polysaccharide production in hairy roots has remained constant over the past 3-4 years, and can be used as a biological model for studies of metabolism regulation, including enzymology and biotransformation which may affect the expression of biogenetic pathways of desired products. On the other hand, by using modern and very accurate methods of mathematical programming more effective production will be possible.
The excretion of saponins and polysaccharides to the liquid medium offers possibilities for continuous cultures. In general, hairy roots are preferable to callus and suspension cultures, because of the higher genetic stability, higher growth rates, and production of metabolites. Concerning cultured tissues, hairy root cultures may be hopeful as a potential material for saponin and polysaccharide production.
Selections from the book: Medicinal and Aromatic Plants VIII (1995).