The Leguminosae (syn. Fabaceae) family is one of the three largest families of flowering plants. There is still no general agreement regarding the number of genera and species. Estimates vary between 590-690 genera and 12,000-17,000 species. The family is divided into three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae.
The genus Trigonella sensu stricto belongs to the latter subfamily and is composed of 75 species. The name of the genus derives from the Latin Trigonus, “three-angled” in reference to the small, triangular appearance of the flower. Trigonella foenum-graecum L. (fenugreek) is an erect, annual, herbaceous plant widely distributed in many parts of Asia, Africa and Europe. It is 10-50 cm high, sparsely pubescent with leaves pinnately three-foliolate. Leaflets (20-50 x 10-15 mm) are obovate to oblong-oblanceolate and denticulate. Flowers are solitary or in pairs in the axils of the leaves. The calyx is short (6-8mm) and the corolla (12-18mm) is yellowish-white tinged with violet at the base. The fruit (legume) (60-110 x 4-6 mm) is linear, somewhat curved, glabrous or glabrescent with longitudinal veins. The seeds (2-6 x 2-4 mm) are quadrangular, somewhat compressed, yellow or pale-brown, finely tuberculate with punctiform hilum.
Chemical Constituents and Their Medicinal Importance
Fenugreek foliage was one of the components of the celebrated Egyptian kuphi incense, a holy smoke used in fumigation and embalming rites. Crushed fenugreek seeds are used in the preparation of poultices for burns, in cosmetics, hair oils, and in cough, diuretic and laxative preparations. They are mucilaginous when mixed with water and may be consumed raw or boiled. Egyptian and Indian women eat the seeds to promote lactation. They are also used locally as a source of yellow dye and as an ingredient of curry powder. Commercial samples vary according to their geographic origin. Furthermore, the seeds present a pharmaceutical interest as they contain 1-2% steroidal sapogenins, particularly diosgenin and yamogenin in the oily embryo. These sapogenins are of considerable economic importance as they are used as a starting material for the partial synthesis of oral contraceptives, sex hormones and other steroids. Although the diosgenin yield is lower than in the tubers of certain species of Dioscorea (yams), the easy cultivation of fenugreek and its rapid growth, affording seeds 3-4 months after sowing, make the plant a potentially valuable crop for sapogenin production.
Steroidal sapogenins occur in all parts of the plant. Ortuiio et al. reported maximum levels of diosgenin in young leaves of Trigonella foenumgraecum. However, most studies focus on the seeds. Sapogenins occur as their glycosides (saponins) in plant tissues. Besides saponins, the seeds also contain fixed oil, polysaccharides, proteins, flavonoids and various gums which may approximate 23% of the seed content. The alkaloid trigonelline, a nicotinic acid derivative which has been isolated from all plant parts, is converted into nicotinic acid by roasting.
Trigonella: Summary and Conclusion
Diosgenin is a natural sapogenin of prime importance for the pharmaceutical industry as starting material for the partial synthesis of oral contraceptives, sex hormones and other corticosteroids. Steroidal sapogenins have been reported in Agave, Dioscorea, Costus, Solarium and Trigonella species. Trigonella has drawn considerable attention as a source of diosgenin. In vitro callus and cell suspension cultures have been successfully established from seeds, roots and leaves. Optimization of the culture conditions for the growth and productivity of steroidal sapogenins and trigonelline has been achieved. It was demonstrated that the age of the culture, the nature and the dose of auxin and cytokinin and the addition of precursors can have marked effects on the growth and production of secondary metabolites. However, despite a substantial increase in the diosgenin production, the use of tissue culture as a commercial source of steroidal sapogenins does not seem to be competitive with their extraction from yams and sisal.
The Agrobacterium-mediated transformation of Trigonella foenum-graecum is a valuable method for obtaining hairy roots rapidly with a high growth rate and high yield of diosgenin. Compared with cell cultures, hairy roots are more easily established. After optimization of the culture conditions, WP medium was selected. This medium, supplemented with 3% sucrose (WP 3) was found the most appropriate for the root growth, whereas the same medium, diluted in half and supplemented with 1% sucrose (1/2 WP 1) was the most favorable for the production of diosgenin. The addition of 40 mg/1 of chitosan to the 1/2 WP 1 liquid medium allowed the dry weight of diosgenin to reach 0.125% after 35 days of culture. This is three times the amount found in the untreated root cultures and five times the amount detected in the roots of 8-month-old nontransformed plants grown in the field. Cholesterol induced an important decrease in hairy root growth and inhibited the diosgenin production. Whereas light had no effect on the biosynthesis of diosgenin, decreasing the pH increased the sapogenin content significantly.
The growth and diosgenin production have been examined in a 7-1 rotating-drum fermenter in batch fermentation on WP 3 culture medium. This study allowed us to obtain 557 g fresh wt. (about 28 g dry wt.) of hairy roots after 65 days, and a diosgenin content of 0.023% dry wt. Unfortunately, in the Erlenmeyer flasks as well as in the bioreactor, diosgenin was not released from the root cells into the medium. This result suggests that Trigonella foenum-graecum hairy roots present very limited diosgenin excretion, which constitutes a major obstacle to the use of a bioreactor system. Much effort is necessary to improve the release of diosgenin into the culture medium. In particular, the use of per-meabilizing agents may improve diosgenin production in hairy root culture of Trigonella foenum-graecum.
Selections from the book: “Medicinal and Aromatic Plants XII” (2002).