Dioscorea belongs to the monocotyledons, family Dioscoreaceae, subfamily Dioscoreoideae. It comprises ca. 600 species and is divisible into numerous sections according to stem twining, leaf morphology, inflorescences, seed wings, bulbil formation, tuber morphology and chemical content. Bulbils occur in the leaf axis of numerous species of Dioscorea and contribute greatly to vegetative propagation. Flowers are dioecious and seeds often winged.
The plants are usually climbers, with tubers or rhizomes at the base. Underground tubers, vary in shape and are rich in starch. They also contain the poisonous alkaloid dioscorine, and therefore may be eaten only when boiled or roasted. The tubers are given the name yams. The term, however, should not be confused with the sweet potato, Ipomoea batatas (Convolvulaceae), which is also known as yam in the USA. Food tuber yams include D. alata, D. bulbifera, D. rotundata, D. cayenensis, D. esculenta and D. trifida. The plants are propagated vegetatively from tubers. Many species have a low rate of flowering and fruit setting and poor seed germination. Many species of Dioscorea are native to the Old World tropics and warm-temperature regions, some to tropical America. In Europe D. caucasica and D. pyrenaica are found.
In recent years great advances in research on the Dioscorea species have been observed. The species have been the object of numerous studies by botanists, chemists and pharmacists.
Some Dioscorea species play an important role in pharmacy. The steroidal sapogenins, mostly diosgenin, present in the tubers and roots are one of the most valuable commercial sources for synthesis of corticosteroidal drugs (cortisone) and sexual hormones such as progesteron.
The consumption of steroid drugs increases every year. According to Fowler (1984), the steroids derived from diosgenin are among the ten most often prescribed medicines of plant origin. Other authors have described the biological properties of diosgenin. Dhawan et al. (1977) found that D. bulbifera has diuretic and anti-inflammatory activity. Sarma (1980) studied the influence of diosgenin, isolated from D. prazeri, on chromosomes of Allium cepa.
Diosgenin was first isolated by Tsukamoto and Ueno (1936) from the Japanese species D. tokoro with a yield of 1%. Later, Marker et al. (1943) studied many other wild yams from Central America, such as D. composita, D. testudinatia and D. lobata, with high levels of steroidal sapogenin. The greatest development in sapogenin utilization has taken place in Mexico. One of the first plants to be investigated there were D. mexicana, D. floribunda and D. composita. New sources for important sapogenins have been indicated.
Hegnauer (1963) mentioned many Dioscorea species containing diosgenin, but as is known from other authors, only certain Dioscorea have so far been found to contain sufficient diosgenin of economic importance.
Akahori (1965) found an interesting correlation between morphological features and sapogenin content. He analyzed the Japanese Dioscorea species, three of which did not contain sapogenins. He discovered that species with alternative leaves, stems twining to the left and no bulbils contained diosgenin, whereas in species with opposite leaves, stems twining to the right and edible roots sapogenins were not present. The only exception is D. bulbifera, which, although if has stems twining to the left and globose tuber and forms bulbils, was not found to contain sapogenins. Other authors found traces of diosgenin in D. bulbifera.
Coursey (1967) mentioned 53 Dioscorea species containing sapogenin, and a high percentage of diosgenin content (more than 2%) was found in 23 species ineluding D. composita, D. deltoidea and D. floribunda. According to Kaul and Staba (1968), diosgenin is obtained commercially, principally from the tubers of D. composita, D. floribunda, D. tepinapensis, D. prazeri, D. deltoidea, D. sylvatica and D. belizensis.
Quigley (1978) investigated Dioscorea species from Ghana: D. bulbifera, D. burkilliana, D. hirtiflora, D. minutiflora, D. praehensilis, D. preusii, D. togoensis and D. zanzibarensis. Diosgenin content in Dioscorea roots is 4% to 6% of dry weight. The highest level of this compound (16.15%) was found by Ting et al. (1981) in rhizomes of the Chinese D. zingiberensis. Other Chinese species, D. althaeoides, D. colettii and D. septemloba, were the object of studies by Lou and Chen (1983) and Liu and Chen (1984), in which they isolated and identified diosgenin.
Another kind of chemical investigation of Dioscorea species is the biotrans-formation of steroidal saponins. Joly et al. (1969) found that D. floribunda leaf homogenate converted the open-chain glycoside of diosgenin to dioscin. The same authors established that double-labelled cholesterol was converted by D. floribunda to diosgenin.
In the 1970’s, Pal and Sharma (1979) induced mutations by X-irradiation in D. bulbifera and D. alata with subsequent changes of steroidal sapogenin and alkaloid content in the species. Biochemical investigations of D. floribunda following gamma irradiations were studied by Gupta et al. (1984).
Selvaraj and Subhas-Chander (1980) observed that fresh tuber homogenates of D. floribunda incubated at 37 °C for 24 h contained more diosgenin than control and dry tuber homogenates. This may be caused by conversion of endogenous precursors by the enzymes liberated during tuber homogenization. In dry tuber incubation endogenous enzymes are destroyed.
The yield of diosgenin in various species of Dioscorea is presented in Table Diosgenin content in selected Dioscorea species.
Table Diosgenin content in selected Dioscorea species
|Species||Diosgenin yield in %||Plant material||Habitat|
|D. bulbifera||4.5||Tubers||Asia trop.|
|D. burkilliana||0.61||Tubers||Africa trop.|
|D. hirtiflora||0.03||Tubers||Africa trop.|
|D. praehensilis||0.18||Tubers||Africa trop.|
|D. prazeri||2.1||India, Burma|
|D. zanzibarensis||0.05||Tubers||Africa trop.|
|D. sylvatica||0.2-0.7||Tubers||South Africa|
Dioscorea: Conclusions and Perspectives
The studies of Dioscorea were at first prompted by economic reasons, mostly for the species used as food. The other kind of investigations concerned species containing steroidal sapogenins, of which diosgenin is of special pharmaceutical importance.
Numerous studies which have been carried out on Dioscorea species in laboratories interested in medicinal plants prompted the question as how to obtain diosgenin without cultivation of the plants in the field or how to regenerate them to provide a large number of plants. It is important to note that using tissue culture technique the problem of freeing the yams from virus diseases can also be solved.
From the pharmaceutical point of view, the most important has been the finding that diosgenin production in tissue culture can be at least as high as in intact plants. Dougall (1979) showed that the yield of diosgenin found in cell culture of D. deltoidea (26mg/g_1 dry wt) was higher than those found in whole plants (20mg/g_1 dry wt).
Many results suggest that the high diosgenin content can be obtained by biotransformation, and this method should be developed in the future. Better diosgenin production may also be achieved by addition of mycelia of some fungal species to the culture, a technique that seems be very promising.
Recent advances in the areas of diosgenin biosynthesis in tissue culture indicate great scope for obtaining higher yield of steroidal sapogenins in tissue culture, but also for encouraging more interdisciplinary studies in this field.
One should consider the fact that abundant information on diosgenin production in tissue culture on the one hand and rapid development of the biotechnology of the plants on the other, would probably require the application of mathematical programming. By using these modern and very accurate methods, it will be possible to apply plant cell cultures to commercial diosgenin production.
Selections from the book: “Medicinal and Aromatic Plants II”, 1989.