Although diosgenin is the most important source for the production of steroids, alternative raw materials for replacing Dioscorea spp. (Mexican yams) have been sought. In this regard solasodine, aglycone of the glycoalkaloids solamargine and solasonine, has been shown to be a promising replacement for diosgenin. About 100 Solanum species have been tested for their glycoalkaloid content. The same author considered that Solanum aviculare, Solanum khasianum, and Solanum laciniatum were the main candidates for solasodine production.
During the 1970’s, the Organic Chemistry Department of the Universidad Nacional de San Luis (Argentina) started the screening of Argentinian species of the genus Solanum, in order to explore the possibility of local production of the steroids. Table 1 shows the results of this screening, Solanum eleagnifolium Cav. var. leprosum (Ortega) Dunal has a good concentration of glycoalkaloid solamargine.
This chapter deals with the agronomic and ecological aspects related to the variety of Solanum eleagnifolium Cav. that grows in Argentina, the establishment of tissue culture, and also solasodine production from solamargine.
Solanum eleagnifolium Cav. var. leprosum (Ortega) Dunal has been described by several authors. A member of the Solanaceae it is a perennial shrubby herb, 30-50 cm high, with very branched stems, especially near the base, that are covered by hairs and spines. Leaves are numerous, alternate, with both sides of the leaf blade covered by hairs and with spines along petiole and veins. The leaf shape is oval-oblong, sometimes linear-oblong, and with entire or lobed margins. The root system is dimorphic, composed of two kinds of roots. Each plant has a strong vertical root, clearly geotropic, which can reach depths of 2 m; from this root others grow that are more or less horizontal, with diageotropic growth, that can extend to a distance of ca. 2.5-3 m. Along the horizontal roots and creeping rhizomes, adventitious buds usually originate.
Table Glycoalkaloids content of Argentinian Solanum species, screened by members of the Department of Organic Chemistry of the Universidad de San Luis, Argentina
|Species||Organs||Total Glycoalkaloids kg/dry (g/matter)||Glycoalkaloids Found||Diosgenin kg/dry matter|
|S. eleagnifolium Cav. var. leprosum (Ortega) Dun.||Unripe fruits||72.70||Solamargine||1.95|
|S. lorentzii Bitt.||Leaves and stems||37.10||Solamargine
|S. calophyllum Phil.||Leaves and stems||5.76||Solamargine||0.25|
|S. juncallense Reiche||Leaves and stems||5.05||Solamargine
|S. sublobatum Willd. ex Roem. et Schult||Fruits||11.60||Solamargine||0.90|
|S. atriplicifolium Gill. ex Mey.||Fruits||-||-||8.05|
|S. euacanthum Phil.||Leaves and stems||3.50||Solamargine||0.25|
|S. pyretrifolium Griseb.||Leaves and stems||12.50||Solamargine||0.90|
|S. juvenale Thell.||Leaves and stems||—||—||1.90|
The flowers are blue, rarely white, either single or in groups, subterminals or opposite to leaves. Fruits are berries, orange-yellow when ripe, with diameters of 10-13 mm; 40-50 large seeds, agglutinated by a substance rich in saponins. Due to this, the fruits are used for soap, while Indians used them for leather tanning. It is also considered a toxic plant for cattle, horses, and mules.
Solanum eleagnifolium has different common names: quillo, meloncillo, revientacaballo in Argentina; silverleaf nightshade in the U.S.A. It is a plant of American origin, with widespread distribution in Argentina, Chile, Uruguay, SW Brazil, Mexico, and the United States. According to Morton (1976), the South American plants are different from those of North America, and as a consequence he assigned the var. leprosum to the species with Argentinian distribution.
It is a very noxious weed, due to the presence of the bud-forming roots. It has been reported in India, Australia, South Africa, Egypt, Italy, and Greece.
Solanum eleagnifolium is a very ubiquitous species, easily prospering in environments with different types of climates and soils. It can be found in arid and semi-arid zones (200-600 mm rainfall), as well as in humid areas (about 1000 mm). Although it is more common in sandy soils, it can also be found in saline ones. As a consequence of this growth in such diverse environments, different phenotypic characteristics can be found in the species. These characteristics are related to the growth habit, the size and shape of leaves, and number and size of flowers and fruits.
It is a hemicryptophyte, with spring-summer growth of shoots. Senescence of the aerial parts occurs after the first frosts. The regrowth of shoots is produced by buds located at the base of stems, near the soil surface or barely beneath it. Flowering starts in early December and continue until March. Fruiting occurs from January on, and the first ripe fruits can be seen at the end of February.
Reproduction is by seeds, that can germinate from September to December, with another peak in February. These phenological data correspond to the situation in Argentina.
Its ability for invasion is increased by the formation of adventitious buds along the horizontal roots. The formation of these buds is favored by soil management prior to the sowing of either a crop or a pasture. Small pieces of root may form several buds, that can emerge from depths of about 20-40 cm.
Due to the danger as a weed, numerous trials for controlling this plant have been conducted in the United States, Australia, and Argentina.
As has been pointed out, Solanum eleagnifolium is a plant with a wide distribution in different countries. However, it has to be introduced as a crop in order to be a reliable source for solasodine production. Since 1980, in the Instituto Argentino de Investigaciones de las Zonas Aridas (Mendoza-Argentina) a project is developing with the aim of investigating the potentialities for cultivation of this plant. A germplasm bank has already been started, with seeds collected in different areas of Argentina and surrounding countries. Ecological and physiological studies are also underway, headed by S.O. Trione.
Solanum: Summary and Conclusions
The in vitro production of solasodine from Solanum eleagnifolium was studied. Callus culture was initiated from hypocotyl sections of 21-day-old seedling in a MS medium with the addition of myo-inositol (100 mg/1), sucrose (30 g/1), and RT vitamin complex (20 ml/1). The pH was adjusted to 5.6-5.8 and solidified with 8 g/1 agar (MSRT medium). The peak of solasodine production was achieved after 21 days of culture (3 mg/g DW).
Solasodine content (as mg/g DW) was higher than that found in stems and leaves of the plant, but lower than in fruits. Some organogenesis (bud formation) was obtained when the MSRT medium was supplemented with IBA (0.5 mg/1), plus BAP (2 mg/1).
Cell suspension cultures were produced when calli were transferred to liquid conditions in a rotary shaker (120 r.p.m.) in MSRT plus 2,4-D (4.5 mM). After 11 days of culture, the maximum values of solasodine achieved were ca. 6 mg/g DW. Concerning the future of solasodine production, it is evident that the in vitro cell culture (including immobilization techniques, use of elicitors, etc.) should be economically compared with production from the whole plant. The results obtained so far have indicated that it is an attractive possibility which deserves further investigation, although no data for estimating the cost of production are yet available.
Selections from the book: “Medicinal and Aromatic Plants III”, 1991.