Distribution and Importance of the Plant
Solanum khasianum () which originates from India, is a widespread plant in Asia. The stem and leaves of this bushy annual or short-lived perennial plant are packed with spines.
The plant is of commercial and medicinal importance because of its high content of steroidal alkaloids such as solasodine and its triglycosides solasonine and solamargine. For the pharmaceutical industry, solasodine has become increasingly interesting as a valuable starting compound for the synthesis of steroid hormones, such as cortisone and oral contraceptives. It can be readily converted to 16-dehydropregnenolone acetate, a key intermediate in the synthesis of steroid drugs, and therefore might serve as an alternative to the hitherto extensively used diosgenin. In addition to the use of solasodine as raw material for the synthesis of steroid hormones, the solasodine glycosides, solamargine and solasonine, show biological activities, which might be of medicinal value. The antitumoral effects of a Solarium sodomaeum glycoalkaloid extract, mainly consisting of the glycoal-kaloids solamargine and solasonine, have been described and this preparation has been recently used for treatment of certain skin cancers. The antitumoral activity of this preparation is largely due to solamargine, which is thought to be bound by lectins in the cancer cell membranes. Furthermore, the solasodine-based glycoalkaloids can be toxic or inhibitory to a wide range of organisms, e.g., several fungi, the flagellate parasite Trypanosoma cruzi (), larvae of insects, molluscs and the nematode Setaria cervi (). Solamargine has also been reported to exhibit antiviral activities.
The toxicity of glycoalkaloids is mainly due to their lytic effects on biological membranes. The disruption of the membrane appears to result from the ability of the steroidal alkaloids to complex with free sterols of the membranes. With regard to the solasodine-based glycoalkaloids, solamargine is the more disruptive compound compared to solasonine, but together the two compounds showed a marked synergistic effect. In addition, steroidal glycoalkaloids can inhibit the acetylcholinesterase, a key postsynaptic enzyme in the transmission of the nerve impulse. However, solasonine and solamargine were much less inhibitory than oc-solanine and oc-chaconine.
Production of Steroidal Glycoalkaloids
The steroidal alkaloid solasodine and related compounds have been reported to occur in more than 200 species within the Solanaceae. Besides Solanum khasianum, Solanum aviculare and Solanum laciniatum are the most promising species for the commercial production of solasodine (Mann 1978). Solasodine-producing species are cultivated in many parts of the world, especially in the former USSR (Solanum laciniatum), India (Solanum khasianum), New Zealand (Solanum aviculare), eastern Europe and China.
With regard to S. khasianum, the solasodine content ranges from 2.5 to 5% based on dry weight of the fruits, the most promising variety being Chatter-jeeanum which is probably not S. khasianum but S. viarum Dunal.
Only a few data are available on the commercial yield of solasodine. In India the productivity of this plant was reported as 72.4-106 kg/ha. In Indonesia, based on figures from cultivation experiments, the yield of solasodine was reported to amount to 37.5-75kgha-1 year-1.
Solasodine glycosides are mainly found in the fruit of the plant. The vegetative parts of the plant contain only traces of this steroidal alkaloid. In the fruit the solasodine glycosides are present in the extracellular mucilage surrounding the seeds. In contrast to most other species, in S. khasianum the solasodine content does not decrease during fruit ripening. The presence of solasodine in ripe fruits is advantageous for agriculture because the timing of the harvest is not decisive for an optimal glycoalkaloid yield. However, the aforementioned presence of spines at the stem and leaves is an enormous problem for the harvesting of the plant. Therefore, besides improving the solasodine content, reducing spininess is a main aim of plant breeders. The attempts to reduce the spines include techniques like gamma-ray irradiation of seeds and, in recent years, somatic hybridization.
Solanum khasianum: Conclusion and Prospect
Based on published data and the experience gathered in the senior author’s laboratory with S. khasianum and other Solanaceae plants, the former plant represents a good species for cell culture work. Establishment of callus and cell suspension cultures as well as regeneration processes of protoplasts/cells/tissue explants to plants appear to proceed smoothly. Therefore, attempts to achieve the most desired breeding aims (spineless leaves, increased solasodine content, improved resistance towards bacterial and fungal pathogens) can be strongly recommended.
The antimicrobial defence reactions of Solanaceae obviously comprise cell wall modifications by peroxidative polymerization of cinnamoylamides as a prominent feature. Several species when studied in elicitor-treated cell cultures or infected leaves express this pathway at a high level. The adherent mechanisms of gene activation and the full pattern of other induced defense reactions can be studied well at the cell culture level. The availability of het-erotrophic and photosynthetically active cell cultures further allows the investigation of those reactions, which in a plant are only expressed in a particular organ.
With regard to steroids and their biosynthesis, more intensive investigations are required in order for these compounds to accumulate in a cell culture.
Selections from the book: “Medicinal and Aromatic Plants XII” (2002).