Aspilia mossambicensis (Oliv.) Wild (Asteraceae), is widespread in central and eastern tropical Africa (), ranging from Ethiopia through east Africa, the Congo, Zambia, Zimbabwe, Malawi, Mozambique, and Transvaal to Natal (). Various folk uses of this and other Aspilia species have been reported, including its use as a remedy for cystitis and gonorrhoea (), treatment of abdominal pains, intestinal worms, and skin infections (). Previous reports for two other species of Aspilia (A. montevidencis and A. parvifolia) showed the presence of the tridecapentaynene derivative, thiophene A (I) (), in roots (). Methanol and aqueous extracts of Aspilia africana have recently been shown to have antibacterial activity against Gram-positive and Gram-negative bacteria, including Agrobacterium tumefaciens, at concentrations ranging from 0.1 to 0.5 g/ml (). Evidence that wild chimpanzees use Aspilia mossambicensis as a dietary and medicinal supplement () suggested the possibility that the plant could have biocidal activity, and prompted an investigation of the phytochemistry of this species. Thiarubrines A and B (II, IV) and the mono-thiophenic derivatives, thiophenes A and B (I, III), were subsequently isolated from leaves of dried material (). Thiarubrine A has been found to exhibit strong antifungal activity towards the human pathogens Candida albicans and Aspergillusfumigatus at concentrations comparable to those of amphotericin B (). Thiarubrine A and thiophene A have also been shown to be toxic to murine cytomegalovirus and Sindbis virus in the presence of ultraviolet radiation (). In vitro studies conducted at Irvine indicate a concentration of 5 ppm of thiarubrine A is toxic to the nematode Meloidogyne javanica (). This finding has been utilized in a joint project of researchers at Irvine and the University of California at Riverside, and further research has established that thiarubrine-containing plants can effectively suppress plant parasitic nematode infestation under field conditions, and thereby increase crop yields ().
Because of difficulties in obtaining sufficient material from A. mossambicensis for additional studies, and to insure homogeneity of the material, an in vitro propagation system was developed. Previous work showed that thiarubrines can be efficiently produced by root cultures of Eriophyllum lanatum (), by crown gall callus of Chaenactis douglasii (), and by hairy root cultures of C. douglasii (). Applying similar methods to Aspilia mossambicensis, we developed an efficient propagation system and root cultures with good growth characteristics.
Aspilia mossambicensis produces thiarubrines A, B and thiophenes A, B, compounds with reported antibiotic properties. An efficient propagation system has been devised to allow rapid multiplication of selected seedling tissue. Optimum hormone levels for multiplication were 1.0 mg/1 BAP and 1.5 mg/1 GA3 on MS medium. Root induction in 98% of plants was achieved with no hormones and 0.2% activated charcoal. Root cultures were established and grew best on 0.1 mg/1 NAA. Thiarubrine and thiophene levels in cultures exceeded the level in roots of intact plants, but the composition for all media was very similar for cultured and intact roots.
Recent interest in the antibiotic properties of thiarubrines as constituents of the leaves of Aspilia mossambicensis and their use by primates suggest the possibility of a form of self-medication (). The availability of an in vitro culture system for propagation and root culture will provide a system for investigating factors which affect synthesis and metabolism of the compounds and also provide an alternative source for moderate quantities of the compounds.
Selections from the book: “Medicinal and Aromatic Plants V”, 1993.