Importance of Legumes and Legume-Derived Compounds in Medicine and Agriculture

The chemistry of economically important legumes is being studied to understand better the role that natural compounds play in traditional and new uses of legumes by humans.

Tephrosia vogelii is cultivated widely in southern and eastern Africa for use as a natural pesticide, but chemical analyses have reveal two chemotypes, only one of which contains pesticidal rotenoids.

The objective of this project is to identify more of the compounds in legumes that can explain the traditional uses of different species of legumes and to identify chemical traits in plants that could be used to improve our understanding of insect-legume interactions, particularly of crop legumes. The project integrates our knowledge about the ecological role of legume-derived compounds, the traditional uses of the plants and the distribution of secondary metabolites in the legumes. Thus it links closely with the outcomes from the project on the 'Systematic Phytochemistry of Legumes'.

Wild relatives of crop legumes

Pigeonpea (Cajanus) and chickpea (Cicer) are are major protein sources for poor communities in many parts of semi-arid tropical areas of Africa and Asia. They can be totally destroyed by insects and diseases. Their wild relatives, however, are often resistant to these pests and pathogens. We have identified compounds in these wild species that confer this resistance and therefore traditional breeding methods could use these as markers to help introduce resistance into commercial varieties. Specifically, in wild species of chickpea we have identified compounds that confer resistance to one of its most important diseases, Fusarium wilt that additionally have potent anti-insect properties. We have also identified a range of compounds on the pod surfaces of cultivated pigeonpea that stimulate the egg-laying behaviour of the pod-borer Helicoverpa armigera and these may be targets for deselection in breeding. Other compounds from wild species of pigeonpea deter egg-laying, however, it appears to be the relative proportion of these compounds that modulates the behaviour of the adult moths rather than the presence or absence of specific compounds. Thus the value to breeding of this character is less clear. This research is undertaken in collaboration with organizations including International Crop Research Institute for the Semi-Arid Tropics and the Natural Resources Institute (NRI), University of Greenwich.

Pesticidal legumes

Current collaborations with NRI also include research into the optimization of pesticidal plants as environmentally benign alternatives to synthetic pesticides in the control of insect pests of stored products and field pests for poor farmers in sub-Saharan Africa. For example we have validated the anti-insect activity of Tephrosia vogelii a rotenoid containing plant that is used widely in southern Africa attributing the activity to the isoflavonoid deguelin. Furthermore, we have shown that the species occurs naturally as two chemotypes one of which one is not biologically active owing to the absence of rotenoids but instead contains at least 9 flavanones and flavones of which 6 are novel.. Thus chemical analysis is critical in the identification of elite materials for promotion and up-scaling to farmers. T. vogelii is also used for soil enrichment through biological nitrogen fixation and as a green mulch thus its dual human use is particularly compelling. Complimentary work on a related species T. candida obtained from herbarium material has led to the identification of 47 new flavonoid glycodsides from less than 1mg of dried leaf material.

Legumes in human health

Some of the compounds identified as a consequence of our research on insect-legume interactions have potential human health applications. The non-protein amino acid L-dopa (L-3,4-dihydroxyphenylaline) from Mucuna pruriens has potential uses in the treatment of Parkinson's disease and is has been tested against the malaria vector Anopheles gambiae while polydroxyalkaloids isolated from a range of legumes, including Castanospermum, have been investigated for their use in the treatment of cancer, HIV-related conditions and diabetes. As part of a review of 1,700 species of plants used traditionally to treat diabetes we identified over 80 species from 47 genera of Leguminosae that are reported to have anti-diabetic properties. We are studying the active compounds in these species which include species of Phaseolus, Pterocarpus, Indigofera and Cajanus. More recently, as part of wider work on the authentication of traded plants and extracts, we have developed chemical authentication methods for the traditional Chinese medicine ingredient Fructus Sophorae (fruits of Styphnolobium japonicum) and are investigating Mimosa and Glycyrrhiza.

Key publications 2006-2011

• Stevenson, P. C., D’Cunha, R F. & Grzywacz, D. (2010). Inactivation of baculovirus by isoflavonoids on chickpea (Cicer arietinum) leaf surfaces reduces the efficacy of nucleopolyhedrovirus against Helicoverpa armigera. Journal of Chemical Ecology 36: 227–235.
• Stevenson, P.C., Nyirenda, S.P. & Veitch, N.C. (2010) Highly glycosylated flavonoid glycosides from Bobgunnia madagascariensis. Tetrahedron Letters, 51, 4727–4730.
• Kite, G.C., Veitch, N.C., Boalch, M.E., Lewis, G.P., Leon, C.J., & Simmonds, M.S.J. (2009). Flavonol tetraglycosides from fruits of Styphnolobium japonicum (Leguminosae) and the authentication of Fructus Sophorae and Flos Sophorae. Phytochemistry 70: 785-794.
• Stevenson, P.C. Jayasekera,T.K., Belmain, S.R. & Veitch, N.C. (2009). Bisdesmosidic saponins from Securidaca longepedunculata (Polygalaceae) with deterrent and toxic properties to Coleapteran storage pests. Journal of Agricultural and Food Chemistry 57 (19), 8860–8867.
• Kestenholtz, C. Stevenson, P.C. Belmain, S.R. (2007). Comparative study of field and laboratory evaluations of the ethnobotanical Cassia sophera L. (Leguminosae) for bioactivity against the storage pests Callosobruchus maculates (F.) (Coleoptera: Bruchidae) and Sitophilus oryzae (L.) (Coleoptera: Curculionidae). Stored Products Research 43:79-86.
• Sharma, H.C., Gowda, C.L.L., Stevenson, P.C., Ridsdill-Smith, T.J., Clement, S.L., Ranga Rao, G.V., Romies, J., Miles, M. & Bouhssini, M. (2007). Host plant resistance and insect pest management in chickpea. In Yadav, S.S., Redden, R. & Sharma, B. (Eds) Chickpea breeding and management. Oxford: CABI
• Simmonds, M.S.J. & Howes, M.-J., R (2006). Plants used in the treatment of diabetes. In Soumyanath, A. (ed) Traditional medicines for modern times: antidiabetic plants. Boca Raton: Taylor  & Francis.