Humans have through thousands of years used plants as a source for medicines. Today, many drugs are still based on natural products or natural product derivatives despite the technological advancement and development of synthetic medicines during the last 25 years. Moreover, the many unique natural compounds from plants still have a huge potential for investigation for medicinal properties as possible drug leads. Humans have a long history of using plant medicine in treatments and indigenous cultures are a remarkable source for immense knowledge about plants and the uses of them - knowledge that has been transferred from generation to generation. In Australia the Aboriginal people represent one of the world’s oldest living continuous cultures and have inhibited the Australian continent for more than 40,000 years. Many different Aboriginal groups have adapted to the extremely diverse environments across Australia and therefore they have developed traditional medicine from the particular natural resources they have been surrounded with. This has led to the development of unique knowledge of medicinal plants and ways of using them by different Aboriginal groups across Australia. This knowledge often only exists amongst members of communities or groups of the land where it has arisen.

In this PhD project the three Australian plant species Acacia ligulata A.Cunn. ex Benth, Santalum spicatum (R.Br.) A.DC and Santalum lanceolatum R.Br were investigated for their bioactivity and chemistry. These plant species have been traditionally used by Aboriginal Australians for symptoms often associated with bacterial infections. Further, A. ligulata has been described as a food source, where seeds and possible whole pods were used to prepare food. However, people of the Yankunytjatjara language group in Central Australia also had knowledge that eating from this food would “make your hair fall out”. Different tissue parts from the three species were used to prepare crude extracts for an initial screening study using antibacterial, cell cytotoxicity and α-amylase and α-glucosidase enzyme inhibition assays where several promising activities were observed.
The mature pod extract from A. ligulata was found to be the most potent in the cell cytotoxicity assays and was thus further investigated by activity-guided fractionation. This led to the isolation of two novel triterpenoid saponins elucidated using 1D and 2D NMR, LC-MS/MS and GC-MS. Both compounds were also found to be moderately cytotoxic to a melanoma cell line and a normal human skin cell line. Triterpenoid saponins have in general been shown to be cytotoxic towards different cell lines likely due to their amphiphilic structure. An initial study of the mechanism of action using flow cytometry showed that a mixture of the two triterpenoid saponins induce apoptosis in the melanoma cells.
Saponins in plants are often described as defence compounds with ecological roles such as protection against herbivores and fungi. Mass spectrometry imaging revealed that the two triterpenoid saponins together with their corresponding aglycone, a putative saponin pathway intermediate, and four putative saponins were present primarily in the outer parenchyma layer of the pod tissue. This particular storage site indicates a role as defence compounds.

The results obtained in this PhD project have highlighted several bioactivites of crude extracts from A. ligulata, S. spicatum and S. lanceolatum with potential for further investigations. The study has provided Western scientific evidence supporting the traditional knowledge about these plant species. In addition, two novel triterpenoid saponins were isolated from the mature pods of A. ligulata and shown to contribute to the in vitro cytotoxicity of the crude mature pod extract by an apoptotic mechanism. These are the first compounds isolated from A. ligulata and the first triterpenoid saponins from any legume pod (Fabaceae) to be spatial localised in the pod tissue.