Abstract
The Chilean soapbark tree, Quillaja saponaria Molina, grows in the central part of Chile. As its name suggests, the tree is a natural source of soap. Indeed, the name Quillaja is derived from the indigenous Chilean word 'küllay', which means 'soap'. Soapbark is not the only plant to produce natural soaps. Other examples include the perennial soapwort (Saponaria officinalis), which grows in Europe and has historically been used as a source of gentle soap for washing delicate fabrics, including allegedly the Turin shroud; and Sapindus mukorossi (soapberry or soapnut), which grows in temperate and tropical regions of the world. The soapy properties of these plants are due to their production of glycosylated compounds known as saponins. In fact, saponins are found across the Plant Kingdom, not just in species with 'soap' in their names, and are one of the largest groups of plant natural products. The major saponin produced by Q. saponaria, which is known as QS-21, is a potent immunostimulant and has been approved for use in human vaccines. QS-21, which is the first naturally occurring plant glycoside to be used as an adjuvant, is a highly complex molecule that is produced only by Quillaja. This review covers the history of the soapbark tree, the discovery of QS-21 and the potential for making the next generation of saponin vaccine adjuvants using engineering biology approaches.