Influence of environmental biotic factors on the content of saponins in plants

Saponins occur constitutively in many plant species as part of their defense system. However, saponin content in plants seems to be dynamic, responding to many external factors including various biotic stimuli connected to herbivory attack and pathogenic infection, as well as involved in plant mutualistic symbioses with rhizobial bacteria and mycorrhizal fungi. Thus, not only saponins influence the living organisms interacting with plants, but in turn, all these interactions can impact the plant saponin content. According to their constitutive occurrence in plants, saponins are regarded mainly as phytoanticipins. Nevertheless, some presented data clearly point out to induced biosynthesis of saponins, especially in plant response to insect herbivory or inoculation with root symbionts, while the best studied examples of interactions between plants and their microbial pathogens show rather qualitative change of saponin composition based on chemical modifications of preformed, pre-infectional precursors. Simultaneously, despite evident inducibility of saponin production in plant cell cultures, the possible role of these compounds as phytoalexins synthesized in intact plants after pathogen infection is still not well documented. Some practical patterns and ecological consequences of biotic factors influencing saponin content in plants are briefly highlighted, with the special attention paid to microbial inoculants applied for optimisation of saponin synthesis in cultivated medicinal plants.