Saponins – eco-friendly pesticides
Plants and insects have interacted for millions of years with the former evolving specialised defence mechanisms against insect feeding strategies. Saponins are such defence compounds with detergent-like properties that can disrupt the cell membranes of herbivory pests, causing cell death. Despite the promising potential of saponins as bio-pesticides, little is known about how the chemical structures are toxic to specific pests. The EU-funded Marie Skłodowska-Curie Individual Fellowships grant SSAR elucidated the relationship between saponin chemical structure and biological activity. “We wanted to identify the most potent saponin chemical structures for use as biopesticides,″ explains project coordinator Prof. Søren Bak. Insight into the saponin biosynthetic pathway As a first step, SSAR researchers employed the wild species Barbarea vulgaris as a model organism widely used in the field for studying evolution and ecology of plant defence compounds. They selected a number of candidate genes to identify the ones involved in the biosynthetic pathway of saponins. Using state-of-the-art methodology such as pathway reconstitution in tobacco leaves, enzymatic production in vitro, and insect feeding assays, scientists discovered that a cytochrome orthologue CYP72A causes considerable larval mortality. Researchers observed that CYP72A modify saponins at a specific position, indicating that a simple structural modification can strongly augment saponin activity. Interestingly, sequence analysis indicated that this CYP72A evolved through gene duplications after the ancestral split between Arabidopsis and Barbarea species and has been under strong selection pressure. This finding emphasises the importance of chemical modification in plant defence evolution and as Dr Qing Liu – the lead researcher in the project – highlights: “it demonstrates that evolution of chemical novelties by gene duplication and selection is a key driver in appointment of new functions to enzymes.″ Researchers successfully produced saponins in tobacco plants, a plant that naturally does not produce saponins, through metabolic engineering of the identified Barbarea genes and evaluated the biological function of the generated saponins. Finally, using feeding bioassays and a range of insect herbivore pests, they successfully decoded part of the structure-activity relationships of saponins and provided insight into the mechanism of insect defence. “In the SSAR project, we introduced a fast and powerful system for determining structure-activity-relationships, using transient expression of biosynthetic genes in tobacco leaves for compound production and subsequent feeding assays with insects,″ emphasises Prof. Bak. This approach is more physiologically relevant as the tested compounds are produced and stored within leaves, and not applied to the leaf surfaces. It is also faster as it relies on transient compound expression. Compared to the laborious traditional evaluation with stably transformed plants, this transient system takes less than one week from inoculation of the genes to the insect bioassays. The impact of using saponins as biopesticides The SSAR project provided new knowledge on saponin-based bio-pesticides with profound industrial applications. The generated platform for the production of bioactive compounds from plants can be generically applied to identify new biopesticides that can be used as alternatives to existing agrochemicals. From a scientific point of view, the identification of genes involved in the saponin biosynthetic pathway will also open the possibility to engineer and breed crops with tailored saponin-based pest defence capabilities. The SSAR findings will soon be published in a peer-reviewed journal and will hopefully lead to more funding. In view of the future, Prof. Bak believes that “a continuous discovery of novel biopesticides will help address society’s need for more sustainable food production, and help support the bioeconomy.″
Keywords
SSAR, saponin, bio-pesticide, CYP72A, structure-activity relationship, feeding assay, Barbarea vulgaris, pathway reconstitution
