Biosynthesis, localization and ecological role of pyrethrins and linked secondary metabolites in pyrethrum

Summary

The perennial herbaceous plant Tanacetum cinerariifolium, also known as pyrethrum, is a daisy-like flower with an inherent ability to produce considerable amounts of biologically active metabolites, especially pyrethrins, probably intended for self-defence. The discovery of pyrethrin toxicity towards insect pests triggered the exploitation of pyrethrum for commercial purposes in the late 19th century. Despite having a long history of safe and effective use as a source of a versatile botanical insecticide, pyrethrum lost its popularity when, in the mid-20th century, more cost-effective, active and persistent synthetic variants became available. In recent years, a shift in general consumer preferences towards more selective, safer, non-persistent and more environment-friendly pesticides has renewed interest in the use of pyrethrum, renewing pyrethrum's economic significance. Despite the fact that the plant has been under commercial cultivation in many parts of the world for the last 160 years, surprisingly little breeding, ecological and genetic work has been performed to achieve important economic targets of the industry. Increasing the yield of pyrethrins in its natural host, or the mass production of pyrethrins in cultured cells or even a microbial host, would offer new possibilities to the pyrethrin industry that could potentially contribute to placing pyrethrins in a more favourable competitive position in today's insecticide market. Similarly, insights into the biological role of secondary metabolites found in pyrethrum could potentially greatly benefit the economics of the pyrethrum industry. However, in an era in which advanced breeding and genetic modification techniques are not the limiting factor, the lack of basic biochemical information, such as the identification and isolation of key enzymes involved in the formation of pyrethrins and sesquiterpene lactones, constitutes the major hurdle in the genetic engineering of these secondary metabolites, in either the natural host or other species. Genes encoding enzymes involved in the biosynthesis of certain metabolites are expected to be actively transcribed at specific moments and/or specific tissues; hence, the determination of the exact site of accumulation and synthesis of secondary metabolites constitutes a necessary tool to help pick out genes of interest. Developing knowledge around different aspects of pyrethrum secondary metabolism will, therefore, contribute to generating the necessary tools for breeding and/or engineering of varieties with enhanced pyrethrin content and decreased content of unwanted metabolites. Potentially, in the longer run, it will also be possible to engineer the biosynthesis of pyrethrins into other crop species. Ideally, such crops would then no longer require the external application of pesticides to protect them against microbial diseases and pests. Here, we will discuss the most important findings obtained in our lab, ranging from localization and biochemical aspects of the synthesis of pyrethrum defence compounds to their possible biological role in the young emerging seedling as well as in the adult plant.