Limonoids and Quassinoids – Understanding and Harnessing Complex Triterpene Production and Transport in Plants

• Applicant: Professor Dr. Jakob Franke
• Subject Area: Plant Biochemistry and Biophysics ; Biological and Biomimetic Chemistry
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426196231

Specialised metabolites from plants have been used by mankind for thousands of years, many of which are still essential medicines. Sometimes, though, specialised metabolites are unwanted, as they can be bitter or poisonous. Manipulating plant specialised metabolism is therefore highly desirable to produce increased quantities of high-value compounds or to reduce levels of unwanted components in certain plant parts. Such metabolic engineering is nowadays possible in principle. However, a better understanding of the processes underlying the production and the transport of specialised metabolites is key for efficient engineering, which is often missing for chemically complex plant compounds. The aim of this project is to obtain such knowledge using limonoids and quassinoids as an example. Limonoids and quassinoids are highly attractive targets for engineering. These biogenetically related triterpenes are mainly produced in citrus plants (Rutaceae) and in Simaroubaceae, respectively. The most well-known representative of the Simaroubaceae is the cosmopolitan weed Ailanthus altissima, the Chinese tree of heaven. Limonoids are responsible for the bitter taste of citrus fruits, whereas quassinoids are excreted by the Chinese tree of heaven to eliminate competing plants due to their phytotoxic properties. On the other hand, both limonoids and quassinoids possess beneficial properties: Many limonoids are potent insecticides or antiretroviral agents; quassinoids could be used as herbicides or as anti-cancer drugs. The aim of this project is to analyse how plants produce and transport limonoids and quassinoids, to establish a basis for better use of beneficial as well as avoidance of negative properties. Three subprojects will be employed to fulfil this aim: 1) Elucidating the biochemical basis of limonoid production by identifying biosynthesis genes from citrus plants. 2) Understanding and exploiting limonoid pathway logistics. Transporters will be identified by gene silencing, tested biochemically using an oocyte expression system and studied further using fluorescent substrate analogues. 3) Profiling quassinoid pathway in Chinese tree of heaven. Metabolome and transcriptome data from different tissues will be obtained and analysed. This project will provide a better understanding how limonoids and quassinoids are produced and transported in plants, and how this machinery can potentially be affected by metabolic engineering. Additionally, it will help to expand the knowledge about plant biochemistry and plant specialised metabolism in general.

• DFG Programme: Independent Junior Research Groups