Molecular analysis of fatty acid export from plastids of oilseed plants and green microalgae - towards developing tools to improve plant biofuel production

• Applicant: Professorin Dr. Katrin Philippar
• Subject Area: Plant Physiology ; Plant Biochemistry and Biophysics
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 270310439

Fatty acids (FAs) build the majority of cellular lipids, which are essential not only for membrane function, but also for growth and development of organisms. For example, triacylglycerol lipids (TAGs) in seeds of oilseed plants represent the major form of carbon energy storage, or waxes on the plant surface restrict loss of water and provide pathogen protection. Since in plants de novo FA synthesis occurs in plastids, transport to the endoplasmic reticulum for further lipid assembly is required. Although it is generally agreed that free FAs are shuttled across plastid envelope membranes, the mode of plastid FA-export, however, still remains enigmatic. We isolated FAX1 (fatty acid export 1), a novel membrane protein in the plastid inner envelope, which in Arabidopsis thaliana is crucial for biomass production, male fertility, and synthesis of FA-derived compounds such as lipids, ketone waxes or pollen cell walls. Whereas ER-derived lipids decrease when FAX1 is missing, levels of plastid-produced lipids increase. FAX1 over-expressing lines show the opposite, including a TAG increase in leaves and flowers. Since in yeast, FAX1 can complement for FA-transport, we conclude that FAX1 mediates FA-export across the plastid inner envelope. In Arabidopsis, FAX1 belongs to a family of seven proteins. Four are predicted to be in plastids, three most likely integrate into membranes of the secretory pathway. The green microalga Chlamydomonas reinhardtii contains one member of each of these subgroups. In vertebrates, FAX relatives are mitochondrial membrane 'TMEM14' proteins, linked to heme metabolism and apoptosis. However, their biological function is unknown. Thus, this protein family represents a powerful tool not only to increase lipid/biofuel production in the green lineage, but also to explore novel transport systems with impact on mitochondrial dynamics and/or apoptosis in vertebrates.The project aims to characterise FAX proteins in Arabidopsis and Chlamydomonas. Research should clarify their impact on cellular metabolism, in particular on FAs, lipids and FA/lipid-derived compounds (waxes, cutin or oxilipin hormones) as well as on carbon energy metabolites. A spotlight will be on synthesis of storage oils in seeds, i.e. the significance of FAX in TAG production. In Chlamydomonas, over-production of FAX proteins (FA-transport) in combination with enhanced expression of FA/lipid biosynthesis enzymes will lead to development of strategies for biofuel production. The structure/function correlation of FAX1 is completely novel and not related to 'classical' FA or lipid transporters. Thus, research will enable us to study plant-specific but also general aspects of a new intracellular FA-transport pathway. Evaluation of FAX topology, structure, and FA/lipid binding, functional characteristics in a heterologous yeast system as well as identification of interaction partners will help to describe the plastid FA-export pathway.

• DFG Programme: Research Grants