Phytyl ester and triacylglycerol synthesis in plants, green algae and cyanobacteria

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Plants, green algae and cyanobacteria convert light into chemical energy by oxygenic photosynthesis employing chlorophyll for the collection of photons. Chlorophyll is subject to turnover processes resulting in the release of free phytol which is toxic to the cell. We recently isolated two phytyl ester synthases (PES1, PES2) from Arabidopsis which belong to the esterase/lipase/thioesterase (ELT) family of acyltransferases. PES1 and PES2 convert free phytol into fatty acid phytyl esters, thereby removing phytol from metabolism. Therefore, this pathway represents a novel mechanism involved in the regulation of chlorophyll turnover in plants. PES1 and PES2 produce phytyl esters and triacylglycerol in vitro, but it remained unclear whether they also produce triacylglycerol in vivo. Arabidopsis contains four additional ELT genes with sequence similarity to PES1/PES2, and three orthologous genes are found in the green alga Chlamydomonas. In Synechocystis, two genes covering parts of PES1/PES2 are present. In this project, we will study the role of these genes for phytyl ester and triacylglycerol metabolism in Arabidopsis, Chlamydomonas and Synechocystis. We will address the question, whether triacylglycerol synthesis in plant chloroplasts is dependent on PES1/PES2 or other ELT genes. We will study the contribution of the PES genes for triacylglycerol synthesis in Chlamydomonas, and analyze whether these genes can represent targets for biotechnological strategies to improve the oil content in green algae. Furthermore, we will address the question whether the phytyl ester pathway is of cyanobacterial origin.