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

植物、绿藻和蓝藻利用叶绿素收集光子，通过氧合光合作用将光转化为化学能。叶绿素在周转过程中会释放出对细胞有害的游离植物酚。我们最近从拟南芥中分离到两个植酸酯合成酶(PES1，PES2)，它们属于酯酶/脂肪酶/硫代酯酶(ELT)家族的酰基转移酶。PES1和PES2将游离植醇转化为脂肪酸植酸酯，从而将植醇从代谢中移除。因此，这一途径代表了一种参与植物叶绿素周转调控的新机制。PES1和PES2在体外产生植酸酯和三酰甘油，但尚不清楚它们在体内是否也产生三酰甘油。拟南芥含有另外四个与PES1/PES2序列相似的ELT基因，在绿藻衣藻中发现了三个同源基因。在聚球藻中，存在两个覆盖PES1/PES2部分的基因。在这个项目中，我们将研究这些植酸酯和三酰甘油代谢基因在拟南芥、衣藻和集胞藻中的作用。我们将解决这个问题，植物叶绿体中三酰甘油的合成是否依赖于PES1/PES2或其他ELT基因。我们将研究PES基因对衣藻三酰甘油合成的贡献，并分析这些基因是否可以作为提高绿藻含油量的生物技术策略的靶点。此外，我们将解决植酸酯途径是否起源于蓝藻的问题。

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.