Knock Down of Chlamydomonas reinhardtii Phytyl Ester Synthase α Triggers DGAT3 Overexpression and Triacylglycerol Accumulation Under Low-Light Conditions

Evidence indicates that light can trigger an increase in triacylglycerol (TAG) accumulation in eukaryotic microalgae without reducing cell division. In connection with this, we have recently reported that the expression of the chloroplast enzyme diacylglycerol acyltransferase 3 (DGAT3) is induced by light in concert with TAG accumulation in Chlamydomonas reinhardtii. In this work, we report the identification of two phytyl ester synthases (PES) in C. reinhardtii, named PESα and PESβ. These are homologous to chloroplast PES1 and PES2 of Arabidopsis thaliana, which play a role in the synthesis of fatty acid phytyl esters (FAPEs) and TAGs. We demonstrate that PESα and PESβ transcript levels are transiently induced upon transferring cell cultures from a growth condition of low light to high light, and this occurs in parallel to an increase in TAG levels. In a pesα knockdown mutant, DGAT3 transcripts and TAG levels are significantly higher than in the parental strain at the end of the low-light period, and remain elevated after shifting pesα cells to the high-light condition. On the contrary, in a pesβ knockdown mutant, TAG levels, as well as DGAT3 expression, are similar to those of the control strain. These results suggest that PESα and PESβ are non-redundant in TAG metabolism and that PESα is functionally related to DGAT3.