Triacylglycerol accumulates exclusively outside the chloroplast in short-term nitrogen-deprived Chlamydomonas reinhardtii

In microalgae, triacylglycerol (TAG) biosynthesis occurs by parallel pathways involving both the chloroplast and endoplasmic reticulum. A better understanding of contribution of each pathway to TAG assembly facilitates enhanced TAG production via rational genetic engineering of microalgae. Here, using a UPLC-MS(/MS) coupled with TLC-GC-based lipidomic platform, the early response of the major glycerolipids to nitrogen stress was analyzed at both the cellular and chloroplastidic levels in the model green alga Chlamydomonas reinhardtii. Subcellular lipidomic analysis demonstrated that TAG was accumulated exclusively outside the chloroplast, and remained unaltered inside the chloroplast after 4 h of nitrogen starvation. This study ascertained the existence of the glycolipid, digalactosyldiacylglycerol (DGDG), outside the chloroplast and the betaine lipid, diacylglycerol-N,N,N-trimethylhomoserine (DGTS), inside the chloroplast. The newly synthesized DGDG and DGTS prominently increased at the extra-chloroplastidic compartments and served as the major precursors for TAG biosynthesis. In particular, DGDG contributed to the extra-chloroplastidic TAG assembly in form of diacylglycerol (DAG) and DGTS in form of acyl groups. The chloroplastidic membrane lipid, monogalactosyldiacylglycerol (MGDG), was proposed to primarily offer DAG for TAG formation outside the chloroplast. This study provides valuable insights into the subcellular glycerolipidomics and unveils the acyl flux into the extra-chloroplastidic TAG in microalgae.