Triacylglycerol (TAG) is the major storage lipid in most terrestrial plants and microalgae, and has great nutritional and industrial value. Since the demand for vegetable oil is consistently increasing, numerous studies have been focused on improving the TAG content and modifying the fatty‐acid compositions of plant seed oils. In addition, there is a strong research interest in establishing plant vegetative tissues and microalgae as platforms for lipid production. In higher plants and microalgae, TAG biosynthesis occurs via acyl‐CoA‐dependent or acyl‐CoA‐independent pathways. Diacylglycerol acyltransferase (DGAT) catalyzes the last and committed step in the acyl‐CoA‐dependent biosynthesis of TAG, which appears to represent a bottleneck in oil accumulation in some oilseed species. Membrane‐bound and soluble forms of DGAT have been identified with very different amino‐acid sequences and biochemical properties. Alternatively, TAG can be formed through acyl‐CoA‐independent pathways via the catalytic action of membrane‐bound phospholipid:diacylglycerol acyltransferase (PDAT). As the enzymes catalyzing the terminal steps of TAG formation, DGAT and PDAT play crucial roles in determining the flux of carbon into seed TAG and thus have been considered as the key targets for engineering oil production. Here, we summarize the most recent knowledge on DGAT and PDAT in higher plants and microalgae, with the emphasis on their physiological roles, structural features, and regulation. The development of various metabolic engineering strategies to enhance the TAG content and alter the fatty‐acid composition of TAG is also discussed.

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The D5 desaturase mutant (P127) of the microalga Lobosphaera incisa is a promising organism for large-scale production of the valuable LC-PUFA dihomo-g-linolenic acid (DGLA, 20:3 n-6). We examined the potential of P127 for DGLA production under nitrogen (N) starvation conditions, triggering the deposition of DGLA in triacylglycerols, and developed a strategy for optimization of the DGLA productivity in highdensity cultures. Towards this end, the effects of initial biomass concentration (1, 2, and 4 g L À1 ) and PAR irradiance (170 and 400 mmol m À2 s À1 ) on DGLA and total fatty acid (TFA) production were studied. The highest DGLA and TFA percentages (10 and 38% of dry weight, respectively) were displayed by the cultures initiated at 1 g L À1 and grown under a moderate irradiance. Higher irradiances and lower starting biomass content facilitated oleic acid accumulation at the expense of DGLA. Maximum volumetric productivities of TFA and DGLA were recorded in the cultures started at 2 g L À1 biomass and grown under 400 mmol PAR m À2 s À1 . We show that a sufficiently high starting culture density should be combined with a mild light stress to facilitate the production of biomass enriched in DGLA-containing triacylglycerols.Practical applications: The D5 desaturase mutant of L. incisa, P127, is a rare green source of DGLA, a LC-PUFA with valuable pharmaceutical and neutraceutical properties. We report on the optimization of DGLA production by the nitrogen-starving indoor cultures of P127. Stresses, such as N starvation and/or high irradiances, promote accumulation of lipids by microalgal cells. On the other hand, excessively severe stress is deteriorative for the target LC-PUFA accumulation. An important outcome of the present work is establishing physiological constrains of DGLA productivity and finding an approach to balance starting cell density vs. irradiance in order to maximize DGLA yields. Findings of the present work lay a foundation of the efficient production of DGLA using upscaled cultures of P127.