The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime

The target of rapamycin (TOR) signaling pathway regulates fundamental intracellular functions critical for cell viability and proliferation. Manipulation of TOR in high lipid-producing microalgae may help overcome the trade-off between biomass production and lipid yield that still impairs the viable production of biofuel from microalgae. In this study, we inhibited the TOR kinase in the model diatom Phaeodactylum tricornutum using the selective TOR inhibitor AZD-8055, and analyzed cell proliferation, chlorophyll content, lipid synthesis and carbon metabolism. AZD-8055 inhibits cell proliferation in a dose-dependent manner compared to N deprivation which stops growth. Microscopy, flow cytometry, and quantitative analyses of lipids also demonstrated that AZD-8055 treatment strongly promotes triacylglycerol (TAG) accumulation while decreasing the quantity of sterols. The TAG productivity of AZD-8055 treated cultures was significantly higher than for N deprived cultures. Measurement of the activities of the key metabolic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH), glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase (MDH) revealed opposite effects for AZD-8055 treatment and N-starvation on the activity of the glycolytic enzyme GAPDH. This suggests that TOR inhibition and N starvation may have distinct impacts on general metabolism and lipid accumulation. Our main finding is that treating cultures with AZD-8055 results in higher TAG productivity than N starvation in P. tricornutum. The chemical or genetic manipulation of the TOR signaling pathway in P. tricornutum and other diatoms may lead to the development of strains or approaches suitable for the enhanced production of TAGs for biofuel.