Target of rapamycin regulates the accumulation of fatty acid but not of triterpene-derived hydrocarbons in microalgae of the Botryococcus genus

Target of rapamycin (TOR) is a conserved eukaryotic kinase that regulates cell growth and metabolism in coordination with the energy and nutrient status. Although the role of TOR signalling in starch and triacylglycerol accumulation has been established in microalgae, whether TOR also regulates the production of hydrocarbons in photosynthetic organisms is largely unknown. Colonial green microalgae of the genus Botryococcus have been studied due to their ability to produce large amounts of liquid hydrocarbons, suitable for use as biofuel feedstocks. Here, we studied TOR signalling in B. alkenealis (formerly A race), a fatty acid-derived hydrocarbons producer, and B. braunii (formerly B race), a triterpene hydrocarbons producer, by using the selective TOR inhibitor AZD-8055, with/without glucose supplementation. Although AZD-8055 treatment effectively reduced Botryococcus biomass production in both cultures, it did not significantly affect B. braunii hydrocarbon production under photoautotrophic and mixotrophic conditions. However, B. alkenealis cultures treated with AZD-8055 showed an increase in extracellular hydrocarbon accumulation in a dose-dependent manner. Chemical TOR inhibition downregulated the expression of Ba- and BbTOR and a core cell-cycle gene, Ba- and BbCYCD, but their expression levels were upregulated upon glucose addition. Our results indicate that TOR modulates fatty acid-derived hydrocarbon production from B. alkenealis in response to nutrient availability, but not triterpene hydrocarbons from B. braunii.