The Target of Rapamycin kinase is a positive regulator of plant fatty acid and lipid synthesis

Abstract

In eukaryotes, Target of Rapamycin (TOR), a conserved protein sensor kinase, integrates diverse environmental cues, including growth factor signals, energy availability, and nutritional status, to direct cell growth. In plants, TOR is activated by light and sugars and regulates a wide range of cellular processes, including protein synthesis and metabolism. Fatty acid synthesis is key to membrane biogenesis that is required for cell growth. To elucidate the primary regulatory role(s) of TOR in lipid metabolism, we followed fatty acid and lipid changes in plants with altered TOR protein levels or activity for short durations, using Nicotiana benthamiana leaves, Arabidopsis seedlings, and Brassica napus cell suspension cultures. Transient expression of TOR significantly elevated the levels of total fatty acids (TFAs) in Nicotiana benthamiana leaves. Conversely, treatment of Arabidopsis seedlings with the TOR-specific inhibitor Torin 2 for one day caused significant reductions in fatty acids and membrane lipids. Similarly, incubating oil-producing Brassica napus suspension culture cells with Torin 2 for eight hours led to significant decreases in the levels of TFAs and triacylglycerol. The results from three independent systems presented here establish that TOR positively regulates lipid synthesis in plants, consistent with its role in animals. Furthermore, RNA-seq analysis of Torin 2-treated Arabidopsis seedlings showed that TOR promotes the upregulation of several genes involved in de novo fatty acid synthesis while downregulating genes involved in lipid turnover, which we propose as a mechanistic explanation for its promotion of lipid synthesis and accumulation.