Most microalgae produce triacylglycerol (TAG) under stress conditions such as nitrogen depletion, but the underlying molecular mechanism remains unclear. In this study, we focused on the role of target of rapamycin (TOR) in TAG accumulation. TOR is a serine/threonine protein kinase that is highly conserved and plays pivotal roles in nitrogen and other signaling pathways in eukaryotes. We previously constructed a rapamycin-susceptible Cyanidioschyzon merolae, a unicellular red alga, by expressing yeast FKBP12 protein to evaluate the results of TOR inhibition (Imamura et al. in Biochem Biophys Res Commun 439:264-269, 2013). By using this strain, we here report that rapamycin-induced TOR inhibition results in accumulation of cytoplasmic lipid droplets containing TAG. Transcripts for TAG synthesis-related genes, such as glycerol-3-phosphate acyltransferase and acyl-CoA:diacylglycerol acyltransferase (DGAT), were increased by rapamycin treatment. We also found that fatty acid synthase-dependent de novo fatty acid synthesis was required for the accumulation of lipid droplets. Induction of TAG and up-regulation of DGAT gene expression by rapamycin were similarly observed in the unicellular green alga, Chlamydomonas reinhardtii. These results suggest the general involvement of TOR signaling in TAG accumulation in divergent microalgae.

No abstract

Summary Cells of land plants contain several kinds of plastids such as chloroplasts, etioplasts, proplastids, leucoplasts, and amyloplasts. Among them, the chloroplast, the most well characterized type of plastid, relies on expression of plastid-encoded photosynthetic genes. The genome of plastids encodes many photosynthetic genes that are mainly transcribed by the plastid-encoded plastid RNA polymerase (PEP). Transcriptional activity of PEP is controlled by nuclear-encoded sigma factors that are important for transcription initiation and promoter selectivity. Arabidopsis thaliana possesses 6 sigma factor genes, SIG1 to 6. Here, we analyzed the function of SIG6, a gene related to chloroplast differentiation. The null mutant (sig6-1) of the SIG6 gene exhibited a pale green phenotype in the cotyledons and in the basal part of emerging true leaves at early stages of development. Interestingly, as leaves matured, the color of cotyledons and true leaves changed to green. In the wild-type and sig6-1 mutant, plastids visualized by green fluorescent protein (GFP) were observed under the epifluorescence microscope. In cotyledons of 3-day-old seedlings, chloroplasts of the sig6-1 mutant showed small and irregular morphology compared with that of the wild-type chloroplasts. However, amyloplasts and leucoplasts in root tissues showed no obvious differences between the wild-type and the sig6-1 mutant. These results suggest that SIG6 plays a key role during the early stages of chloroplast differentiation, but not in differentiation into other types of plastids such as leucoplasts and amyloplasts.Key words Arabidopsis thaliana, Sigma factor, Chloroplast differentiation, Pale green phenotype.All life on earth depends on photosynthesis by chloroplasts in plants for energy, food, and oxygen. Chloroplasts arose from an endosymbiotic cyanobacterial ancestor, and they proliferate by division (reviewed in Kuroiwa et al. 1998). Whereas most land plants possess several subtypes of plastids, most algae have only the chloroplast, a subtype of plastid with a specific function and morphology. In land plants, plastids including chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts differentiate from small non-green proplastids in meristematic cells in response to