Comparative transcriptome analysis reveals a potential photosynthate partitioning mechanism between lipid and starch biosynthetic pathways in green microalgae

Comparative transcriptome analysis provides a tool to top-view regulations of biochemical pathways in the cells and predicts the key enzymes by comparing gene expression changes in different conditions. Transcriptomes of a green microalga Neodesmus sp. UTEX 2219-4 treated under different stresses favorable or unfavorable for lipid biosynthesis were sequenced and compared to elucidate the photosynthate partitioning mechanism between fatty acid and starch biosynthesis pathways. This microalga contains 20,556 isotigs, and 8387 of these have homologs in the Non-Redundant (NR) database. The fatty acid and starch biosynthetic pathways of this microalga were reconstructed. The expression levels of the isotigs related to these pathways and involved in photosynthesis were calculated. The photosynthetic light reaction genes were down-regulated but electron carriers were up-regulated under osmotic and salt stresses, suggesting that ATP production was supported by cyclic electron transport in these conditions. In lipid producing conditions, expression of multiple isotigs in the fatty acid biosynthesis pathway was up-regulated, indicating that overexpression of acetyl-CoA carboxylase alone was not sufficient to enhance lipid biosynthesis. Results from this study suggested that triose phosphate isomerase is the key enzyme regulating photosynthate partitioning between fatty acid and starch biosyntheses in green microalgae.