Reprogramming alanine catabolism to triacylglycerol biosynthesis for carbohydrate hyper-accumulation in microalgae

Abstract

Abstract Branched-chain amino acids can be catabolized for triacylglycerol (TGA) biosynthesis in microalgae. To identify the role of alanine in nitrogen (N) starvation-induced TGA accumulation, we performed integrative genetic and pharmaceutical manipulation to decipher the regulation of alanine towards TAG and starch biosynthesis in the alga Desmodesmus sp. CNW-N. Alanine aminotransferase (ALT) catabolized alanine towards lipid biosynthesis, which was blocked by ALT inhibitors without impact on already accumulated carbohydrate. Acetyl-CoA carboxylase (ACC) inhibitor (clethodim) suppressed lipid accumulation whilst enlarged carbohydrate pool. Microalgae consist of abundant alanine and higher carbohydrate/lipid buildup under N starvation appeared similar responses, but not for those which alanine was not a major amino acid. It shows that alanine catabolism to lipid is restricted to Chlorophyceae species containing plentiful alanine. The ALT-overexpressing Desmodesmus lines driven by Chlorella vulgaris N Deficiency Inducible (CvNDI1) promoter promoted lipid accumulation, which was suppressed by clethodim treatment along with a substantial increase of carbohydrates as bioethanol production feedstock. The discovery of ALT as the genetic targets together with pharmaceutical inhibition of ACC facilitates targeted engineering strategies to mediate carbohydrate hyper-accumulation for cost-effective liquid biofuel manufacture in Chlorophyceae microalgae.