The Unicellular Red Alga Cyanidioschyzon merolae, an Excellent Model Organism for Elucidating Fundamental Molecular Mechanisms and Their Applications in Biofuel Production.

Imran Pancha,Kan Tanaka,Sousuke Imamura,Kazuhiro Takaya

doi:10.3390/plants10061218

Abstract

Microalgae are considered one of the best resources for the production of biofuels and industrially important compounds. Various models have been developed to understand the fundamental mechanism underlying the accumulation of triacylglycerols (TAGs)/starch and to enhance its content in cells. Among various algae, the red alga Cyanidioschyzon merolae has been considered an excellent model system to understand the fundamental mechanisms behind the accumulation of TAG/starch in the microalga, as it has a smaller genome size and various biotechnological methods are available for it. Furthermore, C. merolae can grow and survive under high temperature (40 °C) and low pH (2–3) conditions, where most other organisms would die, thus making it a choice alga for large-scale production. Investigations using this alga has revealed that the target of rapamycin (TOR) kinase is involved in the accumulation of carbon-reserved molecules, TAGs, and starch. Furthermore, detailed molecular mechanisms of the role of TOR in controlling the accumulation of TAGs and starch were uncovered via omics analyses. Based on these findings, genetic engineering of the key gene and proteins resulted in a drastic increment of the amount of TAGs and starch. In addition to these studies, other trials that attempted to achieve the TAG increment in C. merolae have been summarized in this article.

Highlights

The significant challenges in the 21st century include the increasing human population, decline in fossil energy reserves, global warming, and other environmental issues.Primary research in modern times is devoted to developing sustainable and renewable biofuel production
To further identify how the expression of acyl-ACP reductase results in TAG accumulation in C. merolae, the authors conducted gene expression analysis and capillary electrophoresis mass spectrometry-based metabolome analysis, and the results indicated that the acyl-ACP reductase expression upregulated several vital steps and metabolic pathways in the accumulation of TAGs within cells
The results indicated that target of rapamycin (TOR) regulates various metabolic processes in cells, such as starch metabolism and photosynthesis

Summary

Introduction

The significant challenges in the 21st century include the increasing human population, decline in fossil energy reserves, global warming, and other environmental issues. TAG productivity in this alga (Figure 1) The authors used this strategy because the overexpression of acyl-ACP reductase reportedly resulted in higher fatty acid accumulation in cyanobacteria [27]. Under nitrogen depletion conditions, the expression levels of genes involved in the pentose-phosphate pathway are upregulated, but in AAR-3HA cells, the expression levels of genes such as G6PD were downregulated, and those of chloroplast ACCase were upregulated, indicating that an additional supply of carbon source does not result from the degradation of chlorophyll and proteins [28] These results indicate that heterologous expression of the cyanobacterial gene in C. merolae markedly improves. Further studies on the expressions of other genes from cyanobacteria and other oleaginous organisms might lead to improvements in the biomass production of this alga Such metabolic engineering strategies are important for the production of sustainable biofuels and valuable compounds from C. merolae.

Role of TOR Signaling in TAG Accumulation

Findings

Conclusions and Prospects