Identification and Biotechnical Potential of a Gcn5-Related N-Acetyltransferase Gene in Enhancing Microalgal Biomass and Starch Production.

Kang-sup Yoon,Danxiang Han,Yi Chen,Zhongze Li,Qiang Hu,Li Cao,Liang Zhao,Lihua Yu

doi:10.3389/fpls.2020.544827

Abstract

Microalgae are promising feedstocks for starch production, which are precursors for bioenergy and chemicals manufacturing. Though starch biosynthesis has been intensively studied in the green alga Chlamydomonas reinhardtii, regulatory mechanisms governing starch metabolism in this model species have remained largely unknown to date. We proposed that altering triacylglycerol (TAG) biosynthesis may trigger intrinsic regulatory pathways governing starch metabolism. In accordance with the hypothesis, it was observed in this study that overexpression of the plastidial lysophosphatidic acid acyltransferase gene (i.e. LPAAT1) in C. reinhardtii significantly enhanced TAG biosynthesis under nitrogen (N)-replete conditions, whereas the starch biosynthesis was enhanced in turn under N depletion. By the exploitation of transcriptomics analysis, a putative regulatory gene coding Gcn5-related N-acetyltransferase (GNAT19) was identified, which was up-regulated by 11–12 times in the CrLPAAT1 OE lines. Overexpression of the cloned full-length CrGNAT19 cDNA led to significant increase in the starch content of C. reinhardtii cells grown under both N-replete and N-depleted conditions, which was up to 4 times and 26.7% higher than that of the empty vector control, respectively. Moreover, the biomass yield of the CrGNAT19 OE lines reached 1.5 g L-1 after 2 days under N-depleted conditions, 72% higher than that of the empty vector control (0.87 g L-1). Overall, the yield of starch increased by 118.5% in CrGNAT19 OE lines compared to that of the control. This study revealed the great biotechnical potentials of an unprecedented GNAT19 gene in enhancing microalgal starch and biomass production.

Highlights

Many unicellular green algae can store carbon in the form of starch under adverse environmental conditions, such as high light and nutrient starvation (Ball et al, 1990)
The soluble SS is generally believed to be responsible for synthesis of amylopectin, a type of highly branched starch molecules that constitute the major component of starch granules in green algae and plants
The enhanced TAG biosynthesis in CrLPAAT1 OE lines grown under N-replete conditions may in turn stimulate starch biosynthesis

Summary

Introduction

Many unicellular green algae can store carbon in the form of starch under adverse environmental conditions, such as high light and nutrient starvation (Ball et al, 1990). The ADPglucose pyrophosphorylase (AGPase, EC 2.7.7.23) is the first committed enzyme that catalyzes the formation of ADP-glucose, the nucleotide sugar donor for starch biosynthesis, from Glc-1-P and ATP in the chloroplasts. The soluble SS is generally believed to be responsible for synthesis of amylopectin, a type of highly branched starch molecules that constitute the major component of starch granules in green algae and plants. The GBSS is involved in biosynthesis of amylose, which is less branched starch molecules and is the minor component of starch granules. Synthesis of a(1,6) branches in amylopectin is mediated by the starch branching enzyme (SBE, EC: 2.4.1.18), which hydrolyzes the a-(1,4) linkage and catalyzes the formation of a-(1,6) linkage between the reducing end of the cut glucan and another glucosyl moiety of ADP-glucose. Debranching enzymes (DBE), including isoamylase (ISA, EC: 3.2.1.68) and limit-dextrinase (LDA, EC: 3.2.1.142), are involved in cleaving the branch points of amylopectin molecules, which play important role in shaping the structure of starch molecules

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