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Engineering Properties of Sweet Potato Starch for Industrial Applications by Biotechnological Techniques Including Genome Editing

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International Journal of Molecular Sciences Review Engineering Properties of Sweet Potato Starch for Industrial Applications by Biotechnological Techniques including Genome Editing Ruiqing Lyu 1,2,† , Sulaiman Ahmed 1,† , Weijuan Fan 3 , Jun Yang 3 , Xiaoyun Wu 1, Wenzhi Zhou 1, Peng Zhang 1,4,* , Ling Yuan 2 and Hongxia Wang 1,3,* 1 National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences, Chinese Academy of Sciences, Shanghai 200032, China; [email protected] (R.L.); [email protected] (S.A.); [email protected] (X.W.); [email protected] (W.Z.) 2 Department of Plant and Soil Sciences and Kentucky Tobacco Research and Development Center, University of Kentucky, Lexington, KY 40546, USA; [email protected] 3 Shanghai Key Laboratory of Plant Functional Genomics and Resources, Shanghai Chenshan Plant Science Research Center, Chinese Academy of Sciences, Shanghai 201602, China; [email protected] (W.F.); [email protected] (J.Y.) 4 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (P.Z.); [email protected] (H.W.) † These authors contributed equally to this work. Abstract: Sweet potato (Ipomoea batatas) is one of the largest food crops in the world. Due to its abundance of starch, sweet potato is a valuable ingredient in food derivatives, dietary supplements, Citation: Lyu, R.; Ahmed, S.; Fan, W.; and industrial raw materials. In addition, due to its ability to adapt to a wide range of harsh climate Yang, J.; Wu, X.; Zhou, W.; Zhang, P.; and soil conditions, sweet potato is a crop that copes well with the environmental stresses caused Yuan, L.; Wang, H. Engineering by climate change. However, due to the complexity of the sweet potato genome and the long Properties of Sweet Potato Starch for breeding cycle, our ability to modify sweet potato starch is limited. In this review, we cover the recent Industrial Applications by development in sweet potato breeding, understanding of starch properties, and the progress in sweet Biotechnological Techniques potato genomics. We describe the applicational values of sweet potato starch in food, industrial including Genome Editing. Int. J. Mol. Sci. 2021, 22, 9533. https://doi.org/ products, and biofuel, in addition to the effects of starch properties in different industrial applications. ijms22179533 We also explore the possibility of manipulating starch properties through biotechnological means, such as the CRISPR/Cas-based genome editing. The ability to target the genome with precision Academic Editor: provides new opportunities for reducing breeding time, increasing yield, and optimizing the starch Endang Septiningsih properties of sweet potatoes. Received: 26 July 2021 Keywords: sweet potato; molecular genetics; starch metabolism; crop improvement; genome editing; Accepted: 29 August 2021 biotechnology; CRISPR/Cas9 Published: 2 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Sweet potato (Ipomoea batatas) is one of the largest food crops in the world (Figure1a) . iations. Although it originated from Central or South America, China is now the leading sweet potato producer in the world (Figure1b). Sweet potato has become one of the most important food crops globally due to its superior stress tolerance and high yields [1,2]. Due to its high starch content and sustainable production, sweet potato provides raw materials Copyright: © 2021 by the authors. for starch and starch-derived food, biofuel, and industrial products [3]. Licensee MDPI, Basel, Switzerland. Starch biosynthesis requires four classes of core enzymes: ADP-glucose (Glc) py- This article is an open access article rophosphorylase (AGPase), starch synthases (SSs; EC 2.4.1.21), starch branching enzymes distributed under the terms and (SBEs), and starch debranching enzymes (DBEs; EC 3.2.1.70) [4,5] (Figure2). AGPase conditions of the Creative Commons catalyzes the formation of ADP-Glc for the elongation of α−1,4-glucosidic chains [6]. SSs Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ are categorized into five groups as granule-bound starch synthase (GBSS), SSI, SSII, SSIII, 4.0/). and SSIV [7]. In cereal crops, GBSSI is a key enzyme for amylose synthesis [4]. SSI, SSII Int. J. Mol. Sci. 2021, 22, 9533. https://doi.org/10.3390/ijms22179533 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 9533 2 of 20 and SSIII are responsible for the elongation of amylopectin [7]. SBE functions to generate 1,6-branch linkages, and acts as a key enzyme controlling starch granule structure and Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 21 physicochemical properties [8]. The isoamylase and pullulanase activities of DBE play important roles in amylopectin synthesis [9]. Figure 1. Global production of major crops including sweet potato. (a) The production of the top eight major crops in the Figureworld; 1. ( bGlobal) the global production distribution of major and crops the top including ten sweet sweet potato-producing potato. (a) The countries production in 2018. of the The top darkness eight major of the crops color in reflects the world; (b) the global distribution and the top ten sweet potato-producing countries in 2018. The darkness of the color the production volumes. The production numbers were generated according to the Food and Agriculture Organization reflects the production volumes. The production numbers were generated according to the Food and Agriculture Organ- (FAO) Statistics 2019. ization (FAO) Statistics 2019. Biochemical properties of starch vary among plant species, largely because of the Starch biosynthesis requires four classes of core enzymes: ADP-glucose (Glc) pyro- ratio of amylose and amylopectin. The ability to modify starch properties for novel uses phosphorylaseis of significant (AGPase), agricultural starch and synthases industrial (SSs; importance. EC 2.4.1.21), Genetic starch engineering branching has enzymes emerged (SBEs),as a highly and starch practical debranching and cost-effective enzymes approach(DBEs; EC to 3.2.1.70) alter the [4,5] properties (Figureof 2). starch, AGPase produc- cat- alyzesing unique the formation starch types of ADP-Glc for different for the industrial elongation applications of α−1,4-glucosidic [10]. The chains starch biosynthetic[6]. SSs are categorizedpathway has into been five studiedgroups as via granule-bound genetic transformation starch synthase of major (GBSS), crops, SSI, including SSII, SSIII, rice and [11 ], SSIVpotato [7]. [In12 ],cereal and maizecrops, [GBSSI13]. In is sweeta key potato,enzyme the for coreamylose genes synthesis involved [4]. in SSI, starch SSII biosyn- and SSIIIthesis, are includingresponsibleAGPase for the[ 14elongation,15], GBSSI of [amyl16], SSIopectin[7], SSII[7]. SBE[17], functions and SBEII to[ 16generate], have 1,6- been branchinvestigated linkages, (Table and1 acts). as a key enzyme controlling starch granule structure and phys- icochemical properties [8]. The isoamylase and pullulanase activities of DBE play im- portant roles in amylopectin synthesis [9]. Int. J. J. Mol. Sci. Sci. 20212021,, 22,, x 9533 FOR PEER REVIEW 3 of 21 20 Figure 2. 2. SchematicSchematic presentation presentation of ofamyl amyloseose and and amylopectin amylopectin structure structure (a) (anda) and core core enzymes enzymes for starchfor starch biosynthesis biosynthesis (b). There (b). There are two are major two major glycosidic glycosidic bonds bonds in the instarch the starchmolecules, molecules, α-1,4-glyco-α-1,4- sidicglycosidic bond and bond α-1,6-glycosidic and α-1,6-glycosidic bond. The bond. branched The branched amylop amylopectinectin chain is chainbuilt from is built mostly from short mostly α- 1,4-glucanshort α-1,4-glucan chains (as chains presen (ast in present amylose) in amylose)linked by linkedα-1,6-glycosidic by α-1,6-glycosidic branching branchingpoints (a). pointsStarch (bi-a). osynthesis in sweet potato storage roots requires a multitude of enzyme activities (b). Starch bio- Starch biosynthesis in sweet potato storage roots requires a multitude of enzyme activities (b). Starch synthesis starts with the conversion of sugar adenosine diphosphate glucose (ADP-Glu) from glu- biosynthesis starts with the conversion of sugar adenosine diphosphate glucose (ADP-Glu) from cose 1-phosphate (Glu-1-P), catalyzed by ADP-glucose pyrophosphorylase (AGPase). ADP-Glc is theglucose precursor 1-phosphate of amylose (Glu-1-P), and amylopectin catalyzed by biosyn ADP-glucosethesis. Granule-bound pyrophosphorylase starch (AGPase).synthase (GBSS) ADP- elongatesGlc is the the precursor linear α-(1,4)-glucan of amylose and chains amylopectin by adding biosynthesis.a glucose unit Granule-boundfrom ADP-Glc to starch the non-reduc- synthase ing(GBSS) end, elongates whereas the the soluble linear αstarch-(1,4)-glucan synthase chains (SS) and by starch adding bran a glucoseching enzyme unit from (SBE) ADP-Glc catalyze to am- the ylopectinnon-reducing production. end, whereas Debranching the soluble enzymes, starch isoamy synthaselase (ISA), (SS) and andstarch pullulanase branching (PUL) enzyme maintain (SBE) the correctcatalyze assembly amylopectin of the production. starch granule Debranching by efficiently enzymes, hydrolyzing isoamylase the α-(1,6)-linkage (ISA), and pullulanase in amylopectin. (PUL) Themaintain activities the correctbetween assembly the branching of the starch and debran granuleching by efficiently enzymes hydrolyzing determine
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