Plant Gene and Traits 2024, Vol.15, No.3, 108-117 http://genbreedpublisher.com/index.php/pgt 110 Starting with sucrose from the cytosol, the pathway utilizes enzymes like sucrose synthase (SuSy) to break down sucrose into fructose and uridine diphosphate glucose (UDPG). Other enzymes, such as UDPG pyrophosphorylase (UGPase), are involved in further transformations that form critical intermediates like glucose-1-phosphate (G1P). The figure also includes various other enzymes located in both the cytosol and plastid, such as phosphoglucomutases (cPGM and pPGM), fructokinase (FRK), and phosphoglucose isomerase (cPGI), highlighting their roles in manipulating glucose molecules into forms suitable for starch synthesis. The core of the starch synthesis occurs in the plastid, where adenosine diphosphate glucose (ADPG) produced by ADP glucose pyrophosphorylase (AGPase) is polymerized into starch by enzymes such as granule-bound starch synthase (GBSS) and other starch synthases (SS). The pathway complexity is increased by the actions of starch branching enzymes (SBE) and de-branching enzymes (DBE), which modify the structure of the growing starch molecule (Li, 2024). Additionally, the illustration includes membrane transporters such as sucrose transporters (SUT), glucose 6-phosphate/phosphate transporter (GPT or G6PPT), ATP/ADP transporter (AATP), and ADPG translocator, which are crucial for moving molecules like glucose 6-phosphate (G6P) and ADP across the plastid membrane, facilitating the flow of substrates necessary for starch biosynthesis. 2.2 Enzymatic roles and pathways 2.2.1 Specific enzymes involved in cassava starch synthesis In cassava (Manihot esculenta Crantz), a total of 45 genes have been identified to participate in starch biosynthesis. These include isoforms of AGPase, granule-bound starch synthase (GBSS), SS, SBE, DBE, and glucan, water dikinase (GWD). Each of these enzymes plays a specific role in the synthesis and modification of starch, contributing to the unique properties of cassava starch that are important for food processing and industrial applications (Tappiban et al., 2019). 2.2.2 Pathway details from sucrose breakdown to starch assembly The pathway of starch synthesis in cassava starts with the breakdown of sucrose into glucose and fructose. Glucose is then phosphorylated to glucose-6-phosphate and further isomerized to glucose-1-phosphate. AGPase catalyzes the conversion of glucose-1-phosphate to ADP-glucose, which is the substrate for starch synthases. GBSS is primarily responsible for amylose synthesis, while SS, SBE, and DBE are involved in amylopectin synthesis. The orchestrated action of these enzymes leads to the assembly of starch granules in the plastids (Ihemere et al., 2006; Tappiban et al., 2019). 2.3 Regulatory mechanisms: factors influencing enzyme activity and starch biosynthesis rate The rate of starch biosynthesis in cassava is influenced by various factors, including gene regulation, enzyme activity, and environmental conditions. The expression of genes involved in starch biosynthesis is regulated throughout root development, and quantitative trait loci (QTLs) associated with starch content and pasting properties have been identified. Genetic modification techniques, such as CRISPR-Cas9 mediated targeted mutagenesis and transgenic breeding, have been used to alter the expression of key genes like GBSS and PTST1, thereby modifying the starch content and properties in cassava roots (Bull et al., 2018; Tappiban et al., 2019). Additionally, the domestication of cassava has led to the selection of genes that increase carbon flux towards starch accumulation, which is a reflection of the natural selection and breeding efforts to enhance starch production (Wang et al., 2014). 3 Genetic Bases of Starch Synthesis in Cassava 3.1 Genetic variability in starch synthesis genes Cassava (Manihot esculenta Crantz) is a staple food crop whose starch composition is crucial for both consumption and industrial applications. The genetic variability in starch synthesis genes is significant, as evidenced by the identification of 110 quantitative trait loci (QTLs) associated with starch content and pasting properties (Tappiban et al., 2019). This genetic diversity is a valuable resource for breeding programs aimed at improving starch quality.
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