Rice Genomics and Genetics 2025, Vol.16, No.4, 219-236 http://cropscipublisher.com/index.php/rgg 220 the structure of amylopectin (such as starch synthase and branched enzymes) mutate, it will affect the digestion rate of starch and the formation of resistant starch (Zhou et al., 2022; Miura et al., 2024). Therefore, in-depth research on the gene functions and regulatory mechanisms in the starch synthesis pathway is conducive to precisely improving these quality traits in molecular breeding. In recent years, the development of new technologies such as genome editing has enabled us to specifically modify these genes and create high-quality functional alleles. This method can break through the limitations of long breeding cycles and strong randomness in traditional breeding, and cultivate new rice varieties that are both high-yielding and of high quality. It is evident that systematic research on the starch synthesis pathway in rice not only holds significant scientific importance but also has high application value. This research will introduce the starch synthesis pathway at different levels, covering everything from biochemical mechanisms, genetic regulation to breeding applications. This research will first describe the biochemical reaction process and main metabolic links of starch synthesis in rice grains, then explain the key enzymes involved in starch synthesis and their functions, and also introduce the expression of these related genes during grain development. It will summarize the genetic mechanisms regulating starch synthesis and discuss how to adjust the ratio of amylose to amylopectin through genetic means. And the ultrastructure of starch granules, analyze the key loci and allelic variations that affect the physicochemical properties of starch. By combining some typical cases of genes and mutants, such as functional variations of the Wx gene and other starch synthase genes, to illustrate their impact on rice quality and the utilization of these superior alleles in high-quality rice breeding. The objective of this study is to review the significant progress made in rice starch synthesis and quality control in recent years, identify the current technical difficulties and scientific issues, and make prospects for future research directions and application prospects. It is hoped that this will provide theoretical basis and technical reference for accelerating the cultivation of high-quality new rice varieties that are both high-yielding and meet consumer demands. 2 Overview of the Starch Biosynthesis Pathway in Rice 2.1 Biochemical reactions and major metabolic steps in starch biosynthesis The biosynthesis of rice grain starch is accomplished in the amyloid bodies (powdery bodies) of endosperm cells, involving a series of continuous enzymatic reactions. Firstly, sucrose, a product of photosynthesis, is converted into the substrate glucose-1-phosphate (G1P). Subsequently, under the action of ADP-glucose pyrophosphorylase (AGPase), a key initiating enzyme in starch synthesis, G1P reacts with ATP to form ADP-glucose (ADPG), which is the first rate-limiting step in starch synthesis. AGPase is composed of a heterotetramer consisting of two large subunits and two small subunits, and its activity regulation has a significant impact on the rate of starch synthesis (Dawar et al., 2013). The generated ADP-glucose serves as a glycoside donor and is utilized by subsequent enzymes to extend the glucan chain: Granulose-binding starch synthase (GBSS, also known as Wx protein) mainly catalyzes the synthesis of amylose, that is, adding the glucose group to the non-reducing end of the chain through an α-1, 4-glycosidic bond to form a long chain that is basically non-branched. Soluble starch synthases (SS, including ISO-SSIV equivalent enzymes) assist in prolonging the linear portion of amylopectin. When the glucan chain is extended to a certain length, starch branching enzymes (SBE, including SBEI, SBEIIb, etc.) will cut part of the α-1,4 bonds and connect the chain segments to another glucan chain through α-1,6-glycosidic bonds, forming side chains, thereby constructing the branched structure of amylopectin (Han et al., 2019). Meanwhile, starch debranching enzymes (DBE, including isoamylase ISA and starch debranching enzyme PUL) selectively hydrolyze inappropriate α-1,6 bonds, modifying the structure of amylopectin and making the branching chain arrangement more ordered and dense. These synthetic and modifying enzymes work in synergy to construct the semi-crystalline starch granule structure in rice grains. In addition, auxiliary enzymes such as starch phosphorylase and dismutase are involved in the conversion and balance of starch synthesis precursors, but their roles are relatively secondary. The rice starch synthesis pathway is a highly coordinated biochemical network, from substrate synthesis, chain extension to branching and modification, each step is interlinked and jointly determines the final starch yield and structural properties.
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