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ADP-D-glucose is converted into (1,4-alpha-D-glucosyl)(n+1) by the enzymatic activity of Starch Synthase. Starch Synthase (SS) catalyzes the elongation of the ?-glucan polymer by adding Glc residues from ADP-Glc to the nonreducing ends via ?-1,4-glucosidic linkages. In rice, 10 isoforms of SS exist, including soluble isoforms SSI, SSIIa (SSII-3), SSIIb (SSII-2), SSIIc (SSII-1), SSIIIa (SSIII-2), SSIIIb (SSIII-1), SSIVa (SSIV-1), SSIVb (SSIV-2), and two granule-bound isoforms (GBSSI)/(Waxy) and GBSSII. These isoforms exhibit tissue-specific and temporal expression patterns (Toyosawa et al., 2016). The soluble SSs and starch branching enzymes catalyze the formation of amylopectin. The SSI isoform accounts for ?70% of the total starch synthase activity in cereal endosperm; however, the SSI knockout mutant shows no significant defects in rice seeds and starch granules (Hayashi et al., 2018). Furthermore, the Expression assay indicated that SSI has a constitutive expression pattern in all tested tissues of rice, including germinating seeds, roots, leaves, leaf sheaths, and panicles (Hirose and Terao, 2004). In addition to starch biosynthesis enzymes, several other proteins are known to regulate starch biosynthesis. For example, an amyloplast-localized protein, SBDCP1, encoded by the rice Os01g0856900 gene, inhibits SSIIIa activity and indirectly lowers SSIIIa protein levels, thus impacting starch (amylopectin) synthesis (Cakir et al., 2019).
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