Function and Characterization of Starch Synthase I Using Mutants in Rice

Abstract

(A.M., H.H.) Four starch synthase I (SSI)-deficient rice (Oryza sativa) mutant lines were generated using retrotransposon Tos17 insertion. The mutants exhibited different levels of SSI activities and produced significantly lower amounts of SSI protein ranging from 0% to 20% of the wild type. The mutant endosperm amylopectin showed a decrease in chains with degree of polymerization (DP) 8 to 12 and an increase in chains with DP 6 to 7 and DP 16 to 19. The degree of change in amylopectin chain-length distribution was positively correlated with the extent of decrease in SSI activity in the mutants. The structural changes in the amylopectin increased the gelatinization temperature of endosperm starch. Chain-length analysis of amylopectin in the SSI band excised from native-polyacrylamide gel electrophoresis/SS activity staining gel showed that SSI preferentially synthesized DP 7 to 11 chains by elongating DP 4 to 7 short chains of glycogen or amylopectin. These results show that SSI distinctly generates DP 8 to 12 chains from short DP 6 to 7 chains emerging from the branch point in the A or B 1 chain of amylopectin. SSI seemingly functions from the very early through the late stage of endosperm development. Yet, the complete absence of SSI, despite being a major SS isozyme in the developing endosperm, had no effect on the size and shape of seeds and starch granules and the crystallinity of endosperm starch, suggesting that other SS enzymes are probably capable of partly compensating SSI function. In summary, this study strongly suggested that amylopectin chains are synthesized by the coordinated actions of SSI, SSIIa, and SSIIIa isoforms.

Gradually Decreasing Starch Branching Enzyme Expression Is Responsible for the Formation of Heterogeneous Starch Granules

“…The different amounts of SBEI, SBEIIa, and SBEIIb distributed in the four heterogeneous starch granules caused different binding amounts of SSI. It has been reported that SSI plays a critical role in generating chains of DP 8-12 from chains of DP 6-7 that emerge from the branch points of amylopectin, carried out by SBE (Fujita et al, 2006). This finding greatly helped to elucidate the gradually decreasing ratio of DP 6-12 in the four TRS heterogeneous starch granules (Fig.…”

“…However, when diverse ranges of genes are mutated, distinct effects on granule morphology in a seed are detected. For example, once the soluble starch synthase OsSSIIIa/Flo5, which is responsible for the extension of relatively long chains of both amylopectin B2 and B3 to B4 (Gao et al, 1998;Jane et al, 1999;Fujita et al, 2006), is eliminated from the genome, a phenotype of loosely packed central portions exhibiting a floury white-core endosperm is detected. The granules located in this region are smaller than those of the parental control and exhibit round morphology with no sharp edges, while the peripheral region in ssIIIa/flo5 contains normal compound starch (Ryoo et al, 2007).…”

Functional Interactions between Starch Synthase III and Isoamylase-Type Starch-Debranching Enzyme in Maize Endosperm

“…Thus, SSIII appears to affect the entire range of chains that constitute crystalline lamellae as well as those that extend through multiple crystalline clusters. In contrast, SSI and SSII are required for the normal abundance of discrete, continuous spans of chain length, which can be explained by substrate preference Zhang et al, 2004Zhang et al, , 2008Delvallé et al, 2005;Nakamura et al, 2005;Fujita et al, 2006).…”

Studies on rice seed quality through analysis of a large-scale T-DNA insertion population

“…Previous studies have focused on genes directly involved in the biosynthesis/metabolism of starch, protein and fat [66][67][68][69][70][71][72][73]; our analysis suggests that many other genes may also be involved in seed quality control. Analysis of the underlying complex regulatory network should broaden our understanding of the relevant regulatory mechanisms.…”