Abstract
Soluble starch synthase I (SSI) is a key enzyme in the biosynthesis of plant amylopectin. In this study, the gene named IbSSI, was cloned from sweet potato, an important starch crop. A high expression level of IbSSI was detected in the leaves and storage roots of the sweet potato. Its overexpression significantly increased the content and granule size of starch and the proportion of amylopectin by up-regulating starch biosynthetic genes in the transgenic plants compared with wild-type plants (WT) and RNA interference plants. The frequency of chains with degree of polymerization (DP) 5–8 decreased in the amylopectin fraction of starch, whereas the proportion of chains with DP 9–25 increased in the IbSSI-overexpressing plants compared with WT plants. Further analysis demonstrated that IbSSI was responsible for the synthesis of chains with DP ranging from 9 to 17, which represents a different chain length spectrum in vivo from its counterparts in rice and wheat. These findings suggest that the IbSSI gene plays important roles in determining the content, composition, granule size and structure of starch in sweet potato. This gene may be utilized to improve the content and quality of starch in sweet potato and other plants.
Highlights
In plants, starch consists of amylose and amylopectin
Two mutants generated by T-DNA insertion into the Arabidopsis synthase I (SSI) gene demonstrated that AtSSI was most active on glycogen with an average outer chain length (OCL) of 7–8 compared with maize amylopectin (OCL = 12–14) and maize β-limit dextrin (OCL = 2.5)
These findings suggest that isolated the soluble starch synthase I gene (IbSSI) may contain less regulatory sequences than StSSI, which may be responsible for their differences in expression patterns and biological functions
Summary
Starch consists of amylose and amylopectin. Amylose mainly comprises linear chains that are linked by α-1, 4 O-glycosidic bonds, whereas amylopectin is highly branched and contains 5–6% α-1,6 O-glycosidic bonds to generate glucan branches of various lengths[1]. Size is increased in Arabidopsis mutants that carry defective SSIV, which suggests that this gene is involved in the initiation stages of starch granule biosynthesis rather than in the elongation of amylopectin chains[18]. Two mutants generated by T-DNA insertion into the Arabidopsis SSI gene demonstrated that AtSSI was most active on glycogen with an average outer chain length (OCL) of 7–8 compared with maize amylopectin (OCL = 12–14) and maize β-limit dextrin (OCL = 2.5). This length is consistent with the preferable DP value of 6–7 that is often modified by rice SSI. Improving the content and quality of sweet potato starch remains an urgent demand, especially in the field of biotechnology
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