Abstract
Studies in Arabidopsis and rice suggest that manipulation of starch synthase I (SSI) expression in wheat may lead to the production of wheat grains with novel starch structure and properties. This work describes the suppression of SSI expression in wheat grains using RNAi technology, which leads to a low level of enzymatic activity for SSI in the developing endosperm, and a low abundance of SSI protein inside the starch granules of mature grains. The amylopectin fraction of starch from the SSI suppressed lines showed an increased frequency of very short chains (degree of polymerization, dp 6 and 7), a lower proportion of short chains (dp 8-12), and more intermediate chains (dp 13-20) than in the grain from their negative segregant lines. In the most severely affected line, amylose content was significantly increased, the morphology of starch granules was changed, and the proportion of B starch granules was significantly reduced. The change of the fine structure of the starch in the SSI-RNAi suppression lines alters the gelatinization temperature, swelling power, and viscosity of the starch. This work demonstrates that the roles of SSI in the determination of starch structure and properties are similar among different cereals and Arabidopsis.
Highlights
Starch is the major component of wheat grain
Three independent synthase I (SSI)-RNAi suppressed lines (SSI-RNAi A, E, and F) were identified with a noticeably reduced SSI enzyme activity (Fig. 2). These lines were selfed to create T2 plants, and homozygous transgenic and negative segregant T3 SSIRNAi A and SSI-RNAi F lines were analysed by polymerase chain reaction (PCR) to identify whether the SSI-RNAi transgene was present or absent in all twenty of the T3 transgenic plants originating from one T2 transgenic plant
Changes in the endosperm starch of the SSI-RNAi transgenic lines in this work included an increase in the abundance of very short chains and intermediate chains, and a reduction in the abundance of short chains of amylopectin
Summary
Starch is the major component of wheat grain (approximately 60–70%). Starch granules in higher plants contain two main classes of glucan polymer, amylose and amylopectin. Amylopectin is a large and highly branched polymer (containing 5–6% α-1,6 linkages) reaching a dp of about 5000–50 000 (see reviews from Kossmann and Lloyd, 2000; Rahman et al, 2000; Ball and Morell, 2003; James et al, 2003; Tetlow et al, 2004; Morell et al, 2006).
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