Simultaneous antisense inhibition of two starch-synthase isoforms in potato tubers leads to accumulation of grossly modified amylopectin

Abstract

A chimaeric antisense construct was used to reduce the activities of the two major starch-synthase isoforms in potato tubers simultaneously. A range of reductions in total starch-synthase activities were found in the resulting transgenic plants, up to a maximum of 90% inhibition. The reduction in starch-synthase activity had a profound effect on the starch granules, which became extremely distorted in appearance compared with the control lines. Analysis of the starch indicated that the amounts produced in the tubers, and the amylose content of the starch, were not affected by the reduction in activity. In order to understand why the starch granules were distorted, amylopectin was isolated and the constituent chain lengths analysed. This indicated that the amylopectin was very different to that of the control. It contained more chains of fewer than 15 glucose units in length, and fewer of between 15 and 80 glucose units. In addition, the amylopectin contained more very long chains. Amylopectin from plants repressed in just one of the activities of the two starch-synthase isoforms, which we have reported upon previously, were also analysed. Using a technique different to that used previously we show that both isoforms also affect the amylopectin, but in a way that is different to when both isoforms are repressed together.

Mutations Affecting Starch Synthase III in Arabidopsis Alter Leaf Starch Structure and Increase the Rate of Starch Synthesis

“…This indicates that loss of SSIII activity directly or indirectly results in increased phosphorylation of transient starch as well as storage starch. This result is supportive of the previous finding that antisense suppression of SSIII in potato tubers (Solanum tuberosum) results in higher phosphate content of the tuber starch (Lloyd et al, 1999), although in the case of the Arabidopsis and maize mutants the relative increase is greater than was observed in potato.…”

“…This is in line with studies in transgenic plants that indicate starch phosphorylation depends on the structure of the starch. The specific suppression of SSIII in transgenic potato causes a 70% increase in the phosphorylation frequency, most likely due to increases in the lengths of the longest Ap chains (Abel et al, 1996;Lloyd et al, 1999). Antisense suppression of BE activity, which causes formation of longer length Ap chains, also resulted in significantly increased levels of starchbound phosphate (Schwall et al, 2000).…”

The Structure and Expression of the Wheat Starch Synthase III Gene. Motifs in the Expressed Gene Define the Lineage of the Starch Synthase III Gene Family

“…Differences in the phenotypes of mutants affecting SS expression, and differences in kinetic properties between SS isoforms, suggested that each enzyme tends to contribute to the extension of specific subsets of the available non-reducing ends within amylopectin. Evidence from two similar studies in potato, in which the expression of both SSII and SSIII were reduced by chimeric antisense approaches, provides further support for this hypothesis (Edwards et al, 1999;Lloyd et al, 1999).…”

Starch accumulation, activities of key enzyme and gene expression in starch synthesis of wheat endosperm with different starch contents

“…3c, d, e). Starches produced by the combined reduction in SS-II and SS-III activities in potato revealed that different SS isoforms make distinct contributions to amylopectin biosynthesis (Edwards et al 1999), and also plants lacking SS-III in potato resulted in changes of glucan chain-length distribution, and cracking and distortion of granule shapes (Lloyd et al 1999). We found SS-III had the similar expression profiles as SS-I, and SS-II exhibited higher expression than SS-III from 6 to 18 DAP, but significantly down-regulated after 18 DAP.…”