Role of Granule-bound Starch Synthase in Determination of Amylopectin Structure and Starch Granule Morphology in Potato

Daniel C Fulton,Anne Edwards,Emma Pilling,Helen L Robinson,Brendan Fahy,Robert Seale,Lisa Kato,Athene M Donald,Peter Geigenberger,Cathie Martin,Alison M Smith

doi:10.1074/jbc.m111579200

Abstract

Reductions in activity of SSIII, the major isoform of starch synthase responsible for amylopectin synthesis in the potato tuber, result in fissuring of the starch granules. To discover the causes of the fissuring, and thus to shed light on factors that influence starch granule morphology in general, SSIII antisense lines were compared with lines with reductions in the major granule-bound isoform of starch synthase (GBSS) and lines with reductions in activity of both SSIII and GBSS (SSIII/GBSS antisense lines). This revealed that fissuring resulted from the activity of GBSS in the SSIII antisense background. Control (untransformed) lines and GBSS and SSIII/GBSS antisense lines had unfissured granules. Starch analyses showed that granules from SSIII antisense tubers had a greater number of long glucan chains than did granules from the other lines, in the form of larger amylose molecules and a unique fraction of very long amylopectin chains. These are likely to result from increased flux through GBSS in SSIII antisense tubers, in response to the elevated content of ADP-glucose in these tubers. It is proposed that the long glucan chains disrupt organization of the semi-crystalline parts of the matrix, setting up stresses in the matrix that lead to fissuring.

Highlights

Reductions in activity of SSIII, the major isoform of starch synthase responsible for amylopectin synthesis in the potato tuber, result in fissuring of the starch granules
The fragments of the granule-bound isoform of starch synthase (GBSS) and SSIII cDNAs used in the construct had both been used previously to obtain plants expressing antisense RNA for either SSIII (11, 13: referred to as SSIII antisense lines) or GBSS (24: referred to as GBSS antisense lines)
The SSIII/GBSS lines were compared with three SSIII and three GBSS antisense lines described previously (11, 13, 24) and with control lines that were either untransformed Desiree or a line transformed with the vector pBIN19 alone

Summary

Introduction

Reductions in activity of SSIII, the major isoform of starch synthase responsible for amylopectin synthesis in the potato tuber, result in fissuring of the starch granules. Little is known about the processes that determine the morphology of starch granules, but important clues have been gained from studies of mutant plants with altered granule morphology Many such plants carry mutations in genes encoding isoforms of starch synthase and starch-branching enzyme responsible for the synthesis of amylopectin, the branched ␣1,4, ␣1,6 glucan, which forms the semi-crystalline matrix of the granule. The r and rug mutations of pea and the amylose-extender mutation of maize affect the proportion of short and long chains in amylopectin, the chain-length distribution within the short-chain population, and the ratio within the granule of amylopectin to amylose, the essentially linear glucan that makes up 20 –30% of storage starches (2, 6 –9) It is not known which of these changes in granule composition and polymer structure leads to the changes in granule morphology. We reasoned that the altered granule morphology in SSIII antisense lines could be caused either by the altered chainlength distribution of short chains of amylopectin, or by the increased proportion of very long chains in amylopectin, or by the interaction of amylose molecules within the granule with one or both of these abnormal features of amylopectin

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