Food Biochemistry and Food Processing (2 edition)

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Starch Synthesis in the Potato Tuber

P. Geigenberger and A.R. Fernie

Introduction

What Is Starch?

Routes of Starch Synthesis and Degradation and Their

Regulation

Manipulation of Starch Yield

Manipulation of Starch Structure

Conclusions and Future Perspectives

References

INTRODUCTION

Starch is the most important carbohydrate used for food and

feed purposes and represents the major resource of our diet.

The total yield of starch in rice, corn, wheat, and potato ex-

ceeds 10^9 tons/year (Kossmann and Lloyd 2000, Slattery et al.

2000). In addition to its use in a nonprocessed form, due to the

low cost incurred, extracted starch is processed in many dif-

ferent ways. Processed starch is subsequently used in multiple

forms, for example in high-fructose syrup, as a food additive,

or for various technical processes based on the fact that as a

soluble macromolecule it exhibits high viscosity and adhesive

properties (Table 32.1). The considerable importance of starch

has made increasing the content and engineering the structural

properties of plant starches major goals of both classical breed-

ing and biotechnology over the last few decades (Smith et al.

1997, Sonnewald et al. 1997, Regierer et al. 2002). Indeed, since

the advent and widespread adoption of transgenic approaches

some 15 years ago gave rise to the discipline of molecular plant

physiology, much information has been obtained concerning the

potential to manipulate plant metabolism. For this chapter, we

intend to review genetic manipulation of starch metabolism in

potato (Solanum tuberosum). Potato is one of the most important

crops worldwide, ranking fourth in annual production behind the

cereal species rice (Oryza sativa), wheat (Triticum aestivum),

and maize (Zea mais). Although in Europe and North America

the consumption of potatoes is mainly in the form of processed

foodstuffs such as fried potatoes and chips, in less developed

countries it represents an important staple food and is grown by

many subsistence farmers. The main reasons for the increasing

popularity of the potato in developing countries are the high nu-

tritional value of the tubers combined with the simplicity of its

propagation by vegetative amplification (Fernie and Willmitzer

2001). Since all potato varieties are true tetraploids and display

a high degree of heterozygosity, genetics have played only a mi-

nor role in metabolic studies in this species. However, because

the potato is a member of theSolanaceaefamily, it was amongst

the first crop plants to be accessible to transgenic approaches.

Furthermore, due to its relatively large size and metabolic homo-

geneity, the potato tuber represents a convenient experimental

system for biochemical studies (Geigenberger 2003a).

In this chapter we will describe transgenic attempts to mod-

ify starch content and structure in potato tubers that have been

carried out in the last two decades. In addition to describing

biotechnologically significant results we will also detail funda-

mental research in this area that should enable future biotech-

nology strategies. However, we will begin by briefly describing

starch, its structure, and its synthesis.

WHAT IS STARCH?

For many years, it has been recognized that the majority of

starches consist of two different macromolecules, amylose and

amylopectin (Fig. 32.1), which are both polymers of glucose and

are organized into grains that range in size from 1μmtomore

than 100μm. Amylose is classically regarded as an essentially

linear polymer wherein the glucose units are linked through

α-1-4-glucosidic bonds. In contrast, although amylopectin con-

tainsα-1-4-glucosidic bonds, it also consists of a high pro-

portion ofα-1-6-glucosidic bonds. This feature of amylopectin

makes it a more branched, larger molecule than amylose, hav-

ing a molecular weight of 10^7 –10^8 as opposed to 5× 105 –10^6.

Food Biochemistry and Food Processing, Second Edition. Edited by Benjamin K. Simpson, Leo M.L. Nollet, Fidel Toldr ́a, Soottawat Benjakul, Gopinadhan Paliyath and Y.H. Hui.

©C2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc.

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