Use of Biotechnology to Engineer Starch in Cereals

Abstract

Use of Biotechnology to Engineer Starch in Cereals Diane M. Beckles and Maysaya Thitisaksakul Department of Plant Sciences, University of California—Davis, Davis, California, U.S.A. Abstract The starch accumulated in rice, wheat, maize, sorghum, and millet grain is indispensible for human survival as it accounts for most of the consumed calories. In addition, cereal starch is used in its natural or modified state as a healthful food, and as an environmentally friendly additive or replacement for petroleum-derived fuel and polymers. Therefore, developing cereals that accumulate higher endosperm starch and starches with novel polymeric properties could help to meet the dual challenges of sustaining human population growth needs while minimizing some of the harmful environmental impacts. Despite its fundamental impor- tance, comparatively little is known about the mechanistic basis of starch biosynthesis. This entry provides a basic overview of the “starch field” for the beginner. First, the various uses of starch, its structural organiza- tion and biosynthesis, and how its functionality relates to its structure are outlined. Second, how recent bio- technological advancements are leading to the discovery of new genes that modulate starch and to novel starches generated through genetic engineering are described. Finally, some of the remaining questions and challenges that must be tackled in order to meet the goal of increasing starch production for use as a food, feed, fiber, and polymer in the next 50 years and beyond are illustrated. INTRODUCTION Starch is a natural polymer of glucose. It is deposited as water-insoluble granules in most plant tissues, but in cereal it makes up ∼70–90% of endosperm dry weight, and serves as a dense and rich source of carbon and energy. [1] Depend- ing on the nutritional status of the plant, starch biosynthesis can be triggered, enhanced, slowed down, or even inhibited to suit plant growth requirements. The judicious allocation of carbon from photosynthetic tissues to the seeds for long- term storage enhances survivability in the next generation and thereby increases plant fitness. [2] Storage starch also plays a critical role in the human diet, accounting for 50–80% of all calories consumed. In many economically advanced countries, cereal starch is abundant and cheap and as a result consumers expect starch products to provide not only basic nutrition but also health- ful benefits and high hedonistic value. For example, in many Asian markets, few compromises can be made with respect to the sensorial property of rice starch where texture and translucency are critical, and in North America and Europe, there is a surge of interest in slow-release, low- digestible starches. [3] These starches behave in a physiolog- ically similar way to fiber by creating an enhanced prebiotic effect that is associated with reduction in the occurrence of colon cancer. [4] The abundance, low cost, and physico-chemical versatil- ity of starch also makes it competitive as a raw material for the large-scale biomaterial processing sector. [5] About 35% of all starch in Western countries is used in its native or modified form as biopolymers in the food, textile, and paper manufacturing industries (Fig. 1). Maize starch use in the United States exemplifies this; most of it is not directly con- sumed, but is converted into a diverse set of products, rang- ing from animal feeds to sweeteners, to polymers, and fuels. [3] Many value-added starch-based biopolymers and starch-derived biofuels are projected to be less harmful to the environment than those derived from petrochemicals. [5] What these examples illustrate is that there is a need for cere- als producing a wide array of starches for non-food uses, and also, for increased starch production to ensure global food security. The goal of this entry, therefore, is to examine the many attempts to engineer starch to optimally meet its grow- ing and diverse end-uses. ORGANIZATION OF THE STARCH GRANULE Starch is composed of two large glucose polymers called amylose and amylopectin. Amylose is smaller, comprising 20–30% of the dry weight of normal starch, while amylo- pectin may be up to 100 times larger than amylose and makes up 60–70% of starch. Both polymers consist of glucose molecules connected by α-1-4-linkages creating glucan chains which are occasionally branched by α-1-6- linkages. The frequency of branching is higher in amylo- pectin (Fig. 2), [1] and is highly ordered so that chains of 3–4 signature lengths are created. These chains are arrayed to form clusters interspersed by regions with branch points (Fig. 2C). [2] The specific arrangement and architecture of these glucan chains permits their molecular self-assembly into a semicrystalline macromolecule and eventually to gran- ules of distinct sizes and morphologies at maturity (Fig. 2). [1] The morphology, sizes, and relative numbers of the granules Encyclopedia of Biotechnology in Agriculture and Food DOI: 10.1081/E-EBAF-120051354 Copyright © 2014 by Taylor & Francis. All rights reserved.