Introduction

Starch is the second-largest form of biomass produced by plants, yet is one of the least studied biopolymers, especially in terms of our understanding regarding the nature and regulation of biosynthesis. In nature, starch is present as partially crystalline granules encompassing two major homopolymers of glucose. The larger and more branched of the two homopolymers is amylopectin and the smaller, and essentially linear homopolymer is amylose. In both amylopectin and amylose, the linear chain is formed by a-1, 4 glycosidic linkages and the branches are formed by a-1, 6-glycosidic linkages.

The ever-increasing need for starch possessing specific properties by diverse industries [5] is fueling starch biotechnology research. The unfolding biotechnological revolution over the last decade is enabling us to dissect starch metabolism at the molecular level and bioengineer plants to produce starch types that are not normally found in nature. Study of the regulation of starch synthesis using transgenic plants is now supplanting the study of well-characterized plant-starch mutants [6] used in the past.

* Address for communication: USDA-CCRU, 501 Walnut Street, Madison, WI 53705, USA. E-mail: [email protected]. Prof. C. Jansson and Dr. C. Sun of Department of Plant Biology, Swedish University of Agricultural Sciences, Uppsala, introduced the topic of plant starch biochemistry to P. Sathish, and are gratefully acknowledged. This work was supported by a grant from the USDA-ARS to H.F. Kaeppler.

It is generally accepted that the core processes central to starch biosynthesis (Figure 1) begin with the generation of ADPglucose by the enzyme Adenosine 5'-diphosphoglucose pyrophosphorylase (AGPase) [8], Using ADPglucose as the substrate, a host of starch synthases, branching enzymes and debranching enzymes structure the final starch granule. However, changes anywhere in the photosynthetic pathway, sucrose transport pathway, or metabolic flux due to environmental constraints can and do alter the quantity and/or quality of starch produced in vivo. To date, many transgenic plant lines have been created to determine the role and function of enzymes involved in the starch metabolic pathway. We will describe the role of these enzymes in the regulation of starch synthesis in transgenic plants.

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