Abstract
Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.
Highlights
Since the discovery around 1940 that starch is composed of the two polysaccharides amylose and amylopectin, the molecular branched structure of the latter has been a subject of intense research
Starch granules are deposited inside the plastids of higher plant cells as discrete, semi-crystalline granules that to the most part consist of the two polyglucans amylose and amylopectin, of which the latter constitutes the major component, and importantly, the component which defines the fine structure of the granule [8]
We argue that a holistic approach to understanding starch biosynthesis using structural analysis data to guide enzymology studies, and vice versa, mutually benefits both fields
Summary
Since the discovery around 1940 that starch is composed of the two polysaccharides amylose and amylopectin, the molecular branched structure of the latter has been a subject of intense research. This review attempts to reconcile current knowledge of starch structure with our evolving view of the complex biochemical pathway responsible for its synthesis in relation to the building block backbone model proposed by Bertoft [10]. An overview of the structural components of the starch granule, amylose and amylopectin are presented in relation to granule architecture and their relationship to the backbone model. This is followed by an outline of the starch biosynthesis pathway occurring in plastids, with an attempt at integration of biological activities within the context of the backbone model. It will become apparent that current knowledge does not yet extend to explaining how the higher-order elements of granule architecture arise from the various enzyme activities responsible for the synthesis of building blocks
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