Designing starch: harnessing carbohydrate polymer synthesis in plants
Final Report Abstract
Starch is produced in most green plants as energy storage, and constitutes the main source of carbohydrate in human diets. It is highly abundant in most staple foods like rice, maize, potatoes and cassava. Beyond its nutritional role it has been used for centuries in several non-food products: paper, adhesive, additive in pharmaceutical products etc. The value of starch, both as food and non-food commodity is still continuously growing, because of its central role in securing food security and its potential for industrial applications. The variety of uses of starch contrasts with the simplicity of its elementary component: glucose. Its remarkable properties emerge from its microscopic structure. Naturally produced starch granules range in size between approximately 1 and 100 µm. Starch granules are composed of two types of polysaccharides, organized in concentric layers: amylose and amylopectin (which constitutes the main part of starch). Both amylose and amylopectin are macromolecules consisting of glucose monomers that are connected by two different kinds of glucosidic bonds. One type is responsible for linear chains, the other for the formation of branching points. Amylose contains mainly long linear chains, while amylopectin exhibits a remarkable, highly packed and regular structure of rather short and moderately branched chains. The regularity of amylopectin and its spatial arrangement result in a semi-crystalline structure. Starch has been studied for decades and most of the biochemical processes and the respective enzymes responsible for starch synthesis and degradation have been well characterised. However, their interplay and their mode of action on a structure as complex as a starch granule are still poorly understood. Hence, in our project we developed a threefold approach to investigate how the ordered structure of starch emerges during starch biogenesis. We developed in vitro assays to characterise the reactivity of individual enzymes, e.g. how fast linear chains are elongated, or how double helices are formed between glucose chains, which are responsible for the crystalline properties. Experiments have also been performed with yeast that was genetically modified to produce starch (instead of glycogen). Transferring selected plant genes into yeast, a set of mutants was generated and investigated with respect to their ability to produce starch-like sugars. These results informed us on the role of the single genes in starch production. The third strategy is a theoretical approach to simulate the formation of polysaccharides in computer models. We translated biological and biochemical knowledge on starch synthesis into mathematical equations and computer code, which allowed us to systematically vary parameters in order to reproduce polysaccharides displaying characteristics of native starch. This virtual approach, also allows reproducing in silico mutants and test any enzymatic combination, such that experimental and theoretical results can be compared. In summary, our interdisciplinary approach has considerably advanced our fundamental understanding about the processes that lead to the formation of the highly complex, but still regular structure of starch granules.
Publications
- (2016). Formation of starch in plant cells. Cellular and Molecular Life Sciences, 73, 2781–2807
Pfister, B., & Zeeman, S. C.
(See online at https://doi.org/10.1007/s00018-016-2250-x) - 2016). Recreating the synthesis of starch granules in yeast. eLife, 5, 1–29
Pfister, B., Sánchez-Ferrer, A., Diaz, A., Lu, K., Otto, C., Holler, M., … Zeeman, S. C.
(See online at https://doi.org/10.7554/eLife.15552) - (2017). Design starch: stochastic modeling of starch granule biogenesis. Biochemical Society Transactions, 45(4), 885-893
Raguin, A., & Ebenhöh, O.
(See online at https://doi.org/10.1042/BST20160407) - (2018). Distinct functions of STARCH SYNTHASE 4 domains in starch granule formation. Plant Physiology, 176, 566–581
Lu, K.-J., Pfister, B., Jenny, C., Eicke, S., & Zeeman, S. C.
(See online at https://doi.org/10.1104/pp.17.01008)
