Browsing by Author "Gouws, Nina Ellen"

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    Investigating starch from transgenic potato plants lacking starch branching enzyme I and/or glucan, water dikinase 1
    (Stellenbosch : Stellenbosch University, 2024-03) Gouws, Nina Ellen; Lloyd, James; Van der Vyver, Christell; Stellenbosch University. Faculty of AgriSciences. Department of Genetics & Institute of Plant Biotechnology.
    ENGLISH ABSTRACT: Starch is the principal storage carbohydrate in plants where it is found in granules in plastids. It is the major calorie in the human diet and is an important feedstock in various industries. To improve the usefulness of native starch for different industrial applications, it must first undergo expensive chemical, physical or enzymatic treatments. Understanding starch biosynthesis can help to introduce modifications in planta with the aim of reducing such postharvest costs. Starch consists of two polymers, amylose and amylopectin, which are synthesised by multiple enzymes and are found in differing ratios where amylopectin normally constitutes the majority of the granule. Starch synthases elongate α-1,4 linked glucose chains to produce amylose. Starch branching enzymes can introduce α-1,6 branchpoints to form amylopectin which can be phosphorylated by glucan, water dikinases. There seems to be a relationship between the branching structure and phosphate incorporation as repression of STARCH BRANCHING ENZYME genes leads to increased granule bound phosphate. In this project, STARCH BRANCHING ENZYME I (SBEI) and GLUCAN, WATER DIKINASE 1 (GWD1) were repressed individually or simultaneously in potato using RNAi constructs. Repression of these enzymes resulted in changes to the amylose and phosphate contents. These alterations influenced granule morphology and gelling properties, but not freeze-thaw stability or starch digestibility. Rapid viscoamylography demonstrated that repression of SBEI increased phosphate content and peak viscosity. Starch pastes from SBEI transgenic plants also had improved paste clarity. GWD1 repression resulted in a decrease in phosphate content and peak viscosity. Simultaneous repression of SBEI/GWD1 resulted in higher amylose content and altered granule morphology with some multilobed starch granules being present. Despite the increase in amylose, a carbohydrate more resistant to digestion, no differences in digestibility of purified starch were found. This study adds to what is known about starch biosynthesis in potato tubers and demonstrates that starch can be modified in planta by targeting starch biosynthetic genes. Whether these changes ultimately result in improvement to end-products still needs to be investigated.