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Evolution of the molecular structure of starch in developing wheat grain

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Title: Evolution of the molecular structure of starch in developing wheat grain
Author: Kalinga, Danusha Nilakshi
Department: Department of Food Science
Program: Food Science
Advisor: Seetharaman, Koushik
Abstract: Starch is the major constituent of matured wheat grain. The details of subtle localized differences in the evolution of the structure of starch are important for an understanding of starch biosynthesis. However, the distinct stages involved in the formation and transformation of the molecular structure of starch during starch biosynthesis are still not fully understood. In this study, starches extracted from wheat grains harvested at 3, 7, 14, 28, and 49 days after anthesis (DAA) were used as a means of examining the molecular structure of starch from developing wheat grain. Gel-permeation chromatography and high-performance anion-exchange chromatography were employed for the analysis of the structure of both whole starch and its isolated amylopectin (AP) component. Scanning electron microscopy of 3 DAA wheat grain cross-sections revealed the absence of endosperm but the presence of spherical transitory-type small starch granules in the pericarp. Endosperm was present at 7 DAA and contained lenticular-shaped developing large granules. From 14 DAA onward, spherical-shaped small granules coexisted with large granules in the endosperm. The structure of transitory pericarp starch (PS) was compared with that of matured endosperm storage starch (ES). The composition of PS and ES differed: PS granules contained 14 % apparent amylose (AAM), whereas ES granules contained 33 % AAM. The AAM fractions of PS showed characteristics similar to those of intermediate-type materials with short branches, whereas ES contained both linear and branched amylose (AM). Differences in the amylopectin component of PS and ES were also apparent, especially in their internal structures. PS amylopectin had longer chains and fewer A-chains, resulting in a structure less branched than that of ES amylopectin. Starches isolated from 7 DAA to 49 DAA were studied with respect to endosperm development. The AAM of both large and small granules increased with increasing maturity. The AAM fraction of starch granules at early maturity (7 DAA and 14 DAA) consisted of intermediate-type materials in addition to linear AM, whereas starch at later maturity stages (28 and 49 DAA) contained linear and branched AM. During granule development, the fine structure of AP varied with the maturity level as well as with the size of the granule. During the post-physiological maturity stage, when the net accumulation of sugars ceases, the grain dries out; however, structural changes occurred in AP at this stage, possibly due to the action of starch branching and debranching enzymes. In both large and small granules, the external AP structure was more organized at post-physiological maturity (49 DAA) than at pre-physiological maturity (7 DAA to 28 DAA). Compared to their characteristics at post-physiological maturity, at the pre-physiological maturity stage, isolated clusters of AP were larger with more branches and building blocks. In addition to the time-dependent discrepancies in the AP structure of developing starch, differences were also evident between large and small granules with regard to glucan trimming and the type of new chains produced. The clusters isolated from small starch granules were more tightly branched than those isolated from large granules
URI: http://hdl.handle.net/10214/6651
Date: 2013-05


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