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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.

Barley spikes of the cultivars/breeding lines Gustav, Karmosé and SLU 7 were harvested at 9, 12 and 24 days after flowering in order to study starch structure in developing barley endosperm. Kernel dry weight, starch content and amylose content increased during development. Structural analysis was performed on whole starch and included the chain-length distribution of the whole starches and their β-limit dextrins. Karmosé, possessing the amo1 mutation, had higher amylose content and a lower proportion of long chains (DP ≥38) in the amylopectin component than SLU 7 and Gustav. Structural differences during endosperm development were seen as a decrease in molar proportion of chains of DP 22-37 in whole starch. In β-limit dextrins, the proportion of Bfp-chains (DP 4-7) increased and the proportion of BSmajor-chains (DP 15-27) decreased during development, suggesting more frequent activity of starch branching enzymes at later stages of maturation, resulting in amylopectin with denser structure., (Copyright © 2015 Elsevier B.V. All rights reserved.)

In this study, starches extracted from wheat grains harvested at 7, 14, 28, and 35 days after anthesis (DAA) were used as a means of examining the molecular structure of amylopectin (AP) from developing wheat grain. Scanning electron microscopy of wheat grain cross-sections revealed the presence of endosperm at 7 DAA and contained lenticular-shaped developing large (A-type) granules. From 14 DAA onward, spherical-shaped small (B-type) granules coexisted with large granules in the endosperm. During granule development, the fine structure of AP varied with maturity in both large and small granules. Towards the end of the pre-physiological maturity stage (28 DAA), AP in small and large granules had shortest external chain length (ECL), longest internal chain length (ICL) and lowest amount of A-chains. At physiological maturity (35 DAA), these changes in ECL, ICL and amount of A-chains were reversed when compared to 28 DAA. In both large and small granules, the external AP structure was apparently more organized at physiological maturity than at pre-physiological maturity., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Changes in internal structure of amylopectin (AP) during wheat endosperm development were studied by isolating clusters and building blocks of AP from both large A-type and small B-type starch granules at different maturity stages up to harvest time at 49 days after anthesis (DAA). Clusters isolated from B-type granules had a degree of branching (DB) of 16.5-16.8% and were more tightly branched than those isolated from A-type granules (DB 15.7-16.2%). The degree of polymerization (DP) of the clusters increased in both types of granules during the pre-physiological maturity stage up to 28 DAA. Clusters at maturity were smaller with less branches and building blocks than at the end of the pre-maturity stage. It is suggested that this was due to a continuous trimming of the cluster structure after the active period of starch synthesis. Differences were evident between A- and B-type granules with regards to glucan trimming and the type of new chains produced., (Copyright © 2014 Elsevier Ltd. All rights reserved.)