Contact:
Dr Nikolaus Wellner
T: +44 (0)1603 255012
E: wellner@ifr.ac.uk

Granule structure of starch mutants

Starch granule structures are affected by starch synthesis mutations

Starch is one of the most important food components. It consists of two polysaccharides. Amylopectin is a highly branched molecule and it is considered to comprise the crystalline structure within the granule: the branches of the amylopectin molecules are present as radially-orientated crystalline lamellae. Amylose is an essentially linear molecule which is found predominantly in the amorphous regions within the starch granule. Mutants which affect the starch synthesis can alter the structures of these two components and their relative amounts. This has profound implications for the physical properties of the starches, and also for human nutrition, because many of the mutant starches are less digestible and thus give rise to a lower glycaemic index.

A method developed for in situ imaging of starch granule structure in dry seeds has been applied to compare the starch granule structures found in wild type and amylose extender (ae) mutant maize kernels. In the isogenic ae mutant the activity of the starch branching enzyme IIb is inhibited, which gives rise to a high amylose starch. The granule structures in the wild type samples have been found to be homogeneous, whereas those in the ae mutant are grossly heterogeneous within individual granules, between granules within individual cells, and between cells across the endosperm. The level of heterogeneity observed in situ appears to be more marked than that previously reported for studies on isolated ae mutant starches. Iodine/potassium iodide staining and polarised light microscopy have been used together with Raman microscopy, which has allowed high-resolution mapping of the composition and physical state of the structures within the granules, to probe the origins of the heterogeneity of the starch structures.

Although the mutation inhibits the activity of the branching enzyme within the granules, and both the composition and level of crystallinity within and between granules is variable, the major origin of the heterogeneity of the granule architecture appears to result from significant changes in the assembly and packaging of the crystalline structures within the granule. It is suggested that this arises due to the mutation of the starch branching enzyme introducing defects into the self-assembly of the crystalline structure, resulting in an accumulation of defects and increased randomisation of the granule structure.

N. Wellner, D.M.R. Georget, M.L. Parker, V.J. Morris, In situ Raman Microscopy of Starch Granule Structures in Wild Type and ae Mutant Maize Kernels. 2011, Starch, 63, 128-138.