Atomic force microscopy of pea starch: Granule architecture of the rug3-a, rug4-b, rug5-a and lam-c mutants

Atomic force microscopy has been used to visualise the internal structures of sectioned, encased starch granules isolated from near-isogenic pea starch mutants (rug3-a, rug4-b, rug5-a, and lam-c). A mutation at the locus rug4 was found to have little effect on the granule ultrastructure. However, mutations at rug3 and lam, which give rise to low-amylose starches, led to granules that showed banding (growth rings) in which individual blocklets could not easily be seen. High-amylose (rug5) mutants formed granules ranging in shape from simple ellipsoids through to quite complex, convoluted structures. The internal granule structure was found to be heterogeneous. In some regions, normal banding was visible and the underlying ‘hard’ blocklets were embedded in a ‘soft’ matrix. In other regions of the granule, the banding structure was absent and the matrix in which the blocklets are embedded contained a fine hard network structure. It is proposed that this fine, hard structure is due to the presence of a crystalline amylose network.