Starch is vital for humankind as a major source of calories in our diets, but we still do not fully understand the mechanism of starch biosynthesis in plants. Starch is synthesised in amyloplasts of the wheat endosperm as insoluble, semi-crystalline starch granules. Wheat and other Triticeae are unique in that they have a bimodal distribution of starch granules in the endosperm, containing large, lenticular “A-type” granules, and small spherical “B-type” granules.

These arise from a distinct spatiotemporal pattern of granule initiation during grain development, where A-type granules are initiated in each amyloplast at early developmental stages (by approx. 6-8 days post anthesis (dpa)), followed by B-type granule initiation at 15-20 dpa. We recently discovered that A- and B-type granule initiations occur via distinct biochemical mechanisms.

The initiation of A-type granules is mediated by Starch Synthase 4 (SS4), while B-type granules are initiated by the plastidial alpha-glucan phosphorylase (PHS1). Wheat mutants defective in SS4 produce highly aberrant A-type starch granules, while mutants defective in PHS1 have a dramatic reduction in the number of B-type granules. In addition to these proteins that control granule initiation, we discovered that the structure of the amyloplast plays an important role in determining the number and morphology of starch granules in the endosperm.

Using this knowledge, we can now manipulate granule size and shape in wheat to control the relative amounts of A- and B-type granules, and create diverse granule morphologies that resemble those typically found in other crops (e.g., rice and maize).

The discovery of these gene targets for modulating starch granule structure has major implications on improving the nutritional and functional qualities of starch.