Initiation of B-type starch granules in wheat endosperm requires the plastidial α-glucan phosphorylase PHS1

PHS1 is a plastidial -glucan phosphorylase that can elongate and degrade maltooligosaccharides (MOS), but its exact physiological role in plants is poorly understood. Here, we discover a specialised role of PHS1 in establishing the unique bimodal characteristic of starch granules in the wheat endosperm. Wheat endosperm contains large A-type granules that initiate at early grain development, and small B-type granules that initiate in later grain development. We demonstrate that PHS1 interacts with BGC1 - a carbohydrate-binding protein essential for normal B-type granule initiation. Mutants of tetraploid durum wheat deficient in all homeologs of PHS1 had normal A-type granules, but fewer and larger B-type granules. Grain size and starch content were not affected by the mutations. Further, by assessing granule numbers during grain development in the phs1 mutant, and using a double mutant defective in both PHS1 and BGC1, we demonstrate that PHS1 is exclusively involved in B-type granule initiation. The total starch content and number of starch granules per chloroplast in leaves were not affected by loss of PHS1, suggesting that its role in granule initiation in wheat is limited to the endosperm. We therefore propose that the initiation of A- and B-type granules occur via distinct biochemical mechanisms, where PHS1 plays an exclusive role in B-type granule initiation.