Amyloplast formation and starch granule development in hard red winter wheat

Plastids in the coenocytic endosperm of young wheat caryopses were mostly in the form of pleomorphic proplastids with a few of the plastids containing small starch granules. Following cellularization of the coenocytic cytoplasm, the outer one or two endosperm cell layers became meristematic and continued to divide until about 14 days after flowering (DAF). During the first week of endosperm development, newly divided cells had plastids that were pleomorphic in shape, while subaleurone cells interior to the meristernatic region contained amyloplasts that contained a single-size class of starch granules (incipient A-type starch granules). The pleomorphic plastids exhibited tubular protrusions that extended a considerable distance through the cytoplasm. Amyloplasts in cells interior to the meristernatic region did not exhibit protrusions. Both subaleurone and central endosperm cells had amyloplasts that exhibited protrusions at 10-12 DAF, and some of the protrusions contained small starch granules (incipient B-type starch granules). Protrusions were not observed in endosperm amyloplasts at 14 DAR Two sizes of starch, large A-type and smaller B-type granules were present within the cells, however. Amyloplast protrusions were numerous again at 17 DAF in both subaleurone and central endosperm cells; at 21 DAF, a third size class of small C-type starch granules was observed in the cytoplasm. Amyloplasts in the endosperm of wheat apparently divided and increased in number through protrusions because binary fission typical of plastid division was never observed. Protrusions were observed in the coenocytic cytoplasm, in dividing cells, in subaleurone and central endosperm cells at 10-12 DAF, and in subaleurone and central endosperm cells at 17 DAR The results suggest that there are three sizes of starch granules produced at specific times during wheat endosperm development.