Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 11, 2002
Publication Date: March 1, 2003
Citation: Bechtel, D.B., Wilson, J.D. 2003. Amyloplast formation and starch granule development in hard red winter wheat. v80:175-183. Cereal Chemistry. Interpretive Summary: Although starch comprises up to 62% by mass of the wheat grain, little is known about how starch forms within the seed. We used transmission electron microscopy to follow starch formation in organelles called amyloplasts in developing wheat from the day of flowering through grain maturation. Amyloplasts increased in number at specific times during wheat grain development and resulted in three distinct size classes of starch granules at maturity. Amyloplasts increased in number via a process in which protrusions emanated from the amyloplasts. New starch granules were initiated within the protrusions. The research will provide a means for identifying and predicting starch size distributions for specific wheat end-use qualities, such as, rheological properties, baking characteristics, processing parameters and its adaptation in non-food uses.
Technical Abstract: 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 endosperm outer layer or two 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 meristematic region contained amyloplasts that contained a single size class of starch granules (type A starch granules). The pleomorphic plastids exhibited protrusions that extended a considerable distance through the cytoplasm. Amyloplasts in the interior to the meristematic region did not exhibit the protrusions. Both subaleurone and central endosperm cells had amyloplasts that exhibited protrusions at 10-12 DAF, with some protrusions containing small starch granules. By 14 DAF, endosperm amyloplasts lacked protrusions and two sizes of starch, large type A and small type B granules, were present in the cells. Amyloplast protrusions were numerous again at 17 DAF in both subaleurone and central endosperm cells and by 21 DAF a third size class of small type C starch granules was present in the cytoplasm. Amyloplasts in the endosperm of wheat apparently divided and increased in number via protrusions, since 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 DAF. The results suggest that there are three sizes of starch granules produced at specific times