Early Stages in Wheat Endosperm Formation and Protein Body Initiation

Abstract

The early stages of endosperm formation and protein body initiation are described for hard red winter wheat using light and transmission electron microscopy. Two days after flowering (DAF) the endosperm was a thin layer of coenocytic cytoplasm lining the embryo sac. By 4 DAF the endosperm had cellularized and completely filled the embryo sac. Enough differentiation had occurred by 6 DAF to distinguish cells destined to become the aleurone layer, sub-aleurone region and central endosperm. Protein bodies were initiated at about 6–7 DAF and were first found near the Golgi apparatus. Wheat was ready for combine harvest at 34 DAF. Enlargement of the small protein bodies near the Golgi apparatus occurred by several mechanisms: (1) fusion with one or more of the dense Golgi vesicles or fusion with other protein bodies, (2) fusion with small electron-lucent Golgi-derived vesicles, (3) pinocytosis of a portion of the adjacent cytoplasm into the developing protein body and (4) fusion of large protein bodies with one another at later stages of grain development. Of the four mechanisms described, the pinocytotic vesicles and fusion of protein bodies were the most frequent and consistent processes observed. Direct connections between rough endoplasmic reticulum (RER) and protein bodies were not observed. The results suggest a rôle for the Golgi apparatus in the initiation of protein bodies. Also, the lack of RER derived vesicles suggests a soluble mode of secretion of storage proteins involved in the enlargement of protein bodies.