Early Stages in the Development of Wheat Endosperm. II. Ultrastructural Observations on Cell Wall Formation

Abstract

In the first 4-5 days after anthesis, the central cell of the wheat embryo sac undergoes transformation from a multinucleate syncytium to the cellular endosperm. This is accomplished initially by the centripetal growth of wall projections from the central cell wall and the formation of cylindrical, highly vacuolate alveoli. Growth is mediated through the production and planar aggregation of vesicles at the distal tip of the developing projections. The innermost ends of the alveoli are closed by a thin layer of cytoplasm which is bounded on the inner side by the vacuolar membrane of the central cell. Cell wall material is not found in this thin layer of cytoplasm and the alveoli therefore are not complete cells. Following the division of the alveolar nucleus a cross-wall is laid down between the daughter nuclei by a process which is similar to normal cytokinesis and a layer of endosperm cells is formed from the peripheral portions of the alveoli. This pattern of centripetal growth of alveoli and the formation of complete cells from the proximal portions continues until cellularization is completed by the confluence of alveoli originating from opposite sides of the central cell. Further growth of the cellular endosperm is accomplished by the meristematic activity of the peripheral layer of cells.

The ultrastructure of the early stages of partitioning of the central cell is discussed in relation to current views on the ontogeny of wheat endosperm.