Cytokinesis in the developing wheat grain; Division with and without a phragmoplast.

Cell wall formation during the transition from free-nuclear to cellular endosperm of wheat (Triticum aestivum L. cv. Heron) was investigated using correlated light and electron microscopy. Partitioning of the multinucleate syncytium that lines the inner periphery of the embryo sac is initiated 1-2 days after anthesis. Wall ingrowths, at first recognizable in sections as minute wall pegs, furrow inward from the edge of the embryo sac through the vacuolate cytoplasm which, to the inside, is clearly delimited by the central vacuole. Growth of the walls at this stage is independent of a phragmoplast and in this respect is reminiscent of the cleavage processes of lower plant cells. Intense fluorescence of the walls after staining with aniline blue suggests that callose may be a principal component. The growing walls branch and eventually meet on the side nearest the central vacuole. Cellularization of the peripheral layer of endosperm cytoplasm is thus complete about 2 days after anthesis. Between 2 and 3 days after anthesis, the peripheral layer of cells commences to divide both radially and tangentially and by 4 days the entire embryo sac is cellular. Cytokinesis during this phase entails the formation of a cell plate between sister nuclei. At the periphery of a forming cell plate, "vesicles" appear scattered amongst an array of phragmoplast microtubules. This mechanism of wall growth differs markedly from the initial infurrowing of the first-formed walls. The overall timing and the manner of cell wall deposition vary in a number of important respects from the model recently proposed by Mares et al. whose work was based largely on light microscopy (D.J. Mares; K. Norstog; A.B. Stone: Aust. J. Bot. 23, 311-326, 1975).