Endosperm Morphogenesis in Wheat: Termination of Nuclear Division1
- A. G. Huber and
- D. F. Grabe2
The morphogenesis of endosperm formation between the 5th and 15th days postanthesis (PA) was studied in wheat (Triticum aestivum L.) by light microscopy. This study was necessary because of lack of information about endosperm cell formation during the period of cellularization of the upper embryo sac. Our objective was to make a description of the changes, visible by light microscopy, in the endosperm and surrounding tissues before the time nuclear divisions end. Developing caryopses were collected daily after anthesis, fixed, embedded in paraffin, sectioned, double stained, and examined under bright-field and phase-contrast light. By the fifth day PA, the dense endosperm cytoplasm in the lower embryo sac and in front of the nucellar projection had been separated into cells by walls growing in from the nucellus. After that time, endosperm cells in the crease of the kernel were distinguishable as transfer cells by their greater content of cytoplasm. Between the fifth and seventh days PA, the central vacuole of the embryo sac became partitioned into a netlike reticulum by the endosperm nuclei that had previously surrounded the upper embryo sac. The nuclei were separated by material that stained as protein. The nuclei continued rapid divisions through the ninth day PA; they were associated with very little cytoplasm, compared to the endosperm cells with dense cytoplasm in the lower embryo sac. Where space was available, nuclear divisions occurred both in the peripheral layer of endosperm and throughout the upper embryo sac. The maternal inner integument and nucellar epidermis, surrounding the embryo sac, developed cuticles and stopped expanding between the 9th and 12th days PA. Endosperm nuclear divisions stopped between the 12th and 14th days PA. The reticulum of nuclei was partitioned into nondividing cells by carbohydrate-staining walls growing inward from the maternal tissues. The aleurone layer of endosperm cells became distinguishable after nuclear divisions had ceased. Starch and protein formation was also observed only after endosperm nuclei stopped dividing. Four distinct cell types were observed during development of the wheat endosperm: (i) cells with dense cytoplasm of the lower embryo sac; (ii) transfer cells next to the crease; (iii) starch and protein storing cells formed by partitioning the upper embryo sac; and (iv) the outer layer of aleurone cells.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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