Ultrastructure of the developing and fertilized embryo sac of Amaranthus hypochondriacus L.

Amaranthus hypochondriacus embryo sac development was investigated before a nd after fertilization. During the early stages of development, the young e mbryo sac displays three antipodal cells at the chalazal pole that degenera te very early in the maturation process, before the synergids and egg cell are completely differentiated. The mature embryo sac is composed only of th e female germ unit. The synergid cells organize a filiform apparatus accomp anied by the presence of mitochondria and dictyosomes with numerous vesicle s. The involvement of the synergids in transport and secretory functions re lated to pollen tube attraction and guidance, are discussed. The egg cell i s located at the micropylar pole near the synergids and displays exposed pl asma membranes at the chalazal pole. The fertilized egg cell does not exhib it marked changes after fertilization except for the closure of the cell wa ll. The central cell is the largest cell of this very long embryo sac. The fused nucleus is close to the egg apparatus before fertilization and displa ys a remarkable chalazal migration after gamete delivery. The ultrastructur e of the central cell cytoplasm and the numerous wall ingrowths around this cell suggest an important role in nutrient transportation. After gamete de livery, the embryo sac displays electron dense bodies that aggregate within the intercellular space between the synergids, egg cell and central cell. These bodies, that appear in the embryo sac of several plants, are probably involved in gamete delivery for double fertilization; The possibility of b iparental inheritance of mitochondria in this plant is also discussed. (C) 1999 Annals of Botany Company.