EMBRYO SAC LACKING ANTIPODAL CELLS IN ARABIDOPSIS-THALIANA (BRASSICACEAE)

Ultrastructure of the embryo sac lacking antipodals in prefertilizatio n stages in Arabidopsis thaliana has been examined 2 hr before and 5 h r after manual cross pollination. The cytoplasm of both synergids befo re fertilization is rich in ribosomes, mitochondria, and rough endopla smic reticulum, and also contains several microbodies and spherosomes. The filiform apparatus includes electron-dense material and a fibrous part. Many cortical microtubules appear in the filiform apparatus are a. One of the two synergids degenerates before fertilization. The syne rgids, the egg cell, and central cell have a rich cytoskeleton of micr otubules; only the synergids appear to contain microfilaments. At the chalazal end, the antipodals are initially present but degenerate by t he time of pollination in most embryo sacs in the starchless line stud ied. The embryo sac is completely surrounded by a wall containing an e lectron-dense layer, separating it from the nucellus, including the ch alazal end. When the antipodals have degenerated, the electron-dense l ayer disappears at the chalazal end only, and the wall between the cen tral cell and the nucellus is homogeneous. Between the central cell an d nucellar cells no plasmodesmata are found. The membranes of both ant ipodal cells at the chalazal end of the embryo sac appear sinuous, lik e those of transfer cells. The central cell has plastids preferentiall y distributed around the nucleus, but the other organelles are randoml y distributed. The central cell in the embryo sac and the adjacent cha lazal nucellar cells show a transfer-cell function in the embryo sac a fter the antipodals degenerate.