Ultrastructural examination of embryogenesis inCrepis capillaris (L.) Wallr.: 1. The synergid before and after pollination

Abstract

The ultrastructure and composition of the mature synergids in the embryo sac ofCrepis capillaris (L.) Wallr. before and after the pollination were studied. Two symmetrical synergids occupy the micropylar portion and show evidence of a high degree of differentiation: large numbers of dictyosomes and remarkably, high level of RER, which appears swollen and fragmented, characterize the cytoplasm of the synergids and distinguish them from the egg and the central cell. All other cytoplasmic organelles are embedded in these assemblies of ER and dictyosomes. After pollination, one of the two synergids degenerates before the arrival of the pollen tube at the embryo sac. The first sign of the degeneration is the darkening and loss of definition of the membrane systems of one of the synergids. Staining by the silver methenamine reaction reveals many electron-translucent, poorly defined structures. The membranous aspects of these structures can subsequently be visualized by UA-Pb staining. The denaturation progresses and the pollen tube always enters the degenerating rather than the intact synergid. These data imply that the preliminary denaturation of one synergid is required before the arrival of the pollen tube and that highly differentiated ultrastructure, with abundant RER and dictyosomes, is somehow related to the synthesis or the secretion of some protein or other product(s) that subsequently prompts the entrance and discharge of the pollen tube.