Early Events in the Penetration of the Embryo Sac in Torenia fournieri(Lind.)

Abstract

At maturity, Torenia fournieri(Lind.) has an embryo sac which protrudes through the micropyle placing the synergids, egg cell and part of the central cell within the ovary locule adjacent to the placenta. The present study utilized this unique attribute in combination with confocal and light microscopy to characterize the timing and associated structural changes during pollination events leading to double fertilization. The observation of sperm nuclei in living gametophyte tissue is an important advance in the identification, in real time, of stages leading to fertilization in angiosperms. A continuum of fertilization occurred between 12 and 16h after pollination (hap), with peak frequency of egg and sperm fusion at 14 hap (43%). Movement of the sperm cells through the degenerated synergid took several hours and fusion between sperm and their respective female nuclei occurred simultaneously. Changes in embryo sac structure were also documented. Cell walls in the region between the synergids and egg cell were poorly developed prior to pollen tube penetration. Thickened cell walls were observed around the periphery of the synergids and egg cell following pollination, and in the central cell where it lay within the body of the ovule. Starch was observed in the cells of the embryo sac, although the number and distribution of granules varied before and after pollination. These temporal and spatial observations of the embryo sac inTorenia fournieri provide a basis for further research to determine control mechanisms operating during specific double fertilization events in angiosperms.