image: A simplified model describing the mutual gamete activation during double fertilization in Arabidopsis. Before fertilization EC1 proteins accumulate in secretory vesicles of the egg cell (A). Likewise, the sperm-expressed HAP2/GCS1, a putative fusogen, is mainly endomembrane-associated but not present on the sperm surface before the sperm cells reach the site of gamete fusions. Regulated secretion of EC1 by the egg cell takes place when the sperm cells arrive at the site of gamete fusions (B). Secreted EC1 proteins activate the sperm endomembrane system (B, C) and induce the re-distribution of HAP2/GCS1 to the plasma membrane (C). The triggered secretion of EC1 by the egg cell upon sperm-egg interaction may therefore ensure the appropriate localization of the cell-fusion machinery in distinct sperm membrane domains to achieve full fertilization competence and to accomplish sperm-egg fusion (D). This image relates to a paper that appeared in the Nov. 23, 2012, issue of Science, published by AAAS. The paper, by Stefanie Sprunck at University of Regensburg in Regensburg, Germany, and colleagues was titled, "Egg Cell–Secreted EC1 Triggers Sperm Cell Activation During Double Fertilization." view more
Credit: Image courtesy of Stefanie Sprunck