The ultimate goal for the survival of all species on earth is to reproduce. This uncompromising principle has triggered the evolution of numerous adaptations. One strategy commonly employed by sexually reproducing eukaryotes is the production of tremendous amounts of sperm to maximize the likelihood of an egg becoming fertilised. High sperm to egg ratios are, however, associated with an increased risk of supernumerary sperm fusion. This so-called polyspermy is lethal in many organisms. Accordingly, eukaryotes have evolved polyspermy barriers, which are implemented at different levels in the reproductive process. Flowering plants tightly control the number of sperm-transporting pollen tubes approaching a single ovule by a so-called pollen tube block. We have recently shown that the pollen tube block is relaxed in ethylene hyposensitive plants. Capitalizing on these results, this project aims at identifying and characterising the molecular mechanisms underlying plant polyspermy barriers.