Gamete attachment requires GEX2 for successful fertilization in Arabidopsis

Fertilization requires recognition, attachment and membrane fusion between gametes. In metazoans, rapidly evolving surface proteins contribute to gamete recognition and adhesion. Flowering plants evolved the double fertilization process where two immotile sperm cells are delivered to female gametes by the pollen tube, guided by elaborate communications between male and female reproductive organs. Once released, the sperm cells contact female gametes directly prior to gamete fusion. It remains unclear whether active gamete recognition and attachment mechanisms are required for double fertilization. Here, we provide functional characterization of Arabidopsis GAMETE EXPRESSED 2 (GEX2), which encodes a sperm specific protein of unknown function. GEX2 is located to sperm cells plasma membrane and contains extracellular immunoglobulin like domains, similar to gamete interaction factors in algae and mammals. Using a new in vivo assay, we demonstrate that GEX2 is required for gamete attachment, in absence of which double fertilization is compromised. Ka/Ks analyses indicate that GEX2 is under positive selection as other proteins involved in male and female interactions. We conclude that surface proteins involved in gametes recognition exist in plants with immotile gametes, similar to algae and metazoans. This unexpected conservation broadens the repertoire of research for plant reproduction factors to mechanisms demonstrated in animals.