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 [1]. Flowering plants evolved a double fertilization process wherein two immotile sperm cells are delivered to female gametes by the pollen tube, guided by elaborate communications between male and female reproductive organs [2-7]. 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-expressed protein of unknown function [8]. GEX2 is localized to the sperm membrane and contains extracellular immunoglobulin-like domains, similar to gamete interaction factors in algae and mammals [9, 10]. Using a new in vivo assay, we demonstrate that GEX2 is required for gamete attachment, in the absence of which double fertilization is compromised. Ka/Ks analyses indicate relatively rapid evolution of GEX2, like other proteins involved in male and female interactions [1, 3]. We conclude that surface proteins involved in gamete attachment and recognition exist in plants with immotile gametes, similar to algae and metazoans [11, 12]. This conservation broadens the repertoire of research for plant reproduction factors to mechanisms demonstrated in animals.