Osmoregulation determines sperm cell geometry and integrity for double fertilization in flowering plants

Distinct from the motile flagellated sperm of animals and early land plants, the non-motile sperm cells of flowering plants are carried in the pollen grain to the female pistil. After pollination, a pair of sperm cells are delivered into the embryo sac by pollen tube growth and rupture. Unlike other walled plant cells with an equilibrium between internal turgor pressure and mechanical constraints of the cell walls, sperm cells wrapped inside the cytoplasm of a pollen vegetative cell have only thin and discontinuous cell walls. The sperm cells are uniquely ellipsoid in shape, although it is unclear how they maintain this shape within the pollen tubes and after release. In this study, we found that genetic disruption of three endomembrane-associated cation/H⁺ exchangers specifically causes sperm cells to become spheroidal in hydrated pollens of Arabidopsis. Moreover, the released mutant sperm cells are vulnerable and rupture before double fertilization, leading to failed seed set, which can be partially rescued by depletion of the sperm-expressed vacuolar water channel. These results suggest a critical role of cell-autonomous osmoregulation in adjusting the sperm cell shape for successful double fertilization in flowering plants.