Kinesin proteins HUG1 and HUG2 are essential for the formation and transportation of male germ units in Arabidopsis pollen tubes.

Flowering plants rely on double fertilization for their sexual reproduction. A pair of immotile sperm cells (SCs) are conveyed through a pollen tube into the female gametophyte, where they fertilize the egg cell and central cell to give rise to the embryo and endosperm in seeds, respectively. The SCs and the pollen vegetative nucleus (VN) move as a male germ unit (MGU) in pollen tubes. The tryptophan-proline-proline (WPP) domain-interacting tail-anchored proteins (WITs) and WPP domain-interacting proteins (WIPs) are involved in VN migration, and the cytoskeleton is required for MGU transportation in pollen tubes. Here we report that two kinesins, referred to as HUG1 and HUG2, localize at the VN envelope in a WIT- and WIP-dependent manner and surround the SCs in pollen. Mutation of HUG1 and HUG2 leads to disconnected VN and SCs, impaired MGU transportation and reduced plant fertility, supporting key roles of HUG proteins in MGU formation and transportation in pollen.