Kinesins control male germ unit assembly for sperm delivery in Arabidopsis

Unlike flagellated sperm in animals, which use molecular motors for motility, immotile sperm cells of angiosperms rely on cytoplasmic transport within pollen tubes to reach female gametes for fertilization. However, the mechanism underlying sperm cell transport in angiosperms remains unknown. Since the 1970s it has been observed that the two sperm cells, or their progenitor generative cell, are transported together with the pollen vegetative nucleus as part of an aggregated structure called the male germ unit, which forms within the pollen cytoplasm. Here, using super-resolution and live-cell imaging, we show that two kinesins, HUG1 and HUG2, form a kinesin cage encasing a microtubule cage around the generative cell or sperm cells and vegetative nucleus, tethering them into a single unit during Arabidopsis pollen development. Loss of HUG proteins disrupts male germ unit organization, leading to failed sperm delivery and complete plant sterility. These findings uncover the genetic and cellular basis of male germ unit organization and highlight its essential role in sperm transport for plant fertilization.