KAKU4-mediated deformation of the vegetative nucleus controls its precedent migration over sperm cells in pollen tubes

A putative nuclear lamina protein, KAKU4, modulates nuclear morphology in Arabidopsis thaliana seedlings but its physiological significance is unknown. KAKU4 was strongly expressed in mature pollen grains, each of which has a vegetative cell and two sperm cells. KAKU4 protein was highly abundant on the envelopes of vegetative nuclei (VNs) and less abundant on the envelopes of sperm cell nuclei (SCNs) in pollen grains and elongating pollen tubes. VN is irregularly shaped in wild-type pollen. However, KAKU4 deficiency caused it to become more spherical. These results suggest that the dense accumulation of KAKU4 is responsible for the irregular shape of the VNs. After a pollen grain germinates, the VN and SCNs migrate to the tip of the pollen tube. In the wild type, the VN preceded the SCNs in 91–93% of the pollen tubes, whereas in kaku4 mutants, the VN trailed the SCNs in 39–58% of the pollen tubes. kaku4 pollen was less competitive than wild-type pollen after pollination, although it had an ability to fertilize. Taken together, our results suggest that controlling the nuclear shape in vegetative cells of pollen grains by KAKU4 ensures the orderly migration of the VN and sperm cells in pollen tubes. Highlight The nuclear envelope protein KAKU4 is involved in controlling the migration order of vegetative nuclei and sperm cells in pollen tubes, affecting the competitive ability of pollen for fertilization.