A putative component protein of the nuclear lamina, KAKU4, modulates nuclear morphology in Arabidopsis thaliana seedlings but its physiological significance is unknown. KAKU4 was highly 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. VNs are irregularly shaped in wild-type pollen. However, KAKU4 deficiency caused them to become more spherical. After a pollen grain germinates, the VN and sperm cells enter and move along the pollen tube. In the wild type, the VN preceded the SCNs in more than 90% of the pollen tubes, whereas in kaku4 mutants, the VN preceded the SCNs in only about half of the pollen tubes. kaku4 pollen was less competitive for fertilization than wild-type pollen after pollination. These results lead us to hypothesize that the nuclear shape in vegetative cells of pollen grains affects the orderly migration of the VN and sperm cells in pollen tubes.