Microtubules ensure transport of vegetative nuclei and sperm cells by fine-tuning their home positions

The pollen tube plays a pivotal role in double fertilization by delivering sperm cells (SCs) to the ovule. In Arabidopsis thaliana , a pair of SCs tightly connects with the vegetative nucleus (VN) to form the male germ unit (MGU), which is located in the apical region during pollen-tube growth, keeping the VN ahead of the SCs. MGU transport relies on independent motility of VN and SC pairs. However, the complexity of this dual motive force has hindered our understanding of MGU behavior, including its positioning and nuclear order. We used Arabidopsis mutants or transgenic plants that produced semi-motile MGUs with aberrant VNs or SCs to analyze the positioning of VN or SCs after stochastically disconnecting the MGU. In pollen tubes with an immotile SC pair, the VN was ∼70 μm away from the tip, whereas in pollen tubes with an immotile VN, the SC pair was ∼100 μm away from the tip, implying that the VN and SCs have independent home positions. The position of MGU moved forward owing to the loss of the microtubule-destabilizing kinesin KINESIN-13A . Conversely, microtubule depolymerization by oryzalin treatment or introducing mutations in TUBULIN BETA 4 ( TUB4 ) deregulated the position of the MGU and shifted its position backward. In addition, tub4 plants exhibited reduced fertility. These data indicate a significant role of microtubules in stable MGU positioning to ensure reproductive success.