Three-dimensional visualization of tip-vesicles in growing pollen tubes by electron tomography

The life cycle of flowering plants starts when a zygote is formed following a double fertilization event. To achieve fertilization, sperm cells are delivered to the female gametes within the embryo sac by the tip-growing pollen tube. The fast-growing pollen tube is characterized and regulated by abundant transport vesicles responsible for both exocytosis and endocytosis in the tip region. Visualization of these tip-vesicles has been challenging owing to their small size, high dynamics, and complexity. In this study, we illustrated the three-dimensional (3D) ultrastructure of tip-vesicles in growing pollen tubes of lily, tobacco, and Arabidopsis. Five major types of tip-vesicles, including secretory vesicles (SVs), electron-dense vesicles (DVs), clathrin-coated vesicles (CCVs), mini vesicles (MVs), and extracellular vesicles (EVs), can be distinguished using room-temperature electron tomography (RT-ET) based on their ultrastructural features. We also demonstrated the extensive distribution of tubular endoplasmic reticulum (ER) structures at the apex of growing pollen tubes and vesicles budding from the tip-localized ER. Cryo-ET further revealed the tip-localized tubular ER with budding coat protein complex II (COPII) vesicles. Our study thus offered a structural basis for a deeper comprehension of vesicular trafficking in the tip growth of the pollen tube, aiding future research on vesicle-mediated membrane trafficking in polarized cell growth.