Dynamics of Cell Membrane and Cell Wall Development during Generative Cell Engulfment by the Pollen Tube Cell in Liriope muscari

Abstract

During the ontogeny of angiospermous pollen, a small generative cell (GC) detaches from the inner pollen wall (intine) and is engulfed within the cytoplasm of the pollen tube cell (PC). This engulfment process is considered to be a type of endocytosis; however, the type is currently unknown. We investigated morphological changes in the cell membrane and cell wall, and the dynamics of cell wall ingredients and cytoskeletal elements, during GC engulfment by the PC in Liriope muscari pollen. Three-dimensional imaging using field-emission scanning electron microscopy revealed that the GC membrane detached from the intine in all directions. Simultaneously, the GC became spherical, and the PC membrane progressed centripetally while covering the GC detachment area. Finally, the GC was released into the PC cytoplasm, and the exfoliated area was completely covered by the PC membrane. Immediately after microspore division, the cell wall between the GC and PC contained methyl-esterified pectin and callose, which subsequently disappeared, thereby thinning the cell wall. During GC engulfment, actin filaments were distributed throughout the PC, particularly in the GC detachment area. Myosin was distributed around the GC and granules immediately after microspore division, until the completion of GC engulfment. Cytochalasin B prevented GC engulfment. These results suggest that cell wall thinning through the degradation of pectin and callose is important in GC engulfment and that actin filaments are involved, similar to their role in endocytosis in animal cells.