We have investigated the process of somatic-type cytokinesis in Arabidopsis (Arabidopsis thaliana) meristem cells with a three-dimensional resolution of ∼7 nm by electron tomography of high-pressure frozen/freeze-substituted samples. Our data demonstrate that this process can be divided into four phases: phragmoplast initials, solid phragmoplast, transitional phragmoplast, and ring-shaped phragmoplast. Phragmoplast initials arise from clusters of polar microtubules (MTs) during late anaphase. At their equatorial planes, cell plate assembly sites are formed, consisting of a filamentous ribosome-excluding cell plate assembly matrix (CPAM) and Golgi-derived vesicles. The CPAM, which is found only around growing cell plate regions, is suggested to be responsible for regulating cell plate growth. Virtually all phragmoplast MTs terminate inside the CPAM. This association directs vesicles to the CPAM and thereby to the growing cell plate. Cell plate formation within the CPAM appears to be initiated by the tethering of vesicles by exocyst-like complexes. After vesicle fusion, hourglass-shaped vesicle intermediates are stretched to dumbbells by a mechanism that appears to involve the expansion of dynamin-like springs. This stretching process reduces vesicle volume by ∼50%. At the same time, the lateral expansion of the phragmoplast initials and their CPAMs gives rise to the solid phragmoplast. Later arriving vesicles begin to fuse to the bulbous ends of the dumbbells, giving rise to the tubulo-vesicular membrane network (TVN). During the transitional phragmoplast stage, the CPAM and MTs disassemble and then reform in a peripheral ring phragmoplast configuration. This creates the centrifugally expanding peripheral cell plate growth zone, which leads to cell plate fusion with the cell wall. Simultaneously, the central TVN begins to mature into a tubular network, and ultimately into a planar fenestrated sheet (PFS), through the removal of membrane via clathrin-coated vesicles and by callose synthesis. Small secondary CPAMs with attached MTs arise de novo over remaining large fenestrae to focus local growth to these regions. When all of the fenestrae are closed, the new cell wall is complete. Few endoplasmic reticulum (ER) membranes are seen associated with the phragmoplast initials and with the TVN cell plate that is formed within the solid phragmoplast. ER progressively accumulates thereafter, reaching a maximum during the late PFS stage, when most cell plate growth is completed.

Summary: Cervical cancer is the second most prevalent cancer in women worldwide. It is linked to infection with human papillomavirus (HPV). As the virus cannot be propagated in culture, vaccines based on virus‐like particles have been developed and recently marketed. However, their high costs constitute an important drawback for widespread use in developing countries, where the incidence of cervical cancer is highest. In a search for alternative production systems, the major structural protein of the HPV‐16 capsid, L1, was expressed in tobacco chloroplasts. A very high yield of production was achieved in mature plants (~3 mg L1/g fresh weight; equivalent to 24% of total soluble protein). This is the highest expression level of HPV L1 protein reported in plants. A single mature plant synthesized ~240 mg of L1. The chloroplast‐derived L1 protein displayed conformation‐specific epitopes and assembled into virus‐like particles, visible by transmission electron microscopy. Furthermore, leaf protein extracts from L1 transgenic plants were highly immunogenic in mice after intraperitoneal injection, and neutralizing antibodies were detected. Taken together, these results predict a promising future for the development of a plant‐based vaccine against HPV.

The cell plate of higher plants is formed within a ribosome-excluding cell plate assembly matrix. Phragmoplast microtubules facilitate cell-plate formation by forming a scaffold that directs Golgi-derived vesicles to the forming cell plate. Here, we analyse the effects of the cell-plate assembly matrix on phragmoplast microtubule plus-end geometry by electron tomography of cryogenically fixed Arabidopsis meristem cells. Five distinct microtubules plus-end geometries are seen - blunt, extended, horned, flared and hybrid extended/horned. We have quantified and mapped these types of plus-end morphology during the different stages of cell-plate formation and analysed the effects of cell-plate assembly matrix association on microtubule plus-end morphologies. Our results show that somatic-type phragmoplast microtubules do not interdigitate at the cell plate mid-line. The cell-plate assembly matrix is shown to stabilize microtubule plus ends, as evidenced by the fact that of these microtubules that do not terminate in such a matrix, 40-80% are horn-shaped (shrinking), whereas of those that end in such a matrix, 50-70% are blunt (metastable). Also, a third of the blunt-ended microtubules within the cell-plate assembly matrix end at a distance of ∼30 nm from the cell plate.