Fig. 1. Initiation of cellularisation is coupled to mitosis. (A-F) Time series visualising nuclear division and cellularisation in the PEN (see movie: http://dev.biologists.org/supplemental/). The in vivo progression of cellularisation was monitored by time-lapse confocal microscopy of living seeds from the GFP-expressing line KS22. GFP accumulates at sites of forming cell walls between sister nuclei (arrowheads). This series is part of an 8 hour recording of endosperm development. Frames were acquired every 10 minutes. Images in A and F mark the beginning and end of the series, respectively, and were recorded with a decreased acquisition rate to give better resolution. 200 minutes after the beginning of recording, a mitotic wave was initiated at the anterior pole (upper side of the section) and crossed the PEN (B,C). Immediately after mitosis cell plates are visible between sister nuclei (D, arrowhead) and slightly later between non-sister nuclei (E, arrowhead). Cellularisation appears to be complete 6 hours after the mitotic wave (F). (G-J) Detail of the formation of a cell plate between sister nuclei following the 8th mitotic cycle in the PEN observed in fixed material. (G) Metaphase, (H) anaphase, (I) telophase, and (J) the clear formation of a cell plate. Scale bar: 20 µm (A-F); 5 µm (G-J).