Experimental Obliteration of the Preprophase Band Alters the Site of Cell Division, Cell Plate Orientation and Phragmoplast Expansion in Adiantum Protonemata

Division sites in higher plant cells are known to be determined before mitosis, and cell plates are precisely inserted into the predetermined division sites at the end of cytokinesis by unknown mechanisms. When apically growing protonemal cells of the fern, Adiantum capillus-veneris L., grown under red light are transferred to the dark, apical growth ceases and the protonemal cells then divide. However, this mitotic event can be influenced by subsequent exposure to light. If red-light pre-cultured protonemata are incubated in the dark and then transferred back to red light (after 28–36 h), apical growth resumes and the nuclei migrate toward the growing tips; interestingly, mitosis still occurs, although in an altered fashion. In the re-irradiated cells, timing of the premitotic nuclear positioning is delayed markedly and irregularly oriented cell plates are frequently observed. Re-irradiation with red light also causes an increase in cells without preprophase bands (PPBs) at prophase and the irregular expansion of the phragmoplast at late telophase, while early phragmoplast microtubule (MT) organization takes place normally. These data suggest the indirect involvement of PPBs in the guidance of phragmoplast expansion.