Preprophase-band positioning in isolated tobacco BY-2 cells: evidence for a principal role of nucleus-cell cortex interaction in default division-plane selection.

In some plant tissue types, new cross-walls tend to divide parental cells equally and to meet parental walls at right angles while tending to have minimal surface area. A previously proposed model that I call the reach model suggests that this feature originates from the tendency of premitotic division-plane selection or of the positioning of microtubule preprophase bands (PPBs) which predict the cortical division site, and that default division-plane selection involves nuclear centering and subsequent PPB microtubule assembly on the cell wall parts closest to the nucleus. In an initial effort to characterize truly default division-plane selection, the present study quantified division orientation and PPB positioning in protoplast-derived isolated elongate tobacco BY-2 cells. In this system, PPB-predicted and actual division planes were mostly oriented transversely, as predicted based on the reach model. Some sample elongate cells had asymmetric shapes that came from clear terminal-size differences and, in those cells, PPB-marked planes tended to be displaced from the centers of centrally located nuclei toward the narrower cell end, again as predicted based on the reach model. Such PPB positioning typically forecasted volumetrically asymmetric transverse division that would produce a smaller daughter cell from a parental cell part including the narrower cell end. These results provide experimental evidence that default division-plane selection tends to be close to or the same as the selection using the reach model's criterion, and that it does not use any criterion that specifically prioritizes the equality or verticality of division.