Spiral phyllotaxis in the moss Physcomitrium patens emerges from simple division rules of the apical cell.

In many plants, leaves are arranged around the stem in a pattern called Fibonacci spiral phyllotaxis. These patterns have been well studied in flowering plants and are thought to arise from a spacing mechanism based on the cell-to-cell transport of the plant hormone auxin. This causes new primordia to emerge as far as possible from previous ones in the available space on a multicellular meristem. However, it is not clear how a spacing mechanism can create spirals in plants with a unicellular meristem. Through time-lapse imaging, quantification, and computer modeling, we study the single tetrahedral apical stem cell of the moss Physcomitrium patens and the emergence of a spiral pattern of leaf-like structures. We find that the apical cell divides asymmetrically in a spiral pattern, giving rise to a leaf progenitor daughter cell and another apical cell; thus, phyllotaxis in P. patens is controlled by cell division orientation. Apical cell divisions are asymmetric both in fate and geometry, the latter being explained through displacement of the new wall from the centroid of the apical cell. Modeling shows that incorporation of displacement from the centroid with the default division plane selection by the minimal wall area ("shortest wall" rule) is sufficient to explain the spiraling division planes leading to phyllotaxis. Thus, the whole architecture of the shoot is defined by the orientation of cell division. Some cell types in flowering plants undergo a similar spiraling division plane pattern, suggesting that this may be a common mechanism across phyla.