A Plausible Model of Phyllotaxis Patterning from a Single Apical Cell

Abstract

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 multicellular meristems. However, it is not clear how a spacing mechanism can create spirals in a plant 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. Phyllotaxis, and ultimately the whole architecture of the shoot, is thus defined by the orientation of this cell division. The default division plane selection in plants (the 'shortest wall rule') is to choose a wall of minimal area through the cell centroid. Modeling shows that dividing the cell with a wall of minimal area through a displaced centroid is sufficient to explain the division asymmetry, as well as the division plane patterns leading to spiral phyllotaxis. Some cell types in flowering plants undergo a similar spiraling division plane pattern, suggesting this may be a common mechanism.

Keywords: Phyllotaxis, Apical Cell, Nuclear Movement, Shortest Wall, Division Plane, 3D Model