Abstract

One pattern generated by the shoot apical meristem of flowering plants has held a fascination for generations of biologists and mathematicians. This is the phyllotactic pattern, the pattern of leaves and flowers around the stem. The most common such pattern is the spiral phyllotactic pattern, which creates the highly recognizable organization of compound fruits such as pineapples, of flowers like roses, and of inflorescences such as sunflowers. The model plant Arabidopsis thaliana also has a spiral phyllotaxis, and we have used genetic, genomic, and cell biological approaches to learn in detail how the cells of the meristem collaborate to generate this pattern.

The major chemical signal is auxin, which has a specific transport system, in which a family of plasma membrane proteins directs the efflux of the hormone from cells, while both diffusion and a group of importers directs influx. The efflux proteins are not uniformly distributed, hence they cause efflux directionally, leading to a net flow of auxin in complex patterns across the surface of the meristem. Auxin not only induces new primordia of leaves and flowers, but also changes the physical properties of the cell wall. These physical changes alter the stress pattern in the meristem surface, which in turn regulates the position of the auxin efflux carrier in anisotropically stressed cells. The feedback between auxin concentration and physical stress creates the dynamic auxin patterns that cause successive auxin peaks at positions approximately 130-140 degrees around the stem, creating the spiral (and other patterns of) phyllotaxis. The stress pattern in the meristem may also determine aspects of cell wall synthesis, direction of cell growth, and plane of cell division.