Fig. 6.

Bud activation at the cellular level: simulations (A–F) and in planta data (G and H). (A) Schematic representation of a cell. Auxin concentration is shown as the intensity of blue, PIN as the intensity of red, and the dominant direction and magnitude of auxin flux is indicated by a black arrow. (B–F) Selected stages of the simulation. A longitudinal section of a stem with 2 buds is represented schematically as a grid of cells, approximating the shape of the section. Source cells are outlined in green; the sink is outlined in red. In the initial state there is small background production of auxin in every cell, and a single sink cell is present at the base (B). Following the placement of an auxin source at the top of the main stem, a vascular strand running through the stem emerges (C). Subsequent placement of auxin sources in the 2 buds does not trigger formation of lateral vasculature (D) until the auxin source at the top of the main stem is removed (E). Removal of the source in the upper bud causes the lower bud to be activated (F). (G and H) PIN1 localization in Arabidopsis axillary bud stems. Hand sections through the stems of inhibited (G) and active (H) axillary buds carrying the PIN1 protein fused to GFP under the control of the native PIN1 promoter. Arrows indicate an example of basally localized GFP in a file of cells. (Scale bar: 50 μm.)