Fig. 3.

Schematic of PIN-mediated auxin transport between two cells. Auxin (red circles) is transported from cell i to cell j by the efflux protein PIN (blue rectangles). In mathematical models, the concentration of auxin in cell i is denoted by $a_i$, and the concentration of PIN localised to the portion of cell i’s membrane which neighbours cell j is denoted $P_{\mathit{ij}}$. The flux of auxin from cell i to cell j is denoted by $J_a^{\mathit{ij}}$ and is assumed to positively feedback on the localisation of PIN to membrane portion ij between cells i and j (Color figure online)