Figure 1.

Models of auxin transport in the shoot apex. (A) At the cell level, auxin is actively transported by PIN (efflux) and AUX (influx) proteins, in addition to the natural influx of protonated auxin. Abbreviation: IAAH, indole-3-acetamide hydrolase. The polar localisation of PIN on the membranes is believed to be auxin-dependent, but the exact mechanism is unknown, as indicated by the question mark. Black arrows represent chemical reactions (thickness indicates relative rates). Coloured arrows represent transport of the substance bearing the same colour. (B) Experimental data, which often consists of microscope images of fluorescent reporters for auxin response and/or antibody-based localisation of PIN subcellular distribution (in red), are incorporated into multicellular computational models (C) to make predictions about auxin distribution patterns (denoted in green). In (C) left, white arrows represent PIN polarity, green circles are auxin sources, and blue triangles auxin sinks. In (C) right, white dots mark cells from the central zone. (B) and (C) are reproduced, with permission, from [25], indicated by (*) and [27], indicated by (**).