Figure 4. Auxin Transport Model of Lateral Root Initiation.

Polarly localized auxin export proteins (red arrows) pump auxin toward the quiescent center at the tip of the root, creating an auxin maximum there (blue). Auxin is then returned upwards by polarly localized auxin exporters in the outer layers of cells in the root. A portion of the auxin returns into the reflux stream due to lateral orientation of some of the exporters. In the model of Laskowski et al., bends in the root caused by anisotropic growth in the elongation zone induce a mini-reflux loop, causing auxin to accumulate preferentially on the outside of the curve. This induces cells to produce additional import carriers (small green arrows), which creates an auxin maximum (blue cells) by skimming auxin out of the transport stream. This auxin maximum then triggers lateral root formation.