Figure 5.

The auxin autocatalytic cycle. While Turing proposed a model based on the interaction between simple diffusion gradients of two morphogens, we suggest the interaction of three ions: PIN proteins boost the uphill diffusion of auxin against the concentration gradient while protons the fastest diffusing ions, dissociate AGP-Ca²⁺ thus enhancing exocytosis of PIN proteins (and Ca²⁺channels) that propagate the Ca²⁺ message. This is summarized by the canalization theory [12] in which “small local differences in auxin concentration are amplified by a self-reinforcing accumulation mechanism, resulting in local auxin elevation and auxin depletion in the surrounding tissue.”