Figure 1.

Auxin feedback on PIN levels and polarity as a driver of plant auxin patterning. A, Phase plane depiction of a two-variable auxin PIN model (for more details see e.g. Van Berkel et al., 2013). A, Positive nonlinear feedback from auxin on PIN levels results in bistability, with a stable low auxin-low PIN equilibrium (lower left black dot) separated from a stable high auxin-high PIN equilibrium (upper right black dot) by an instable intermediate auxin-intermediate PIN equilibrium (middle dot). B, In models, up-the-gradient polarization of PINs results in the regularly spaced auxin maxima observed during phyllotaxis. C, In models, with-the-flux polarization of PINs results in the formation of auxin canals observed during venation. D, Paciorek et al. (2005) reported a negative effect of auxin on PIN endocytosis. If the auxin effect is exerted by external, apoplastic auxin this results in a positive feedback and PIN polarization. E, Narasimhan et al. (2021) instead reported a positive, PIN2-specific effect on endocytosis. Here, to obtain a positive feedback between auxin and PIN that results in PIN polarization, the auxin effect would need to be exerted by an internal auxin.