FIGURE 2 | Molecular mechanism of auxin polar transport.

Asymmetrically transported auxin-efflux carriers (PIN proteins) cause a polarized flow of auxin, which leads to the formation of continuous columns of procambial cells. AtPIN1 is recycled in cells — it is transported from endomembranes to the plasma membrane by endosome-like AtPIN1-specific vesicles (AtPINSVs). A guanine-nucleotide-exchange factor for the ADP-ribosylation factor G protein, GNOM/EMB30, and the serine/threonine kinase PINOID are thought to promote the asymmetrical transport of AtPINSVs by the activation of ADP-ribosylation factor and the phosphorylation of an unknown component of AtPINSVs, respectively. This process might be initiated by the natural auxin indoleacetic acid (IAA) to create a positive-feedback loop of IAA flow, which is consistent with the IAA-flow canalization model.

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