The intercellular transport of auxin is driven by PIN-formed (PIN) auxin efflux carriers. PINs are localized at the plasma membrane (PM) and on constitutively recycling endomembrane vesicles. Therefore, PINs can mediate auxin transport either by direct translocation across the PM or by pumping auxin into secretory vesicles (SVs), leading to its secretory release upon fusion with the PM. Which of these two mechanisms dominates is a matter of debate. Here, we addressed the issue with a mathematical modeling approach. We demonstrate that the efficiency of secretory transport depends on SV size, half-life of PINs on the PM, pH, exocytosis frequency and PIN density. 3D structured illumination microscopy (SIM) was used to determine PIN density on the PM. Combining this data with published values of the other parameters, we show that the transport activity of PINs in SVs would have to be at least 1000× greater than on the PM in order to produce a comparable macroscopic auxin transport. If both transport mechanisms operated simultaneously and PINs were equally active on SVs and PM, the contribution of secretion to the total auxin flux would be negligible. In conclusion, while secretory vesicle-mediated transport of auxin is an intriguing and theoretically possible model, it is unlikely to be a major mechanism of auxin transport in planta.

• Keywords
• 3D-SIM microscopy, PIN transporters, auxin, mathematical modeling, polar auxin transport, secretion,
• MeSH
• Arabidopsis metabolism   MeSH
• Models, Biological * MeSH
• Biological Transport MeSH
• Endocytosis MeSH
• Indoleacetic Acids metabolism   MeSH
• Cell Membrane Permeability MeSH
• Arabidopsis Proteins metabolism   MeSH
• Secretory Vesicles metabolism   MeSH
• Green Fluorescent Proteins metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Arabidopsis Proteins MeSH
• Green Fluorescent Proteins MeSH

• MeSH
• Indoleacetic Acids * MeSH
• Secretory Vesicles * MeSH
• Publication type
• Letter MeSH
• Comment MeSH
• Names of Substances
• Indoleacetic Acids * MeSH