Figure 5. auxin fluxes and PIN trafficking during the gravitropic root response.

A) Auxin fluxes and PIN2/PIN3 localization in the primary root tip before gravistimulation. Note the position of sedimented amyloplasts in the collumela. The arrow with the ‘g’ letter indicates the gravity vector. B) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 0 to 30 min after gravistimulation. Note the movement of amyloplasts according to the new gravity vector, which triggers PIN3 transcytosis to the lower part of the cell and initiates the establishment of an asymmetric auxin maximum between the upper and lower part of the root. C) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 30 to 120 min after gravistimulation. High auxin in the lower part of the root inhibits PIN2 endocytosis, which promotes its localization at the plasma membrane and reinforce asymmetric auxin localization. Accumulation of auxin on the lower part of the root locally inhibits cell elongation, which triggers root bending. D) Auxin fluxes and PIN2/PIN3 localization in the primary root tip from 120 to 240 min after gravistimulation. Inhibition of endocytosis by auxin is transient and PIN2 endocytosis is re-established in the lower part of the root. In the meantime, low auxin on the upper part of the root triggers PIN2 degradation in a TIR1/AFBs-dependent manner. Note that the times indicated are compiled between those reported by Band et al., (2012) and Baster et al., (2013), but may vary according to the experimental setup and are only indicative. It is unknown at what point during the gravitropic response PIN3 localization is reset to apolar, hence the purple question mark in C and D.