Modelling auxin efflux carrier phosphorylation and localization

Regulation of the activity and localization of PIN-FORMED (PIN) membrane proteins, which facilitate efflux of the plant hormone auxin from cells, is important for plants to respond to environmental stimuli and to develop new organs. The protein kinase PINOID (PID) is involved in regulating PIN phosphorylation, and this is thought to affect PIN localization by biasing recycling towards shootwards (apical) (rather than rootwards (basal)) membrane domains. PID has been observed to undergo transient internalization following auxin treatment, and it has been suggested that this may be a result of calcium-dependent sequestration of PID by the calcium-binding protein TOUCH3 (TCH3). We present a mathematical formulation of these processes and examine the resulting steady-state and time-dependent behaviours in response to transient increases in cytosolic calcium. We further combine this model with one for the recycling of PINs in polarized cells and also examine its behaviour. The results provide insight into the behaviour observed experimentally and provide the basis for subsequent studies of the tissue-level implications of these subcellular processes for phenomena such as gravitropism. (C) 2012 Elsevier Ltd. All rights reserved.