PIN and AUX/LAX proteins: their role in auxin accumulation

There is mounting evidence that plant tissues specialized for auxin transport coincide with local maxima in the auxin concentration. This result is difficult to reconcile with the traditional model of auxin transport, which relies on high levels of auxin efflux carrier expression. Because transporting cells maintain high levels of auxin efflux, one naively expects a depletion of auxin relative to surrounding tissues. Here I use a computer model of auxin transport in a background of parenchyma cells to evaluate the possible roles of the PIN and AUX/LAX families of putative auxin carriers in auxin accumulation. I describe two effective accumulation strategies and review the evidence that these strategies are used by plants.