PIN and AUX/LAX proteins: their role in auxin accumulation Eric M. Kramer Physics Department, Simon’s Rock College, 84 Alford Road, Great Barrington, MA 01230, USA There is mounting evidence that plant tissues special- ized 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 accumu- lation strategies and review the evidence that these strategies are used by plants. In the original chemiosmotic theory of polar auxin transport, the path of auxin transport spans both the cell protoplast and the cell wall [1,2]. Auxin enters the cytoplasm from the cell wall by diffusion across the cell membrane and possibly also via membrane-bound influx carriers. Auxin exits the cytoplasm via membrane-bound efflux carriers, whose asymmetric localization on the cell membrane generates the net flux through the tissue. In the subsequent 30 years, advances in molecular biology have substantially confirmed and refined this picture. This includes the demonstration that proteins involved in auxin efflux are indeed localized asymmetrically in transporting cells [3,4], the discovery of a family of puta- tive auxin efflux carriers – the PIN gene family [4–6], and the discovery of a family of putative auxin influx carriers – the AUX/LAX gene family [7]. Although the molecular biology of auxin transport has been clarified in recent years, the quantitative relation- ship between carrier expression and the distribution of auxin within plant tissues remains vague. Recent measurements of the auxin distribution with cell-scale resolution [5,8–14] have repeatedly shown that provas- cular and cambial tissues co-localize with a maximum in the auxin concentration (although there are some open questions regarding the interpretation of auxin reporter gene constructs, see for example Ref. [15]). This concen- tration maximum is of more than academic interest. It is likely that the auxin concentration gradient conveys positional information to developing cells [5,10,13,16,17]. In addition, the ability to accumulate auxin against a concentration gradient is important to the auxin economy of the plant. The transporting tissue must be able to recruit auxin from its sites of synthesis and also maintain a significant concentration against losses resulting fom diffusion. Several authors have suggested that AUX/LAX and PIN carriers play a role in maintaining the high concentration of auxin [5–7,17,18]. Indeed, carrier mutants and plants treated with transport inhibitors show a diminished ability to accumulate auxin. A model plant tissue Given the importance and ubiquity of auxin transport, it is perhaps surprising that there has not been any attempt to model auxin transport at the cellular scale since the early 1980s [19–21]. Mathematical and computer models are a helpful step towards a synthesis of the disparate data on carrier expression and localization patterns, and they allow one to test the plausibility of various proposals for carrier function. I have modeled a simplified plant tissue composed of three cell types – parenchyma cells, canal cells and border cells – distinguished by their pattern of PIN carrier localization (Figure 1; Box 1). Parenchyma cells have a uniform distribution of PIN carriers on the cell membrane. Figure 1. Schematic representation of auxin transporting tissue. Cell walls are depicted in yellow, cytoplasm in green and vacuoles in white. The localization of PIN carriers is indicated in plum. The figure shows two files of canal cells, bordered on each side by one file of border cells. The remaining files are parenchyma. Fine details of the PIN distribution are not shown. Corresponding author: Eric M. Kramer (ekramer@simons-rock.edu). Available online 2 November 2004 Opinion TRENDS in Plant Science Vol.9 No.12 December 2004 www.sciencedirect.com 1360-1385/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2004.10.010