Characterization of Anion Channels in the Plasma Membrane of Arabidopsis Epidermal Root Cells and the Identification of a Citrate-Permeable Channel Induced by Phosphate Starvation^{<a href="#fn1" rid="fn1" class=" fn">1</a>}

Abstract

Anion channels in the plasma membrane of plant cells catalyze anion fluxes both into and out of the cell and serve a variety of functions. They have been implicated in stomatal function, where their activation is thought to be one of the rate-limiting steps in the loss of salts (and thus cell turgor) from guard cells leading to stomatal pore closure (Roelfsema et al., 2004). In less specialized cells, anion channel activation is a likely step in the transduction of signals modulating hypocotyl growth, including blue light (Cho and Spalding, 1996) and possibly auxin (Thomine et al., 1997); these signal transduction events arise from depolarizations resulting from anion channel activation. Anion channels are also thought to facilitate the release of organic acids from higher plant roots. Al^{-activated anion channels (ALAACs) in the tips of wheat (}Triticum aestivum) and maize (Zea mays) roots have been shown to be permeable to malate and/or citrate (Ryan et al., 1997; Kollmeier et al., 2001; Pineros and Kochian, 2001; Zhang et al., 2001), a function of which is thought to reduce Al ^{stress by chelating this cation. Thus activation of ALAACs by Al and their pharmacological profile, which resembles that for Al-induced organic-acid efflux from cereal roots, makes ALAACs likely candidates for mediating Al-induced organic-acid secretion from roots.}

The biophysical properties of plant anion channels have been best characterized in guard cells. Two types of anion channels have been extensively investigated: rapidly activating (R-type) and slowly activating (S-type) anion channels (e.g. Hedrich et al., 1990; Schroeder and Keller, 1992; Schmidt and Schroeder, 1994). The R-type anion channel exhibits activation/deactivation kinetics in the millisecond range and inactivates in response to prolonged membrane depolarization. Thus, R-type channels in guard cells are thought to mediate transient anion efflux and membrane depolarization. In contrast, S-type channels activate and deactivate slowly (with a time constant of seconds), and they do not inactivate. S-type channels may possibly mediate prolonged anion efflux. R- and S-type anion channels also have distinct gating properties. The typical R-type whole-cell current voltage relationship is exemplified by a pronounced peak-current magnitude at relatively positive voltages and complete deactivation at relatively negative (resting) membrane voltages; in contrast, S-type whole-cell current voltage relationships display a less pronounced peak current and exhibit inward rectification in hyperpolarized conditions (Schroeder and Keller, 1992; Roelfsema et al., 2004). Similar R- and S-type anion channels have also been characterized in the plasma membrane of hypocotyl cells (Thomine et al., 1995, 1997; Frachisse et al., 2000).

Anion channels in roots have not been well characterized compared to those in guard cells, despite their potential importance in regulating acquisition from soil solution. The ALAACs of wheat and maize roots (see above) resemble S-type channels in that they display slow activation kinetics (Pineros and Kochian, 2001; Zhang et al., 2001) and exhibit inwardly rectifying current voltage relationships. Other reports of anion channel activity in roots are limited to an outwardly rectifying anion-selective channel in wheat and maize, which allows anion influx from the soil solution (Skerrett and Tyerman, 1994; Pineros and Kochian, 2001) and three different anion conductances in the xylem parenchyma cells of barley (Hordeum vulgare) roots, which are thought to mediate anion efflux to the xylem vessels during salt delivery to the shoot (Kohler and Raschke, 2000; Kohler et al., 2002). In particular, there has been no systematic study of anion channels in roots with respect to root soil interaction and nutrient acquisition.

In this study we address this dearth of knowledge and use the patch clamp technique to investigate anion channel activity in the epidermis of Arabidopsis (Arabidopsis thaliana) roots. We show two types of voltage-dependent channel activity, which resemble the R-type anion channel activity described in guard cells and hypocotyls. One of these channels was ubiquitously expressed in the epidermal cells and was permeable to the inorganic anions, SO₄^{, NO}₃^{, and Cl but was impermeable to organic-acid anions, citrate and malate. The second anion channel was less frequently observed, was induced by phosphate starvation, and mediated the efflux of organic-acid anions. It is suggested that the phosphate-regulated anion channel mediates organic-acid anion efflux from Arabidopsis roots, which is thought to be an important strategy for efficient phosphate acquisition by higher plants (}Narang et al., 2000).

All measurements were in SBS. Pipette solution was based on standard pipette solution and modified by replacing Cs₂SO₄ with appropriate anion (as a cesium salt) and adjusted to 700 mosmol kg ^{using sorbitol when necessary.}

Notes

^{This work was supported by the Biotechnology and Biological Sciences Research Council (grant no. BRE13629 to S.K.R., D.S., and M.R.).}

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.046995.