Citrate-Permeable Channels in the Plasma Membrane of Cluster Roots from White Lupin

Abstract

White lupin (Lupinus albus) is well adapted to phosphorus deficiency by developing cluster roots that release large amounts of citrate into the rhizosphere to mobilize the sparingly soluble phosphorus. To determine the mechanism underlying citrate release from cluster roots, we isolated protoplasts from different types of roots of white lupin plants grown in phosphorusreplete (1P) and phosphorus-deficient (2P) conditions and used the patch-clamp technique to measure the whole-cell currents flowing across plasma membrane of these protoplasts. Two main types of anion conductance were observed in protoplasts prepared from cluster root tissue: (1) an inwardly rectifying anion conductance (IRAC) activated by membrane hyperpolarization, and (2) an outwardly rectifying anion conductance (ORAC) that became more activated with membrane depolarization. Although ORAC was an outward rectifier, it did allow substantial inward current (anion efflux) to occur. Both conductances showed citrate permeability, with IRAC being more selective for citrate 32 than Cl 2 (P Cit /P Cl 5 26.3), while ORAC was selective for Cl 2 over citrate (P Cl /P Cit 5 3.7). Both IRAC and ORAC were sensitive to the anion channel blocker anthracene-9-carboxylic acid. These currents were also detected in protoplasts derived from noncluster roots of 2P plants, as well as from normal (noncluster) roots of plants grown with 25 mM phosphorus (1P). No differences were observed in the magnitude or frequency of IRAC and ORAC currents between the cluster roots and noncluster roots of 2P plants. However, the IRAC current from 1P plants occurred less frequently than in the 2P plants. IRAC was unaffected by external phosphate, but ORAC had reduced inward current (anion efflux) when phosphate was present in the external medium. Our data suggest that IRAC is the main pathway for citrate efflux from white lupin roots, but ORAC may also contribute to citrate efflux.

White Lupin Cluster Root Acclimation to Phosphorus Deficiency and Root Hair Development Involve Unique Glycerophosphodiester Phosphodiesterases

“…The formation of cluster roots having abundant root hairs results in a striking increase in surface area available for Pi uptake from the rhizosphere (Keerthisinghe et al, 1998;Neumann et al, 1999). Cluster roots also excrete large amounts of protons, citrate, and acid phosphatase (LaSAP) to the rhizosphere, consequently increasing Pi bioavailability (Dinkelaker et al, 1989;Neumann et al, 1999;Watt and Evans, 1999;Miller et al, 2001;Wasaki et al, 2003;Zhang et al, 2004).…”

Soil Phosphorus Uptake by Continuously Cropped Lupinus albus: A New Microcosm Design

“…At the cellular level, Jones (1998) hypothesized two main routes of organic acid efflux from roots to the soil solution: (i) a slow passive diffusion across the lipid bilayer and, (ii) an efflux through a plasma membrane channel protein. In white lupin, carboxylates are secreted in the unprotonated form through recently identified channels (Sasaki et al, 2004;Zhang et al, 2004), whereas acidification is due to the activation of the plasma membrane proton pump (Yan et al, 2002). Depending on the dissociation properties and number of the carboxylic groups, organic anions may complex metal cations like Al 3+ , Fe 3+ in acid soils and/or Ca 2+ and Mg 2+ in alkaline soils.…”

Interactive effects of phosphorus deficiency and exogenous auxin on root morphological and physiological traits in white lupin (Lupinus albus L.)

“…Further investigation is required, however, to elucidate the relationship between citrate exudation and H + -ATPase activity. Citrate efflux from white lupin CRs probably occurs through anion channels permeable to citrate [30,62,63]. The increased citrate exudation observed when auxin was applied in our study may be partly attributed to anion channel activation mediated by auxin [64,65].…”