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ENVIRONMENTAL STRESS AND ADAPTATION

Citrate-Permeable Channels in the Plasma Membrane of Cluster Roots from White Lupin¹

Wine and Horticulture, School of Agriculture and Wine, The University of Adelaide, Waite Campus, Glen Osmond, South Australia 5064, Australia (W.-H.Z., S.D.T.); and Commonwealth Scientific and Industrial Research Organization Plant Industry, Canberra, Australian Capital Territories 2601, Australia (P.R.R.)

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 phosphorus-replete (+P) and phosphorus-deficient (–P) 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^3– than Cl^– (P_Cit/P_Cl = 26.3), while ORAC was selective for Cl^– over citrate (P_Cl/P_Cit = 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 –P plants, as well as from normal (noncluster) roots of plants grown with 25 µM phosphorus (+P). No differences were observed in the magnitude or frequency of IRAC and ORAC currents between the cluster roots and noncluster roots of –P plants. However, the IRAC current from +P plants occurred less frequently than in the –P 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.

² Present address: Institute of Botany, Chinese Academy of Sciences, Beijing 100093, People's Republic of China.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.046201.

^* Corresponding author; e-mail steve.tyerman{at}adelaide.edu.au; fax 81–8–8303–7116.

Received May 11, 2004; returned for revision June 12, 2004; accepted June 22, 2004.

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