Plant Physiol, February 2002, Vol. 128, pp. 388-399

Nonselective Currents and Channels in Plasma Membranes of Protoplasts from Coats of Developing Seeds of Bean¹

School of Biological Sciences, The Flinders University of South Australia, G.P.O. Box 2100, Adelaide, South Australia 5001, Australia (W.-H.Z., M.S., S.D.T.); Biophysics Department, School of Physics, The University of New South Wales, Kensington, New South Wales 2052, Australia (N.A.W.); and School of Biological and Chemical Sciences, The University of Newcastle, Newcastle, New South Wales 2308, Australia (W.-H.Z., J.W.P.)

In developing bean (Phaseolus vulgaris) seeds, phloem-imported nutrients move in the symplast from sieve elements to the ground parenchyma cells where they are transported across the plasma membrane into the seed apoplast. To study the mechanisms underlying this transport, channel currents in ground parenchyma protoplasts were characterized using patch clamp. A fast-activating outward current was found in all protoplasts, whereas a slowly activating outward current was observed in approximately 25% of protoplasts. The two currents had low selectivity for univalent cations, but the slow current was more selective for K⁺ over Cl (P_K:P_Cl = 3.6-4.2) than the fast current (P_K:P_Cl = 1.8-2.5) and also displayed Ca²⁺ selectivity. The slow current was blocked by Ba²⁺, whereas both currents were blocked by Gd³⁺ and La³⁺. Efflux of K⁺ from seed coat halves was inhibited 25% by Gd³⁺ and La³⁺ but was stimulated by Ba²⁺ and Cs⁺, suggesting that only the fast current may be a component in the pathway for K⁺ release. An "instantaneous" inward current observed in all protoplasts exhibited similar pharmacology and permeability for univalent cations to the fast outward current. In outside-out patches, two classes of depolarization-activated cation-selective channels were observed: one slowly activating of low conductance (determined from nonstationary noise to be 2.4 pS) and another with conductances 10-fold higher. Both channels occurred at high density. The higher conductance channel in 10 mM KCl had P_K:P_Cl = 2.8. Such nonselective channels in the seed coat ground parenchyma cell could function to allow some of the efflux of phloem-imported univalent ions into the seed apoplast.