Calcium-dependent K current in plasma membranes of dermal cells of developing bean cotyledons


W. H. Zhang; Fax: +61 88303 7116; e-mail:


In developing seeds of bean (Phaseolus vulgaris L.), phloem-imported assimilates (largely sucrose and potassium) are released from coats to seed apoplasm and subsequently retrieved by the dermal cell complexes of cotyledons. To investigate the mechanisms of K+ uptake by the cotyledons, protoplasts of dermal cell complexes were isolated and whole-cell currents across their plasma membranes were measured with the patch-clamp technique. A weakly rectified cation current displaying a voltage-dependent blockade by external Ca2+ and acidic pH, dominated the conductance of the protoplasts. The P haseolus v ulgaris Cotyledon Dermal-cell pH and Calcium-dependent Cation Conductance (Pv-CD-pHCaCC) was highly selective for K+ over Ca2+ and Cl. For K+ current through Pv-CD-pHCaCC a sigmoid shaped current–voltage (IV) curve was observed with negative conductance at voltages between −200 and −140 mV. This negative K+ conductance was Ca2+ dependent. With other univalent cations (Na+, Rb+, NH4+) the currents were smaller and were not Ca2+ dependent. Reversal potentials remained constant when external K+ was substituted with these cations, suggesting that Pv-CD-pHCaCC channels were non-selective. The Pv-CD-pHCaCC would provide a pathway for K+ and other univalent cation influx into developing cotyledons. These cation influxes could be co-ordinated with sucrose influx via pH and Ca2+dependence.