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

Abstract

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 Phaseolus vulgaris 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 (I-V) 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.