Plant Physiol, February 2001, Vol. 125, pp. 1142-1150

Potassium-Efflux Channels in Extensor and Flexor Cells of the Motor Organ of Samanea saman Are Not Identical. Effects of Cytosolic Calcium¹

Department of Agricultural Botany, Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel

Leaflet movements in the mimosa-family tree Samanea saman stem from coordinated volume changes of cells in the leaf motor organs in the adaxial and abaxial motor cells ("flexors" and "extensors"). Shrinking, initiated by dissimilar light signals in extensors and in flexors, depends in both cell types on K⁺ efflux via depolarization-dependent potassium (K_D) channels. To compare between flexor and extensor K_D channels and to test for a possible interaction of these channels with the Ca²⁺-mobilizing phosphoinositide cascade evoked in these motor cells by the "shrinking signals," we probed the channels with varying (5 nM-3 mM) cytosolic free-Ca²⁺ concentration ([Ca²⁺]_cyt) in patch-clamped inside-out excised membrane patches. Ca²⁺ was not required for K_D channel activation. [Ca²⁺]_cyt of 600 nM decreased the mean number of open K_D channels in flexors, as monitored at 30 mV. Detailed analysis revealed that in flexors millimolar [Ca²⁺]_cyt decreased the maximum number of open channels, but simultaneously increased K_D channel opening probability by negatively shifting the half-maximum-activation voltage by 40 to 50 mV. Thus, the promoting and the inhibitory effects at millimolar [Ca²⁺]_cyt practically cancelled-out. In contrast to flexors, none of the gating parameters of the extensor K_D channels were affected by [Ca²⁺]_cyt. Irrespective of [Ca²⁺]_cyt, the steady-state gating of extensor K_D channels was slightly but significantly more voltage sensitive than that of flexors. The unitary conductances of flexor and extensor K_D channels were similar and decreased by approximately 20% at millimolar [Ca²⁺]_cyt. It is intriguing that the extensor K_D channels were significantly less K⁺ selective than those in flexors.