Different pH-dependences of K+ channel activity in bundle sheath and mesophyll cells of maize leaves

The isolation of bundle sheath protoplasts from leaves of Zea mays L. for p atch clamp whole-cell experiments presents special problems caused by the s uberin layer surrounding these cells. These problems were overcome by the i solation technique described here. Two different types of whole-cell respon se were found: a small response caused by MB-1 (maize bundle sheath conduct ance type 1) which was instantaneously activated, and another caused by MB- 2 (maize bundle sheath conductance type 2) consisting of an instantaneous r esponse (maize bundle sheath K+ instantaneous current type 2; MB-KI2) simil ar to but stronger than the current through MB-1 plus a small time-dependen t outward rectifying component (maize bundle sheath activated outward recti fying current; MB-AOR) with voltage-dependent delayed activation. The occur rence of MB-AOR was often accompanied by a smaller contribution from an inw ard rectifying channel at negative potentials. Activation of MB-2 required ATP. It is suggested that MB-1 and MB-2 are related to bundle sheath cells with and without direct contact with the xylem vessels. In mesophyll cells, only one type of response caused by MM-2 (maize mesophyll conductance type 2) was found with an instantaneous (maize mesophyll Kt instantaneous curre nt type 2, MM-KI2) and a voltage-dependent delayed component (maize mesophy ll activated outward rectifying current, MM-AOR). The most striking differe nce between bundle sheath and mesophyll cells was the pH dependence of K+ u ptake. At pH 7.2, uptake of K+ by MB-2 was identical to that by MM-2 over t he whole voltage range. However, acidification stimulated K+ conductance in bundle sheath cells, whereas a decrease was found for MM-2. At pH 6.15, th e bundle sheath channel MB-2 had more than a 10-fold higher K+ uptake at po sitive and negative potentials than MM-2. The channel MB-1, too, was stimul ated by low pH. This seems to indicate a putative role for MB-1 and MB-2 in charge balance during uptake of nutrients via cotransport from the xylem i nto the symplasm.