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 patch clamp whole-cell experiments presents special problems caused by the suberin layer surrounding these cells. These problems were overcome by the isolation technique described here. Two different types of whole-cell response were found: a small response caused by MB-1 (maize bundle sheath conductance type 1) which was instantaneously activated, and another caused by MB-2 (maize bundle sheath conductance type 2) consisting of an instantaneous response (maize bundle sheath K+ instantaneous current type 2; MB-KI2) similar to but stronger than the current through MB-1 plus a small time-dependent outward rectifying component (maize bundle sheath activated outward rectifying current; MB-AOR) with voltage-dependent delayed activation. The occurrence of MB-AOR was often accompanied by a smaller contribution from an inward 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 K+ instantaneous current type 2, MM-KI2) and a voltage-dependent delayed component (maize mesophyll activated outward rectifying current, MM-AOR). The most striking difference between bundle sheath and mesophyll cells was the pH dependence of K+ uptake. At pH 7.2, uptake of K+ by MB-2 was identical to that by MM-2 over the whole voltage range. However, acidification stimulated K+ conductance in bundle sheath cells, whereas a decrease was found for MM-2. At pH 6.15, the bundle sheath channel MB-2 had more than a 10-fold higher K+ uptake at positive and negative potentials than MM-2. The channel MB-1, too, was stimulated 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 into the symplasm.
Keunecke
M
M
Institut für Angewandte Physik, Christian-Albrechts-Universität, Kiel, Germany.
Hansen
U P
UP
D000255:Adenosine Triphosphate / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D018515:Plant Leaves / Q000166:cytology / Q000502:physiology*; D015221:Potassium Channels / Q000502:physiology*; D011523:Protoplasts / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D003313:Zea mays / Q000166:cytology / Q000502:physiology*
D015221:Potassium Channels; D000255:Adenosine Triphosphate