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Plant Physiol, November 1999, Vol. 121, pp. 977-986
Magnesium Sensitizes Slow Vacuolar Channels to Physiological
Cytosolic Calcium and Inhibits Fast Vacuolar Channels in Fava Bean
Guard Cell Vacuoles1
Zhen-Ming
Pei,*
John M.
Ward,2 and
Julian I.
Schroeder
Department of Biology and Center for Molecular Genetics, University
of California, San Diego, La Jolla, California 92093-0116
Vacuolar ion channels in guard cells
play important roles during stomatal movement and are regulated by many
factors including Ca2+, calmodulin, protein kinases, and
phosphatases. We report that physiological cytosolic and luminal
Mg2+ levels strongly regulate vacuolar ion channels in fava
bean (Vicia faba) guard cells. Luminal Mg2+
inhibited fast vacuolar (FV) currents with a
Ki of approximately 0.23 mM in a
voltage-dependent manner at positive potentials on the cytoplasmic
side. Cytosolic Mg2+ at 1 mM also inhibited FV
currents. Furthermore, in the absence of cytosolic Mg2+,
cytosolic Ca2+ at less than 10 µM did not
activate slow vacuolar (SV) currents. However, when cytosolic
Mg2+ was present, submicromolar concentrations of cytosolic
Ca2+ activated SV currents with a
Kd of approximately 227 nM,
suggesting a synergistic Mg2+-Ca2+ effect. The
activation potential of SV currents was shifted toward physiological
potentials in the presence of cytosolic Mg2+
concentrations. The direction of SV currents could also be changed from
outward to both outward and inward currents. Our data predict a model
for SV channel regulation, including a cytosolic binding site for
Ca2+ with an affinity in the submicromolar range and a
cytosolic low-affinity Mg2+-Ca2+ binding site.
SV channels are predicted to contain a third binding site on the
vacuolar luminal side, which binds Ca2+ and is inhibitory.
In conclusion, cytosolic Mg2+ sensitizes SV channels to
physiological cytosolic Ca2+ elevations. Furthermore, we
propose that cytosolic and vacuolar Mg2+ concentrations
ensure that FV channels do not function as a continuous vacuolar
K+ leak, which would prohibit stomatal opening.
1
This work was supported by the National Science
Foundation (grant no. MCB-9506191 to J.I.S.).
2
Present address: Center for Plant Molecular
Biology, University of Tübingen, D-72076 Tübingen, Germany.
*
Corresponding author; e-mail zpei{at}biomail.ucsd.edu; fax
858-534-7108.
© 1999 American Society of Plant Physiologists
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