Abstract
myo
-Inositol hexakisphosphate (InsP
6
) is the most
abundant inositol phosphate in cells, yet it remains the most enigmatic of
this class of signaling molecule. InsP
6
plays a role in the
processes by which the drought stress hormone abscisic acid (ABA) induces
stomatal closure, conserving water and ensuring plant survival. Previous work
has shown that InsP
6
levels in guard cells are elevated in response
to ABA, and InsP
6
inactivates the plasma membrane inward
K
+
conductance (
I
K,in
) in a cytosolic
calcium-dependent manner. The use of laser-scanning confocal microscopy in
dye-loaded patch-clamped guard cell protoplasts shows that release of
InsP
6
from a caged precursor mobilizes calcium. Measurement of
calcium (barium) currents
I
Ca
in patch-clamped protoplasts
in whole cell mode shows that InsP
6
has no effect on the
calcium-permeable channels in the plasma membrane activated by ABA. The
InsP
6
-mediated inhibition of
I
K,in
can also be
observed in the absence of external calcium. Thus the InsP
6
-induced
increase in cytoplasmic calcium does not result from calcium influx but must
arise from InsP
6
-triggered release of calcium from endomembrane
stores. Measurements of vacuolar currents in patch-clamped isolated vacuoles
in whole-vacuole mode showed that InsP
6
activates both the fast and
slow conductances of the guard cell vacuole. These data define
InsP
6
as an endomembrane-acting calcium-release signal in guard
cells; the vacuole may contribute to InsP
6
-triggered
Ca
2
+
release, but other endomembranes may also be
involved.