Abstract
(
RS
)-2-
cis
, 4-
trans
-abscisic acid (ABA), a naturally occurring plant stress hormone, elicited rapid agonist-specific changes in
myo
-inositol hexakisphosphate (InsP
6
) measured in intact guard cells of
Solanum tuberosum
(
n
= 5); these changes were not reproduced by (
RS
)-2-
trans
, 4-
trans
-abscisic acid, an inactive stereoisomer of ABA (
n
= 4). The electrophysiological effects of InsP
6
were assessed on both
S. tuberosum
(
n
= 14) and
Vicia faba
(
n
= 6) guard cell protoplasts. In both species, submicromolar concentrations of InsP
6
, delivered through the patch electrode, mimicked the inhibitory effects of ABA and internal calcium (Ca
i
2+
) on the inward rectifying K
+
current,
I
K,in
, in a dose-dependent manner. Steady state block of
I
K,in
by InsP
6
was reached much more quickly in
Vicia
(3 min at ≈1 μM) than
Solanum
(20–30 min). The effects of InsP
6
on
I
K,in
were specific to the
myo
-inositol isomer and were not elicited by other conformers of InsP
6
(e.g.,
scyllo
- or
neo
-). Chelation of Ca
2+
by inclusion of 1,2-bis(2-aminophenoxy)ethane-
N,N,N
′
,N
′-tetraacetic acid or EGTA in the patch pipette together with InsP
6
prevented the inhibition of
I
K,in
, suggesting that the effect is Ca
2+
dependent. InsP
6
was ≈100-fold more potent than Ins(1,4,5)P
3
in modulating
I
K,in
. Thus ABA increases InsP
6
in guard cells, and InsP
6
is a potent Ca
2+
-dependent inhibitor of
I
K,in
. Taken together, these results suggest that InsP
6
may play a major role in the physiological response of guard cells to ABA.