Inositol hexakisphosphate is a physiological signal regulating the K+-inward rectifying conductance in guard cells

Abstract

(RS)-2-cis, 4-trans-abscisic acid (ABA), a naturally occurring plant stress hormone, elicited rapid agonist-specific changes in myo-inositol hexakisphosphate (InsP6) 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 InsP6 were assessed on both S. tuberosum (n = 14) and Vicia faba (n = 6) guard cell protoplasts. In both species, submicromolar concentrations of InsP6, 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 InsP6 was reached much more quickly in Vicia (3 min at ≃1 μM) than Solanum (20-30 min). The effects of InsP6 on I(K,in) were specific to the myo-inositol isomer and were not elicited by other conformers of InsP6 (e.g., scyllo- or neo-). Chelation of Ca2+ by inclusion of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or EGTA in the patch pipette together with InsP6 prevented the inhibition of I(K,in), suggesting that the effect is Ca2+ dependent. InsP6 was ≃ 100-fold more potent than Ins(1,4,5)P3 in modulating I(K,in). Thus ABA increases InsP6 in guard cells, and InsP6 is a potent Ca2+-dependent inhibitor of I(K,in). Taken together, these results suggest that InsP6 may play a major role in the physiological response of guard cells to ABA.