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

(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_i²⁺) 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.