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 myoinositol hexakispho sphate (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. tuberosom (n = 14) and Vicia faba (n = 6) guard cell protoplasts. In both species, submicromolar concentrations o f InsP(6). delivered through the patch electrode, mimicked the inhibitory e ffects of ABA and internal calcium (Ca-i(2+)) on the inward rectifying K+ c urrent. I-K,I-in, in a dose-dependent manner. Steady state block of I-K,I-i n by InsP(6) was reached much more quickly in Vicia (3 min at approximate t o 1 mu M) than Solanum (20-30 min). The effects of InsP(6) on I-K,I-in were specific to the myo-inositol isomer and were not elicited by other conform ers of InsP(6) (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 patc h pipette together with InsP(6) prevented the inhibition of I-K,I-in, sugge sting that the effect is Ca2+ dependent. InsP(6) was approximate to 100-fol d more potent than Ins(1.4.5)P-3 in modulating I-K,I-in. Thus ABA increases InsP(6) in guard cells, and InsP(6) is a potent Ca2+-dependent inhibitor o f I-K,I-in. Taken together, these results suggest that InsP(6) may play a m ajor role in the physiological response of guard cells to ABA.