REVERSIBLE INACTIVATION OF K+ CHANNELS OF VICIA STOMATAL GUARD-CELLS FOLLOWING THE PHOTOLYSIS OF CAGED INOSITOL 1,4,5-TRISPHOSPHATE

RECENT investigations suggest that cytoplasmic D-myo-inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger in plants, as in animals, coupling environmental and other stimuli to intracellular Ca2+ release1,2. Cytoplasmic levels of InsP3 and the turnover of several probable precursors in plants are affected by physiological stimuli-including light, osmotic stress and the phytohormone indoleacetic acid3-5- and InsP3 activates Ca2+ channels6 and Ca2+ flux across plant vacuolar7 and microsomal membranes8. Complementary data also link changes in cytoplasmic free Ca2+ to several physiological responses, notably in guard cells which regulate gas exchange through the stomatal pores of higher plant leaves. Recent evidence indicates that guard cell K+ channels and, hence, K+ flux for stomatal movements9 may be controlled by cytoplasmic Ca2+ (ref. 10). So far, however, direct evidence of a role for InsP3 in signalling in plants has remained elusive. Here we report that InsP3 released from an inactive, photolabile precursor, the P5-l-(2-nitrophenyl)ethyl ester of InsP3 (caged InsP3)11 reversibly inactivates K+ channels thought to mediate K+ uptake by guard cells from Vicia faba L. while simultaneously activating an apparently time-independent, inward current to depolarize the membrane potential and promote K+ efflux through a second class of K+ channels12,13. The data are consistent with a transient rise in cytoplasmic free Ca2+ (ref. 9) and demonstrate that intact guard cells are competent to use InsP3 in signal cascades controlling ion flux through K+ channels. © 1990 Nature Publishing Group.