Abscisic acid-induced stomatal closure mediated by cyclic ADP-ribose

Abscisic acid (ABA) is a plant hormone involved in the response of plants t o reduced water availability. Reduction of guard cell turgor by ABA diminis hes the aperture of the stomatal pore and thereby contributes to the abilit y of the plant to conserve water during periods of drought. Previous work h as demonstrated that cytosolic Ca2+ is involved in the signal transduction pathway that mediates the reduction in guard cell turgor elicited by ABA, H ere we report that ABA uses a Ca2+-mobilization pathway that involves cycli c adenosine 5'-diphosphoribose (cADPR), Microinjection of cADPR into guard cells caused reductions in turgor that were preceded by increases in the co ncentration of free Ca2+ in the cytosol, Patch clamp measurements of isolat ed guard cell vacuoles revealed the presence of a cADPR-elicited Ca2+-selec tive current that was inhibited at cytosolic Ca2+ greater than or equal to 600 nM, Furthermore, microinjection of the cADPR antagonist 8-NH2-cADPR cau sed a reduction in the rate of turgor loss in response to ABA in 54% of cel ls tested, and nicotinamide, an antagonist of cADPR production, elicited a dose-dependent block of ABA-induced stomatal closure. Our data provide defi nitive evidence for a physiological role for cADPR and illustrate one mecha nism of stimulus-specific Ca2+ mobilization in higher plants. Taken togethe r with other recent data [Wu, Y,, Kuzma, J,, Marechal, E,, Graeff, R,, Lee, H, C,, Foster, R, & Chua, N,-H, (1997) Science 278, 2126-2130], these resu lts establish cADPR as a key player in ABA signal transduction pathways in plants.