Putative primary involvement of Arabidopsis phosphoinositide-specific phospholipase C1 within abscisic acid-induced stomatal closing

Stomatal closing to abscisic acid (ABA) was studied in leaf epidermal peels of a dexamethasone (Dex)-inducible transgenic line expressing the phospholipase C AtPLC1 antisense in the Columbia genetic background. In the absence of Dex, the Ca2+ buffer, ethylene glycol-bis(b-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and the phopholipase C inhibitor, 1-[6-{[17β-3-methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl]-1H-pyrrole-2,5-dione (U73122) specifically inhibited the response to 20 µM ABA, whereas the Ca2+ buffer, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) inhibited the response to 20 or 30 µM ABA. Neither EGTA nor BAPTA increased the U73122 effect. Applying 30 µM Dex specifically affected 20 µM ABA-induced stomatal closing through reducing its magnitude as well as suppressing the EGTA, BAPTA and U73122 inhibitory effects. Neither Dex nor U73122 changed the specific inhibitory effects of both the antagonist of cyclic ADP-ribose synthesis, nicotinamide and the GTP-binding protein (G protein) modulators, pGlu-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH2 (GP Ant-2) and mas17 on 30 µM ABA-induced stomatal closing. When tested in combination, substituting nicotinamide for mas17, but not for GP Ant-2, enhanced their inhibitory effect to an extent that BAPTA did not increase. These results supported that AtPLC1 primarily mediates the Ca2+-dependent stomatal closing response to 20 µM ABA as much as 30 µM Dex did not affect 20 µM ABA-induced stomatal closing when tested on the wild type Columbia-4 ecotype. Furthermore, the present study suggested that Ca2+ mobilization did not involve any dependency between AtPLC1 and a putative G protein-coupled ADP-ribosyl cyclase at the tested ABA concentrations.