Involvement of phospholipase C-independent calcium-mediated abscisic acid signalling during Arabidopsis response to drought

The present study investigated whether Ca(2+) mobilization independent of phosphoinositide-specific phospholipase C (PI-PLC) would delay wilting in Arabidopsis thaliana (L.) Heynh. cv. Columbia through mediating stomatal closure at abscisic acid (ABA) concentrations rising beyond a drought-specific threshold value. In wild type (WT) epidermis, the PI-PLC inhibitor (U73122) affected the stomatal response to 20 A mu M ABA but not to 30 A mu M ABA. Disruption in GTP-binding protein Inot sign subunit 1 (GPA1) affected the stomatal response to 30 A mu M ABA, but not to 20 A mu M ABA. In the gpa1-4 mutant, the inhibitory effects of the Ca(2+) buffer, 1,2-bis(0-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), the inactive mastoparan analogue, mas17 and the antagonist of cyclic ADP-ribose synthesis, nicotinamide, were differentially attenuated on 30 A mu M ABA-induced stomatal closure. By contrast, the NADPH oxidase atrbohD/F double mutation fully suppressed inhibition of 20 A mu M ABA-induced stomatal closure by BAPTA or U73122 as well as inhibition of 30 A mu M ABA-induced stomatal closure by BAPTA, mas17 or nicotinamide. On the contrary, The Al resistant alr-104 mutation modulated ABA-induced stomatal closure by a stimulatory effect of U73122 and an increased sensitivity to mas17, nicotinamide and BAPTA. Compared to WT, the atrbohD/F double mutant was more hypersensitive than the gpa1-4 mutant to wilting under the tested water stress conditions, whereas wilting was delayed in the alr-104 mutant. Since the atrbohD/F mutation breaks down ABA-induced Ca(2+) signalling through fully preventing apoplastic Ca(2+) to enter into the guard cells, these results showed that a putative guard cell GPA1-dependent ADP-ribosyl cyclase activity should contribute to drought tolerance within PI-PLC-independent-Ca(2+)-mediated ABA signalling.