Cytosolic acidification precedes nitric oxide removal during inhibition of ABA-induced stomatal closure by fusicoccin

The role of nitric oxide (NO) and the relationship between NO and cytosolic pH during inhibition of ABA effect by fusicoccin (FC) in guard cells of Vicia faba were analyzed. ABA induced NO generation and stomatal closure, but FC inhibited the effects of ABA. Treatment with 2-(4-carboxyphenyl)-4,4,5,5-tetra-methylimidazoline-1-oxyl-3-oxide (cPTIO) and N^G-nitro-L-Arg-methyl ester (L-NAME) mimicked the effects of FC. These data suggest that inhibition of ABA effect by FC is possibly related to the decreasing in the NO level. Furthermore, like cPTIO, FC not only suppressed stomatal closure and NO level in guard cells treated with NO donor sodium nitroprusside (SNP), but also reopened stomata, which had been closed by ABA, and reduced the level of NO in guard cells that had been produced by ABA, indicating that FC caused NO removal. Butyric acid simulated the effects of FC on the stomatal aperture and increased NO levels in guard cells treated with SNP and had been closed by ABA, and both FC and butyric acid surely reduced cytosolic pH, which demonstrates that cytosolic acidification mediates FC-induced NO removal. Taken together, our results show that FC induces NO removal and reduces NO level via cytosolic acidification in guard cells, thus inhibiting ABA effect.