Nitric oxide (NO)-dependent and NO-independent signaling pathways act in ABA-inhibition of stomatal opening

We investigated the role of nitric oxide (NO) in ABA-inhibition of stomatal opening in Vicia faba L. in different size dishes. When a large dish (9 cm diameter) was used, ABA induced NO synthesis and the NO scavenger reduced ABA-inhibition of stomatal opening. When a small dish (6 cm diameter) was used, ABA induced stomatal closure and inhibited stomatal opening. The NO scavenger was able to reduce ABA-induced stomatal closure, but unable to reverse ABA-inhibition of stomatal opening. Furthermore, NO was not synthesized in response to ABA, indicating that NO is not required for ABA-inhibition of stomatal opening in the small dish. These results indicated that an NO-dependent and an NO-independent signaling pathway participate in ABA signaling pathway. An NO-dependent pathway is the major player in ABA-induced stomatal closure. However, in ABA-inhibition of stomatal opening, an NO-dependent and an NO-independent pathway act: different signaling molecules participate in ABA-signaling cascade under different environmental condition.