Reactive Oxygen Species Are Involved in Gibberellin/Abscisic Acid Signaling in Barley Aleurone Cells

Reactive oxygen species (ROS) act as signal molecules for a variety of processes in plants. However, many questions about the roles of ROS in plants remain to be clarified. Here, we report the role of ROS in gibberellin (GA) and abscisic acid (ABA) signaling in barley (Hordeum vulgare) aleurone cells. The production of hydrogen peroxide (H₂O₂), a type of ROS, was induced by GA in aleurone cells but suppressed by ABA. Furthermore, exogenous H₂O₂ appeared to promote the induction of a-amylases by GA. In contrast, antioxidants suppressed the induction of a-amylases. Therefore, H₂O₂ seems to function in GA and ABA signaling, and in regulation of a-amylase production, in aleurone cells. To identify the target of H₂O₂ in GA and ABA signaling, we analyzed the interrelationships between H₂O₂ and DELL A proteins Slender 1 (SLN1), GA-regulated Myb transcription factor (GAmyb), and ABA-responsive protein kinase (PKABA) and their roles in GA and ABA signaling in aleurone cells. In the presence of GA, exogenous H₂O₂ had little effect on the degradation of SLNl, the primary transcriptional repressor mediating GA signaling, but it promoted the production of the mRNA encoding GAMyb, which acts downstream of SLNl and involves induction of a-amylase mRNA. Additionally, H₂O₂ suppressed the production of PKABA mRNA, which is induced by ABA: PKABA represses the production of GAMyb mRNA. From these observations, we concluded that H₂O₂ released the repression of GAMyb mRNA by PKABA and consequently promoted the production of α-amylase mRNA, thus suggesting that the H₂O₂ generated by GA in aleurone cells is a signal molecule that antagonizes ABA signaling.