Abscisic acid (ABA) and gibberellins (GAs) control several developmental processes including seed maturation, dormancy, and germination. The antagonism of these two hormones is well-documented. However, recent data from transcription profiling studies indicate that they can function as agonists in regulating the expression of many genes although the underlying mechanism is unclear. Here we report a rice WRKY gene, OsWRKY24, which encodes a protein that functions as a negative regulator of both GA and ABA signaling. Overexpression of OsWRKY24 via particle bombardment-mediated transient expression in aleurone cells represses the expression of two reporter constructs: the beta-glucuronidase gene driven by the GA-inducible Amy32b alpha-amylase promoter (Amy32b-GUS) and the ABA-inducible HVA22 promoter (HVA22-GUS). OsWRKY24 is unlikely a general repressor because it has little effect on the expression of the luciferase reporter gene driven by a constitutive ubiquitin promoter (UBI-Luciferase). As to the GA signaling, OsWRKY24 differs from OsWRKY51 and -71, two negative regulators specifically function in the GA signaling pathway, in several ways. First, OsWRKY24 contains two WRKY domains while OsWRKY51 and -71 have only one; both WRKY domains are essential for the full repressing activity of OsWRKY24. Second, binding of OsWRKY24 to the Amy32b promoter appears to involve sequences in addition to the TGAC cores of the W-boxes. Third, unlike OsWRKY71, OsWRKY24 is stable upon GA treatment. Together, these data demonstrate that OsWRKY24 is a novel type of transcriptional repressor that inhibits both GA and ABA signaling.

To investigate the crosstalk of abscisic acid (ABA) and gibberellin (GA) signaling in wheat (Triticum aestivum), we have focused on the transcription factor TaABF1. TaABF1 (a member of the ABA response element binding factor family) physically interacts with PKABA1, a signaling component in the ABA-suppression of GA-induced gene expression in cereal grains. Constitutive expression of TaABF1 in aleurone cells of imbibing grains completely eliminated GA-induced expression from the Amy32b promoter. In addition to its effect on Amy32b, TaABF1 strongly stimulated expression from the ABA-inducible HVA1 and HVA22 promoters. Overexpression of TaABF1 fully substituted for exogenous ABA in the induction of these two promoters. The introduction of a construct directing RNA inhibition (RNAi) of TaABF1 did not prevent either ABA-mediated or PKABA1-mediated suppression of Amy32b expression. Similarly, the RNAi construct did not prevent ABA-induction of HVA1. These results suggest that another protein may act redundantly with TaABF1 during cereal imbibition. Although TaABF1 mRNA was downregulated during imbibition of afterripened grains, transcript levels were not significantly altered by exogenous GA or ABA, suggesting that upregulation of TaABF1 at the mRNA level is not required for its role in ABA signaling. We propose a model in which TaABF1 is involved in two separate branches of ABA signaling. In this model, TaABF1 acts downstream of PKABA1 in ABA-suppression of GA-induced gene expression, and participates (independently of PKABA1) in the stimulation of ABA-induced genes.