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In the germinated cereal aleurone layer, gibberellic acids (GA) induce expression of a number of genes encoding hydrolytic enzymes that participate in the mobilization of stored molecules. Previous analyses suggest that the key events controlling the GA-regulated gene expression in the aleurone are formation of active transcription machinery referred to as the GA responsive complex, followed by recruiting GAMYB. In general, bipartite promoter contexts composed of the GA-responsive element and the pyrimidine box are observed within the regulatory regions of cereal GA-responsive genes. Protein factors that recognize each promoter sequence were identified and distinct effects on the GA-mediated activation of gene expression have been also investigated; however, the connection and intercalation between two promoter motifs remain obscure. In this study, I have evaluated cooperative function of GAMYB and a pyrimidine box-binding protein OsDOF3 that influenced the promoter activity of the most predominant GA-responsive gene (RAmy1A) of rice (Oryza saliva). Transient expression of OsDOF3 in the germinated aleurone prolonged GAMYB function on the reporter expression in the absence of GA. The synergistic effect required a set of DNA bindings of two proteins on the RAmy1A promoter region. The yeast two-hybrid assay showed the physical interaction of GAMYB and OsDOF3 in yeast cells, indicating that the association of GAMYB and OsDOF3 may be a functional unit in transcription regulation. The results showed the accessory function of OsDOF3 responsible for a dosage-dependent mediation of GA signaling that leads to high-level expression of physiological target genes.
Gibberellins (GAs) are diterpenoid hormones that play crucial roles in plant growth and development, including seed germination, leaf expansion, stem elongation, and flower and fruit development (Hooley, 1994). During the germination of cereal seeds, the de novo-synthesized GAs are transported from the embryo to the aleurone layer where GAs trigger the expression of hydrolytic enzymes through transcription activation of corresponding genes (Jacobsen et al., 1985). The induced enzyme activities are secreted into the starchy endosperm and mobilize the reserved nutrients to support the seedling growth.
Detailed analyses of the GA-regulated promoters have provided the basis for the discovery of transcription factors involved in GA actions in the aleurone. The research has culminated in determining the function of a R2R3-type Myb transcription factor GAMYB (Gubler et al, 1995). GAMYB induces the expression...