Protein Binding to the Abscisic Acid - Responsive Element 1 s lndependent of VIVIPAROUSI in Vivo

The plant hormone abscisic acid and the transcriptional activator VIVIPAROUSl have a synergistic effect on transcript ion during embryo development. An abscisic acid-responsive element (ABRE) mediates induction by abscisic acid and VlVlPAROUSl, but the mechanism involved has not been determined. In this study, we explore the interaction between abscisic acid and VIVIPAROUSl and i ts effect on the ABRE from the maize rab28 gene. In transient transformation experiments, abscisic acid stimulated transcription via several elements, whereas activation by VIVIPAROUSl was mediated exclusively through the ABRE. In vivo footprinting showed only minor differences in binding to the ABRE between wild-type and VlVlPAROUSl -deficient embryos, suggesting that VlVlPAROUSl stimulates transcription through the ABRE without major changes in protein-DNA interactions. A factor that bound to the ABRE in electrophoretic mobility shift assays was present at the same developmental stages as rab28 mRNA and had binding characteristics similar t o those observed by in vivo footprinting. This suggests that the factor binds to the ABRE in the rab28 promoter in vivo. We discuss the constraints that our results put on the possible mechanism for action of VlVlPAROUSl in vivo.