PROTEIN-BINDING TO THE ABSCISIC ACID-RESPONSIVE ELEMENT IS INDEPENDENT OF VIVIPAROUS1 IN-VIVO

The plant hormone abscisic acid and the transcriptional activator VIVI PAROUS1 have a synergistic effect on transcription during embryo devel opment. An abscisic acid-responsive element (ABRE) mediates induction by abscisic acid and VIVIPAROUS1, but the mechanism involved has not b een determined. In this study, we explore the interaction between absc isic acid and VIVIPAROUS1 and its effect on the ABRE from the maize ra b28 gene. In transient transformation experiments, abscisic acid stimu lated transcription via several elements, whereas activation by VIVIPA ROUS1 was mediated exclusively through the ABRE. In vivo footprinting showed only minor differences in binding to the ABRE between wild-type and VIVIPAROUS1-deficient embryos, suggesting that VIVIPAROUS1 stimul ates transcription through the ABRE without major changes in protein-D NA interactions. A factor that bound to the ABRE in electrophoretic mo bility shift assays was present at the same developmental stages as ra b28 mRNA and had binding characteristics similar to 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 res ults put on the possible mechanism for action of VIVIPAROUS1 in vivo.