Protein binding to the abscisic acid-responsive element is independent of VIVIPAROUS1 in vivo

The plant hormone abscisic acid and the transcriptional activator VIVIPAROUS1 have a synergistic effect on transcription during embryo development. An abscisic acid-responsive element (ABRE) mediates induction by abscisic acid and VIVIPAROUS1, but the mechanism involved has not been determined. In this study, we explore the interaction between abscisic acid and VIVIPAROUS1 and its effect on the ABRE from the maize rab28 gene. In transient transformation experiments, abscisic acid stimulated transcription via several elements, whereas activation by VIVIPAROUS1 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 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 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 results put on the possible mechanism for action of VIVIPAROUS1 in vivo.