β-Phaseolin gene activation is a two-step process: PvALF- facilitated chromatin modification followed by abscisic acid-mediated gene activation

We have shown previously that a rotationally and translationally positioned nucleosome is responsible for the absence of transcriptional expression from the phaseolin (phas) gene promoter in leaf tissue and that the repressive chromatin structure is disrupted on transcriptional activation during embryogenesis. To investigate how the chromatin structure is modified, we ectopically expressed PvALF, a putative seed-specific phas activator, in leaf tissue of a tobacco line transgenic for a chimeric phas/uidA construct. DNase I footprinting in vivo revealed that the ectopic expression of PvALF resulted in remodeling of the chromatin architecture over the TATA region of the phas promoter but did not lead to transcriptional activation in the absence of abscisic acid (ABA). Treatment of the transgenic tobacco leaves with ABA in the absence of PvALF neither alleviated the repressive chromatin architecture nor activated transcription. However, in the presence of PvALF, high levels of β-glucuronidase expression were obtained on exposure of leaves to ABA. These results reveal that expression from the phas promoter involves at least two discrete steps: chromatin potentiation by PvALF followed by ABA-mediated transcriptional activation.