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PLANT PHYSIOLOGY , Vol 107, Issue 2 523-534, Copyright © 1995 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Phytochrome-Mediated Light Regulation of PHYA- and PHYB-GUS Transgenes in Arabidopsis thaliana Seedlings
D. E. Somers and P. H. Quail
Department of Plant Biology, University of California, Berkeley, Berkeley, California 94720
Phytochrome wild-type gene-[beta]-glucuronidase (PHY-GUS) gene fusions were
used in transgenic Arabidopsis to compare the activity levels and light
regulation of the PHYA and PHYB promoters and to identify the
photoreceptors mediating this regulation. In dark-grown seedlings, both
promoters are 4-fold more active in shoots than in roots,but the PHYA
promoter is nearly 20-fold more active than that of PHYB in both organs. In
shoots, white light represses the activities of the PHYA and PHYB promoters
10- and 2-fold, respectively, whereas in roots light has no effect on the
PHYA promoter but increases PHYB promoter activity 2-fold. Consequently,
PHYA promoter activity remains higher than that of PHYB in light in both
shoots (5-fold) and roots (11-fold). Experiments with narrow-waveband light
and photomorphogenic mutants suggest that no single photoreceptor is
necessary for full white-light-directed PHYA repression in shoots, but that
multiple, independent photoreceptor pathways are sufficient alone or in
combination. In contrast, phytochrome B appears both necessary and
sufficient for a light-mediated decrease in PHYB activity in shoots, and
phytochrome A mediates a far-red-light-stimulated increase in PHYB promoter
activity. Together, the data indicate that the PHYA and PHYB genes are
regulated in divergent fashion at the transcriptional level, both
developmentally and by the spectral distribution of the prevailing light,
and that this regulation may be important to the photosensory function of
the two photoreceptors.
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