Both phyA and phyB mediate light-imposed repression of PHYA gene expression in arabidopsis

The negatively photoregulated PHYA gene has a complex promoter structure in Arabidopsis, with three active transcription start sites. To identify the photoreceptors responsible for regulation of this gene, and to assess the r elative roles of the three transcription start sites, we analyzed the chang es in PHYA transcript levels in wild-type and photoreceptor mutant seedling s under various irradiation conditions. Continuous far-red or red light exp osures each induced a significant decline in transcript levels in wild-type etiolated seedlings. Analysis of mutants specifically lacking either phyA or phyB protein demonstrated that these phytochromes are required for the n egative regulation induced by far-red and red light, respectively. Ribonucl ease protection experiments showed further that this negative regulation is confined almost exclusively to the shortest, most abundant PHYA transcript , and occurs predominantly in shoots. By contrast, both of the other minor transcripts in shoots, and all three transcripts in roots, exhibit near con stitutive expression. This complex expression pattern indicates that the PH YA gene is subject to regulation by multiple signals, including environment al, developmental, and organ-specific signals.