TEMPORAL AND SPATIAL EXPRESSION PATTERNS OF PHYA AND PHYB GENES IN ARABIDOPSIS

Phytochromes A and B have discrete photosensory functions in Arabidops is. To determine whether differential temporal or spatial expression p atterns of the PHYA and PHYB genes contribute to this phenomenon the e xpression of PHYA-GUS and PHYB-GUS reporter genes has been examined in transgenic Arabidopsis. Histochemical and quantitative biochemical an alyses indicate that both transgenes are expressed extensively through out the plant, including roots, shoots and flowers, during the entire life cycle, but with strong differences between the two in expression level and photoregulation, and more limited differences in spatial exp ression patterns. The data indicate that regulation is at the transcri ptional level. In dry seed, PHYB-GUS is expressed throughout the embry o at three-fold higher levels than PHYA-GUS, which is confined primari ly to the embryonic root tip. By contrast, PHYA promoter activity, des pite strong negative regulation in shoots by light, is consistently hi gher than PHYB (two- to 20-fold) in both the light and dark in most ti ssues during all subsequent developmental phases, from seedling to mat ure adult. At the tissue level, most cells appear to express both tran sgenes at some level at all stages examined, with highest apparent act ivity in vascular tissue and root tips. With the notable exception of pollen, where high PHYB-GUS but not PHYA-GUS expression occurs, few ma jor differences are observed in the quantitative spatial distribution pattern between the two transgenes. The strongly similar spatial and t emporal expression patterns of PHYA-GUS and PHYB-GUS transgenes sugges t that the differential photosensory activity of these two phytochrome s occurs largely through differences in their (i) intrinsic biochemica l activities, (ii) relative abundances, and/or (iii) independent and s eparate reaction partners, rather than through discrete, developmental ly controlled expression patterns.