Overexpression of Phytochrome B Induces a Short Hypocotyl Phenotype in Transgenic Arabidopsis.

The photoreceptor phytochrome is encoded by a small multigene family in higher plants. phyA encodes the well-characterized etiolated-tissue phytochrome. The product of the phyB gene, which has properties resembling those of "green tissue" phytochrome, is as yet poorly characterized. We have developed a phytochrome B overexpression system for analysis of the structure and function of this protein. Using newly generated polyclonal and monoclonal antibodies that are selective for phytochrome B, we have demonstrated high levels of expression of full-length rice and Arabidopsis phytochrome B under the control of the cauliflower mosaic virus 35S promoter in transgenic Arabidopsis. The overexpressed phytochrome is spectrally active, undergoes red/far-red-light-dependent conformational changes, is synthesized in its inactive red light-absorbing form, and is stable in the light. Overexpression of phytochrome B is tightly correlated with a short hypocotyl phenotype in transgenic seedlings. This phenotype is strictly light dependent, thus providing direct evidence that phytochrome B is a biologically functional photoreceptor. Based on similarities to phenotypes obtained by overexpression of phytochrome A, it appears that phytochromes A and B can control similar responses in the plant.