Obligate heterodimerization of Arabidopsis phytochromes C and E and interaction with the PIF3 basic helix-loop-helix transcription factor.

Phytochromes are dimeric chromoproteins that regulate plant responses to red (R) and far-red (FR) light. The Arabidopsis thaliana genome encodes five phytochrome apoproteins: type I phyA mediates responses to FR, and type II phyB-phyE mediate shade avoidance and classical R/FR-reversible responses. In this study, we describe the complete in vivo complement of homodimeric and heterodimeric type II phytochromes.

Heterodimerization of type II phytochromes in Arabidopsis.

Coimmunoprecipitation of members of the phytochrome red/farred photoreceptor family from plant extracts has been used to analyze their heteromeric binding interactions. Phytochrome (phy)B or phyD apoproteins with six myc epitopes fused to their N termini are biologically active when expressed in Arabidopsis. Immunoprecipitation of either of these tagged proteins from seedling extracts coprecipitates additional type II phytochromes: six myc (myc6)-phyB coprecipitates phyC-phyE; and myc6-phyD coprecipitates phyB and phyE.

Differential activities of the Arabidopsis phyB/D/E phytochromes in complementing phyB mutant phenotypes.

The Arabidopsis phyB, phyD, and phyE phytochromes regulate plant developmental and growth responses to continuous red light and to the ratio of red to far-red light. They are also more highly related in sequence to each other and more recently derived evolutionarily than phyA and phyC. In order to directly compare the signaling activities of these three photoreceptor apoproteins, an assay was developed based upon complementation of the phyB-1 null mutant phenotype with transgenes consisting of the PHYB promoter (PB) driving expression of the PHYB, PHYD, or PHYE coding sequences.

Signaling activities among the Arabidopsis phyB/D/E-type phytochromes: a major role for the central region of the apoprotein.

The Arabidopsis phyB, phyD, and phyE phytochromes regulate plant developmental and growth responses to continuous red light (R) and to the ratio of R to far-red (FR) light. The activities of these three photoreceptors in the control of seedling growth have been compared using a transgenic assay based upon induction of R-hypersensitivity of hypocotyl elongation by overexpression of the apoproteins from the 35S promoter. 35S-phyB, 35S-phyD, and 35S-phyE lines expressing similar levels of the respective phytochromes were isolated.

Patterns of expression and normalized levels of the five Arabidopsis phytochromes.

Using monoclonal antibodies specific for each apoprotein and full-length purified apoprotein standards, the levels of the five Arabidopsis phytochromes and their patterns of expression in seedlings and mature plants and under different light conditions have been characterized. Phytochrome levels are normalized to the DNA content of the various tissue extracts to approximate normalization to the number of cells in the tissue. One phytochrome, phytochrome A, is highly light labile.