| Summary: | Phytochromes (phys) are a family of dimeric chromoprotein photoreceptors that modulate plant physiological and developmental processes in response to red (R) and far-red (FR) light. In Arabidopsis thaliana, these fall into two functional groups, type I phyA and type II phyB-E. Previous findings have shown that heterodimerization occurs in type II phytochromes and suggest that diverse dimer forms may have specific functions. The first objective of this study was to characterize the activities of individual phytochrome dimer combinations by developing a novel in vivo protein engineering system. Either obligate homodimers or heterodimers of phytochrome N-terminal regions were produced in phyB mutant plants. With this system, a highly active phyB/D heterodimeric form was shown to rescue the phyB mutant phenotype. Dimers of phyB/achromo-phyB, phyB/C, and phyB/E mediated organ-specific growth in de-etiolation by functioning differentially in cotyledons but not in hypocotyls. Light labile phyA is critical in the plant transition from skotomorphogenic to photomorphogenic growth. To investigate possible in vivo phyA heterodimerization with type II phys and the relationship between phy quaternary structure and signaling mechanisms, transgenic plants were generated that express different myc- tagged N- or C-terminal end fusion phyA proteins in a Landsberg erecta (Ler) phyA mutant or a wild-type background. Co-immunoprecipitation showed that phyA only forms homodimers with itself. Compared with fully active one myc epitope (myc1)-tagged phyAs, six myc epitopes attached to the ends of the N- or C- terminus of phyA impaired phyA-mediated far-red high irradiance (FR-HIR) signaling and also attenuated degradation in the light, indicating that alteration of phyA architecture may damage protein-protein interaction both in phyA downstream signaling and in its protein turnover. Overall, these findings have expanded the structurally complex R/FR sensing systems in plants and have implications for how plant growth and development may be fine-tuned through phy heterodimer-mediated tissue-specific growth or phy-modified activity.
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