Modification of distinct aspects of photomorphogenesis via targeted expression of mammalian biliverdin reductase in transgenic Arabidopsis plants

The phenotypic consequences of targeted expression of mammalian biliverdin IX alpha reductase (BVR), an enzyme that metabolically inactivates the line ar tetrapyrrole precursors of the phytochrome chromophore, are addressed in this investigation. Through comparative phenotypic analyses of multiple pl astid-targeted and cytosolic BVR transgenic Arabidopsis plant lines, we sho w that the subcellular localization of BVR affects distinct subsets of ligh t-mediated and light-independent processes in plant growth and development. Regardless of its cellular localization, BVR suppresses the phytochrome-mo dulated responses of hypocotyl growth inhibition, sucrose-stimulated anthoc yanin accumulation, and inhibition of floral initiation. By contrast, reduc ed protochlorophyll levels in dark-grown seedlings and fluence-rate-depende nt reduction of chlorophyll occur only in transgenic plants in which BVR is targeted to plastids. Together with companion analyses of the phytochrome chromophore-deficient hy1 mutant, our results suggest a regulatory role for linear tetrapyrroles within the plastid compartment distinct from their as sembly with apophytochromes in the cytosol.