To determine the functional homology between phytochromes from evolutionarily divergent species, we used the cauliflower mosaic virus 35S promoter to express a monocot (oat) phytochrome cDNA in a dicot plant (tomato). Immunoblot analysis shows that more than 50% of the transgenic tomato plants synthesize the full-length oat phytochrome polypeptide. Moreover, leaves of light-grown transgenic plants contain appreciably less oat phytochrome than leaves from dark-adapted plants, and etiolated R1 transgenic seedlings have higher levels of spectrally active phytochrome than wild-type tomato seedlings in direct proportion to the level of immunochemically detectable oat polypeptide present. These data suggest that the heterologous oat polypeptide carries a functional chromophore, allowing reversible photoconversion between the two forms of the molecule, and that the far-red absorbing form (Pfr) is recognized and selectively degraded by the Pfr-specific degradative machinery in the dicot cell. The overexpression of oat phytochrome has pleiotropic, phenotypic consequences at all major phases of the life cycle. Adult transgenic tomato plants expressing high levels of the oat protein tend to be dwarfed, with dark green foliage and fruits. R1 transgenic seedlings have short hypocotyls with elevated anthocyanin contents. We conclude that a monocot phytochrome can be synthesized and correctly processed to a biologically active form in a dicot cell, and that the transduction pathway components that interact with the photoreceptor are evolutionarily conserved.