The phytochrome chromophore precursor, 3E-phytochromobilin, and the phycobiliprotein chromophore precursors, 3E-phycocyanobilin and 3E-phycoerythrobilin, are enzymatically converted to novel rubinoid products by purified rat liver biliverdin reductase. Phytochromobilin and phycocyanobilin are particularly good substrates for biliverdin reductase with Km and Vmax values very similar to those of the natural substrate, biliverdin IX alpha. Phycoerythrobilin is the least preferred of the three bilin substrates. 1H NMR spectroscopy of phycocyanorubin, the product of phycocyanobilin catalysis by biliverdin reductase, and comparison of absorption spectra of all three rubinoid products reveal that the C10 methine bridge is selectively reduced by biliverdin reductase without altering the A-ring ethylidene substituent. In vitro phytochrome assembly experiments demonstrate that the phytorubin products do not form photoactive adducts with recombinant apophytochrome. These results suggest that ectopic expression of biliverdin reductase in plants will prevent assembly of the functional photoreceptor and thus will potentially alter light-mediated plant growth and development.