Light Regulation of Phytochrome Content in Wild‐type and Aphototropic Mutants of the Moss Ceratodon purpureus

ABSTRACT In filaments of the moss Ceratodon purpureus, phototropism is controlled by the photoreceptor phytochrome. Thirty‐three aphototropic mutants with a proposed defect in phytochrome‐chromophore biosynthesis were isolated and analyzed. The phototropic response of those mutants was rescued with the precursor of the phytochrome chromophore, biliverdin. Phytochrome spectral activity was measured in 19 arbitrarily chosen mutants. All contained low but still measurable quantities of photoactive phytochrome; the highest level was around 15% of the wild‐type. The level of total phytochrome (apophytochrome and holophytochrome) as assayed by immunoblotting was indistinguishable from wild‐type. The content of photoactive phytochrome in Ceratodon is light‐regulated. Phytochrome of wild‐type kept for 24 h in red light was reduced to 50% as compared to dark controls but was unaffected by blue. The red‐light‐induced decrease was partially reversible by far‐red light, indicating that phytochrome itself is the photoreceptor for this response. This regulation was further analyzed with the mutant ptr114, which contains 15% photoactive phytochrome as compared to the wild‐type. In this mutant, continuous red light given for 6 days decreased the level of spectrally active phytochrome down to 25% of dark controls, whereas the amount of phytochrome found on immunoblots was hardly reduced. This indicates that the loss of phytochrome affects only the holoprotein and implies that Ceratodon phytochrome is specifically degraded as a far‐red‐absorbing phytochrome.