Distribution of cytochrome b photoreductions mediated by endogenous photosensitizer or methylene blue in fractions from corn and cauliflower

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Abstract
Light‐induced absorbance changes [LIAC; measured as Δ(A428–A410)] reflecting the reduction of a b‐type cytochrome and mediated by an endogenous blue light absorbing receptor have been proposed to be related to blue light physiology of fungi and higher plants. It has also been suggested that the same cytochrome specifically can be reduced by red light in the presence of methylene blue. We have investigated the distribution of LIAC between different membrane fractions from corn (Zea mays L.) coleoptiles and cauliflower (Brassica oleracea L.) inflorescences. The membrane fractions were obtained by differential centrifugation followed by partition in an aqueous polymer two‐phase system. By this procedure fractions rich in plasma membrane were obtained from both mitochondrial and microsomal fractions obtained by centrifugation. LIAC was by far most enriched in fractions also enriched in plasma membranes (identified by silicotungstic acid staining), but LIAC could be obtained also in other fractions. Our conclusion is that LIAC undoubtedly is caused by a b‐cytochrome bound to the plasma membrane, but that LIAC also may be due to other b‐cytochromes, one of which is probably located in the endoplasmic reticulum. Thus, the two assay procedures used for LIAC (blue and red light induced) could not disciminate between different b‐cytochromes giving rise to LIAC.