BLUE-LIGHT PERCEPTION BY ENDOGENOUS REDOX COMPONENTS OF THE PLANT PLASMA-MEMBRANE

b-Type cytochromes of the higher plant plasma membrane may be reduced by irradiation with actinic blue light (light-induced absorbance change). Although this reaction has been reported to depend on the presence of an exogenous oxygen-scavenging system, significant cytochrome reduction was obtained in bean hook (Phaseolus vulgaris L. cv. ''Limburgse Vroege'') plasma membranes without any addition. An endogenous oxygen-consuming reaction is apparently sufficient to achieve a proper redox balance. A blue light-mediated absorbance change with absorbance minima at 450 and 475 nm precedes cytochrome b reduction and indicates the presence of a flavoprotein in the plasma membrane fraction. Cytochrome b reduction by blue light in the absence of an oxygen scavenger is highly sensitive to flavin photosensitizers. Glucose oxidase, which has previously been used to lower the oxygen concentration in membrane samples, was demonstrated to have a photosensitizing effect. Inhibitors of flavin photochemical reactions (KI and phenylacetic acid) were highly effective in preventing cytochrome b reduction. These results indicate that the blue light-mediated reaction probably involves an endogenous plasma membrane flavoprotein as the photoreceptor. As plasma membrane NADH-dependent oxidoreductases potentially are flavoproteins these experiments raise the question whether a plasma membrane cytochrome b and a flavin-enzyme may cooperate in blue light reactions. Evidence is also discussed, suggesting the possible involvement of oxygen radicals in the blue light-induced cytochrome b reduction.