Spectroscopic and Photochemical Characterization of Flavoproteins and Carotenoproteins as Blue Light Photoreceptors

Do plant photoreceptors act at the membrane level?

All of the photoreceptors involved in the absorption and transduction of light energy in photosynthesis are integral (carotenoid, chlorophyll) or peripheral (phycobilin) membrane proteins. The informational photoreceptors (phytochrome) and the flavoprotein (carotenoprotein?) cryptochrome, could be integral (carotenoprotein, flavoprotein) or peripheral or soluble (phytochrome, flavoprotein) pigment-protein complexes. The primary activity of the informational photoreceptors is unlikely to involve energization of primary active transport: the solute fluxes produced in this way would not form a quantitatively significant link in the perception-transduction-response sequence. By contrast, regulation of mediated solute fluxes at the plasmalemma could effect a substantial amplification of the absorbed photon signal, i.e. a large change in moles of solute transported could result from the absorption of 1 mol of photons. Modulation of the passive influx (or active efflux) of protons or calcium ions at the plasmalemma are likely targets for regulation by photoreceptors. Calcium flux regulation is particularly attractive in view of the ubiquity of calmodulin activity in eukaryotes, although problems could arise in maintaining the uniqueness of phytochrome messages vis-à-vis cryptochrome messages. Temporal analysis of the relation between photoreceptor changes and electrical effects resulting from changes in ion fluxes cannot, in general, rule out the involvement of intermediates between the redox or conformational change in the photoreceptor and the observed change in ion flux. Although slow in terms of the potential rate of change on solute fluxes resulting from direct interaction of a photoreceptor and a solute porter, the observed rates of signal transduction are well in excess of any obvious ‘need’ on the part of the plant in terms of rates of response to environmental changes.

Blue light photoreception in Neurospora circadian rhythm: evidence for involvement of the flavin triplet state

The mechanism of the photoreceptor acting on the circadian conidiation rhythm of Neurospora crassa was studied, with the following results: (1) the efficiency of 8-haloflavins as sensitizers increased with their triplet yields. (2) Phase shifts were not abolished by removal of oxygen prior to illumination. (3) Oxygen inhibited phase shifts when introduced into the cultures after light treatment. It is proposed that the blue light photoreceptor for the circadian clock of Neurospora crassa acts (1) from its triplet state, but (2) not via singlet oxygen; (3) signal transduction involves (an) oxygen-sensitive intermediate(s).

Electrophoretic analysis of proteins from Phycomyces mutants with abnormal tropisms

Certain phototropism mutants of Phycomyces blakesleeanus show defective bending responses (tropisms) to stimuli besides light, such as gravity, wind, and barriers. These so-called "stiff" mutants are affected in four genes (madD to madG). Using two-dimensional gel electrophoresis, we have analyzed polypeptides from microsomal and soluble fractions obtained from the wild type, four single mutants, and six double mutants affected in all pairwise combinations of the four genes. Consistent differences in spot patterns for madE and madF mutants were found in microsomal fractions but not in soluble fractions. In madE mutants, two spots designated E1 (52 kDa, pI 6.65) and E2 (50 kDa, pI 6.65) were altered. E1 appeared denser in the wild type than in the madE mutants, while the reverse was true for E2. The spots E1 and E2 are probably under regulatory control by madE, perhaps involving posttranslational modification. A protein spot, F1 (53 kDa, pI 6.1), was present on the wild-type gels but absent from all gels for madF mutants. The F1 polypeptide probably represents the madF gene product.

The blue-light reaction in plasmodia of Physarum polycephalum is coupled to respiration

The influence of inhibitors of energy metabolism (2-deoxy-D-glucose, monoiodoacetate, KCN) as well as various substrates for respiration (sodium acetate, glycine, glutamine, α-ketoglutarate, pyruvate) were investigated with respect to the effect of blue light (450 nm) on contractile behaviour of plasmodial strands of Physarum polycephalum. When the energy metabolism is not experimentally modified, blue light induces a prolongation of the period of the contraction-relaxation cycle. This effect appears within 2-3 min and seems to represent the primary reaction of this organism to blue light. Inhibition of respiration by KCN completely abolished this response to blue-light irradiation. In contrast, an impediment of glycolysis enhanced the effect. This indicates that the reaction to blue light is related to respiration, i.e., to the function of mitochondria. Among different substrates for respiration only α-ketoglutarate combined with pyruvate and applied in the presence of inhibitors of glycolysis showed an enhancement of the photoresponse, i.e., a prolongation of the period and an increase of the amplitude of the force oscillations. This indicates that the pyruvate and α-ketoglutarate-dehydrogenase complexes functioning in mitochondrial respiration are involved in the primary blue-light reaction of plasmodia of Physarum polycephalum.