Biphasic fluence-response curves and derived action spectra for light-induced absorbance changes in Phycomyces mycelium

Light reception of Phycomyces revisited: several white collar proteins confer blue- and red-light sensitivity and control dynamic range and adaptation

The giant-fruiting body, sporangiophore, of the fungus Phycomyces blakesleeanus grows toward near-UV/blue-light (phototropism). The blue-light photoreceptor, MadA, should contain FAD bound to the LOV domain, and forms a complex with MadB. Both proteins are homologs of white collar proteins WC-1 and WC-2 from the fungus Neurospora crassa and should be localized in nuclei, where they function as a light-sensitive transcription factor complex. The photoreceptor properties of two further Wc proteins, WcoA and WcoB, remain unclear because of lack of mutants. We propose that WcoA and/or WcoB play essential roles in photoreception by enlarging the dynamic range that help explain complex stimulus-response relationships. Even though red light does not elicit photo-movement or -differentiation in Phycomyces, it affects the effectiveness of blue light which indicates an underlying photochromic receptor. Protein sequence searches show that other fungal red-light receptors are absent in Phycomyces. The solution to the red-light riddle is thus sought in the ability of Wc complexes to generate after blue-light irradiation a neutral flavosemiquinone radical that absorbs red light and functions as primary photochemical signal. Phototropism requires Ras-GAP (MadC) as part of the signal transduction cascade and, we propose, to allocate photoreceptors in the plasmalemma of the growing zone, which allows for receptor dichroism, range adjustment and contrast recognition for spatial orientation. Phototropic signal chains must entail transduction networks between Wc receptors and small G-proteins and their associated Ras-GAP and Ras-GEF proteins. The interactions among these proteins should occur in trans-Golgi vesicles and the plasmalemma of the growing zone.

Action spectrum for subliminal light control of adaptation in Phycomyces phototropism

Adaptation processes enable phototropism and other blue light responses of Phycomyces to operate over a 10-decade range of fluence rate. Phototropic latency, used routinely to monitor the kinetics of sensitivity recovery after a step down in fluence rate, can be shortened by application of dim light for 35 min during the early part of the latency period. This light is termed subliminal, because it does not elicit phototropism under these experimental conditions; rather, it exerts its influence on the underlying adaptation kinetics. Fluence rate-response data for this latency reduction, obtained at 17 wavelengths of subliminal light from 347 to 742 nm, showed a variety of shapes that could be fit by zero, one, or two sigmoidal components, plus a constant term. At most wavelengths, the fluence-rate threshold for latency reduction by subliminal light tended to be well below the absolute threshold for phototropism, indicating that this effect is highly sensitive. An action spectrum for the sensitivity of the subliminal light effect, derived from the fluence rate-response curves, shows major peaks around 400 and 500 nm and a broad band from 570 to 670 nm, followed by a steep absorption edge. The sensitivity in the near ultraviolet region is relatively very low. The magnitude of the latency reduction also depends strongly on wavelength with a maximum at about 450 nm. The fluence-rate response data and the action spectrum--which is markedly different from that for phototropism and other blue-light responses of Phycomyces--indicate the participation of multiple pigments, or pigment states, in the photocontrol of adaptation.

Light-controlled adaptation kinetics in Phycomyces: evidence for a novel yellow-light absorbing pigment

When sporangiophores of the fungus Phycomyces blakesleeanus adapt from high to low fluence rate, dark adaptation (sensitivity recovery) can be accelerated by dim subliminal light [Galland et al. (1989) Photochem. Photobiol. 49, 485-491]. We measured fluence rate-response curves for this acceleration under the following conditions. After sporangiophores were initially adapted symmetrically to a fluence rate of 1 W m-2 (447 nm), they were exposed to unilateral subliminal light (subthreshold for phototropism) of variable wavelength and fluence rate, and then to unilateral test light (447 nm) of fluence rate either 10(-3) or 10(-5) W m-2. The duration of the subliminal light was chosen so that phototropism would not occur during this period. Phototropic latencies could be shortened by subliminal light that was less intense than the test light by several orders of magnitude. In experiments with the final unilateral light of fluence rate 10(-3) W m-2, the 447 nm subliminal light had a threshold (for the acceleration effect) of about 10(-11) W m-2. Yellow light of wavelength 575 nm, which itself is extremely ineffective for phototropism was extremely effective in shortening phototropic latencies in response in response to the test light. At 575 nm, the threshold was about 2 x 10(-12) W m-2. Conversely, near-UV light of wavelength 347 nm, which is highly effective for phototropism, was relatively ineffective (threshold approximately 7 x 10(-8) W m-2) in shortening the phototropic latency. Our results suggest the presence of a novel yellow-light absorbing pigment in Phycomyces that specifically regulates dark adaptation.(ABSTRACT TRUNCATED AT 250 WORDS)

Photomorphogenesis inPhycomyces: Fluence-response curves and action spectra

Blue light regulates vegetative reproduction inPhycomyces blakesleeanus Bgff. by inhibiting the development of microphores and stimulating that of macrophores. Fluence-response curves were obtained at twelve different wavelengths. Each response exhibits a two-step ("biphasic") dependence on fluence, as if it resulted from the addition of two separate components with different thresholds, midpoints, and amplitudes. The absolute threshold is close to 10 photons·μm(2). The threshold fluence of the low-intensity component is about 10(4) times smaller than that of the high-intensity component. The action spectra for each of the two components of the two responses share general similarities, but exhibit significant differences that might be taken to favour four separate photosystems. Additional complexity is indicated by the wavelength dependence of the saturation levels.