INTRACELLULAR PROCESSING OF A SPATIALLY NONUNIFORM STIMULUS - CASE-STUDY OF PHOTOTROPISM IN PHYCOMYCES

The phototropic reaction is manifested in asymmetrical (non-vertical) growth caused by unilateral irradiation in organs which under symmetrical irradiation (or in darkness) grow vertically. Thus, the spatial non-uniformity of both stimulus and response is the core of this phenomenon. A comprehensive theory should, therefore, treat phototropic signal processing as a sequence of pattern formations. Some steps toward such a theory have been carried through recently in the study of phototropism of giant sporangiophores (fruit bodies) of the lower fungus Phycomyces. Among other spatial factors, this classic phototropic object is also characterized by dichroism of the non-uniformly spaced membrane bound photoreceptor(s). Quantitative treatment of the spatial aspects - light pattern, pattern of excitation (energy absorbed per second on the site of photoreceptor location), pattern of adaptation - revealed some previously ignored general problems of spatially distributed signal processing. Among these are severe restrictions of the conventional interpretation of the action spectra, as well as of the purely temporal experimental kinetics of response and adaptation. Furthermore, this approach has provided new information about the location of the photoreceptor(s) and about the mechanism of negative tropism. New effects have been predicted and confirmed experimentally, and the physiological and genetic dissection of the presumably branched phototropic transduction chain now appears feasible. An intriguing question now is whether the following combined problem can be approached: on the basis of theoretical calculations, and measurements under incident light of varying polarization states, of the phototropic responses of sporangiophores immersed in media with varying refractive index, can one find both the in situ orientation of the receptor molecules (more exactly, their transition dipoles) and the spatial perceptivity of the phototropic mechanism? Some general aspects of spatially distributed signal processing and adaptation are also discussed.