Phototactic orientation in plasmodia of the acellular slime mold, Physarum polycephalum [Myxomycetes]

Phototactic orientation to lateral light was studied in plasmodia of the yellow-pigmented strain (CL) and a white mutant of the slime mold Physarum polycephalum. The orientation changes were performed by either a turn or a reversal of the internal polarity of the plasmodium. Fluence rate-response curves for white and monochromatic light showed positive phototaxis with low fluence rates and negative phototaxis with higher ones. The wavelength dependences for both strains were similar and indicated the predominant role of a UV/blue light-absorbing photoreceptor, which was not the same as one of the yellow pigments found in the wild-type strains. Wavelengths >500 nm induced only an insignificant effect. The control of the motor apparatus and the possible linkage of the photoresponse with the other sensory processes known in Physarum are discussed.