Abstract
Abstract Plasmodia of Physarum polycephalum are giant single cells that respond to UV and visible light by photomovement and photomorphogenesis. Light sensing is mediated by the Physarum phytochrome and at least three additional blue and UV light photoreceptors that converge into signal transduction pathways to control basic cellular functions. One pathway targets the cytoskeleton resulting in changes of the cell shape, the periodicity of protoplasmic streaming and the direction of cell migration during photomovement. Other pathways target gene expression and mediate photomorphogenetic responses like sporulation of starving plasmodia. The developmental program of sporulation occurs upon irreversible commitment and includes the activation of a gene expression cascade, differential regulation of the cell cycle and changes in cell motility including its regulation by external stimuli. Factors found to be involved in signal transduction are Ca2+, cyclic nucleotides and metabolic intermediates. Phylogentically, Physarum is more close to the protozoa than to any other phylum and its sensory physiology displays motifs found in both, plant and animal cells: the synergism of phytochrome and blue light photoreceptors in Physarum is a recurring theme in higher plant development and the sensory control of differentiation, cell cycle and cell motility is fundamental in developing animal cells as well. Over the years, a considerable body of mainly physiological data on photosensing in Physarum plasmodia, namely photoreceptors involved, signal transduction mechanisms and downstream cellular targets has accumulated. This review puts these data together to draw a clearcut picture of the Physarum photobiology. It is meant to provide an up-to-date basis for new start-ups employing the genetic tools nowadays available for Physarum polycephalum, gene replacement and time-resolved somatic complementation analysis. These tools together with the extreme experimental manipulatability of plasmodial cells make Physarum a unique cellular model which promises not only answers to old and new questions in photobiology and but also new general insight into central pathways of cell control.
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