LIGHT-ORIENTED CHLOROPLAST POSITIONING - CONTRIBUTION TO PROGRESS IN PHOTOBIOLOGY

Intracellular rearrangements of chloroplasts with respect to the direction of light, eg. as found in Vaucheria, Adiantum, Mougeotia and Mesotaenium, are model systems for investigating the processing of light signals from perception through transduction to the terminal response. Two photosensory pigments, namely blue-/UV-absorbing pigment and - in a few exceptional species - the red/far-red reversible phytochrome system, are localized in the cytoplasm outside the chloroplasts. Dichroic orientation of the pigments is important for the detection of light direction via generation of an intracellular absorption gradient. Chloroplast movement, resulting in altered positioning, is based on the actomyosin system. Light-governed reorganization of the cortical and/or subcortical actin cytoskeleton results in biased chloroplast translocation. Besides polymerization or anchoring of actin to cell structures, the activity of myosin may be a point of control in some species. Local changes in membrane properties, including transient modulation of the membrane potential, appear to be the earliest steps in signal tranduction. The role of calcium and calmodulin, obviously involved in some species, is not clear as yet.