Chapter 26 Light-controlled chloroplast movement

This chapter discusses the theory of light-controlled chloroplast movement. Chloroplast movement is an excellent model system to study signal transduction pathways in photobiology, because the response is rather simple and occurs in the cell periphery without involvement of gene expression. Not many components might be involved. Moreover, the first step is photoperception by a photoreceptor and the last step is movement by the acto-myosin system; that means the first and last components could clearly be focused, although the molecular species are not yet identified. Light-directed chloroplast movement is a widespread phenomenon in plant cells that need to perform highly efficient photosynthesis both under low and high fluence rate conditions. Chloroplasts in a mesophyll cell of a seed plant, for example, spread over the surface of the cell faced toward the light source under low and moderate light conditions, but under very high fluence rate, they move to the cell sides, i.e. anticlinal walls, to avoid photodamage of the chloroplasts (high fluence rate response, HFR). This phenomenon is called “chloroplast relocation”. In the case of some algal cells, such as Mougeotia and Mesotaenium, which have one large ribbon-shaped chloroplast in the midst of the cell between two large vacuoles, the chloroplast turns in the cell to face the light source at low fluence rates but does not move toward the light source. This phenomenon is called “photoorientation.”