Membrane Excitation and Cytoplasmic Streaming as Modulators of Photosynthesis and Proton Flows in Characean Cells

Abstract

Internodal cells of Chara corallina represent a unique model system to study interactions between photosynthesis, membrane excitation, and cytoplasmic streaming, as well as the role of these processes in generation and regulation of functional patterns in green cells and tissues. It is established that the inflow of cytoplasm from darkened cell parts promotes photosynthetic activity of chloroplasts residing at intermediate irradiance, whereas the arrival of cytoplasm from illuminated regions suppresses this activity and enhances nonphotochemical quenching. The vectorial movement of the “irradiated” cytoplasm induces functional asymmetry around the light spot (pattern formation) both in the chloroplast layer and in the plasma membrane. The messenger transported between illuminated and shaded cell parts was found to move at the velocity of cytoplasmic streaming. The effects of membrane excitation (action potential) on photosynthesis and membrane H+ transport are area specific; they are mediated by different mechanisms under physiological conditions and in the presence of some redox-cycling compounds. The influence of action potential on chlorophyll fluorescence under spot illumination appears to involve the activation of Ca2+-mediated pathways and the suppression of metabolite exchange between darkened and illuminated cell parts due to the stoppage of cyclosis. The cytoplasmic flow from darkened to illuminated cell parts seems to enhance interactions between respiratory and light-dependent metabolism, which promotes photosynthesis and protects chloroplasts from photooxidative damage under excess light.