Detection of Cell-to-Cell Transport with Chlorophyll Microfluorometry: Selectivity of Metabolite Passage and Sensitivity to Sodium and Potassium Ions

Pulse-amplitude-modulated chlorophyll microfluorometry was used to quantify the permeation of photometabolites across the plasmodesmata in characean algae. The method is based on cyclosis-mediated sensitivity of chlorophyll fluorescence in Chara australis R. Br. internodes to illumination of a remote cell area. The application of a local light (LL) pulse to one internode in combination with measurement of fluorescence changes in the neighbor internode provided the means to assess connectivity between two adjoining internodal cells. The cytoplasmic flow seems to contain two types of photometabolites that produce opposite changes in chlorophyll fluorescence. The cytoplasm irradiated by LL pulse is enriched with reducing equivalents that produce a transient increase in F ' fluorescence owing to plastoquinone/Q A reduction; these metabolites were found to pass freely across the plasmodesmata. At high background irradiance, the “irradiated” cytoplasm contains an additional component that strongly quenches the maximal ( F_m^' ) fluorescence; this component did not permeate across the transnodal barrier. Thus, the plasmodesmata behaved like a selective filter that allowed penetration of reducing metabolites and restricted the passage of photoproducts accumulated under excessive light. The transnodal passage of permeable metabolites was further examined at various concentrations of NaCl and KCl in the medium. Both solutes inhibited the plasmodesmal conductivity but the inhibitory effect of KCl under short-term exposures was stronger than that of NaCl. In addition, the intracellular signaling was substantially suppressed upon the increase of KCl content in the medium. A possible relation between the plasma-membrane K + transporters and photosystem II operation is considered.