Plasma membrane-chloroplast interactions activated by the hyperpolarizing response in characean cells

Signaling pathways in plant cells often comprise electrical phenomena developing at the plasma membrane. The action potentials in excitable plants like characean algae have a marked influence on photosynthetic electron transport and CO₂ assimilation. The internodal cells of Characeae can also generate active electrical signals of a different type. The so called hyperpolarizing response develops under the passage of electric current whose strength is comparable to physiological currents circulating between nonuniform cell regions. The plasma membrane hyperpolarization is involved in multiple physiological events in aquatic and terrestrial plants. The hyperpolarizing response may represent an unexplored tool for studying the plasma membrane-chloroplast interactions in vivo. This study shows that the hyperpolarizing response of Chara australis internodes whose plasmalemma was preliminary converted into the K⁺-conductive state induces transient changes in maximal (F_m') and actual (F') fluorescence yields of chloroplasts in vivo. These fluorescence transients were light dependent, suggesting their relation to photosynthetic electron and H⁺ transport. The cell hyperpolarization promoted H⁺ influx that was inactivated after a single electric stimulus. The results indicate that the plasma membrane hyperpolarization drives transmembrane ion fluxes and modifies the ionic composition of cytoplasm, which indirectly (via envelope transporters) affects the pH of chloroplast stroma and chlorophyll fluorescence. Remarkably, the functioning of envelope ion transporters can be revealed in short-term experiments in vivo, without growing plants on solutions with various mineral compositions.