Light-induced changes of cytosolic pH in Eremosphaera viridis: recordings and kinetic analysis

Abstract

ATPase in the plasmalemma. Thus recovery from acid load in the cytosol can be performed by proton translocaShort-term light-induced pH changes were measured tion across one of the adjacent membranes, tonoplast or by means of pH-selective microelectrodes in the uni- plasmalemma, or, alternatively, by a biochemical pH-stat cellular green alga Eremosphaera viridis. Cytosolic pH mechanism. It is still unknown which mechanisms contribchanges were always transient and reversible within ute to the pH-stat mechanism under varying conditions. 1‐3 min. Taking into account the low-pass filtering by One way to approach this question is to investigate the the experimental set-up, the light-dependent cytosolic light-induced changes of cytosolic pH. They provide a pH changes could be described by a sum of two expo- physiological approach to disturbing cytosolic pH and to nential functions. By mathematical analysis it is dem- investigate short-term pH regulation. These changes have onstrated that the transient nature of light-dependent been observed in a number of plant species and share pH changes was due to a consecutive pH-stat common characteristics. Darkening of green plant cells regulation and not to a parallel light-triggered reaction. induces a transient acidification and illumination causes The short-term pH regulation depended linearly on the a transient alkalization (Felle and Bertl, 1986; Steigner pH-deviation with no indication of a feedback control et al., 1988; Okazaki et al., 1994). These pH deviations loop. Vacuolar light-dependent pH changes were were shown to be caused by the light-dependent proton measured to investigate further which pH-stat mech- translocation across the thylakoid membrane. Proton anisms are responsible for the back regulation of the fluxes across the chloroplast envelope propagate the pH light-dependent cytosolic pH changes. Vacuolar pH deviations from the chloroplast stroma to the cytosol changes were too small to explain the recovery of the ( Thaler et al., 1992; Hansen et al., 1993). cytosolic pH after darkening or illumination by H+ The transient character of these light-dependent pH fluxes across the tonoplast. changes is not understood. It is generally assumed that eYcient short-term pH-control mechanisms are respons