LIGHT-DEPENDENT SIGNAL-TRANSDUCTION AND TRANSIENT CHANGES IN CYTOSOLIC CA2-ALGA( IN A UNICELLULAR GREEN)

The physiological function and the molecular mechanisms of Ca2+-mediat ed signal transduction processes were studied in the unicellular green alga Eremosphaera viridis by different electrophysiological and micro fluorimetric techniques. A sudden blockage of photosynthetic electron transport by darkening or inhibitors causes a transient hyperpolarizat ion of the plasma membrane, For the alga this transient hyperpolarizat ion seems to be an important mechanism to release monovalent ions and to drive the uptake of divalent cations. The transient hyperpolarizati on is due to the opening of K+ channels and is caused by a rapid trans ient elevation of the cytosolic free Ca2+ concentration ([Ca2+](cy) sp ike). Different agonists like caffeine or InsP(3) which are known to r elease Ca2+ from internal stores in animal cells, also cause a transie nt hyperpolarization and a [Ca2+](cy) spike, similar to darkening, In Eremosphaera the transient hyperpolarization can be used as an indicat or for [Ca2+](cy) spikes. The InsP(3) gated and the ryanodine/cADPR ga ted Ca2+ channels which obviously both mediate Ca2+ release from inter nal stores in Eremosphaera do not seem to be involved in the dark-indu ced [Ca2+](cy) spikes. Besides single [Ca2+](cy) spikes, the addition of Sr2+ (or caffeine in the absence of divalent cations) causes repeti tive [Ca2+](cy) spikes which may last hours and resemble [Ca2+](cy) os cillations observed in excitable animal cells. These observations sugg est that some principal molecular mechanisms causing single or repetit ive [Ca2+](cy) spikes are conserved from animal to plant cells.