ON THE SIGNAL-TRANSDUCTION CHAINS OF TWO Pfr- MEDIATED SHORT-TERM PROCESSES: INCREASE OF ANCHORAGE and MOVEMENT OF Mougeotia CHLOROPLASTS

Abstract— We examined two published hypotheses on the signal-transduction chain of the light-oriented chloroplast movements in the fresh-water alga Mougeotia. One hypothesis postulates a Ca²+-influx controlled by a tetrapolar gradient of phytochrome in its far-red light absorbing form (Pfr). The other hypothesis postulates anchorage sites for actin-filaments even at those areas of the plasmalemma where phytochrome is in its inactive form (Pr). Calmodulin and Ca²+-sequestering vesicles are assumed to be essential links of this transduction chain.
To test these hypotheses we have studied the effects of Ca²+-entry blockers, Ca²+ deficiency and calmodulin antagonists on chloroplast movements and on chloroplast anchorage. None of our results support the Ca²+/calmodulin hypotheses mentioned above. The results and their implications (with regard to the role of Ca²', calmodulin and anchorage sites) are discussed.