Light‐Dependent Chloroplast Reorientation in Mougeotia and Mesotaenium: Riased by Pigment‐Regulated Plasmalemma Anchorage Sites to Actin Filaments?

Conjugatophycean green algae, such as Mougeotia and Mesotaenium , are presumably the most ancient organisms to show phytochrome‐mediated photomodulatory processes, i.e. chloroplast reorientational movements. Experiments have provided striking evidence for a dichroic mode of light absorption by the phytochrome molecules located at the periphery of the cylindrical cell; in addition, the transition moment of the chromophoric group of phytochrome has been shown to change by a fixed angle upon conversion of P r to P fr and vice versa . Consequently, a hypothesis has been put forward involving a tetrapolar phytochrome gradient at the plasmalemma. This presumed pigment pattern precisely controls chloroplast reorientation in the low‐irradiance response. Intriguingly, a blue‐light absorbing pigment is expressed in Mougeotia as well, which also mediates low‐irradiance response via a presumed tetrapolar gradient, apparently independent of the phytochrome. Two hypotheses for the controlling mechanism of chloroplast reorientation have been put forward: Coupling of the influx of calcium through the plasmalemma to the tetrapolar gradient of the sensor pigment proper, resulting in a tetrapolar gradient of calcium in the cytoplasm. This is the “reorientation via calcium” hypothesis. Coupling of actin anchorage sites on the plasmalemma to the tetrapolar gradient of the sensor pigment proper, resulting in a tetrapolar gradient of actin anchorage sites. Cytoplasmic calcium, released from internal stores or taken up through the plasmalemma, triggers actomyosin interaction. This is the “reorientation via anchorage sites” hypothesis. Consistent with the latter hypothesis, photoregulation by two steps seems to be indicated, (i) cytoplasmic initiation of actomyosin interaction, (ii) the graded formation of plasmalemma anchorage sites for actin filaments.