RED‐BLUE‐INTERACTION IN Mesotaenium CHLOROPLAST MOVEMENT: BLUE SEEMS TO STABILIZE THE TRANSIENT MEMORY OF THE PHYTOCHROME SIGNAL

Fast chloroplast orientation in Mesotaenium from profile position to face position cannot be induced by either red (R) or blue (BL) light (in contrast to Mougeotia ). Rather interaction of light signals mediated by phytochrome and blue‐light photoreceptor(s) is essential for the response. If both light treatments are separated in time, the irradiation sequence R‐BL is much more effective than BL‐R, i. e. a gradient of the far‐red (FR)‐absorbing form of phytochrome (Pfr) renders following BL highly effective, but BL cannot increase the responsiveness to following Pfr. The memory of a R irradiation before BL holds only for some minutes, indicating that the physiological activity of Mesotaenium ‐Pfr and its photoproducts is very short‐lived. This transient signal mediated by Pfr can be transformed to a more stable internal signal by interaction with BL. The interaction process does not occur at the level of photoperception. Rather, early products of the phytochrome‐initiated signal transduction chain interact with excited cryptochrome or an early product of it; Pfr can be removed by FR before the onset of BL. The internal signal stores the directional information of the Pfr‐gradient, so that BL is now fully effective and induces chloroplast movement.