Life of activated photoreceptors

The chloroplast accumulation response can be induced even by a short (a few seconds) pulse of blue or red light in a dark-adapted cell if the light intensity is high enough. The information on light perception persists in the light irradiated area for a period of time after the light pulse ends, and chloroplasts move towards the light irradiated area in the dark. This observation raises the question whether (1) the photo-activated photoreceptor continues to work as an active form after removing the light source and releasing the signals or (2) the light signal is transferred from the photoreceptors to the next components and the photoreceptors are already non-functional. The red/far-red photo-reversibility of phytochrome that mediates the red light induced chloroplast movement is an ideal system to answer this question.

When a short microbeam pulse of red light was aimed at the center of a dark-adapted gametophyte cell, chloroplasts along the anticlinal walls moved to the beam-irradiated area and remained there for a period of time and then returned to the anticlinal walls (Kagawa and Wada, 1994). Similar experiments were performed using protonemata (Yatsuhashi and Kobayashi, 1993; Kagawa et al., 1994; Figure 1.17). When chloroplast movement was induced by a pulse of polarized light applied horizontally, chloroplasts moved toward the upper and lower sides of the protonemal cell and stayed there for a period of time and returned to their original position. If far-red light was applied after chloroplast movement was induced by red light, all chloroplasts that were still moving or had already reached the upper side of the cell immediately started to move back (Kagawa et al., 1994). This indicates that without far-red light, phytochrome molecules activated by red light remained in the far-red light absorbing form (Pfr) and retained chloroplasts at the site of highest Pfr concentration. In addition, the signal transferred from the light irradiated area to the anticlinal walls is not the photoreceptor itself but represents other factors (or components) activated by Pfr. It is not possible to conduct similar experiments to confirm the results using blue light, because we do not have any means of converting activated blue light receptors to an inactive state to cancel the blue light effect. However, considering that the blue light receptor for chloroplast movement and

Figure 1.17 Effects of non-polarized far-red light, applied at various times after irradiation with polarized red light, on the orientation of chloroplast movement. Protonemata were irradiated with vertically polarized red light (3 W m-2, 10 min) and then with non-polarized far-red light (5 W m-2, 10 min) immediately after the red light, or after an intervening dark period of 10 or 40 min. Solid symbols represent the response after red light irradiation without far-red light irradiation. Open symbols represent the response after far-red light irradiation. The upper panel shows the experimental schedule for the light treatment. (Modified from Kagawa et al., 1994.)

Figure 1.17 Effects of non-polarized far-red light, applied at various times after irradiation with polarized red light, on the orientation of chloroplast movement. Protonemata were irradiated with vertically polarized red light (3 W m-2, 10 min) and then with non-polarized far-red light (5 W m-2, 10 min) immediately after the red light, or after an intervening dark period of 10 or 40 min. Solid symbols represent the response after red light irradiation without far-red light irradiation. Open symbols represent the response after far-red light irradiation. The upper panel shows the experimental schedule for the light treatment. (Modified from Kagawa et al., 1994.)

the red light receptor described here as phytochrome belong to the same family of proteins (see Section 1.10), a similar situation should be working in both systems.

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