Signals transferred from a photoreceptor to chloroplasts

Signal transduction pathways of photoreceptors are not well understood but it is well established that phytochrome and cryptochrome move into nuclei and control the expression of light-mediated genes (Sakamoto and Nagatani, 1996; Guo et al., 1999; Kleiner et al., 1999). However, the phototransduction pathway of chloroplast movement has not been clarified. To discover whether gene expression was involved in chloroplast movement, a long protonemal cell was

Figure 1.16 Chloroplast movement induced by microbeam irradiation. (a) Weak light adapted gametophytes of Adiantum capillus-veneris, (b) irradiated with a microbeam of strong light. (c) Chloroplasts move away from the beam (1.5 hour after the microbeam irradiation) and (d) return when the light is switched off. (e) A dark-adapted gametophyte kept in the dark for 3 days, (f) partly irradiated with a strong microbeam light. (g) Chloroplasts show an accumulation response towards the microbeam but remain outside the beam (photograph taken 1.5 hour after the start of irradiation) and (h) do not enter until the light is switched off, because of the high fluence rate of the beam light. The fluence rate of both microbeams was 15 W m-2, the diameter of the microbeam was 20 |im.

Figure 1.16 Chloroplast movement induced by microbeam irradiation. (a) Weak light adapted gametophytes of Adiantum capillus-veneris, (b) irradiated with a microbeam of strong light. (c) Chloroplasts move away from the beam (1.5 hour after the microbeam irradiation) and (d) return when the light is switched off. (e) A dark-adapted gametophyte kept in the dark for 3 days, (f) partly irradiated with a strong microbeam light. (g) Chloroplasts show an accumulation response towards the microbeam but remain outside the beam (photograph taken 1.5 hour after the start of irradiation) and (h) do not enter until the light is switched off, because of the high fluence rate of the beam light. The fluence rate of both microbeams was 15 W m-2, the diameter of the microbeam was 20 |im.

separated into two parts and the portion of the cell containing the nucleus was removed. Even the enucleated cells showed chloroplast photorelocation under either red or blue light irradiation, indicating that signals from photoreceptors were transferred directly to chloroplasts (Wada, 1988b). Thus, chloroplast photorelocation is a very simple system for studying signal transduction pathways that may occur through the cell surface along the plasma membrane.

When the center of a prothallial cell cultured under weak white light was irradiated with a microbeam (e.g., 10 |im in width) of high fluence blue light (e.g., 10 W m-2), chloroplasts in the beam migrated outside of the beam. However, as soon as they reached the edge of the beam they stopped moving and remained along the border of the beam spot (Kagawa and Wada, 1999). When a similar experiment was performed using dark-adapted prothallial cells, in which all chloroplasts had moved to the anticlinal walls, chloroplasts began to move toward the beam, displaying an accumulation response, but they stopped moving at the border of the beam and remained outside the beam (Kagawa and Wada, 1999). These two experiments indicate that the signal for an accumulation response can be transmitted over a long distance, at least to the edge of the cell, but the signal for an avoidance response can only be transmitted a short distance. As soon as the microbeam of the strong light was switched off, chloroplasts rushed into the former beam-irradiated area (Figure 1.16), indicating that the lifetime of the avoidance signal was very short but that of the accumulation response was long. Moreover, under strong light conditions, the accumulation response signal could be released simultaneously with that of the avoidance response. We do not know what the signals are for the two responses, nor do we know whether they are the same or different substances. There have been long debates on whether the signal for chloroplast movement is a calcium ion (see Wada et al., 2003 for more details), but we do not have any positive evidence so far on the calcium ion theory.

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