Induction of chloroplast movement

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When part of a long protonemal cell (Yatsuhashi et al., 1985) or a two-dimensional gametophyte (Kagawa and Wada, 1994) was irradiated with a red or blue light microbeam (either slit, spot, or rectangular in shape, from a few micrometers to 10 or 15 ^m in diameter) at a weak fluence rate (e.g., 1 W m-2), chloroplasts outside the beam moved towards the light irradiated area. When the fluence rate of blue light was increased above 10 W m-2, chloroplasts moved away from the beam. Red light does not induce an avoidance response within the range of fluence rate which is reasonable for physiological function (Yatsuhashi et al., 1985).

Polarized light is also very effective at inducing chloroplast photorelocation in fern protonemata (Figure 1.13) (Yatsuhashi et al., 1987a, 1987b). When the sides of protonemal cells sandwiched between an agar surface and a cover slip were irradiated with polarized red or blue light, chloroplasts moved depending on the vibration plane of the polarized light. If the vibration plane was perpendicular to the protonemal axis, chloroplasts moved towards the light source. But if the vibration plane was parallel to the cell axis, theoretically the chloroplasts should not have moved in any direction but should stay in their original position. To obtain these results, however, polarized light should be applied exactly from the side of the protonema as a parallel light beam hitting the cell perpendicularly. If the light source is a long fluorescent lamp, for example, protonemata can easily be irradiated obliquely or, in extreme cases, from their tip or base, so the chloroplasts move on both sides of the protonemata.

When polarized light was applied from the direction of the protonemal tip or base along the growing axis, i.e., parallel to the cell axis, chloroplasts moved to the cell wall parallel to the vibration plane of the polarized light (Yatsuhashi et al., 1987a, 1987b). The reason why polarized light induces such an effect on chloroplast movement is that the photoreceptors mediating chloroplast movement are localized on the plasma membrane or attached to the plasma membrane through other membrane proteins. In this case a transition moment of the photoreceptor is arranged parallel, perpendicular, or obliquely with the plasma membrane as in the case of polarotropism (Yatsuhashi et al., 1987a, 1987b). Because the photoreceptors mediating chloroplast movement and polarotropism were identified as those mentioned in Section 1.10, the intracellular arrangement of photoreceptors should be the same. The distinction between the two phenomena is the different sites where photoperception and the response occur in the protonemata. The apical part of protonemata is the site for polarotropism and the linear part is the site for chloroplast movement. This can be explained easily with the drawing shown in Figure 1.14. In the case of phytochrome it

Figure 1.13 Chloroplast photorelocation movement induced by polarized red light with different vibration planes irradiated from different directions as shown in each scheme. The chloroplast distribution can be explained by tetrapolar distribution of high (H) and low (L) densities of accelerated photoreceptor molecules. The transition moments of photoreceptors shown in this diagram occur before light absorption. Circles are cross-sections of protonemata. Arrows and bars indicate the direction of polarized light and their vibration planes. The chloroplast distributions shown in the photographs were those observed from the z axis. (Modified from Wada and Kagawa, 2001.)

Figure 1.13 Chloroplast photorelocation movement induced by polarized red light with different vibration planes irradiated from different directions as shown in each scheme. The chloroplast distribution can be explained by tetrapolar distribution of high (H) and low (L) densities of accelerated photoreceptor molecules. The transition moments of photoreceptors shown in this diagram occur before light absorption. Circles are cross-sections of protonemata. Arrows and bars indicate the direction of polarized light and their vibration planes. The chloroplast distributions shown in the photographs were those observed from the z axis. (Modified from Wada and Kagawa, 2001.)

has been proposed that the red light absorbing form of phytochrome (Pr) has its transition moment parallel to the plasma membrane, but the far-red light absorbing form of phytochrome (Pfr) is perpendicular (Etzold, 1965). All dichroic effects observed in ferns, mosses, and some algae could be explained with these arrangements of Pr and Pfr. But the transition moments of Pr and Pfr are not necessarily at exactly 90°, i.e., perpendicular with or parallel to the cell walls. If the transition moment of Pr molecules in protonemata is close to parallel rather than perpendicular to the plasma membrane as shown in Figure 1.14, parallel-polarized light has a tendency to be absorbed more than perpendicular light, and vice versa.

Figure 1.14 Schematic illustrations of Acneo1 arrangement in protonemata of Adiantum capillus-veneris. The transition moment of the red light absorbing form (Pr) is parallel to the plasma membrane and that of the far-red light absorbing form (Pfr) is perpendicular. Cross-sections (top) and longitudinal sections (bottom) of protonemata are shown. Double arrowhead lines indicate the vibration plane of polarized light. The transition moment of Acneo1 parallel to the vibration plane of polarized light can absorb the light efficiently and convert between Pr and Pfr repeatedly.

Figure 1.14 Schematic illustrations of Acneo1 arrangement in protonemata of Adiantum capillus-veneris. The transition moment of the red light absorbing form (Pr) is parallel to the plasma membrane and that of the far-red light absorbing form (Pfr) is perpendicular. Cross-sections (top) and longitudinal sections (bottom) of protonemata are shown. Double arrowhead lines indicate the vibration plane of polarized light. The transition moment of Acneo1 parallel to the vibration plane of polarized light can absorb the light efficiently and convert between Pr and Pfr repeatedly.

These polarized light effects cannot be seen clearly unless protonemal cells are submerged in water, because polarized light cannot penetrate into a cell evenly by refraction. The vibration plane may be randomized, and polarized light does not reach some parts of the other side of the cells because of the lens effect of the cylindrical protonemata when cells are held above the agar surface.

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