J. Plant Res. 109: 139-146, 1996 Journal of Plant Research (~ by The Botanical Society of Japan 1996

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Photoregulation Systems for Light-Oriented Chloroplast Movement

Hiroko Yatsuhashi

Department of Biology, Faculty of Education, Miyazaki University, Gakuen-Kibanadai-Nishi, Mtyazaki, 889-21 Japan

Movements of the chloroplasts induced by a directional stimulus of light are found to occur in various plant materials ranging from algae to terrestrial angiosperms. Depending on the fluence rate of light, chloroplasts move toward the area of the maximum light absorption and escape from it, which are named as low- and high-fluence-rate responses, respectively. In most materials the effective wavelengths are exclusively found in the blue-UV region of spectrum, (a) flavin pigment(s) being considered as the photoreceptor, while in a few species of plants phytochrome is involved. The arrangement of chloroplasts as a result of the move- ment depends on the orientation of the electrical vector of light, which reflects the dichroic orientation of the photor- eceptor for perception of light direction. Photosystems involved in these responses, however, are not only for perception of light direction, but also for realization of the movement and for holding of the chloroplasts in the reached site. Possible interactions and dual roles of photosystems in regulation of chloroplast movement are discussed.

Key words: Blue-light photoreceptor m Chloroplast movement m Dichroic orientation (photoreceptors)-- Multi- ple photosystems-- Phytochrome

Light-oriented chloroplast movement is a phenomenon in which lateral light induces directional movement of chloro- plasts. One of the characteristics of the response, most clearly shown by local irradiation experiments of a cell, is that chloroplasts respond to a restricted area where light is absorbed, or to light-absorption gradient in a cell. Accord- ingly, we can estimate the area of light absorption in a cell under a certain light condition from the response of chloro- plasts, which gives us important information about dichroic arrangement of physiologically active photoreceptor mole- cules. Furthermore, various interactions of photosystems to modify the response are observed. Another feature of the response that occurs within a cell and in a relatively short periods after the signal perception should be valuable for investigation of the signal transduction chain.

Many detailed reviews for this field are available now. Some of them are as follows: Zurzycki (1980) compiles the responses induced by BL. There are various reviews by

Abbreviations: BL, blue light; E-vector, electrical vector; FR, far- red light; Pfr, far-red-light-absorbing form of phytochrome; Pr, red- light-absorbing-form of phytechrome; R, red light

Haupt and his coworker especially on the responses mediat- ed by phytochrome in Mougeotia and Mesotaenium (Haupt 1982, 1983a, 1983b, 1983c, Haupt and Wagner 1984, Haupt 1987, Haupt and Scheuerlein 1990, Haupt 1991a). Reviews by SchSnbohm (1980) and Haupt (1991b) on the interaction with phytochrome and BL-specific photoreceptor(s) are also valuable. Wada et al. (1993) review the recent progress in the mechanisms of photoperception and movement in detail.

This article places a special emphasis on photosystems which control the response of chloroplasts. First, I will outline the patterns of the response, active wavelengths, and the photoreceptors involved. Second, I will describe the arrangement of photoreceptors to detect light direction, which has been revealed by the chloroplast response, and discuss the interaction of photosystems to modify the response, especially possible coaction of photosystems in fern protonemata.

Effective wavelengths and photoreceptors in low- and high-fluence-rate responses

In the first extensive article on this phenomenon, Senn (1908) shows that chromatophores including chloroplasts and phaeoplasts behave differently to light of low and high fluence rates. Usually, under unidirectional white light, chloroplasts migrate or rotate so that they can receive more light in "low-fluence-rate response", while in "high-fluence- rate response" they behave so as to avoid light. In Adiantum protonemata as well as Vaucheria, which has many small chloroplasts in a cell, Iow-fluence-rate light accumulates chloroplasts in the area of the front and the rear to the light source, and high-fluence-rate light assem- bles them at both flanks of a cell, respectively. In a cylindri- cal cell of Mougeotia or Mesotaenium a single flat rectangu- lar chloroplast rotates on their long axis so as to expose its face to Iow-fluence-rate light and its edge to high-fluence- rate light. The ecological significance of these two responses may be considered to optimize photosynthesis and to protect photosynthetic pigments from photodestruc- tion. In fact, in a brown alga Dictyota dichotoma photodes- truction of photosynthetic pigments is less when phaeoplasts (often called chloroplasts) are in the high-fluence-rate arrangement than in Iow-fluence-rate one (Hanelt and Nultsch 1991), and the efficiency of light utilization in photo- synthesis is closely related to chloroplast arrangement in Oxalis, Marah and Cyrtomium (Brugnoli and Bj6rkman 1992).

What kinds of photoreceptors are involved in chloroplast

]40 H. Yatsuhashi

movement has attracted our interest for many years. Senn (1908) reports wavelength dependency of these responses: blue light (BL, the light passed through copper oxide/ammo nia solution) is exclusively effective in both low- and high- fluence-rate responses of various materials with some exceptions as Mesocarpus (=Mougeotia) in which only orange-red light (the light passed through potassium di- chromate solution) can induce the Iow-fluence-rate response.

Precise action spectra are now available in various materials from algae to terrestrial higher plants, most of which show that the light of the wavelengths from UV to 500 nm are effective with peaks at around 360 nm, 450 nm and/ or 480 nm, and in Selaginella and Funaria the maximum action is shown around 270 nm (Zurzycki 1980). Similar shapes of action spectra are obtained in both low- and high- fluence-rate responses. The photoreceptors involved in these responses is assumed to be flavin pigments. Then a question may arise: How the response is switched from Iow- fluence-rate one to high-fluence-rate one ? However, at present there is no clear answer to this question.

A distinguished feature is reported in Mougeotia and Mesotaenium, in which red light (R) is predominantly effective in Iow-fluence-rate response as has been shown by Senn, being mediated by phytochrome (Haupt 1959), while only BL can induce high-fluence-rate response (Sch6nbohm 1963, Haupt and G&rtner 1966). In addition to a R peak, an action spectrum for Iow-fluence-rate response in Mougeotia has a minor BL peak which is ascribed to a BL-specific photorece- ptor (Gabrys et al. 1985).

The involvement of phytochrome has been considered to be restricted in those algae. It turns out, however, that chloroplast movement in protonemata of some ferns is mediated by both phytochrome and BL photoreceptor. Red light is half as effective as BL in an action spectrum for the Iow-fluence-rate response in Adiantum protonemata (Yatsu- hashi et al. 1985), although under some culture conditions R is more effective than BL (cf. Yatsuhashi et al. 1987b). The effect of R is suppressed by simultaneous FR, while the effect of BL is not. Similar effects of R and BL are shown in Dryopteris sparsa (Yatsuhashi and Kobayashi 1993). In Adiantum protonemata, a high-fluence-rate response of chloroplasts is induced by R as well as BL, although only local irradiation with R of extremely high fluence rates of 230 to 500 Wm -2, compared with 10 Wm -2 in BL, can induce light-avoiding response of chloroplasts (Yatsuhashi et al. 1985). This R response is ascribed to lower Pfr level in the beam area than in the vicinity resulted from partial photodes- truction of phytochrome in the beam area, and the conver- sion of Pr to Pfr by light scattered around the beam (Yatsuha- shi and Wada 1990). In another fern Pteris protonemata, however, BL solely induces the response (Kadota et al. 1989). More data are required to reveal phylogenetic relationship of photoreceptors for this response.

Localization and dichroic orientation of photoreceptors for perception of light direction

Arrangement of chloroplasts as a result of the oriented movement depends on the polarization of the inductive light, suggesting a dichroic orientation of the photoreceptor molecules. Because it is a response to a local gradient of light absorption in a cell, chloroplast response caused by appropriate use of local irradiation and polarized-light irradi- ation serves for analysis of the localization and the dichroic orientation of the photoreceptors for perception of light direction.

Local irradiation of a cell has revealed that, in most cases, a chloroplast itself has no system of light perception for chloroplast movement, and has suggested that photorece- ptors are localized in the periphery of a cell (Haupt 1982). This conclusion is also supported by action dichroism of light, which implies association of the photoreceptor mole- cules with a membranous structure as mentioned below.

Obviously one of the most important contributions of the investigation into chloroplast movement to photobiology is demonstration of the dichroic orientation of the photorece- ptors. The following paragraphs show a model for estima- tion of the photoreceptor orientation in a cylindrical cell of Adiantum or Dryopteris protonema, which is summarized in Figs. 1 and 2.

As the first step one should know the behavior of chloro- plasts in response to light-absorption gradient of photorece- ptors generated by local irradiation of a cell with a mi- crobeam. In the next step, a cell is irradiated with polarized light having the electrical vector (E-vector) in a particular orientation. Since light is propagated as transverse waves oscillating in electric and magnetic fields, the polarization axis, usually expressed with the E-vector, is perpendicular to the propagation direction. The light absorption by a photor- eceptor molecule is proportional to cos28 where ~ is an angle between the E-vector of the incident light and the absorption axis, i.e. the electronic transition moment, of the molecule. Accordingly the maximum and the minimum absorption by the molecule are caused when the E-vector is parallel and perpendicular to the absorption axis of the molecule, respectively. Although the direction of light prop- agation may have some effects on the absorption gradient in a cell through refraction or attenuation of light, in the follow- ing case it hardly affects the pattern of chloroplast accumu- lation. Therefore it is enough to consider the spatial rela- tionship between the E-vector of light and the cell.

In Adiantum or Dryopteris, it is reported that local irradia- tion of a protonema with a R microbeam of a low fluence rate (0.01 to 10 Wm -2 in Adiantum) causes chloroplasts to move from a distance to the irradiated area and to accumulate in it (Yatsuhashi et al. 1985, Yatsuhashi and Kobayashi 1993) as shown in Fig. 1 (2nd row, left). Because the effect of a R beam is suppressed by simultaneous FR irradiation, and a FR beam combined with whole-cell R irradiation expels chloro- plasts from the beam area (Fig. 1, 2nd row right), it is clear that chloroplasts accumulate in the area of higher Pfr concentration. When whole protonemata of these ferns are

Photoregulation of Chloroplast Movement 141

Dark

Local irradiation with (an) unpolarized beam(s)

R

Q FR

R~--> R~ R~ R:~

" 'rPgra~ ~ ( ~ 0 ( ~ 0 F~

FR

Pr Pfr Onentation ~ of the photoreceptor

Fig. 1. Chloroplast responses induced by local irradiation and polarized-light irradiation with R and FR, and estimated orientation of the photoreceptor, phytochrome, in Adiantum and Dryopteris protonemata. The circles show the cross views of protonemata and the small ellipses represent chloroplasts. E-vectors of light are indicated by double-headed arrows and asterisks (~), the latter denoting vectors perpendicu- lar to the plane of the figure. In the dark, chloroplasts distribute randomly in the periphery of a cell (top row). By local irradiation with an unpolarized R microbeam of low fluence rate, they gather in the irradiated area (2nd row, left), and move out of the area where a FR beam is applied simultaneously with whole-cell R irradiation (2nd row, right). Polarized R which has the E-vector perpendicular to the protonema axis (3rd row, R~) induces chloroplast accumulation along the part of the cell wall parallel to the vector. Polarized R with the E-vector parallel to the cell axis (3rd row, R =~ ) does not induce any particular arrangement of chloroplasts, but when FR with the perpendicular E-vactor (3rd row, FR~, I ) is added to the R, the FR accumulates chloroplasts along the part of the cell wall parallel to the vector. Possible orientations of the absorption axes (dashes) of the photoreceptors, Pr and Pfr, can be estimated from the responses as shown in the bottom row.

irradiated with polarized R of a similar fluence rate, the response of chloroplasts depends on the angle between E- vector of the light and the protonema axis. When the E- vector of R is perpendicular to the protonema axis, chloro- plasts accumulate along the part of the cell wall parallel to the E-vector, while light with the E-vector parallel to the protonema axis does not induce any particular arrangement of chloroplasts (Fig. 1, 3rd row, left). Such an action di- chroism is considered to indicate that the photoreceptor molecules have a rigid dichroic orientation, thus they are associated with stable structures, possibly cytoplasmic membrane (Haupt 1982). The response of chloroplasts leads to the deduction that the R-absorption axes of the photoreceptor, or Pr, molecules are oriented parallel to the cell wall in Adiantum or Dryopteris protonemata (Fig. 1, bottom, left), because the axes are estimated to be nearly parallel to the E-vector in the part where chloroplasts accumulate, and nearly perpendicular to the vector in the part where chloroplasts are dispersed. This conclusion is consistent with the Pr orientation obtained in Mougeotia and Mesotaenium (Haupt 1960, Haupt and Thiele 1961).

The action dichroism of FR given simultaneously with R indicates that rotation of the transition moment of phyto- chrome molecules takes place during photoconversion of phytochrome as shown in Fig. 1, right (Yatsuhashi et al. 1987b, Yatsuhashi and Kobayashi 1993), where the FR- absorption axis of Pfr is preferentially perpendicular to the cell surface. This was first assumed in the polarotropic growth of Dryopteris filix-mas protonemata by Etzold (1965) to interpret the findings that the effective orientation of E- vector of R to convert Pr was different from that of FR to revert Pfr, and directly demonstrated using a microbeam of polarized light to cause chloroplast rotation in Mougeotia (Haupt 1969).

The rotation of phytochrome chromophore is shown in vitro by measuring linear dichroism of Pr and Pfr in non- oriented immobilized molecules (Sundqvist and Bj5rn 1983a, 1983b, Ekelund et al. 1985) or molecules oriented uniaxially (Tokutomi and Mimuro 1989). Although the difference in an angle of the transition moments between Pr and Pfr is estimated at less than 90 ~ i.e. approx. 30 ~ in the former work, and 7 ~ in the latter, it is considered enough to explain the

]42 H. Yatsuhashi

physiological results in Mougeotia (Bj6rn 1984, Tokutomi and Mimuro 1989).

Dichroic orientation of a BL photoreceptor has been assumed also from polarotropism in Dryopteris fi/ix-mas protonemata (Etzold 1965), where the absorption axis for BL is considered to be oriented parallel to the cell surface. The BL photoreceptor(s) for chloroplast movement in Adiantum is demonstrated to have a similar orientation. In the Iow- fluence-rate response by BL at 0.1 to 1.0 Wm -2, the behavior of chloroplasts is similar to that by R (Yatsuhashi et a/. 1985), i.e. a microbeam of these fluence rates attracts chloroplasts in the irradiated area, and polarized light with the E-vector perpendicular to the protonema axis induces chloroplast accumulation along the cell wall parallel to the E-vector (Fig. 2, top and 2nd rows, left). On the other hand, a BL microbeam of higher fluence rates than 10 Wm -2 expels chloroplasts from the beam area, and polarized BL of a corresponding fluence rate causes location of chloroplasts along the cell wall perpendicular to the E-vector of the light

Local irradiation with an unpolarized beam

Low-fluence-rate High-fluence-rate response response

BL(L) BL(H)

BL(L) ~ BL(H) Polarized- ~ G light irradiation

Orientation ~ of the photoreceptor

Fig. 2. Low- and high-fluence-rate responses of chloroplasts induced by local irradiation and polarized-light irradiation with BL, and estimated orientation of the BL- photoreceptor(s) in Adiantum protonemata. Local irradia- tion with a BL microbearn of low fluence rate (top row, left, BL(L)), induces chloroplast accumulation in the beam area, while a BL beam of high fluence rate (top row, right, BL(H)) expels chloroplasts. In the latter case, chloroplast aggre- gation, probably because of light scattered from the beam, is observed in the vicinity of the beam. Polarized BL of low fluence rate with the E-vector perpendicular to the protonema axis accumulates chloroplasts along the part of the cell wall parallel to the E-vector, while high fluence rate polarized BL having the same E-vector expels chloroplasts to the part of cell wall perpendicular to the vector. From these responses orientations of the absorption axes of the corresponding photoreceptors are estimated as shown in the bottom row. Otherwise as in Fig. 1.

(Fig. 2, top and 2nd rows, right, Yatsuhashi and Wada 1990). The deduced orientation of the BL-absorption axes of the photoreceptors for both low- and high-fluence-rate responses is parallel to the cell surface (Fig. 2, bottom row). This is also the case with the BL photoreceptor in Iow- fluence-rate response of Dryopteris sparsa (Yatsuhashi and Kobayashi 1993). In Mougeotia, Kraml (1994)recently showed surface-parallel orientation of the absorbing axis of the BL photoreceptor for the Iow-fluence-rate response.

When one changes the incident direction of polarized light, chloroplast response gives further information about the dichroic orientation of the photoreceptor. In the surface view of Adiantum protonemata, the absorption axes of both Pr and BL photoreceptor for the Iow-fluence-rate response are suggested to be arranged randomly or along double- handed helixes (Yatsuhashi et al. 1987a), in contrast to Mougeotia where the axis of Pr orients along a single- handed helix (Haupt and Bock 1962).

Besides refraction and attenuation of light, surface-paral- lel orientation of the photoreceptor molecules can be an effective mechanism for perception of light direction, even if the light is unpolarized. Under a saturated-light condition for phytochrome, rotation of the chromophore is advanta- geous to establish an Pfr gradient (Haupt 1983c).

Interaction and dual roles of photosystems in regulation of chloroplast movement

More than one photosystem act in the orientation move- ment of chloroplasts, not only to detect light direction, but also to realize the movement or to maintain the chloroplast accumulation. One should note that an action spectrum obtained with single irradiation at varied wavelengths does not necessarily reflect the precise absorption of photorece- ptors actually involved, because such an action spectrum might be a result of an interaction of multiple photoreceptor systems. Some action peaks might stand for overlapping of multiple photoreceptor absorption and other peaks might be hidden because (a) cooperating photoreceptor(s) is/are not excited. Dichromatic or sequential irradiation might give further information on photoreceptors in some cases.

Interaction of phytochrome and a BL photoreceptor sys- tem is first reported in high-fluence-rate response in Mougeotia (Sch6nbohm 1963,1966). Although an action spectrum for the high-fluence-rate response shows the action solely in BL region, simultaneous irradiation with BL and R reveals that phytochrome operates to detect the light direction, and high fluence rate BL reverses the direction of chloroplast rotation with respect to Pfr. Namely, at the high fluence rate of BL, the edge of a chloroplast rotates toward the area of higher Pfr level instead of the area of lower Pfr level as observed in the Iow-fluence-rate response. This BL effect referred to as tonic effect is independent of light direction. Blue light is tonically effective only when it is given prior to or simultaneously with R, thus a product of the excited BL photoreceptor is considered to interact with Pfr (Gabrys et al. 1985, see also a review by Haupt 1991b).

In Iow-fluence-rate response of Mesotaenium, a BL-spe-

Photoregulation of Chloroplast Movement ]43

cific photoreceptor enhances or stabilizes phytochrome action. Unlike in Mougeotia, the Iow-fluence-rate response in Mesotaenium requires continuous R irradiation, for which a R pulse combined with simultaneous or sequential irradia- tion of BL can be substituted (Kraml et al. 1988). After a R pulse, chloroplasts rotate only during the BL irradiation more rapidly than under continuous R irradiation. Also in this case, directional signal is perceived solely by phytochrome. This phenomenon is interpreted as follows: BL interacts with an early product of gradient Pfr to produce more stable signal, and then act for realization of chloroplast movement along this directional information (Kraml and Herrmann 1991). Thus BL has a dual effect in the Iow-fluence-rate response in Mesotaenium.

Involvement of photosynthesis or photosynthetic pigments is also reported. In an action spectrum for high-fluence- rate response in Vallisneria spiralis, an action peak is found in the red region, in addition to the blue region, being ascribed to photosynthesis (Seitz 1967). Interaction of phytochrome and photosynthesis is reported in Iow-fluence- rate response in Vallisneria gigantia where Pfr enhances random movement of chloroplasts and photosynthesis reduces mobility of them, resulting in chloroplast accumula- tion along outer periclinal walls (Nagai 1993, Dong et al. 1993). Accordingly, in this case, phytochrome is not involved in the perception of light direction, but in regulation of rate of the movement.

In Adiantum, although both phytochrome and (a) BL-spe- cific photoreceptor(s) are involved in chloroplast responses, no sequential interaction of these photoreceptors has been observed unlike in Mougeotia. An additive effect of R and BL is found in prothallial cells to induce Iow-fluence-rate response (Kagawa and Wada 1966). In protonemata, how- ever, there is some contradiction in the assumption that concentration-dependent Pfr action by itself mediates the Iow-fluence-rate response induced by R. This problem will be discussed in the following chapter.

Paradox in chloroplast responses in Adiantum and Dryopteris

Local irradiation experiments as shown in Fig. 1 clearly indicate that chloroplasts move along the concentration gradient of Pfr in Adiantum and Dryopteris protonemata. To induce full responses, however, continuous irradiation of R for more than 2 hr is required (Yatsuhashi et al. 1985, Yatsuhashi and Kobayashi 1993). Single R-pulse irradiation can induce response in some extent in Adiantum, but it is low and transient compared with the response by continuous irradiation (Kagawa et al. 1994). Furthermore, the final effect of continuous irradiation depends on its fluence rate. Fluence-rate dependency is also observed when two adja- cent areas of an Adiantum protonema are irradiated continu- ously at different fluence rates. Chloroplasts in the lower fluence rate area move to the area of the higher fluence rate, when the ratio of these fluence rates is higher than 1.5 (Fig. 3, Yatsuhashi et al. 1987). Since the phytochrome in both areas is in photoequilibrium, the chloroplast response is not

W m -2 W m -2

Fig. 3. Chloroplast responses to two adjacent R microbeams of different fluence rates. Protonemata are continuously irradiated for 2 hr with R beams at indicated fluenoe rates (insides the dotted lines) until observation. Chloroplasts originally situated in the Iower-fluence-rate area move to the area of higher fluence rate when the ratio of the fluence rates is 2.0 (top and bottom), but does not move when the ratio is 1.24 (middle), even though the absolute difference is the same as that in the top.

seemed to be induced by Pfr gradient alone. For the requirement of continuous irradiation and the fluence-rate dependency in these responses, there are three possibilities:

(1) So-called "high irradiance reaction" of phytochrome mediates the response.

(2) The response requires continuous supply of "fresh" Pfr.

(2)-1 The life time of Pfr is very short, i.e. it reverts to Pr or degrade rapidly.

(2)-2 Physiological activity of Pfr is lost in a short time, which is called Pfr aging in Mougeotia and Mesotaenium.

] 44 H. Yatsuhashi

(3) In addition to Pfr gradient, some factor which is produced by continuous irradiation depending on the fluence- rate is necessary for chloroplast accumulation. Interconver- sign between Pfr and Pr, or photosynthetic energy supply may be a candidate for the factor. In this case, chloroplasts will move toward the area receiving a higher fluence rate, if the Pfr concentration is uniform.

In Adiantum, an action spectrum for Iow-fluence-rate response determined by 3 hr-irradiation has the R peak at 680 nm (Yatsuhashi et al. 1985) which is much shorter than the wavelength of usual high irradiance reaction peak, 716 nm in the growth inhibition of lettuce hypocotyls (Hartmann 1967), and simultaneous FR suppresses R effect. Thus it may be impossible to ascribe this response solely to the so- called high irradiance reaction of phytochrome. In Mesotaenium, Pfr is considered to be so unstable that continuous irradiation of R is required to induce Iow-fluence- rate response (Haupt and Reif 1979). In Mougeotia, Pfr aging induces "inversion Iow-fluence-rate response", in which a chloroplast reorients according to a gradient of newly formed Pfr rather than that of "aged" one (Haupt 1966). This possibility is examined in Dryopteris sparsa (Yatsuhashi and Kobayashi 1993). After the end of R irradiation, ac- cumulated chloroplasts maintains their arrangement for about 3 hr without further accumulation before they gradually disperses into a random arrangement as are observed in the dark. A FR pulse at the end of R shortens the duration of the maintenance of the arrangement by 2 hr. These results indicate that physiologically active Pfr, at least being active in maintaining the response, remains for 2 hr after the end of the irradiation, while it cannot induce further translocation of chloroplasts. Thus it seems more probable that Pfr alone can determine the orientation of the movement, and hold the accumulated chloroplasts, but induction of the movement requires (an) other factor(s) which can be supplied by contin- uous irradiation as in the possibility (3) rather than (2). In Adiantum protonemata, a 10-min R pulse induces chloroplast translocation during the following dark period up to 60 min, at which time Pfr is still remain and holds chloroplasts, but does not cause increment of the response thereafter (Kagawa et al. 1994) in agreement with the above hypothesis. Johnson and Tasker (1979) propose a model to account for phyto- chrome responses, in which both Pfr concentration and Pr/ Pfr cycling rate are effectors and they interacted multi- plicatively to promote the response. Bartley and Frankland (1982) suggest that phytochrome has a dual role in seed germination of Sinapis, namely Pfr promotes germination in its early stage and subsequently a time- and fluenoe-depen- dent process inhibits germination. For the chloroplast movement in Adiantum and Dryopteris protonemata, a pos- sible explanation may be that full response is induced by synergistic coaction of two processes which depend on Pfr concentration and interconversion rate of Pr and Pfr, respec- tively. The hypothesis should be examined in future investi- gations, in which the complex photoprocesses leading to the final response should be analyzed step by step. Recent investigations have revealed that different molecular species of phytochromes have distinct functions (Smith 1995).

Although a molecular family of phytochrome in ferns has not been fully clarified yet, it would be also possible that different phytochromes play different roles in chloroplast movement in ferns as well.

In Adiantum and Dryopteris protonemata, continuous irra- diation of BL is also required to induce a detectable response (Yatsuhashi et al. 1985, Yatsuhashi and Kobayashi 1993, Kagawa et al. 1994). A fluence-rate dependency in BL action is observed as in R (Yatsuhashi et al. 1985,1987b). In contrast to R, however, chloroplasts accumulated by BL immediately begin to disperse into random arrangement after the end of the irradiation in Dryopteris. Requirement of continuous irradiation is commonly observed in BL-induced chloroplast responses, although a short BL pulse can induce a transient response in several species such as Tradescantia, Chlorophytum, Lemna, (Gabry~ et al. 1981), Dictyota (Hanelt and Nultsch 1989) and Adiantum prothalli (Kagawa and Wada 1994). Short life times of the excited state of photoreceptor and an early product of the transduction chain (Zurzycki et al. 1983) may be a cause of the rapid disappearance of the BL effects. At present, there is no evidence for the involve- ment of additional photosystem in the BL action besides the system to detect light direction in these responses. How- ever, because BL can induce two opposite responses of chloroplasts depending on its fiuence rates as previously mentioned, it might be possible that there is an independent BL photosystem to detect the fluence rate and switch the responsiveness of chloroplasts, just as in the high-fluence- rate response in Mougeotia.

In conclusion, there are some pieces of evidence that light- oriented chloroplast movement is performed by integration of photoresponses which induce the movement, determine its direction and maintain the arrangement of chloroplasts. Interactions and dual roles of photosystems are observed to control these responses.

I gratefully acknowledge the critical reading of the manu- script by Professor T. Hashimoto, Kobe Women's University.

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(Received June 9, 1995: Accepted April 4, 1996)