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First published online November 6, 2003; 10.1104/pp.103.029116 Plant Physiology 133:1471-1479 (2003) © 2003 American Society of Plant Biologists Phytochrome Modulation of Blue Light-Induced Chloroplast Movements in Arabidopsis1Department of Biology, Indiana University, Bloomington, Indiana 47405
Photometric analysis of chloroplast movements in various phytochrome (phy) mutants of Arabidopsis showed that phyA, B, and D are not required for chloroplast movements because blue light (BL)-dependent chloroplast migration still occurs in these mutants. However, mutants lacking phyA or phyB showed an enhanced response at fluence rates of BL above 10 µmol m-2 s-1. Overexpression of phyA or phyB resulted in an enhancement of the low-light response. Analysis of chloroplast movements within the range of BL intensities in which the transition between the low- and high-light responses occur (1.5-15 µmol m-2 s-1) revealed a transient increase in light transmittance through leaves, indicative of the high-light response, followed by a decrease in transmittance to a value below that measured before the BL treatment, indicative of the low-light response. A biphasic response was not observed for phyABD leaves exposed to the same fluence rate of BL, suggesting that phys play a role in modulating the transition between the low- and high-light chloroplast movement responses of Arabidopsis.
Plants have evolved a number of developmental and physiological mechanisms that allow them to adapt to changes in their environment. Many of these pathways are modulated in response to various environmental stimuli such as light, gravity, temperature, and nutrient availability (Hangarter, 1997  ). In the case of light, plants possess a variety of photoreceptor molecules that are sensitive to the quality, quantity, and direction of light within the environment. The known photoreceptors of Arabidopsis include the phytochromes (phys A, B, C, D, and E), which mainly function to detect red light (RL) and far-red light (FRL) and two distinct classes of blue light (BL) photoreceptors, the "photolyase-like" cryptochromes (cry1, cry2, and cry3) and the phototropins (phot1 and phot2; Somers et al., 1991; Ahmad and Cashmore, 1993; Dehesh et al., 1993; Reed et al., 1993, 1994; Clack et al., 1994; Liscum and Briggs, 1995; Guo et al., 1998: Kleine et al., 2003). The crys regulate inhibition of stem elongation, photomorphogenesis, entrainment of the circadian clock, leaf expansion, and anthocyanin production (Ahmad et al., 1995; Somers et al., 1998; Toth et al., 2001; Wang et al., 2001; Mockler et al., 2003). The phots mediate such BL responses as early phase inhibition of hypocotyl elongation (phot1), phototropism, stomatal regulation, and chloroplast movements (Liscum and Briggs, 1995; Christie et al., 1998; Lasceve et al., 1999; Folta and Spalding, 2001a; Kagawa et al., 2001; Kinoshita et al., 2001; Sakai et al., 2001).
Analysis of mutants lacking one or more of these photoreceptors has led to a better understanding of how plants respond to changes in their light environment. It has become increasingly clear that these photoreceptors often act redundantly, synergistically, and/or antagonistically in several different light-mediated pathways (Casal and Boccalandro, 1995
Chloroplast movements are light-directed responses that occur in a number of diverse plant groups including algae, moss, ferns, and angiosperms (Zurzycki, 1961
In nonflowering plant species like algae, moss, and ferns, chloroplast relocation can be induced by either RL or BL with the signals acting synergistically when the plants are exposed to both (Kraml and Herrmann, 1991
It has been determined recently that BL-induced chloroplast movement in angiosperms is mediated by the phots, with phot2 inducing the high-light response and phot1 and phot2 acting redundantly to mediate the low-light response (Sakai et al., 2001
Chloroplast Movements in Phy-Deficient Mutants
Previous studies have established that BL-induced chloroplast movements can be analyzed by measuring the change in RL transmittance through leaves (Inoue and Shibata, 1973
To determine if phy is required for co-activation of BL-induced chloroplast movements, we monitored changes in RL transmittance in leaves from phyABD triple-null mutant leaves (Fig. 1A) and the phy chromophore-deficient mutant hy1-100 (Fig. 1B). BL-induced chloroplast movement responses occurred in both of the phy-deficient mutants, indicating that phys A, B, and D are not required for BL induction of chloroplast movements in Arabidopsis. The involvement of phyC and E could not be completely ruled out by these results because the hy1-100 defect allows some functional phy chromophore to be produced (Davis et al., 1999
Fluence rate response curves were generated for WT and phyABD to determine if phys impact the light intensity dependencies required for the low- or high-light responses. Dark-treated leaves were exposed to various fluence rates of broad-band BL (480 ± 50 nm) for 1 h. Figure 3 shows the overall change in percentage RL transmittance from the initial dark measurement after 1 h. At low-fluence rates of BL, light transmittance decreased in WT leaves with the maximum low-light response occurring at 1.5 µmol m-2 s-1 BL. As the fluence rate of BL was increased from 1.5 to 15 µmol m-2 s-1, the decrease in RL transmittance observed diminished and eventually returned to the level seen in dark-acclimated leaves. Fluence rates of 20 µmol m-2 s-1 or higher were required to cause an increase in RL transmittance above that in dark-acclimated leaves with the magnitude of change in RL transmittance increasing in proportion to the fluence rates of BL over the range we were able to test (up to 60 µmol m-2 s-1 BL). Although exposing WT and mutant leaves to broadband BL (480 ± 50 nm) resulted in slightly less of an overall change in magnitude of percentage RL transmittance compared with the change induced by using a narrower band of BL (450 ± 25 nm; Figs. 1 and 2), the response of the phyABD triple mutant is still altered compared with WT beginning in the transition zone (1.5-15 µmol m-2 s-1) and at higher fluence rates (P < 0.05 for fluence rates 4-60 µmol m-2 s-1; Fig. 3).
The light-dependent chloroplast movements in the pallisade cells of phot mutants (phot1-5 and phot2-1) suggested that phot1 and phot2 have partly redundant roles in mediating the low-light response when the fluence rate of BL is between 2 and 16 µmol m-2 s-1 (Sakai et al., 2001
Because the absence of phyA or phyB resulted in an enhancement of the high-light response, we examined plants overexpressing phyA or phyB (Wagner et al., 1991
To determine if the effect of phy on BL-induced chloroplast movements was dependant upon RL activation in WT plants, we needed to use a region of the spectrum outside the red that could be used to measure the movement responses. Transmittance spectra of an Arabidopsis leaf after dark acclimation and exposure to a high-fluence rate of white light revealed that the regions of greatest change in transmittance were those associated with chlorophyll a, chlorophyll b, and carotenoids with the largest transmittance change occurring in the blue wavelengths (data not shown) similar to that observed in foxtail (Inoue and Shibata, 1973
Light-induced chloroplast movements may be ubiquitous in angiosperms (Zurzycki, 1961 ; Inoue and Shibata, 1973; Brugnoli and Björkman, 1992; Dong et al., 1996; Park et al., 1996; Trojan and Gabrys, 1996; Gorton et al., 1999) and also have been observed in algae, moss, and ferns (Kadota et al., 1989, 2000; Augustynowicz and Gabrys, 1999; Kagawa and Wada, 1999). In nonflowering plant species, chloroplast relocation can be induced by BL or RL. These signals can act additively, but only the RL-induced movement response is FR reversible, indicating roles for both phy and BL receptor systems (Kadota et al., 1989, 2000; Kagawa and Wada, 1996; Augustynowicz and Gabrys, 1999; Kawai et al., 2003). In contrast, induction of chloroplast movement in angiosperms has been proposed to be strictly BL dependent (Inoue and Shibata, 1973; Trojan and Gabrys, 1996) and is dependent on the phot family of photoreceptors (Kagawa et al., 2001; Sakai et al., 2001). According to Sakai et al. (2001), phot1 acts primarily to induce the low-light response at fluence rates of BL less than 2 µmol m-2 s-1. Between 2 and 16 µmol m-2 s-1, phot1 and phot2 function overlap in mediating the low-light response, although it is unclear if the two photoreceptors contribute equally within this range. Above 16 µmol m-2 s-1 BL, phot2 drives the high-light response.
Although RL alone does not induce chloroplast movement in Arabidopsis, it has been suggested that simultaneous irradiation with RL is required for full induction of the responses by BL (Kagawa and Wada, 2000 The most significant finding in this work is that phys A and B can suppress chloroplast movements at fluence rates of BL above 5 µmol m-2 s-1 as shown by a striking enhancement in the change in RL transmittance in phy-deficient mutants (Figs. 1, 2, 3, 4). This apparent enhancement of the high-light response was much more substantial for intermediate fluence rates (20 µmol m-2 s-1; Fig. 2). However, the difference between mutants and WT was still significant at higher fluence rates of BL and in the range of fluence rates in which the transition from the low- to high-light responses occurred in WT leaves (between 2-15 µmol m-2 s-1). In the transition range, there was a general decrease in RL transmittance consistent with a low-light response, yet the magnitude of change was less than that observed when leaves were exposed to lower fluence rates of BL (Fig. 3). The effect of phy is also apparent in the transition between the low- and high-fluence responses as suggested by the kinetic response of the change in RL transmittance we observed when leaves were irradiated with a transitional fluence rate of BL (Fig. 4). In WT leaves, RL transmittance increased above the dark level for about 20 min before decreasing to a level lower than that measured before treatment with BL. This biphasic response was not observed when the phyABD leaves were irradiated with the same "transitional" fluence rate of BL as WT. In the phyABD mutant, RL transmittance increased and remained high for over 2 h (Fig. 4). Phys A and B may modulate this response by either directly inhibiting the high-light signal and/or indirectly by enhancing the low-light response signal. The results with the phy overexpressors tend to support the latter hypothesis because in the presence of excess phyA or phyB, the low-light response was enhanced (Fig. 5).
Although the final effect with the transition fluence rate is indicative of an overall low-light response, initially it appears that there is a transient high-light response. Inoue and Shibata (1973
We were not able to observe a significant effect of RL or FRL on chloroplast migration in WT Arabidopsis when chloroplast movements are measured using the change in percentage transmittance of BL (Fig. 6). Exposure of WT leaves to simultaneous RL and an intermediate fluence rate of BL (25 µmol m-2 s-1) did not result in a significant change in response comparable with leaves exposed to BL only. To try to inactivate phyB, we also exposed WT leaves to FRL during the BL treatments, but we were not able to observe an effect of FRL (Fig. 6). Based on the results with the phy-deficient mutants, FRL might be expected to enhance the high-light response because FR reversibility is used as an indicator of phy action in low-fluence rate responses (Casal et al., 1998
Because chloroplast movements are fluence rate dependent and leaves of some phy mutants develop differently than WT in ways that may increase light conditions within the cell layers (e.g. smaller chloroplasts, smaller cells, and lower chlorophyll content; Chory et al., 1989 Overall, this study suggests that phyA and phyB may contribute to BL-dependent chloroplast movements by modulating the transition between the high- and low-light responses meditated by phot1 and phot2. However, at this time, it is not possible to determine if phyA and phyB are acting directly to enhance the low-light response signal, which in turn inhibits the signal for the high-light response, or if phyA and phyB have different effects, with phyA primarily enhancing the low-light response and phyB inhibiting the high-light response. Analysis of various phy and phot double mutants should help differentiate between these models or suggest an alternative mechanism. Regardless of the exact mechanism, this work suggests that the phys may play a role in the fine-tuning of light transmittance properties of leaves for maintenance of maximal photosynthetic productivity during periods when light levels are transitioning close to the photosynthetic light compensation point.
Growth Conditions Arabidopsis phy-deficient mutant seeds were obtained from Bob Sharrock (Montana State University, Bozeman; single, double, and triple mutants) and Joanne Chory (The Salk Institute, La Jolla, CA; hy1-100). PhyA and phyB overexpressor seeds were obtained from Peter Quail (University of California, Berkeley). All phy null mutants were in the Ler background. Overexpressors were in the RLD background. Seeds were sown in water-soaked Scott's Plug mix (Scotts-Sierra, Marysville, OH) and incubated at 4°C in darkness for 3 to 4 d. Plants were then grown at 23°C in a Percival growth chamber (Percival Scientific, Perry, IA) with a 12-h photoperiod using white light (80-100 µmol m-2 s-1) provided by a mixture of cool-white fluorescent and incandescent bulbs. After 2 weeks of growth, seedlings were fertilized with K-grow All-Purpose Plant Food (Kmart, Troy, MI) and every 2 weeks thereafter.
Leaves from 4- to 6-week-old WT (Columbia, Ler, or RLD), phy-deficient, and phy-overexpressing plants were excised and incubated in a dark humid chamber for 9 to 15 h. Individual leaves were then placed with the leaf blade gently sandwiched between two glass slides, and the petiole, which extended beyond the slides, was wrapped with a water-soaked paper towel to keep the leaves hydrated. The glass slides were placed so that the leaf blade covered a 5-mm hole cut in black electrical tape that covered a red Plexiglas base (Rohm and Haas no. 2423, Dayton Plastics, Columbus, OH). The sensor from a LI-COR 1800 spectroradiometer (LI-COR Inc., Lincoln, NE) was fastened directly under the 5-mm area. A 660-nm RL-emitting diode (LED, Radio Shack, Fort Worth, TX) mounted 2 cm above the 5-mm aperture of the stage provided the RL source (20-25 µmol m-2 s-1) for transmittance measurements. RL transmittance of dark-acclimated leaves was measured for at least 45 min before initiation of the indicated BL treatments. BL was provided by filtering light from a halogen fiber optic light microscope illuminator (Cole Palmer, Chicago) with a blue interference filter (450 ± 25 nm, Melles Griot, 03FIB304, Mellis Griot, Rochester, NY). The fiber optic light guide for the BL was positioned at an angle of 60° relative to the surface of the leaf. The fluence rates of BL were achieved with neutral density filters placed in the light path. RL transmittance though leaves was measured with the LI-COR 1800 spectroradiometer every 2 nm and integrated between 650 and 670 nm at the indicated time intervals. For each leaf, change in percentage RL transmittance was calculated as:
For the experiments that measured chloroplast movements using the change in transmittance of BL, excised WT (Ler) leaves were handled as described above except they were dark acclimated for 18 h and mounted on a clear 1-mm-thick glass stage. The dark-acclimated leaves were then exposed to 25 µmol m-2 s-1 BL (450 ± 25 nm, Melles Griot, 03FIB304) or in some treatments simultaneously with 25 µmol m-2 s-1 RL or 45 µmol m-2 s-1 FRL (750 ± 50 nm) generated by a 660-nm-emitting LED and an RL-/FRL-emitting diode array (Qream-2200, Barnevled, WI). The BL source was mounted perpendicular to the leaf surface, whereas the RL and FRL sources were positioned at angles of 60° and 10° relative to the surface of the leaf, respectively. BL transmittance was measured every 5 min using the LI-COR 1800 spectroradiometer and change in percentage BL transmittance was calculated as:
WT (ecotype Ler) and phyABD triple mutant leaves were excised from 6-week-old plants and dark acclimated for 17 to 24 h sandwiched between an inverted petri dish bottom and lid. Eight leaves were arranged in each petri dish so that their petioles pointed toward the center and were positioned to be on moistened Whatman filter paper (42.5-mm diameter, Whatman, Clifton, NJ). RL transmittance was measured through each leaf using a custom device that consisted of a clear Plexiglas turntable. At one position, an LI-190SA Quantum sensor (connected to a LI-COR LI-189 Quantum Radiometer Photometer) was mounted below the turntable with red LED's positioned directly above the quantum sensor. The turntable was built to hold each inverted petri dish in a specified orientation so when the turntable was rotated to eight precise positions, each of the eight leaves could be located in turn between the red LEDs and the quantum sensor for light transmittance measurements. The device design ensured that each transmittance measurement was made through the same 5-mm-diameter area of each leaf, even after a petri dish was removed and later returned to the turntable. RL transmittance was measured in the dark-acclimated leaves before and after exposure to 1 h of the different fluence rates of broadband BL (480 ± 50 nm) given from above. For each leaf, the change in percentage RL transmittance was calculated as previously described. The data presented are the average change in percentage RL transmittance for a total of eight or 32 phyABD and WT leaves, respectively, for each fluence rate of BL. Fluence rates of BL from 0.3 to 10 µmol m-2 s-1 were provided by filtering light from cool-white fluorescent light bulbs through blue Plexiglas. To obtain fluence rates between 15 and 60 µmol m-2 s-1, light from halogen flood lamps (150-W Quartzline, General Electric, Fairfield, CT) was filtered through 7 cm of 1.5% (w/v) CuSO4·7H2O (Sigma, St. Louis) and blue Plexiglas. Both BL sources provided similar spectral outputs peaking near 480 nm with approximately 100-nm half bandwidth (measured with a LI-COR 1800 spectroradiometer). Received June 24, 2003; returned for revision August 1, 2003; accepted September 18, 2003.
Article, publication date, and citation information can be found at http://www.plantphysiol.org/cgi/doi/10.1104/pp.103.029116.
1 This work was supported by the National Science Foundation (grant no. IBN-0080783) and by the U.S. Department of Agriculture (National Needs Fellowship no. 98-38420-584). * Corresponding author; e-mail rhangart{at}bio.indiana.edu; fax 812-855-6082.
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ABSTRACT
RESULTS
% RL transmittance = ((It/Io) x 100)/IA), where Io and It are the incident and transmitted RL fluence rate, respectively, and IA is the average percentage RL transmittance value measured before the BL treatments. Results are presented as the average change in percentage RL transmittance for the indicated number of leaves. 
