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First published online July 22, 2005; 10.1104/pp.105.062026 Plant Physiology 138:1994-2004 (2005) © 2005 American Society of Plant Biologists Possible Involvement of Phototropins in Leaf Movement of Kidney Bean in Response to Blue Light1Department of Biology, Faculty of Science, Kyushu University, Ropponmatsu, Fukuoka 810–8560, Japan
The leaf of kidney bean (Phaseolus vulgaris) moves in response to blue light. The movement is induced by a decrease in the turgor pressure of pulvinar motor cells on the irradiated side. In this study, we investigated the initial event of the movement with respect to function of phototropin and the plasma membrane H+-ATPase in the motor cells. The results indicated that, in dark conditions, phototropin existed in a dephosphorylated state and the H+-ATPase existed in a phosphorylated state. A pulse of blue light (30 s) induced the phosphorylation of phototropin and the dephosphorylation of the H+-ATPase as determined by the binding behavior of 14-3-3 protein. Phototropin phosphorylation occurred rapidly, followed by the transient gradual dephosphorylation of the H+-ATPase. When the specific flavoprotein inhibitor diphenyleneiodonium and the protein kinase inhibitors K-252a and staurosporine were administered to pulvinar cells, both phototropin phosphorylation and H+-ATPase dephosphorylation were inhibited. The phosphorylation and dephosphorylation exhibited similar fluence rate dependencies to blue light. These results indicated that phototropin may function upstream of the plasma membrane H+-ATPase and decrease the activity of H+-ATPase by dephosphorylation. We provide evidence for the existence of three kinds of phototropins in pulvinar motor cells.
Many leguminous plants orient their leaves in response to blue light, and this response is independent of leaf movement by circadian rhythms (Koller, 1990  ; Coté, 1995; Nishizaki, 1996). In kidney bean (Phaseolus vulgaris), blue light is absorbed by the laminar pulvinus, the leaf-moving site located between the petiole and leaf blade (Fig. 1F), and induces leaf movement through pulvinar bending (Koller, 1990; Coté, 1995). This bending is mediated by a decrease in turgor pressure of pulvinar motor cells, which results in a decrease in cell volume on the irradiated side of the pulvinus (Wang et al., 2001). The decrease in turgor pressure is caused by water efflux from the motor cells, which is brought about by effluxes of K+ and Cl– (Lowen and Satter, 1989; Okazaki et al., 2000). This net K+-salt efflux is driven by depolarization-activated K+ channels in the plasma membrane (Moran et al., 1988; Moran, 1996; Suh et al., 2000), and the depolarization is likely triggered by inactivation of the plasma membrane H+-ATPase in response to blue light (Kim et al., 1992; Nishizaki, 1994; Okazaki et al., 1995; Wang et al., 2001). Recently, Okazaki (2002) used pulvinar protoplasts from Phaseolus to demonstrate that blue light decreased ATP hydrolysis by inactivating the H+-ATPase. However, biochemical evidence for this response is insufficient, and the blue light receptor responsible for the inactivation of H+-ATPase has yet to be identified.
Nishizaki (1988) suggested that a molecular mechanism similar to the blue light response of stomatal guard cells may operate in pulvinar motor cells, although the direction of electrical response is the opposite. Stomatal opening, which facilitates gas exchanges between the leaf and the atmosphere, is driven by increased turgor pressure in guard cells. This increase is induced by membrane hyperpolarization through the activation of the plasma membrane H+-ATPase and the subsequent influx of K+ via hyperpolarization-activated K+ channels (Hedrich and Schroeder, 1989; Schroeder et al., 2001). In guard cells, blue light activates the H+-ATPase via phosphorylation of the C terminus with subsequent binding of 14-3-3 protein to the H+-ATPase (Assmann and Shimazaki, 1999; Kinoshita and Shimazaki, 1999; Palmgren, 2001).
In recent studies using Arabidopsis (Arabidopsis thaliana) mutant plants, phototropins, which are receptor-type protein kinases with two light, oxygen, and voltage (LOV) domains (Huala et al., 1997 In this study, we provide biochemical evidence for inactivation of the plasma membrane H+-ATPase by blue light in pulvinar motor cells and suggest that phototropin may operate in this response. We also found the expression of three phototropin mRNAs in pulvini from kidney bean.
Blue Light Induces Pulvinar Bending of Kidney Bean
To show leaf movement, the plant was kept in the dark for 10 h before being irradiated with continuous blue light at 100 µmol m–2 s–1 for 120 min from above (Fig. 1, A and B). Before the irradiation, the leaves were closed and pointing completely down (Fig. 1A, 0 min) and rose to the horizontal position in response to the blue light, greatly increasing the light-receptive area (Fig. 1, A and B, 120 min). The leaf reached its highest position at 90 min of blue light, and this position was maintained for the rest of the irradiation period (Fig. 1E). When the dark-adapted plant was irradiated at the same intensity of blue light from the side, the leaves oriented to the source of light, although the orientation was limited by the contact of two leaves (Fig. 1D). Such leaf movement maximized the light-receptive area (Fig. 1C). These leaf movements were brought about by the bending of a laminar pulvinus located on the base of each leaf blade (Fig. 1F). The properties of leaf movement may account for those of solar tracking in Phaseolus plant (Ritter and Koller, 1994
Since 14-3-3 protein binds to the C terminus of the H+-ATPase in a phosphorylation-dependent manner and the binding is essential for activation of the H+-ATPase (Fuglsang et al., 1999
Phosphorylation of Phototropin Precedes Dephosphorylation of the H+-ATPase in Response to Blue Light in Pulvini
We investigated the phosphorylation status of phototropin in the pulvinar motor cells through the binding of 14-3-3 protein to phototropin. Using the antibodies for Vicia phototropins, we found initially that phototropin with molecular mass of 118 kD was present in the motor cells (Fig. 2D). When the pulvini were irradiated with a pulse of blue light, 14-3-3 protein rapidly bound to phototropin and reached a maximum within 1 min after the pulse began, after which the binding decreased gradually, returning to the basal levels within 10 min (Fig. 2C). When the pulvini were irradiated with a second blue light pulse 20 min after the first pulse start, 14-3-3 protein bound again (data not shown). The blue light irradiation did not affect the amount of phototropin (Fig. 2D). The phototropin also exhibited a mobility shift to the upper side of the nonirradiated position, suggesting that the phototropin was phosphorylated in response to blue light as described previously (Short et al., 1993
We determined the amounts of 14-3-3 protein bound to the H+-ATPase and to phototropin in pulvini in response to blue light with various fluence rates. Dark-adapted pulvini were irradiated with a pulse of blue light, and the reaction was stopped 1 min after the start of a pulse for phototropin and 5 min after the start for the H+-ATPase. As the fluence rate of blue light increased, the amount of 14-3-3 protein bound to the H+-ATPase decreased (Fig. 3A), whereas the amount bound to phototropin increased (Fig. 3A). Both reactions were saturated near 50 µmol m–2 s–1 when the pulse duration was 30 s (Fig. 3E). The half saturation of the fluence rates for the dissociation of 14-3-3 protein from the H+-ATPase and the binding of 14-3-3 protein to phototropin were 11.2 µmol m–2 s–1 and 7.4 µmol m–2 s–1, respectively, suggesting that both reactions have essentially the same fluence rate dependency (Fig. 3E). The mobility of phototropin on SDS-PAGE decreased with increase in the fluence rate (Fig. 3D), suggesting that the levels of phototropin phosphorylation increased by blue light (Kinoshita et al., 2003
Phosphorylation Levels of the H+-ATPase and Phototropin in Response to Blue Light in Pulvini
We considered that the binding of 14-3-3 protein to the plasma membrane H+-ATPase reflected phosphorylation levels of the H+-ATPase. However, there is no direct evidence for this in pulvinar motor cells. We tried to radiolabel the H+-ATPase by administration of 32Pi to the cells to determine the phosphorylation status, but it was not successful probably due to insufficient incorporation of 32P into the motor cells. We thus tested phosphorylation levels of the H+-ATPase by blue light-dependent mobility shift of the C terminus on SDS-PAGE as shown in Vicia guard cells (Kinoshita and Shimazaki, 1999
Phosphorylation levels of phototropin were also determined by the mobility shift of the protein on SDS-PAGE in response to blue light (Short et al., 1993 ; Salomon et al., 2003; Knieb et al., 2004). In this study, we have already shown the mobility shift of phototropin by blue light (Figs. 2D and 3D). We confirmed the mobility shift of phototropin by blue light and that the shift was strongly suppressed by a Ser/Thr kinase inhibitor K-252a (Fig. 4C). The binding of 14-3-3 protein to phototropin was inhibited by K-252a in the same way (Fig. 4D). The results suggest that the binding of 14-3-3 protein to phototropin depends on the phosphorylation levels of phototropin.
To investigate whether the binding and dissociation of 14-3-3 protein in both the H+-ATPase and phototropin occurs in vivo, we immunoprecipitated these proteins with individual antibodies and inspected the coprecipitated proteins. Microsomal fractions were isolated from the pulvini that were kept in darkness or irradiated with blue light. As shown in Figure 5A, 14-3-3 protein was coprecipitated with the H+-ATPase, and the amount of the 14-3-3 protein decreased by 58% in response to blue light. The result indicates that the 14-3-3 protein dissociated from the H+-ATPase in vivo by blue light. We confirmed that the same amount of H+-ATPase was used in this experiment (Fig. 5B).
We next immunoprecipitated the proteins using antibodies against 14-3-3 protein with microsomal fractions. We found that phototropin was coprecipitated together with 14-3-3 protein only when the pulvini were irradiated by blue light (Fig. 5C). The result demonstrates that the 14-3-3 protein bound to phototropin in vivo by blue light. The same amounts of 14-3-3 protein were used for the immunoprecipitation (Fig. 5D). If we did the immunoprecipitation using antibodies against phototropin, we could not find 14-3-3 protein in the immunoprecipitate in Phaseolus pulvini as shown previously in Vicia guard cells (Kinoshita et al., 2003 ).
If phototropin functions in blue light signaling upstream of the plasma membrane H+-ATPase in pulvinar motor cells, the suppression of phototropin activity would inhibit dephosphorylation of the H+-ATPase. To test this, we investigated the effect of diphenylene iodonium (DPI), a flavoprotein inhibitor (O'Donnell et al., 1993
Cloning of Phototropins from Pulvini of Kidney Bean We cloned phototropins from pulvini of kidney bean by reverse transcription (RT)-PCR and obtained three cDNAs encoding P. vulgaris phototropin 1a (PvPHOT1a), PvPHOT1b, and PvPHOT2. All Pvphots contained LOV1, LOV2, and Ser/Thr kinase domains, which were typically conserved in phototropins of other plants (Fig. 7A). The deduced protein sequences of Pvphot1a, Pvphot1b, and Pvphot2 consisted of 976, 987, and 996 amino acids, respectively, and had molecular masses of 110, 111, and 112 kD, respectively.
We compared the full-length amino acid sequences of Pvphots with those of phototropin homologs in other higher plants, which are classified into two groups, phot1 and phot2, by ClustalX (Fig. 7B). Pvphot1a and Pvphot1b had identities of 68% and 69% with Atphot1, and 57% and 61% with Atphot2, respectively. Pvphot2 had an identity of 72% with both Atphot2 and Osphot2, and 60% with Atphot1. Therefore, Pvphot1a and Pvphot1b were placed into a phot1 group, and Pvphot2 was placed into a phot2 group. Pvphot1a and Pvphot1b have high (77%) identities to Vfphots and Psphot1 in leguminous plants, but lower (about 68%) identities to phot1 in Arabidopsis (Fig. 7B). A phototropin placed in the phot2 group had not been identified in leguminous plants thus far.
To investigate the expression of PvPHOTs, we performed the RT-PCR analysis in leaves, roots, pulvini, and etiolated seedlings using specific primers for PvPHOT1a, PvPHOT1b, and PvPHOT2 (Fig. 7C, left sections). PvPHOT1a transcripts were found in etiolated seedlings, leaves, and pulvini. PvPHOT1b transcripts were found in all tissues investigated. PvPHOT2 transcripts were detected only in green tissue, such as leaves and pulvini.
We studied the effect of light on the expression of PvPHOTs mRNAs (Fig. 7C, right sections). Etiolated seedlings of 4-d-old plants, which were irradiated with white light at 75 µmol m–2 s–1 for 180 min, revealed an increase in PvPHOT2 mRNA with no detectable change in PvPHOT1a and PvPHOT1b mRNAs. This result agrees with the previous observations that light induces the expression of the phot2 group (Kanegae et al., 2000
Biochemical Evidence for the Inactivation of the Plasma Membrane H+-ATPase by Blue Light in Pulvinar Motor Cells
The plasma membrane H+-ATPase is known to be activated by phosphorylation of the penultimate Thr in the C terminus followed by 14-3-3 protein binding on this site (Fuglsang et al., 1999
Inactivation of the H+-ATPase by blue light causes the depolarization of the plasma membrane (Nishizaki, 1994
Photoreceptors that mediate blue light-induced leaf movement in leguminous plants have not been identified. In Arabidopsis, phototropins mediate phototropism (Huala et al., 1997 In this study, we obtained three separate lines of evidence supporting the function of phototropin in the pulvinar motor cells as a photoreceptor in the leaf movement response (Figs. 2, 3, and 6). First, the phosphorylation of phototropin preceded the dephosphorylation of the H+-ATPase in response to blue light (Fig. 2E). Second, the binding of 14-3-3 protein to phototropin and the dissociation of the protein from the H+-ATPase had similar fluence rate dependencies (Fig. 3E). Third, the flavoprotein inhibitor and protein kinase inhibitors, which inhibit the activity of phototropin, also suppressed dephosphorylation of the H+-ATPase (Fig. 6). We recognized phototropin proteins by an immunological method (Figs. 2D, 3D, 4C, and 5C) and identified three PvPHOT cDNAs (Fig. 7C) in pulvini. However, we could not distinguish these isoproteins from each other because of their similar molecular masses.
These results suggest that the phototropin-mediated signal inactivates the H+-ATPase in pulvinar motor cells, while the same signal activates the H+-ATPase in stomatal guard cells (Kinoshita et al., 2001
It is noteworthy that the phosphorylation statuses of the H+-ATPase are different between motor cells and guard cells. In pulvinar motor cells, the greater portion of the H+-ATPase was phosphorylated on the C terminus in the dark (Fig. 4A), but in stomatal guard cells most of the H+-ATPase were dephosphorylated under the same condition (Kinoshita and Shimazaki, 1999
Inactivation of the H+-ATPase by blue light ultimately elicits shrinkage of the motor cells in the pulvinus of kidney bean. If this reaction occurs in plant cells of other tissues, it may bring about growth inhibition. For example, Folta and Spalding (2001)
Plant Materials and Growth Conditions
Plants of kidney bean (Phaseolus vulgaris L.) cv Kentucky Wonder were grown for 2 weeks at 24°C with a relative humidity of 70% under a 12/12-h light-dark cycle or continuous light in a growth chamber (CU-250, Tomy, Tokyo). The light was obtained from fluorescent lamps (FL20SS·W/18, Toshiba, Tokyo) at 100 µmol m–2 s–1 on the leaf surface. Laminar pulvini from a pair of first leaves (Fig. 1F) were excised with a razor blade and were used in all experiments. We used these whole pulvini without dividing them into flexor and extensor cells because blue light induces membrane depolarization in motor cells on both the flexor and extensor sides (Nishizaki, 1988
A pulse of blue light was obtained from a tungsten lamp (EXR 150W, Sylvania, Danvers, MA) by passing the light through a blue glass filter (5-60, Corning, Corning, NY) and was used for the irradiation of excised pulvini. Continuous blue light was obtained from light-emitting photodiodes (Stick-B-32, Eyela, Tokyo; maximum intensity at 470 nm) and was used for the irradiation of the intact plant. Photon flux densities were measured with a quantum meter (model 185A, LI-COR, Lincoln, NE).
We isolated microsomal membrane fractions from pulvini to determine of the phosphorylation statuses of both phototropin and the plasma membrane H+-ATPase. Thirty to 35 pulvini were placed on a wet paper towel in a petri dish and illuminated with a 30-s blue light pulse, and were homogenized at indicated times in an ice-cold buffer (50 mM MOPS-KOH pH 7.5, 2.5 mM EDTA, 100 mM NaCl, 2 mM phenylmethylsulfonyl fluoride, 20 µM leupeptin, 1 mM dithiothreitol, 1 mM ammonium molybdate, 10 mM NaF, 50 nM Calyculin A) using a mortar and pestle. The homogenate was centrifuged at 12,000 rpm for 5 min, and the resulting supernatant was further centrifuged at 45,000 rpm for 45 min. Microsomal membranes obtained as a pellet were suspended in the same buffer. All procedures were performed under dim red light. These microsomal membranes were used for immunoblotting and protein-blot analysis.
Immunoblotting was performed according to a previous method (Kinoshita et al., 2003
Protein-blot analysis was performed according to a previous method (Kinoshita et al., 2003
Immunoprecipitation of the H+-ATPase and 14-3-3 protein was done using the individual antibodies as described previously (Kinoshita and Shimazaki, 1999
The microsomal proteins were digested by CNBr at 100 mg/mL in 70% (v/v) formic acid for 4 h at room temperature with gentle rotation according to the previous method (Kinoshita and Shimazaki, 2001
Total RNA was isolated from pulvini with ISOGEN (Nippon Gene, Tokyo) according to the manufacturer's instructions. The first-strand cDNAs were synthesized from the total RNA of pulvini by the SuperScript III RT First-Strand Synthesis System (Invitrogen, Carlsbad, CA) using Oligo(dT) primer. Two degenerate oligonucleotide primers (5'-CCNGAYAAYCCNATHATHTTYGC-3' and 5'-TAYTCYTCNGTNCCNACRAA-3') were designed on the sequences of consensus regions in phototropin LOV2 and kinase as described previously (Kinoshita et al., 2003
Leaves, roots, and pulvini were harvested from 2-week-old plants. Etiolated seedlings were grown for 4 d in darkness. Total RNA was extracted from these tissues using ISOGEN (Nippon Gene). The first-strand cDNAs were synthesized from 5 µg of total RNAs by the SuperScript III RT First-Strand Synthesis System (Invitrogen) using Oligo(dT) primer. The obtained cDNAs were diluted 10-fold, and 0.5 µL was used in each PCR reaction. PvPHOTs cDNAs were amplified using the following oligonucleotide primers: for PvPHOT1a, 5'-ATTTACAAATATTCTTCACAAGGACCTTAAATTTCC-3' and 5'-GTTACAACTTGGTGACTTGGAAACAT-3'; for PvPHOT1b, 5'-TAAACCAGTAAGTCTCCAGGGAAAG-3' and 5'-CACTAGACTAGACTAATATAATTTGCTTG-3'; and for PvPHOT2, 5'-TCTTGCAGCCAGGCAGTTGATTAATG-3' and 5'-TATGATATTATTGATGTATTGTGGACATGCATG-3'. As an internal standard for RT-PCR, we used ACTIN homolog (AB067722) cDNA of kidney bean using oligoprimers (5'-ATGGGGCAGAAGGATGCGTATGTGG-3' and 5'-TAGCCTTCATAGATGGGGACCGTGTG-3'). PCR reactions were performed in 27 cycles on the left panels and 30 cycles on the right panels. Sequence data from this article have been deposited with the EMBL/GenBank data libraries under accession numbers AB204872 to AB204874.
We thank Y. Okazaki (Osaka Medical College, Osaka, Japan) for critical reading of the manuscript. Received February 28, 2005; returned for revision May 10, 2005; accepted May 15, 2005.
1 This work was supported by the Ministry of Education, Science, Sports and Culture of Japan (grant nos. 13139202 and 16207003 to K.S., and 14704003 to T.K.).  Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.062026. * Corresponding author; e-mail kenrcb{at}mbox.nc.kyushu-u.ac.jp; fax 81–92–726–4758.
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, Fig. 1F) were determined. Averages of eight measurements are presented with SEs. F, The magnification of laminar pulvinus. Arrowhead of white color shows the pulvinus. Arrowheads of blue colors (A and D) are the direction of blue light. White bars in the bottom represent 1 cm.
