Download Osmotic Stress Induces Inactivation of Photosynthesis in Guard Cell

Plant Cell Physiol. 42(10): 1186–1191 (2001)
JSPP © 2001
Osmotic Stress Induces Inactivation of Photosynthesis in Guard Cell
Protoplasts of Vicia Leaves
Chang-Hyo Goh 1, 3, Rainer Hedrich 2 and Ulrich Schreiber 2
1
Division of Molecular and Life Science, Pohang University of Science and Technology, San 31, Nam-Gu, Hyoja-Dong, Pohang, Kyungbuk, 790784 Korea
2
Lehrstuhl für Molekulare Pflanzenphysiologie und Biophysik, Julius-von-Sachs Institut, Universität Würzburg, Julius-von-Sachs Platz 2, D97082 Würzburg, Germany
;
Guard cell protoplasts isolated from Vicia leaves
showed a strong suppression of the photosynthesis under
hypotonic conditions, as reflected by changes in the chlorophyll fluorescence characteristics. The response was reversible as well. Mesophyll cell protoplasts did not show any
lowering of photosynthetic activity under hypo-osmotic
conditions. This result indicates that the response was
guard cell specific.
Key words: Chlorophyll a fluorescence — Guard cell protoplast — Osmotic stress — Photosynthesis — Vicia faba.
Abbreviations: AL, actinic light; ETR, relative electron transport
rate; Fo, dark level of fluorescence yield; Fm, maximum level of fluorescence yield; Fv, variable fluorescence yield, Fm–Fo; GCPs, guard
cell protoplasts; MCPs; mesophyll cell protoplasts; ML, measuring
light; PAM, pulse-amplitude modulation; SP, saturating light pulse; Y,
the effective quantum yield of PSII defined as DF/Fm¢ = (Fm¢–F)/Fm¢.
Stomata are pivotal organs for the exchange of gases
between the outside air and the inside of a leaf for photosynthesis, as well as evaporation of water through transpiration from
the plants. The modulation of the stomatal aperture ensues
from the transduction of environment-dependent changes in the
osmotic potential of guard cells into water fluxes and mechanical forces that determine the dimensions of stomatal pores
(Zeiger 1983, Mansfield 1986). Guard cells sense the osmotic
stress and regulate cell volume (MacRobbie 1995), inducing
changes in stomatal aperture (Assmann 1993). The predominant osmotically active species are K+, malate, Cl–, sucrose,
and possibly other sugars (Assmann 1993). However, the regulation of guard cell turgor in response to osmotic stress is even
more complicated.
Guard cell chloroplasts are considered to be one of the
functional components of stomatal movements. The plasma
membrane H+-ATPase of guard cell, which is activated by blue
and red light, pumps protons out of the cells, thereby inducing
the membrane hyperpolarization that elicits stomatal opening
3
and suppresses stomatal closure (Assmann et al. 1985, Shimazaki et al. 1986, Serrano et al. 1988, Goh et al. 1995). In Vicia
guard cell protoplasts (GCPs), light-induced electrogenic proton pump currents were eliminated by 3-(3,4-dichlorophenyl)1,1-dimethylurea (DCMU), pointing to a requirement of photosynthetically active chloroplasts for stomatal opening in light
(Serrano et al. 1988). Current knowledge suggests that stomatal movements are caused by shrinking or swelling of the
cells and are energy-coupled responses (Willmer and Fricker
1996). This leads to the working hypothesis followed by the
present study that the osmoregulation of guard cell in response
to osmotic stress may be closely correlated with the photosynthetic activity of guard cell chloroplast serving as energy
sources. This study will give us a clue to understand the cellular mechanisms in osmotic response of guard cells. In the
present work, we investigated the effects of osmotic stress on
the photosynthetic activity in Vicia GCPs using sorbitol as the
external osmoticum.
To investigate cell volume changes and its correlation
with photosynthetic activity in Vicia GCPs, we first measured
cell volume under isotonic (400 mOsM), hypotonic (300 mOsM),
and hypertonic (600 mOsM) conditions as a function of time
(Fig. 1). Control experiments showed that there was no significant volume change in GCP during 40 min incubation in isotonic solution in the dark. When cells were exposed to a hypotonic solution, GCP rapidly increased in volume by 55.3%
within 15 min, reaching a plateau in 30 min. When cells were
transferred from an isotonic to a hypertonic solution, the cell
volume decreased by 23.4% in 15 min, after which there was
no further change. These results show that osmotic stress could
be rapidly converted into volume changes in the type of GCP
used in the present study.
Dark-light Chl fluorescence induction kinetics obtained
from single protoplasts (GCPs and mesophyll cell protoplasts
(MCPs)), which were suspended in darkness for 30 min after
being exposed to different osmotic conditions were recorded
(Fig. 2). The induction patterns are similar in GCP and MCP
control samples. In both cases, the maximum fluorescence
yield assessed by saturation pulses during actinic illumination,
Corresponding author: E-mail, [email protected]; Fax: +82-54-279-2199.
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Osmotic stress inhibits guard cell photosynthesis
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Fig. 1 Changes in cell volume upon osmotic stress in Vicia GCP. The
protoplasts were suspended in 400 mOsM before being transferred to
different osmotic conditions in the dark. Symbols indicate cell volumes at 400 mOsM (open squares), 300 mOsM (filled circles), and
600 mOsM (filled triangles) as a function of time. Values indicate
mean ± SE (n = 9, 82<cell<101) of the results from three independent
experiments.
Fm¢, first declines and then slowly rises again. This pattern is
drastically changed by osmotic stress in GCP but not in MCP.
Under hypotonic conditions (300 mOsM) the fluorescence
decline following the peak, P, was severely suppressed in GCP
(Fig. 2b), indicating that the induction of photosynthesis was
inhibited. Both photochemical quenching (reflected by the fluorescence increase during a saturation pulse) and non-photochemical quenching (reflected by the decrease of Fm¢ with
respect to Fm) were strongly suppressed. Here, we note that Fm¢
and Fm indicated the fluorescence from actinic light-adapted
and dark-adapted state during the application of a saturating
pulse of light, respectively (Schreiber et al. 1994). On the other
hand, hypotonic conditions did not affect the induction pattern
of MCP (compare panels d and e). When the protoplasts were
submitted to hypertonic conditions (600 mOsM) for the same
period, the changes of Chl a fluorescence induction kinetics in
GCP were less dramatic, but still indicative of significant inhibition (Fig. 2c). The fluorescence decline from P was slowly
down and non-photochemical quenching was enhanced. In the
steady state, photochemical quenching was decreased and nonphotochemical quenching increased, resulting in an overall
lowering of the effective quantum yield of energy conversion.
Under hypertonic conditions there is also a small change in the
induction pattern of MCP, although less pronounced than in
Fig. 2 Effects of osmotic stress on the fluorescence yields in Vicia
GCP and MCP during dark-light induction with repetitive application
of saturation pulses. Each fluorescence recording shows a typical
response at 15 min after transfer to 400 mOsM (a and d), 300 mOsM
(b and e), and 600 mOsM (c and f). Actinic light (AL, 45 mmol m–2 s–1)
was switched on/off where indicated. Saturation pulses (SP, 0.8 s)
were applied at an intensity of 2.90 mmol m–2 s–1. The fluorescence
yield of single GCPs was estimated as 1/27.4 lower than that of single
MCPs as in a previous report (Goh et al. 1999). The solution for hypotonic and hypertonic medium was adjusted changing D-sorbitol concentration to 300 (or 350) mOsM and to 600 (or 500) mOsM,
respectively.
GCP. As apparent in panel f, the light-induced non-photochemical quenching is enhanced. These results indicate that osmotic
stress severely affects guard cell photosynthesis and the photosynthetic behavior of GCP to hypotonic stress is a cell typespecific response.
The apparent photosynthetic electron transport rate (ETR)
was calculated from the effective quantum yield of energy conversion at PSII reaction centers (Genty et al. 1989, Schreiber et
al. 1994). It gives a relative measure of the overall electron
transport rate, which in the steady state depends not only on the
photochemical activity of PSII, but on the functioning on all
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Osmotic stress inhibits guard cell photosynthesis
Fig. 3 Properties of apparent electron transport rates in Vicia GCP and MCP under different osmotic conditions. (A) Inactivation of the ETR in
GCP in response to osmotic stress. Actinic light (AL, 45 mmol m–2 s–1) was switched on/off where indicated. Control values were obtained at
400 mOsM just before switching to different osmotic conditions. Other readings were recorded at 30 min after switching to isotonic (400 mOsM),
hypotonic (300 mOsM), or hypertonic (600 mOsM) conditions. Values indicate mean ± SE (n = 11) of the results from three independent experiments. (B) a. Effects of hypotonic osmolarities on GCP (n = 8–11) and MCP (n = 6). b. Effects of hypertonic osmolarities on GCP (n = 8–11) and
MCP (n = 6). The protoplasts were incubated for 15 min at different osmotic conditions in the dark before the measurement of Chl fluorescence.
The intensity of AL exposed to the protoplasts was 66 mmol m–2 s–1. Values indicate mean ± SE of the results from two independent experiments.
The ETR was expressed as a relative value, which was calculated from DF/Fm¢ ´ PAR ´ c (where PAR is the photon flux density of incident photosynthetically active radiation, and the constant c corresponds to the fraction of incident quanta being absorbed). The values were determined at
the end of a 4.7 min illumination period with AL using SP. Other experimental conditions were as for Fig. 2.
downstream reactions as well. In Fig. 3A, the averaged values
± SE of the ETR calculated from Chl a fluorescence recordings of single GCPs are shown, which had been suspended in
darkness for 30 min after being transferred to different osmolarities from control conditions (400 mOsM), before they were
illuminated for 4.7 min. In the case of control, ETR value was
also measured just before transferring samples to the various
osmotic conditions. There was no significant change in 30 min
after incubation under isotonic conditions in the dark (relative
value constant at ca. 11 units). On the other hand, when cells
were exposed to hypotonic conditions (300 mOsM) for the
same period, the ETR value significantly decreased to a relative value of 2.20, which translates into 79.5% inhibition.
When the ETR was measured 30 min after switching to hypertonic conditions (600 mOsM), the ETR value showed about
20% inhibition. This confirms the conclusions from Fig. 2 that
osmotic stress causes the inactivation of the photosynthetic
machinery of GCPs.
In Fig. 3B the ETR values of GCP and MCP are plotted as
a function of osmolarity. In this experiment, a higher actinic
intensity was applied, explaining the comparatively higher
ETR values. With increasing hypotonicity (a), the ETR values
of GCP show a steep decline. On the other hand, no significant
changes in the ETR values of MCPs occur. With increasing
hypertonicity (b), the ETR values of GCPs as well as of MCPs
show a gradual decrease, which is somewhat more pronounced
in GCPs than in MCPs.
To examine the recovery of cell volume and of the ETR
value after removal of osmotic stress from the same single
cells, the protoplasts were first incubated with hypotonic (Fig.
4A) and hypertonic (Fig. 4B) conditions for 30 min, then
washed and incubated under isotonic conditions for the same
period. Cells incubated under hypotonic condition showed a
large increase in cell volume (Fig. 4A-a) paralleled by more
than 90% inhibition in the ETR (Fig. 4A-b). After washing of
the cells and incubation under isotonic conditions for 30 min,
cell volumes and the ETR values returned to near original levels. By measuring ETR after various times of exposure to
hypotonic (inhibition phase) and isotonic (recovery phase) conditions it was possible to estimate the rates of stress-induced
inhibition and recovery, which amounted to 0.384 and 0.321
ETR units per min, respectively. When cells were placed under
hypertonic conditions, their cell volumes were significantly
decreased (Fig. 4B-a) and their ETR values became about 45%
Osmotic stress inhibits guard cell photosynthesis
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Fig. 4 Recovery of cell volume and apparent electron transport rates in Vicia GCP after return from hypotonic (A) and hypertonic (B) to isotonic
conditions. Measurements were done at the same single cells. The protoplasts were washed and then incubated under isotonic conditions for further 30 min, after cells previously had been exposed to hypotonic (A) and hypertonic conditions (B) for 30 min. White bars as controls indicate
readings at 400 mOsM just before being transferred to the hypotonic (A) and to hypertonic conditions (B). (a) and (b) indicate cell volume and the
relative ETR, respectively. The ETR was determined as for Fig. 3. Values indicate mean ± SE (n = 4–5) of the results from two independent
experiments (A) and three independent experiments (B). Other experimental conditions were as for Fig. 1 and 2.
inhibited (Fig. 4B-b). After washing and incubation under isotonic conditions for 30 min, cell volumes and ETR values
returned to their original levels. The inhibitory and recovery
rates for photosynthesis were estimated to amount to 0.177 and
0.128 ETR units per min, respectively. These results show that
the osmotic stress induced swelling and shrinkage of guard cells
and their loss in photosynthetic activities are fully reversible.
Our findings show that osmotic stress induced inactivation of guard cell photosynthesis, with hypotonic stress causing particularly dramatic changes of the dark-light induction
kinetics. Phenomenologically, the observed effects of hypotonic conditions were similar to the previously reported effect of
fusicoccin (FC) on pairs of single Vicia guard cells (Goh et al.
1999). As Vicia MCPs are not affected by the hypo-osmotic
stress (Fig. 2e, 3B-a), this response appears to be guard cellspecific reactions or properties. At the present state of information, the actual cause can only be speculated. The uptake and
release of ions in guard cells are under the control of the
cytosolic ATP pool and NAD(P)H levels, since both opening
and closing movements have been described as energyconsuming steps (Willmer and Fricker 1996). Available data
have suggested that ATP and NADPH produced by the light
reactions of photosynthesis in guard cells could be used as
energy sources for the regulation of stomatal movements
(Outlaw et al. 1981, Zeiger et al. 1987, Gotow et al. 1988,
Serrano et al. 1988, Shimazaki et al. 1989). It also has to be
considered that the osmoregulation of guard cells involves activation of inward-rectifying K+ channels, which are ATPdependent (Liu and Luan 1998). Therefore, osmotic stress may
induce the depletion of cytosolic ATP pool in guard cells if cell
volume changes in response to osmotic stress require metabolic components. The increase of K+ concentration in the cells
in response to hypotonic stress might also lead to an increase of
K+ concentration in the intra-thylakoid space (Allakhverdiev et
al. 2000). In this case, it cannot be excluded that the photosynthetic activity in GCP might be affected by a direct effect
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Osmotic stress inhibits guard cell photosynthesis
on the extrinsic proteins in PSII. However, further investigations are needed to analyze the mechanistic cause of the suppression of photosynthetic activity in guard cells by hypoosmotic stress. When protoplasts were exposed to hypertonic
conditions, a completely different inactivation pattern was
observed, which was similar for GCPs and MCPs (Fig. 2c, f
and Fig. 3A). In contrast to hypo-osmotic stress, which eliminated non-photochemical quenching in GCPs, without affecting
MCPs, in both types of protoplasts non-photochemical quenching was stimulated by hyperosmotic stress. The observed effect
is indicative for partial suppression of CO2-fixation, as the
transthylakoidal proton gradient, which can be sustained by
non-assimilatory types of electron transport (Schreiber et al.
1994), will be enhanced when ATP-consumption in the Calvincycle is decreased. A similar phenomenology has been observed
in water-stressed plant leaves (Schreiber and Bilger 1987, Cornic 2000), and in the hypertonically-stressed cyanobacterium
Synechococcus R-2 (Allakhverdiev et al. 2000). Considering the
cited references and the present experimental results, the
inhibitory effect of hypertonic stress on guard cell photosynthesis is likely to be a general response to stress by low water
potential.
In conclusion, this report investigated the changes of
guard cell photosynthetic activity in response to changes in
water potential. Osmotic stress induced two different types of
inactivation of the photosynthetic machinery in Vicia GCPs.
Hypo-osmotic stress is of particular interest, as it selectively
affected GCPs and not MCPs. We suggest that there may be
metabolic coupling between changes in cell volume and the
photosynthetic activity in guard cells to osmotic stress.
We have assessed the photosynthetic activity of single
GCPs in parallel to the osmotic volume changes by applying a
new type of Chl fluorescence method, which has allowed to
specifically analyze the properties of guard cell photosynthesis
from individual healthy cells in mesophyll cell-free conditions
(Goh et al. 1999). The protoplasts from guard cells were enzymatically isolated as previously described (Goh et al. 1999).
For the first and second digestion, the solutions were adjusted
to 250 and 400 mOsM using D-sorbitol, respectively. The cells
were harvested by filtration through a nylon net (20 mm in
mesh size) and collected by centrifugation (110´g, 7 min).
MCPs) were isolated from epidermis-free Vicia leaves. The
procedure was performed at 28°C for 1.5 h as described previously (Sakaki and Kondo 1985). The enzymatic solution was
also adjusted to 400 mOsM using D-sorbitol. The released
GCPs and MCPs were suspended in a solution of 400 mOsM
using D-sorbitol containing 10 mM KCl and 1 mM CaCl2 and
kept in the dark on ice until use. We note here that the unit of
osmolarity (mOsM) is defined as mosmol kg–1.
Cell volume changes upon osmotic stresses were analyzed in weak green light at room temperature. The protoplasts
were suspended in a solution of D-sorbitol (400 mOsM) containing 10 mM MES/Tris (pH 6.1), 10 mM KCl, 1 mM CaCl2,
and 2.5 mM NaHCO3 in the dark. To make hypotonic and
hypertonic conditions, they were adjusted to 300 (or 350)
mOsM and 600 (or 500) mOsM using D-sorbitol as the external solution, respectively. The cell volume was determined by
measuring the diameter assuming that the protoplast was a perfect sphere.
Measurement of Chl a fluorescence of single cells was
performed using a MICROSCOPY-PAM Chlorophyll Fluorometer, which was adapted to an inverted epifluorescence
microscope (model Axiovert 25, Zeiss GmbH, Göttingen,
Germany) as described previously (Goh et al. 1999). The
quenching characteristics of Chl a fluorescence and the effective quantum yield of energy conversion from single protoplasts exposed to an actinic light (45 mmol m–2 s–1) were determined by application of saturating light pulses (SP, 0.8 s width
at 20 s intervals) with a quantum flux density of 2.90 mmol m–2
s–1 (photosynthetically active radiation, PAR), unless stated
otherwise. For comparison, Chl a fluorescence parameters of
single MCP were also determined. In the latter experiments,
actinic intensity amounted to 66 mmol m–2 s–1. Chl a fluorescence measurements from single cell under control conditions
were carried out in a solution of D-sorbitol (400 mOsM) containing 10 mM MES/Tris (pH 6.1), 10 mM KCl, 1 mM CaCl2,
and 2.5 mM NaHCO3.
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(Received May 30, 2001; Accepted August 6, 2001)