JXB Advance Access originally published online on January 10, 2005
Journal of Experimental Botany 2005 56(411):435-447; doi:10.1093/jxb/eri060
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RESEARCH PAPER |
Towards an understanding of photosynthetic acclimation
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK
* Fax: +44 (0)1865 275074. E-mail: robin.walters{at}plants.ox.ac.uk
Received 10 June 2004; Accepted 22 October 2004
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Abstract |
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 Top Abstract IntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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It has long been recognized that higher plants vary the composition and organization of the photosynthetic apparatus in response to the prevailing environmental conditions, with particular attention being paid to the responses to incident light. Under high light conditions there are increases in the amounts of photosystems, electron transport and ATP synthase complexes, and enzymes of the Calvin–Benson cycle; conversely, under low light there is an increase in the relative amounts of light-harvesting complexes (LHC) and in the stacking of thylakoid membranes to form grana. It is believed that these changes are of adaptive significance, and in a few instances evidence has been provided that this is indeed the case; an increase in photosynthetic capacity reduces susceptibility to photodamage, while changes in photosystem stoichiometry serve to optimize light utilization. By contrast, the potential benefit to the plant of other changes in chloroplast composition, such as in the levels of LHC, is far less clear. It is also believed that redox signals derived from photosynthetic electron transport play an important regulatory role in acclimation. However, while there is convincing evidence that such redox signals modulate the expression of many plastidic and nuclear genes encoding photosynthetic components, there is little to demonstrate that such changes are responsible for regulating chloroplast composition. This review discusses the evidence that particular aspects of acclimation are advantageous to the plant, and highlights the significant gaps in our understanding of the mechanisms underlying acclimation.
Key words: Acclimation, environmental conditions, light, photodamage, photosynthetic apparatus
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Introduction |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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The high variability of the natural environment poses significant challenges for higher plants in terms of their ability to grow and reproduce, to compete with neighbours for light and nutrients, and to withstand biotic and abiotic stress. Changes in the environment have a particular impact on the photosynthetic apparatus, since not only is it the plant's source of energy and fixed carbon, but it is also a major site of damage under stress conditions. Accordingly, plants have evolved many methods of responding to changes in their growth conditions. These range from long-term developmental processes occurring at the level of the whole plant or individual leaves (such as altered root/shoot ratios and leaf anatomy) which take effect over periods of weeks or months (Ballaré, 1999
; Weston et al., 2000), to adjustments in the functioning of individual proteins within the photosynthetic apparatus, which operate on timescales ranging from seconds to hours (Demmig-Adams and Adams, 1992).
Lying between these two extremes, both anatomically and temporally, is a further level of response to the environment involving adjustments in the composition of the photosynthetic apparatus within individual cells or perhaps even chloroplasts, a process known as photosynthetic acclimation. According to the conditions under which plants are grown, major differences can be observed in the levels of protein complexes in the thylakoid membrane and of stromal components involved in carbon fixation, with correlating changes in photosynthesis (reviewed by Anderson and Osmond, 1987; Anderson et al., 1995). This does not apply only to the initial growth conditions under which a plant or leaf develops since, within days of a change in growth conditions and without appreciable structural changes in the leaf, there are clear changes in chloroplast composition with corresponding changes in photosynthetic function; these involve not only de novo synthesis of photosynthesis components but also specific degradation (Walters and Horton, 1994; Yang et al., 1998). Thus, photosynthetic acclimation can be viewed as a dynamic process operating on an intermediate timescale compared with short-term regulation and long-term developmental processes.
The main features of acclimation to light have been well-described (Anderson and Osmond, 1987; Anderson et al., 1995). Although the scale and details of the response vary greatly between species, a consistent response to growth in high light is observed: compared with low light, growth in high light leads to increases in levels of PSII, cytochrome b/f complex, ATP synthase, and components of the Calvin cycle (especially ribulose-1,6-bisphosphate carboxylase/oxygenase, Rubisco), while there are reductions in the levels of the major chlorophyll a/b-binding light-harvesting complexes associated with PSII (LHCII); these changes are reflected in increased capacities for oxygen evolution, electron transport and CO2 consumption, and an increased ratio of chlorophyll a to chlorophyll b (Chl a/b) (Leong and Anderson, 1984a, b; Mäenpää and Andersson, 1989; de la Torre and Burkey, 1990a, b; Bailey et al., 2001). Superimposed on this is a distinct pattern of responses to differences in the spectral quality of light such as might be observed when comparing full sunlight with the light available beneath a tree canopy or within dense vegetation: these latter environments lead to relative depletion in blue and red wavelengths, so that there is an enrichment in the proportion of incident light which is preferentially absorbed by PSI; following such changes in spectral quality (without altering the total incident light), acclimation leads to increases in the levels of PSII (and LHCII) relative to PSI, and there are negligible changes in photosynthetic capacity (Chow et al., 1990b; Kim et al., 1993; Walters and Horton, 1994).
Acclimation not only occurs according to the characteristics of incident light, but also is observed to a greater or lesser extent in response to many other aspects of the environment including temperature, drought, atmospheric CO2 levels, pathogen infection, and the harvesting of fruit or leaf tissue. Thus, even treatments which only have indirect effects on photosynthesis are capable of inducing adjustments in chloroplast composition. This ability of photosynthesis to respond to a broad range of stimuli led to the hypothesis that the signal(s) giving rise to acclimation are derived from photosynthesis itself, rather than being dependent on separate direct detection of each possible environmental cue. Such a regulatory strategy would allow the plant to detect a perturbation in photosynthesis no matter what the cause, and to respond so as to compensate for it (Anderson et al., 1995).
Inherent in the proposal that regulatory signals originate from photosynthesis is the idea that acclimation serves as a homeostatic mechanism, reversing the consequences of environmental change and thereby maintaining efficient photosynthesis. Of particular importance in the development of this hypothesis was the finding that, in unicellular algae, an increase in irradiance or a lowering of temperature both gave rise to similar effects on the redox state of the electron transport chain between photosystems II and I (PSII, PSI), and also induced equivalent acclimation responses (Maxwell et al., 1994). This led to the suggestion that acclimation operates through sensing of one or more redox components, currently believed to include plastoquinone/plastoquinol (PQ) (Huner et al., 1998; Pfannschmidt, 2003). Indeed, there is a growing body of evidence that photosynthesis-related redox signals are important in regulating acclimation in unicellular photosynthetic organisms (Durnford and Falkowski, 1997; Im and Grossman, 2002).
However, a similar equivalence of high light and low temperature has not been unambiguously demonstrated when considering acclimation in higher plants; there are often different patterns of acclimation depending on the environmental stimulus (Gray et al., 1996). This raises the question of whether the regulatory processes in single cell organisms necessarily apply in more complex systems. Acclimation in plants is not a cell-autonomous process, since it is inevitably affected by the physiology and developmental stage of the whole plant. Thus, for many treatments, attempts to understand the acclimation response are complicated by interactions with other signalling pathways. For instance, in higher plants the redox changes induced by low temperature or by elevated CO2 are, in each case, accompanied by large increases in carbohydrate levels (Strand et al., 1997; Moore et al., 1999; Backhausen and Scheibe, 1999). These have the potential to provide sugar-sensing signals, confounding attempts to interpret the results of such studies in terms of redox changes alone. Signals from sugars, phytohormones, and nutrients (e.g. phosphate and nitrate) may well be important in the regulation of acclimation, but it is often difficult to interpret the results of experiments in which more than one environmental variable is altered. Although considerations of multiple environmental stimuli will undoubtedly be essential to the understanding of acclimation in the future, consideration of such complexity lies outside the scope of this review, which instead focuses on the acclimation of higher plants to the environmental factor which is directly relevant to the photosynthetic apparatus, light.
The majority of studies on acclimation in higher plants have focused on the effects of different growth light conditions, yet despite a wealth of data describing acclimation in many different species, there remain significant unanswered questions concerning its nature, its purpose, and its regulation. For instance, does light acclimation reflect a single integrated response, or is it the result of a combination of processes? A single environmental stimulus can provoke changes in the levels of multiple proteins, all of which might be due to a single regulatory process or which could alternatively be separate responses to different signals. Moreover, what adaptive benefit to the plant has led to the evolution of acclimation? Although various plausible advantages to the plant have been postulated, in many cases the evidence that these are important is far from compelling. Furthermore, what are the mechanisms by which changes in the environment are sensed by the plant and by which chloroplast composition is controlled? Again, although attractive hypotheses have been presented, the supporting evidence is largely correlative and there is no description at the molecular level. Consideration of such questions reveals that there remain appreciable gaps in the understanding of acclimation, and highlights the need for new investigative approaches.
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Assessing acclimation |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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Differences in chloroplast composition and function in plants grown under different conditions can be quantified in a variety of ways, with particular advantages and disadvantages. For instance, when assessing whole plant photosynthesis, the most appropriate measurement may be photosynthetic capacity per unit leaf area. By contrast, when attempting to determine changes in the composition of the photosynthetic apparatus at a cellular level, it may be more useful to express photosynthesis and chloroplast components relative to leaf chlorophyll, so that the data reflect the balance in investment between light capture and light utilization. Measurements relative to total leaf protein or leaf nitrogen are generally less informative since they tend to be dominated by changes in Rubisco content, which constitutes a major proportion of total leaf protein. A case can also be made for other bases of normalization, for instance, relative to leaf fresh weight or dry weight. Inevitably, the most appropriate representation ultimately depends on the aim of the study in question. Thus, levels of thylakoid membrane components are generally expressed per unit Chl, soluble enzymes including Rubisco are assayed on a Chl, leaf area or protein basis, while rates of photosynthesis are expressed per unit Chl or per unit leaf area (for examples see Chow and Anderson, 1987
; Murchie and Horton, 1997; Savitch et al., 1997; Walters et al., 1999; Strand et al., 1999). Acclimation is commonly monitored via two readily measured parameters: the Chl a/b ratio which reflects the proportion of Chl bound by light-harvesting complexes, and the light-saturated rate of photosynthetic electron transport determined as O2 evolution under CO2-saturating conditions (Pmax). There has in the literature been little consistency in the units used for Pmax, with measurements on a unit Chl or unit leaf area basis being used in a seemingly interchangeable manner.
Under most circumstances any of these measures provides a satisfactory means of assessing the extent to which plants are adjusting to particular environmental stimuli. However, where there is significant variation in leaf Chl content, for example, for plants grown at different temperatures (Gray et al., 1996) or in leaves of different ages (Bailey et al., 2004), potential problems with each of these measurements emerge. This is due to the substantial light gradient which exists within a leaf (Vogelmann and Evans, 2002). Thus, while cells and/or chloroplasts near the adaxial surface experience incident light at its full intensity, those further from the leaf surface are part-shaded due to light absorption and scattering by the upper layers of chloroplasts. The consequences of light gradients through a leaf have been identified in chloroplast ultrastructure (Terashima et al., 1986) and in photosynthetic function (Nishio et al., 1993; Evans and Vogelmann, 2003), demonstrating that there is differential acclimation for individual cells or chloroplasts according to their position within a leaf. As a result, measurements of chloroplast composition or photosynthesis represent the average for a heterogeneous population of chloroplasts; changes in leaf chlorophyll content affect the proportion of ‘shaded’ chloroplasts (Nishio et al., 1993), with consequent changes in measurements of Chl a/b and Pmax on a unit Chl basis. By contrast, Pmax on a leaf area basis has emerged as a robust parameter which is sensitive to growth conditions but which shows little variation in leaves showing wide variations in Chl content (Bailey et al., 2004).
Another problem which can complicate studies of acclimation is the use of inappropriate growth conditions. Certain acclimation responses may be abolished if growth lights are used which lack important regions of the spectrum; for instance, some fluorescent lights are deficient in the blue part of the spectrum, so that they fail to stimulate one or more photoreceptors important in activating acclimation (Walters and Horton, 1995b; Walters et al., 1999). Acclimation may also be hidden if plants are grown under environments which lie at or beyond the limit of that species' acclimation capacity. Detailed analysis shows that for each species (and perhaps even for each ecotype) there is a specific range of environments beyond which there is no further obvious acclimation. For instance, Arabidopsis exhibits little or no increase in Pmax at light levels higher than 600–700 µmol quanta m–2 s–1 (Bailey et al., 2004). It is plainly important to consider such limits to acclimation, so that the growth environments used allow acclimation to be observed.
One further issue, which is of importance not only for studies on acclimation but also more generally in the analysis of photosynthesis, is that actinic lights used for gas exchange or Chl fluorescence analyses are almost always spectrally distinct from sunlight and from lights used in growth chambers. This is very likely to result in an imbalance in the excitation of PSI and PSII: In the case of red or blue LEDs, light is preferentially absorbed by PSII so that PSI excitation is limiting for photosynthesis; conversely, halogen light sources are rich in longer wavelengths preferentially absorbed by PSI, so that PSII excitation is limiting (Walters et al., 1999). As a result of this mismatch between growth and experimental light sources, experimental determinations of photosynthetic efficiency can be appreciably lower than is the case under growth conditions. Indeed, whether PSI or PSII excitation is limiting, measurements of Chl fluorescence in situ routinely show PSII to be markedly more oxidized than is the case under experimental lights of similar irradiance (Walters et al., 2004). Therefore, while gas exchange and Chl fluorescence analyses are invaluable in making comparisons between plants so as to monitor changes which result from acclimation, differences in the spectral quality of growth and experimental lights (as is often unavoidable for technical reasons) mean that such measurements do not necessarily reflect the functioning of the photosynthetic apparatus under growth conditions. This places limits on the extent to which such studies are useful in understanding the consequences of acclimation for photosynthetic function.
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Static versus dynamic acclimation |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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Acclimation has been most thoroughly studied using plants grown over an extended period (often from seed) under controlled environments, with light being constant throughout the photoperiod. While such studies have been instrumental in providing a detailed description of acclimation at the anatomical, physiological, and molecular levels, leading to the identification of qualitatively different responses to specific environmental extremes, the light conditions used in such experiments have frequently lacked many aspects of the natural environment. These include sinusoidal patterns of irradiance during the day corresponding to dawn, noon, and dusk; changes in the angle of incident light as the sun progresses through the sky, which might lead to different faces of a leaf being directly illuminated; and short-term changes due to sunflecks arising from movements in neighbouring vegetation or from changes in cloud cover. Only a few studies have investigated the effects of these aspects of the environment on acclimation, but these have been sufficient to reveal complex responses and have shown that acclimation is a response neither to the maximum incident irradiance nor to incident light integrated over the photoperiod.
Perhaps the clearest demonstration that acclimation reflects dynamic aspects of the environment comes from the response to sunflecks. Growth of plants in fluctuating light has different consequences depending on the characteristics of the periods of high light, with the acclimation response being stronger when there are fewer flecks of a longer duration, even when the total incident light is the same (Yin and Johnson, 2000; Leakey et al., 2003). For some species such acclimation appears to be of an all-or-nothing nature, so that short flecks provoke little or no acclimation response. Even more complexity emerges when levels of individual chloroplast components are considered; Rubisco and cytochrome f levels exhibit different response patterns in plants exposed to sunflecks (Yin and Johnson, 2000).
Another demonstration that dynamic aspects of the light environment are important when considering acclimation comes from the finding that, for many species, photosynthesis is saturated for a large part of the day under field conditions, even in cases of ample nutrient and water supply. Under conditions where light shows diurnal variation, therefore, plants do not acclimate to the high light conditions which occur for several hours around the middle of the day. It is important to emphasize that this is not because they lack the ability to acclimate to higher light; shaded rice plants show a similar saturation of photosynthesis for much of the day, but are able to acclimate to increased light if the shading is removed (Murchie et al., 2002).
Analysis of acclimation is further complicated when considering the effects of a shift between two environments, such as might result from meteorological changes, since it is apparent that plants which are transferred from one growth environment to another do not necessarily exhibit the same acclimation response as those which grow from an early age under the latter growth regime (Yin and Johnson, 2000; Frak et al., 2001; Oguchi et al., 2003). Inspection of simple acclimation profiles constructed for growth under particular regimes gives the impression that acclimation to different environments represents a single continuum of responses; acclimation to a change in the environment would, therefore, involve no more than a shift to a different point on that continuum. However, it is now apparent that while this may be true over limited ranges, at the extremes of a species' acclimation range there are distinct qualitative differences in acclimation strategy; in Arabidopsis, growth under very low light leads to a dramatic increase in levels of PSI, while very high light leads to changes in PSII organization including reductions in the levels of minor LHC components (Bailey et al., 2001).
Such qualitative differences, not only in the acclimation response but also in leaf ultrastructure and chloroplast anatomy (Weston et al., 2000; Oguchi et al., 2003), appear to place limits on the ability of such plants to acclimate to a change in growth conditions. Even when the growth conditions used lie within the normal acclimation range, Arabidopsis grown under the very lowest light only partially acclimate when transferred to high light (Yin and Johnson, 2000; S Bailey, personal communication). It may be that this is in part because the photoinhibition suffered by these plants impacts on their ability to respond to the change in light level (Yin and Johnson, 2000); it is clear that acclimation interacts with short-term photoprotection, whose effects on photosynthesis can modulate potential regulatory signals. However, this cannot explain the failure of high light-grown plants to acclimate fully when transferred to very low light (S Bailey, personal communication).
A likely basis for such observations is that acclimation to a particular environment is limited by a plant's previous growth history. Although de novo synthesis and active degradation of photosynthesis components facilitate acclimation across a wide range of environments (Yang et al., 1998; Suzuki et al., 2001), there may be particular anatomical features or specialized forms of particular complexes which are not readily adjusted once they are in place. Alternatively, acclimation to environments at the limits of a plant's acclimation range may lead to features which attenuate the signals which prompt acclimation, thereby limiting the detection of and response to environmental change (Yin and Johnson, 2000). Thus, while an acclimation profile determined by assessing acclimation in plants grown under uniform conditions may describe the range of environments in which a plant can successfully grow, the composition of the photosynthetic apparatus for a particular plant (or even for individual leaves, cells or chloroplasts) would appear to depend on the aggregate of a variety of acclimation responses over the lifetime of the plant. In the context of a dynamic environment, a full description of acclimation has yet to be obtained.
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The adaptive significance of acclimation |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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There is substantial variation between species in the extent and nature of their acclimation responses. Most species show at least some variation in photosynthetic capacity, but the scale of observed changes in chloroplast composition varies dramatically. Initially this variation was believed to reflect whether plants were ‘sun’ or ‘shade’ species (Boardman, 1977
; Anderson and Osmond, 1987), but more recently it has become clear that acclimation capacity is most pronounced in species which are found growing in both exposed and shaded environments (Murchie and Horton, 1997). The clear inference is that acclimation provides a competitive advantage in locations that are environmentally heterogeneous or subject to environmental change, i.e. plants with strong acclimation are more likely to succeed in variable environments.
Numerous suggestions have been made to provide a basis for such a competitive advantage. The underlying rationale in each case is that adjustments in the photosynthetic apparatus improve the performance of the photosynthetic apparatus by enabling more efficient use of the available light or of other limiting resources, or that they provide a competitive advantage to a plant by reducing photo-oxidative damage (Anderson and Osmond, 1987; Anderson et al., 1995). Thus, under conditions where light is limiting, it is suggested that increases in the abundance of light-harvesting complexes increase the proportion of incident light which is captured, and that adjustments in the relative proportions of photosystems I and II increase the efficiency with which absorbed light is utilized, in each case improving the overall efficiency of photosynthesis. In addition, in low light there may be reduced synthesis of many components involved in electron transport, in the generation of ATP and reducing power and in CO2 fixation, so as to avoid unnecessary over-investment in their respective proteins, thereby releasing potentially limiting resources for use elsewhere in the plant. Conversely, the increases in the levels of these components at higher growth irradiance may allow for efficient photosynthesis while also reducing the susceptibility of the photosynthetic apparatus to environmental stress.
While plausible arguments can be made in support of such hypotheses, the evidence that acclimation directly provides clear adaptive benefits to the plant is often far from convincing. Of course, this is not to argue that acclimation is not advantageous to a plant, simply that there are appreciable gaps in current knowledge. This can partly be ascribed to incomplete understanding in other areas, for instance, the extent of any protective aspects of acclimation is likely to remain in doubt until the detailed mechanistic bases for photodamage are unambiguously established, but there remain significant areas where claims for an adaptive benefit are based more on assumption than proof.
Acclimation to light spectral quality
Adjustments in photosystem stoichiometry in response to changes in light spectral quality have the effect of altering the proportion of incident light which is directed towards each photosystem. Thus, under conditions of natural shade where light reaching the plant is enriched in far-red wavelengths preferentially absorbed by PSI, increases in the relative level of PSII are believed to ensure that the supply of electrons from PSII is sufficient to keep pace with the rate of excitation of PSI so that light reaching PSI is used efficiently. Conversely, in unshaded conditions a decreased PSII/PSI ratio is again believed to ensure that the rates of PSI and PSII excitation are balanced, so that absorbed light is used efficiently.
This is one area where there is strong evidence to support the notion that acclimation has a direct adaptive benefit. Two separate studies have confirmed that adjusting photosystem stoichiometry impacts on the efficiency of photosynthesis in lights of different spectral quality (Chow et al., 1990b; Walters and Horton, 1995a). Measurements of gas exchange and Chl fluorescence were carried out during illumination with light preferentially absorbed by either PSII or PSI. Plants grown in light favouring PSI (which therefore acclimated to give increased PSII/PSI ratios) had more efficient photosynthesis when illuminated with ‘PSI light’; conversely, ‘PSII light’ was used more effectively by plants grown in light favouring PSII (with decreased PSII/PSI ratios).
Thus, it appears that photosynthesis is most efficient when plants are illuminated with light of a similar spectral quality to that in which they were grown. The conclusion is that adjustments in photosystem stoichiometry do indeed serve to optimize electron transport, thereby improving photosynthetic efficiency. In conditions where light is limiting, the resulting improved rate of photosynthesis may provide a competitive advantage for plants acclimating in this way. It should be noted, however, that there has been no demonstration that such marginal increases in photosynthetic efficiency are important in terms of plant growth.
Acclimation to high light
Acclimation to high light results in an increase in the maximum photosynthetic rate, and several lines of evidence make it clear that this is of potential benefit to the plant. Not least of these is the simple observation that an increase in growth light (and therefore in photosynthesis) leads to an increase in growth rate. That a failure to acclimate to high light would result in slower growth is also illustrated by the reduced growth rate of transgenic plants in which there are reduced levels (or activities) of components of the photosynthetic apparatus (Quick et al., 1991a; Price et al., 1998). Such studies have also provided strong evidence that at least some of the changes observed during acclimation to high light are necessary in order for the plant to increase its photosynthesis and growth rates, and are therefore of benefit to the plant. In particular, antisense plants with reduced levels of the cytochrome b/f complex have marked reductions in Pmax, indicating that the large changes in levels of this complex during acclimation are intimately linked to the changes in photosynthetic capacity (Price et al., 1998).
One of the most pronounced changes in chloroplast composition with respect to growth irradiance is in the levels of Rubisco. Although it is frequently present in what appears to be substantial excess, there are nevertheless circumstances under which Rubisco content exerts strong control over photosynthetic capacity, for instance, in low light plants transferred to high light (Lauerer et al., 1993). Furthermore, the high Rubisco content appears to be important in allowing efficient regulation of other aspects of photosynthesis. Antisense plants with greatly reduced levels of Rubisco often maintain photosynthetic rate at wild-type levels, but have been found to suffer imbalances in electron transport and decreased water-use efficiency (Quick et al., 1991b). Thus, changes in Rubisco content appear to have adaptive value in increasing both the capacity for and efficiency of photosynthesis.
Far less clear is the adaptive advantage in adjusting the relative levels of pigment-binding complexes, particularly the substantial reorganization of PSII and its associated antenna involving increases in levels of PSII and concomitant decreases in LHCII in response to high light. This is highly unlikely to be due to a requirement for increased levels of reaction centres to maintain high rates of electron transport, since it is clear from numerous studies that photosynthesis is not limited in vivo by the rate of electron donation from PSII: at high light the acceptor side of PSII is invariably in a reduced state, indicating that limitations on electron transport lie further down the electron transport chain. An alternative is that PSII units with smaller antennae may be somewhat more efficient in energy conversion, since absorbed photons would have a shorter average pathlength before reaching the reaction centre and be less likely to undergo thermal de-excitation or re-emission as fluorescence. However, no significant increases in photosynthetic quantum yield have been reported, although measurements of the Chl fluorescence parameter Fv/Fm do indicate that PSII from high-light-grown plants have a higher intrinsic photochemical efficiency (Walters and Horton, 1999).
Another possibility is that reducing the number of chlorophylls associated with each PSII unit has a photoprotective effect; a smaller absorption cross-section would reduce the rate of excitation and of charge separation for each reaction centre. Several studies have indeed shown that plants acclimated to high light are less susceptible to a range of processes related to photoinhibition and photodamage (Park et al., 1996; Savitch et al., 2000). However, these effects are chiefly explained as a consequence of increases in electron transport rate due to higher rates of CO2 assimilation, photorespiration and/or Mehler-peroxidase reaction, and need not be attributed to the photoprotective effects of altering PSII antenna size (Savitch et al., 2000). Indeed, under particular experimental conditions PSII photodamage has been found to be a stochastic event, with a fixed probability of damage per photon absorbed; although this work showed that a smaller antenna protected individual PSII units (Park et al., 1997), the overall rate of PSII damage would be unaffected since a reduced antenna size is matched by a corresponding increase in the number of PSII units. Thus, although a photoprotective effect of a lower antenna size cannot be ruled out, perhaps by reducing the extent to which PSII is over-reduced during periods of high light (e.g. in the middle of the day), such benefits of this aspect of acclimation have yet to be demonstrated.
One further possible advantage to the plant of antenna reorganization is that it is beneficial in terms of short-term photoprotective processes. High-light-grown plants often have substantially increased capacities for 
pH-dependent protective energy dissipation (qE), which may relate to different energy dissipation characteristics of a larger light-harvesting system (Brugnoli et al., 1994; Park et al., 1996; Bailey et al., 2004). However, this seems unlikely to be a major benefit of a smaller antenna, since the principal determinant of qE capacity appears to be levels of the PsbS protein rather than the precise organization of the PSII antenna (Li et al., 2002; Bailey et al., 2004).
Acclimation to low light
Acclimation to low light presumably has some adaptive benefit, otherwise it would be open to the plant to retain a high-light-acclimated state under all growth conditions. The most obvious possibility is that reducing levels of major chloroplast components avoids unnecessary investment in their synthesis, so that resources (particularly organic nitrogen) are made available for growth in other parts of the plant. It has been suggested that, since LHCII binds pigments in a more N-efficient manner than PSII reaction centres (14 Chl and 3–4 carotenoids per 25 kDa apoprotein, compared with 64 chlorophylls, 4 pheophytins, and 10 carotenoids per 350 kDa core complex), an increase in LHCII content is a mechanism for improving N use efficiency.
Support for the idea that plants balance the demand for limiting resources against acclimation comes from several studies showing that Rubisco levels are reduced under conditions of reduced N supply (Quick et al., 1992). However, this appears to reflect a secondary role for Rubisco as a nitrogen store. Limiting N does not lead to reductions in other chloroplast components, and under extreme N limitation (where photosynthesis is not a major determinant of growth rate) plants respond by reducing levels of all chloroplast components in parallel, rather than by exhibiting acclimation. In particular, those few studies which have investigated the effect of low N on the thylakoid composition of wild-type plants have either found no change in Chl a/b indicating no adjustments in LHCII content (Bungard et al., 2000; E Murchie, personal communication; RG Walters, unpublished data), or increases in Chl a/b where there is extreme nitrogen limitation (Kitajima and Hogan, 2003). Thus, although it remains possible or even likely that acclimation is important in optimizing resource allocation, there is no evidence at present to support this interpretation.
As implied by the name given to them, LHCs are generally held to be important in light-harvesting. Consistent with this view is the increase in levels of LHCs, especially LHCII, in response to low light, conditions where the interception and capture of light are limiting for growth. However, the leaves of most higher plants absorb a high proportion of incident light, irrespective of chloroplast composition, and it does not appear that high levels of LHC are necessary for efficient light capture. In any case, the higher levels of LHCII in low-light-grown plants do not represent an appreciable increase in the total amount of light-harvesting pigments, since there is a compensating reduction in levels of PSII reaction centres (Walters et al., 1999). Therefore, any increase in light capture must be due to LHC-associated Chl b and xanthophylls (which absorb light at different wavelengths from Chl a and ß-carotene), but even this is likely to be of only marginal benefit. Increases in Chl b under low light are only a few per cent of total Chl, and changes in LHC levels have only small effects on xanthophyll content (Andersson et al., 2003; Bailey et al., 2004). It should also be considered that PSII with large antennae may have a lower intrinsic efficiency, as indicated by the lower values of Fv/Fm for low light-grown plants (see above).
Evidence that variations in the levels of LHCII do not have a major effect on light-harvesting comes from antisense Arabidopsis plants lacking most or all of the major LHCII complexes and which hence have marked reductions in PSII antenna size, even after accounting for compensating increases in levels of one of the minor LHCs (Andersson et al., 2003). These had very similar photosynthetic characteristics to the corresponding wild type, small reductions in apparent quantum yield being accounted for by a marginal reduction in light absorption due to a decrease in total leaf Chl. Furthermore, loss of LHCII had no discernible effect on growth rate under low-light conditions, and under shaded field conditions there was no consistent reduction in fitness as determined by seed production. Thus, although wild-type plants show large changes in Chl content in the conditions used in these experiments, there was no evidence to indicate that there is an adaptive benefit related to the ability to increase PSII antenna size during low-light growth.
One other possible advantage of a large PSII antenna is suggested by work investigating PSII photodamage under low light, in which PSII damage was induced with greater efficiency as the irradiance was decreased (Park et al., 1995; Keren et al., 1997). Since the singly-reduced state of the PSII secondary electron acceptor QB has a long lifetime, low rates of PSII charge separation under very low light conditions increase the probability of PSII charge recombination. It was suggested that an associated generation of oxidizing species was responsible for the higher yield of PSII damage (Keren et al., 1997), but such damage is likely to be negligible precisely because light levels are low. A more likely benefit of a large antenna, perhaps, is that under very low light conditions it increases PSII turnover rate. This would reduce the frequency of (harmless) dissipation of energy as heat due to charge recombination, which would otherwise reduce PSII quantum efficiency. It is therefore of significance that antisense LHCII plants had reduced growth rate, but only when grown under extreme low light (<50 µmol quanta m–2 s–1; S Jansson, personal communication). Thus, high levels of LHCII only appear to be important for plants grown in deep shade, and in higher light the identified benefits are, at best, marginal.
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Regulation of acclimation |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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The fact that acclimation responds in a seemingly appropriate manner to many aspects of the environment, often in combination, implies that plants are able to integrate the response to numerous potential signals. Indeed, genes encoding chloroplast components have been shown to be regulated by factors including light, temperature, and sugars (Pfannschmidt et al., 1999
, 2001; Oswald et al., 2001; Pursiheimo et al., 2001; Quail, 2002; Rolland et al., 2002). Furthermore, since each of the major components of the photosynthetic apparatus includes subunits encoded in both the nucleus and in the chloroplast, acclimation entails the co-ordination of expression of genes from both nuclear and plastid genomes. Therefore, it seems certain that there are complex mechanisms regulating chloroplast composition and facilitating acclimation in higher plants (Anderson et al., 1995).
In recent years it has become clear that signals from photosynthesis itself play a major role in regulating acclimation. Early suggestions that light absorption by the photosynthetic apparatus generates acclimation signals were based on analysis of plants and algae subjected to altered growth lights or of mutants deficient in Chl b (Kim et al., 1993; Maxwell et al., 1994). This work was extended by examining changes in mRNA abundance and transcriptional activity of both nuclear and chloroplast genes following treatments with inhibitors of electron transport, and as a result there is growing evidence that the redox state of PQ is a potential regulatory signal (Huner et al., 1998). Since PQ redox signals regulate the LHCII kinase involved in state transitions, it has been suggested that acclimation may involve part of the same regulatory pathway (Pursiheimo et al., 2001).
It is clear, however, that a PQ redox signal is insufficient to account for all observed acclimation responses in plants, since the same change in PQ redox state is associated with different acclimation responses depending on the way in which that change is induced. When PQ is reduced by a change to high light, levels of PSII increase with a corresponding decrease in LHCII levels (Walters et al., 1999; Bailey et al., 2001); but when PQ reduction is as a result of a change in light spectral quality there is, instead, a decrease in PSII levels (Kim et al., 1993; Pfannschmidt et al., 1999). Thus, further signals must be involved in regulating acclimation; candidates include ATP/ADP, stromal redox components, and carbohydrates, all of which are known to influence the expression of genes encoding photosynthetic components (Kim et al., 1999; Shen et al., 2001; Kim and Mayfield, 2002; Pfannschmidt, 2003). In the context of redox signalling, the involvement of signalling by one or more plastid-localized thioredoxins is an attractive possibility, since thioredoxins and PQ would together provide signals concerning the redox states on the acceptor sides of each of PSI and PSII. Thioredoxins have already been shown to interact with PQ signalling; they regulate the LHCII kinase in a manner antagonistic to PQ, and it is possible that acclimation is regulated via similar signalling pathways (Rintamaki et al., 2000; Pursiheimo et al., 2001).
Despite the strong evidence that redox signals affect the expression of both nuclear and chloroplast genes (Karpinski et al., 1997; Pfannschmidt et al., 1999, 2001; Ndong et al., 2001), it remains an open question whether such regulation is relevant to acclimation in higher plants. Figure 1 shows data from a microarray analysis of Arabidopsis grown under low or high light, growth conditions which lead to markedly different chloroplast compositions and photosynthetic characteristics (Fig. 1A; Walters et al., 1999; Bailey et al., 2001, 2004). Remarkably, there are no appreciable changes in the abundance of the nuclear or chloroplast mRNAs encoding any of the components of the photosynthetic apparatus (Fig. 1C–F); the only genes showing clear changes encode early light inducible proteins (ELIP1 and ELIP2). Consistent with this are other microarray studies in which changes in growth conditions (expected to induce significant acclimation) gave rise to only small changes in mRNA levels for most photosynthesis genes (Piippo et al., 2005).
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These findings indicate that post-transcriptional mechanisms play a crucial role in acclimation. Indeed, there are many examples of chloroplast components being regulated post-transcriptionally: LHCII content is not correlated with abundance of the corresponding mRNAs (Flachmann and Kühlbrandt, 1995
; Montane et al., 1998); synthesis of many photosynthesis components is regulated at the levels of mRNA stability and polyribosome loading (Tang et al., 2003); under conditions of high light excess LHCII is subject to proteolytic degradation (Yang et al., 1998); and many chloroplast-encoded proteins are regulated at the levels of translation initiation (Bruick and Mayfield, 1999; Nickelsen, 2003).
Since mRNA abundance clearly does not determine chloroplast composition, there is then a question as to the purpose of redox-mediated regulation of transcription. While it is not necessary for acclimation to occur, it perhaps provides a second layer of regulation which operates in concert with post-transcriptional mechanisms, or which becomes important under conditions which are not optimal for growth; such flexibility in regulation may also be necessary for appropriate acclimation to multiple environmental factors. Alternatively, redox regulation may be important in boosting the capacity for the synthesis of chloroplast components, specifically at times when dynamic acclimation is occurring. Following a change in growth conditions when there may be strong demand for the synthesis of specific components, an increase in transcription may be needed to allow higher rates of protein synthesis to meet the demands of acclimation. Thus, although mRNA levels do not determine LHCII content under normal growth conditions, an increase in the expression of genes encoding LHCII apoproteins may be important in the response to a transition from high to low light (Walters and Horton, 1994), while increased transcription of photosystem components in response to altered light spectral quality (Pfannschmidt et al., 1999, 2001) may be necessary to allow the observed rapid changes in photosystem stoichiometry.
If redox regulation of transcription and/or mRNA stability are not the key to acclimation under stable conditions, what other regulatory mechanisms might be responsible for acclimation in plants? Redox signals are important in chloroplast translation, have been shown to regulate translation of nuclear transcripts, and have also been suggested to influence protein import into chloroplasts (Bruick and Mayfield, 1999; Kuchler et al., 2002; Pfannschmidt, 2003; Tang et al., 2003; Nickelsen, 2003). However, it is important to note that while there is strong evidence that redox signals regulate transcription, the evidence for a role in quantitatively regulating post-transcriptional events is very weak. Studies identifying roles for redox signalling have almost invariably focused on the effects of an inhibitor treatment and/or following a change in growth conditions (Pfannschmidt et al., 1999, 2001); such treatments produce large perturbations in photosynthesis, leading not only to redox changes but also to other potential regulatory signals. By contrast, plants grown under stable environmental conditions exhibit strong acclimation, yet there are no discernible differences in the redox states of PQ and thioredoxins (Fig. 1B), i.e. there are no detectable redox signals which might allow discrimination between different growth conditions. Nevertheless, it is possible that the plants are responding to subtle redox changes which have not been observed, perhaps because they are smaller than can readily be detected, or are in components which have not been measured, or even because acclimation responds to the absolute levels of particular species rather than depending on redox state per se; for instance, a signal might be generated by increased levels of reduced PQ arising from an increase in the total PQ pool under high light growth, while the proportion of reduced PQ was unchanged.
However, there is clearly a strong possibility that redox signals are not directly involved in the regulation of acclimation. The data linking redox changes to acclimation are correlative, and do not exclude possible roles for other signals modulated in parallel. Indeed, evidence is emerging that it is possible to break the apparent correlation between redox signals and acclimation: in antisense tobacco with reduced levels of the cytochrome b/f complex, over-reduction of PQ did not result in changes in chloroplast composition (Anderson et al., 1997); and Arabidopsis lacking the triose-phosphate/phosphate translocator has altered PQ and thioredoxin redox states when grown in high light, but there are no discernible effects on thylakoid composition (Walters et al., 2002, 2004). Rather than having a principal role in acclimation, redox regulation of transcription may instead contribute to the ‘plastid signals’ which are generated during seedling establishment and which are required either to maintain expression of nuclear genes encoding chloroplast components or to repress them under conditions where chlorophyll intermediates accumulate (Rodermel, 2001; Strand et al., 2003). Thus, redox signals transmitted from the chloroplast may not be involved in the fine tuning of chloroplast composition, but instead comprise a global switch which interacts with other regulatory pathways, for example, in overriding sugar-regulated expression of nuclear-encoded photosynthetic genes (Oswald et al., 2001).
There are a number of other metabolic signals which the plant might use to distinguish between different environments, including the ATP:ADP ratio, levels of metabolic intermediates, and the accumulation of sugars. Indeed, the majority of treatments which have been used to manipulate redox signals are expected to have marked effects on these other potential signals. There may also be a role for signalling from trehalose-6-phosphate; recent work from transgenic tobacco with altered trehalose metabolism has shown that manipulating levels of trehalose-6-phosphate leads to appreciable changes in photosynthetic capacity (Pellny et al., 2004).
Another possibility is that photoreceptors have a role in some aspects of acclimation. Although it is clear that phytochromes are not implicated directly in acclimation to light quantity (Chow et al., 1990a; Walters and Horton, 1994; Walters et al., 1999), phyA mutations affecting phytochrome A function disrupt acclimation to high light by limiting the extent to which PSII levels can be increased. A similar effect is observed for mutants lacking either cryptochrome 1 or cryptochrome 2, and there is also evidence for a role for blue light in regulating acclimation (Walters and Horton, 1995b; Walters et al., 1999; RG Walters, unpublished data). CRY1, CRY2, PHYA, and an additional unidentified phytochrome interact in the regulation of chloroplast genes, mediating effects both on overall transcription and on expression from blue light-responsive promoters regulating genes encoding PSII components (Thum et al., 2001). Therefore, responses to changes in the spectral quality of incident light could be mediated via photoreceptor-regulated pathways, due to differential activation of CRY1/CRY2- and phytochrome-responsive pathways. Furthermore, det1, cop1, and hy5 mutations, which are cryptochrome/phytochrome signal transduction components, each cause the plant to acclimate as though experiencing light levels higher than are actually the case; each of these mutants has altered levels of PSII (Walters et al., 1999; RG Walters, unpublished data), so it is significant that blue light-responsive promoters controlling PSII genes have been shown to be regulated by DET1 (Christopher et al., 1997).
A demonstration that a particular signal can regulate expression of genes encoding chloroplast components is not a demonstration of its involvement in acclimation; it may be a prerequisite for gene expression but have no effect on the level of that expression, much as there is a distinction between on-off and dimmer switches for household lights. Thus, while there is strong evidence in support of regulation by redox signalling, sugar levels, energy status, and photoreceptors, it has yet to be established which (if any) of these determine chloroplast composition. Nor is it clear whether the same processes regulate the responses to light quantity and light quality. Furthermore, it is unknown whether the responses to light and to other environmental factors are regulated by the same mechanisms or by distinct but interacting pathways. Such questions have yet to be addressed in a comprehensive manner.
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Conclusions and prospects |
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TopAbstractIntroductionAssessing acclimationStatic versus dynamic...The adaptive significance of...Regulation of acclimationConclusions and prospectsReferences |
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There seems little doubt that photosynthetic acclimation is important to plants, particularly those which habitually grow in variable habitats. The fragmentary nature of the evidence that there are adaptive benefits associated with acclimation does not refute claims for such benefits, but it is at present insufficient to allow a comprehensive understanding of acclimation's significance. Rather more dramatic is the lack of understanding of the regulation of acclimation; there is no description even in general terms of the mechanisms underlying adjustments in chloroplast composition, and even the signals responsible for prompting acclimation have not been unambiguously identified. This is in stark contrast to the situation for unicellular organisms. A key problem has been that photosynthetic metabolism and plant physiology are complex systems where apparently simple treatments can have multiple effects.
The completion of the genome sequences of Arabidopsis (The Arabidopsis Genome Initiative, 2000) and several other plant species, together with recent advances in transcriptomics and proteomics, provide opportunities for new approaches to investigating acclimation, offering the prospect that these gaps in our understanding can be filled. Instead of the analysis of acclimation being a labour-intensive process involving functional, biochemical, spectrophotometric, and antibody-based assays, high throughput quantitative analysis is now possible allowing a description of how levels of all chloroplast proteins change according to the transcriptional activity of their respective genes. Also significant is the availability of collections of Arabidopsis lines carrying insertional mutations (Alonso et al., 2003), allowing reverse genetic approaches to assess proposed roles for putative redox sensors and signalling components.
Perhaps most important is the development of forward genetic screens for mutations affecting acclimation (Walters et al., 2003; Gray et al., 2003; R Wagner and T Pfannschmidt, personal communication). Mutant screen approaches have been the key to unravelling light signal transduction from phytochromes and cryptochromes, and are proving similarly powerful in dissecting sugar signalling (Quail, 2002; Rolland et al., 2002). The recent development of microarray methods to identify rapidly genes which have been disrupted in mutants selected by such screens (Mitra et al., 2004) can only increase the power of such approaches. There is every reason to hope that signalling components will soon be identified which specifically regulate acclimation, enabling an understanding of the mechanistic basis for this process.
Furthermore, mutants specifically defective in acclimation will be critical tools in understanding the adaptive benefits of acclimation. By comparing wild-type plants with acclimation-defective mutants, it will be possible definitively to identify the circumstances in which acclimation provides an adaptive benefit to the plants, and the precise nature of that advantage. At that point, it may finally be possible to consider the ways in which modifying acclimation behaviour in plants might help to improve agricultural productivity, and how acclimation in the natural environment might affect the responses of plants to global environmental change.
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Acknowledgements |
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I would like to thank Professor Peter Horton (Sheffield) for supporting my research on acclimation in Arabidopsis, Drs Giles Johnson (Manchester) and Erik Murchie (Sheffield) for useful and stimulating discussions, and Dr Nick Kruger (Oxford) for constructive comments during the writing of this review.
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