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Optimal photosynthetic performance requires that equal amounts of light are absorbed by photosystem ii (PSii) and photosystem i (PSi), which are functionally linked through the photosynthetic electron transport chain. However, photosynthetic organisms must cope with light conditions that lead to the preferential stimulation of one or the other of the photosystems. Plants react to such imbalances by mounting acclimation responses that redistribute excitation energy between photosystems and restore the photosynthetic redox poise. in the short term, this involves the so-called state transition process, which, over periods of minutes, alters the antennal crosssections of the photosystems through the reversible association of a mobile fraction of light-harvesting complex ii (LHCii) with PSi or PSii. Longer-lasting changes in light quality initiate a long-term response (LTr), occurring on a timescale of hours to days, that redresses imbalances in excitation energy by changing the relative amounts of the two photosystems. Despite the differences in their timescales of action, state transitions and LTr are both triggered by the redox state of the plastoquinone (PQ) pool, via the activation of the thylakoid kinase STN7, which appears to act as a common redox sensor and/or signal transducer for both responses. This review highlights recent findings concerning the role of STN7 in coordinating short- and long-term photosynthetic acclimation responses.
The oxidation of the PQ pool is the rate-limiting step in photosynthetic electron flow1 and its redox poise is maintained by balancing the activity of PSII and PSI, which operate in series within the photosynthetic electron transport chain. Because they differ in polypeptide and pigment composition, the reaction centers of PSII and PSI preferentially absorb light with maxima at 680 and 700 nm, respectively. Therefore, changes in the spectral quality or intensity of incident light will often lead to imbalanced excitation of PSI and PSII. This can happen for instance under shaded or light-limiting conditions, or as a consequence of shifts in the spectral filtering properties of leaf canopies.2 Photosynthetic organisms have evolved ingenious short-term and long-term responses to changing light conditions which permit them to maintain an optimal level of photosynthesis (Fig. 1).2–4 Moreover, despite the fact that they are set in train by changes in light quality, photosynthetic acclimation responses are exclusively modulated by the PQ redox state, without any involvement of photoreceptors.5,6
Rapid changes in light quality, such as flickers of sunlight caused by leaf movement, are accommodated by state transitions, which reconfigure, on a timescale of minutes, the antenna structure of photosystems so as to balance the absorption of incident light.7–9 This is achieved by the lateral movement of the so-called ‘mobile pool’ of LHCII, which can reversibly associate with either PSI or PSII. When PSII is over-stimulated relative to PSI, the redox state of the PQ pool is driven into a more reduced state. These conditions favour the docking of plastoquinol (PQH2) to the Qo site of the cytochrome (Cyt) b6/f complex, which in turn leads to the activation of a thylakoid protein kinase that phosphorylates LHCII. The phosphorylated LHCII (pLHCII) relocates physically from PSII to PSI, allowing PSI to absorb and utilize some of the light that would otherwise be collected by PSII. This corresponds to the so-called State 2. Conversely, when too much energy is allocated to PSI, oxidation of the PQ pool occurs, followed by inactivation of the LHCII kinase and dephosphorylation of pLHCII by a yet unknown protein phosphatase. As a result, the dephosphorylated LHCII detaches from PSI and functionally couples with PSII (State 1), thus favouring energy redistribution towards PSII. Recent findings have shown that the movement of the mobile LHCII fraction during state transitions is due to differences between the two photosystems in their relative affinities for LHCII and pLHCII.10 Phosphorylation of LHCII decreases its affinity for PSII and increases its tendency to dock at a specific site on PSI. Chemical cross-linking and RNA interference approaches have provided evidence for the existence on PSI of a docking domain for LHCII, at the level of the subunits PsaL, PsaH and PsaO.10,11 Thus, Arabidopsis plants devoid of the PsaO core subunit showed 50% reduction in state transitions,12 and plants lacking either PsaH or PsaL are blocked in State 1.10 Importantly, even in the absence of the docking site on PSI, LHCII still undergoes phosphorylation in State 2, but remains attached to PSII. This indicates that pLHCII migration depends on the availability of the binding site on PSI, which has an intrinsically higher affinity for it than the one on PSII.10 A recent study based on gentle mechanical fractionation of the thylakoid membranes has indicated that the lateral movement of pLHCII might be confined to a very limited portion of the thylakoid membranes, at the grana margins, which may represent the sites where balanced allocation of light energy is actually restored.13
Under conditions characterized by persistent gradients in light quality—typical of most parts of dense plant stands—optimal photosynthetic performance is achieved by the long-term response (LTR). Over periods on the order of hours and days, the LTR restores photosynthetic energy balance by re-adjusting photosystem stoichiometry.2 Most species investigated indeed exhibit enhanced expression of the PSI reaction-center genes psaA and psaB (which encode the P700 apoproteins) upon active reduction of the PQ pool or repression of its oxidation. In addition, species that mount LTR responses involving regulation of the expression of the PSII reaction-center gene psbA (encoding the D1 protein) have been also described.14 Besides alterations in photosystem gene expression, several other physiological and molecular parameters change during LTR, including the chlorophyll a/b ratio, steadystate chlorophyll fluorescence, and thylakoid protein phosphorylation, together with the structure of the thylakoid membrane system.15,16 The LTR is also accompanied by dynamic changes in metabolite pools according to the prevailing illumination, as shown more recently.17 In particular, plants propagated under PSI-specific light showed less accumulation of transitory starch compared with plants grown under PSII-specific light,17,18 whereas total protein and fatty acid accumulation was unaffected. Furthermore, metabolic profiling, performed on plants subjected to PSI-PSII light-quality shifts, indicates that a number of important organic acids and several amino acids that are precursor metabolites of secondary metabolism are coregulated in opposite directions, depending on light conditions.17 Thus, the LTR appears to induce two metabolic states that, together with the adjustment of the photosynthetic electron transport chain, are responsible for adapting plant primary productivity to environmental conditions.
Significant advances in our understanding of the molecular components and structural bases for photosynthetic acclimation responses has been made recently, thanks mainly to genetic and biochemical studies in the model organism Arabidopsis thaliana. In particular, mutant analysis revealed that the thylakoid-associated kinase STN7 is essential for state transitions in Arabidopsis.15,19 Mutants devoid of this serine-threonine protein kinase displayed much less phosphorylation of LHCII, and were unable to undergo state transitions, as demonstrated by the lack of the characteristic differences in chlorophyll fluorescence that distinguish between States 1 and 2.15,19 Considerable progress has also been made in elucidating the mechanisms that control the activity of the LHCII kinase. Thus, it is now clear that the phosphorylation of LHCII and the redox state of PQ are not always coupled, as there are numerous reports of downregulation of LHCII phosphorylation at high irradiance, when the PQ pool is reduced.15,20,21 Conversely, maximum phosphorylation of LHCII occurs in vivo at low light intensities. It therefore appears likely that STN7 kinase activity is regulated not by PQ alone, but by a complex network involving co-operative redox control by PQ and the Cyt b6/f complex, as well as by the ferredoxin//thioredoxin system in the stroma of the chloroplasts.21
The recent characterization of the STN7 homologue in C. reinhardtii, STT7, has revealed that this kinase is characterized by a transmembrane helix that separates its stroma-exposed catalytic domain from its lumen-located N-terminal end, and has two conserved cysteine residues that are critical for its activity.22 In addition, coimmunoprecipitation assays have shown that STT7 interacts with Cyt b6/f, PSI and LHCII, suggesting that all these protein complexes might be clustered together, possibly in very restricted areas of thylakoid membranes, such as the grana margins.13,22
Studies on the stn7 mutant uncovered an intriguing connection between state transitions and the LTR, when analyses of photosynthetic parameters, such as the chlorophyll a/b ratio and steady-state chlorophyll fluorescence, indicated that STN7 is also required for the LTR.15,16 Thus, it appears that the STN7 kinase represents a common redox sensor and/or signal transducer for both responses, supporting earlier suggestions that the two processes are subject to regulatory coupling (Fig. 1).23 In this context, the inability of psad1-1 and psae1-3 mutants, which are defective in linear electron transport,24,25 to undergo the LTR appears to be a direct result of the fact that the PQ pool remains permanently reduced and, as a consequence, STN7 is constantly active and no longer subject to regulation by light conditions.26
The regulatory coupling of state transitions to the LTR and the dependence of both processes on STN7 activity are, in principle, compatible with the view that the signal pathways leading to state transitions and LTR represent a hierarchically organized signaling cascade, with changes in PQ redox state first triggering state transitions and then the LTR via an STN7-dependent phosphorylation cascade. However, neither LHCII phosphorylation nor the conformational changes in the thylakoid associated with state transitions themselves appear to play any role in LTR, as shown by the undiminished ability of Arabidopsis mutant lines that are impaired in various components required for state transitions to perform LTR.26 Moreover, Arabidopsis RNAi lines devoid of the TSP9 protein, which is thought to dissociate partially from the thylakoid membrane upon phosphorylation and which has been tentatively suggested to function in the signaling pathway,27,28 exhibited a normal LTR.26 Taking the available data together, it can be concluded that at some point, the PQ redox state reaches a threshold value that induces the reversible STN7-dependent phosphorylation of a specific, as yet unknown, thylakoid-associated protein, which then triggers the signaling events leading to LTR.
The LTR is mainly mediated by changes in the accumulation of thylakoid protein complexes. Indeed, in stn7 mutants exposed to changes in light quality and quantity, photosynthetic para-meters and protein levels behave abnormally,15,16 which is consistent with the idea that STN7 might be involved in prompting changes in plastid and nuclear gene expression that then result in photosynthetic acclimation (Fig. 1). However, only minor differences in the levels of transcripts of photosynthetic genes were detected by comparative microarray analyses of wild-type and stn7 plants that were grown in various light conditions.15,16 Furthermore, a detailed transcriptomic analysis combining light-shift experiments with the application of an inhibitor of thylakoid electron transport revealed that the redox state of the photosynthetic electron transport chain actually controls the expression of very few nuclear genes involved in photosynthesis.5 Similar data were also obtained from a study of the barley mutant viridis zb63, which is depleted of PSI and exhibits a constitutively reduced PQ pool.29 Indeed, although the antenna size was strongly reduced in viridis zb63, levels of transcription of nuclear photosynthesisrelated genes remained unaffected. A comprehensive picture of the effects of the induction of LTR on Arabidopsis nuclear gene expression, under greenhouse conditions, was obtained recently by analyzing the transcriptome profiles of a number of mutants impaired in the LTR process, including the single mutants stn7-1, psad1-1 and psae1-3, and the double mutants stn7-1 psad1-1 and stn7-1 psae1-3.26 Despite the large number of genes found to be deregulated in the different mutants, a small set of only 56 nuclear genes turned out to be co-regulated in the different genetic backgrounds, and these may perhaps represent the direct targets of LTR-dependent regulation. Genes involved in stress responses, and in post-transcriptional, translational, and post-translational regulation of gene expression are among the members of this set, but none of them codes for a protein of the thylakoid electron transport chain. In contrast to the LTR-dependent regulation of nuclear genes, expression analyses of the plastid genes psaAB and psbA encoding the PSI and PSII reaction-center subunits, respectively, revealed that their expression is robustly modulated by changes in the PQ pool redox state, although a few exceptions have been found in other species.2,26
To investigate further the expression behavior of nuclear photosynthesis- related genes during LTR, the transcript profiles of wild-type plants that had been exposed to either strongly reducing (PSI-PSII light shift) or oxidizing conditions (PSII-PSI light shift) have recently been analyzed at different time points during the light-shifts.17 Interestingly, in response to the reducing signal, nuclear photosynthesis-related genes were clearly repressed after two hours, but less than 48 hours later this repression had been reversed, despite the continued presence of the initial signal. Conversely, nuclear photosynthesis-related genes were highly expressed after two hours under oxidizing conditions, whereas they were clearly repressed after 48 hours. The nuclear gene expression response to a reducing signal was also analyzed in wild type and stn7 plants. Of the 937 genes found to be significantly regulated in wild-type plants after 30 minutes of exposure to reducing conditions, 800 showed no reaction in the stn7 mutant, indicating the existence of a consistent fraction of genes whose expression is under redox control.
Thus, a picture emerges in which the LTR-associated regulation of thylakoid protein composition is achieved by modulating the expression of chloroplast photosynthesis-related genes at the transcriptonal level, whereas a multi-step regulatory process seems to be responsible for the regulation of nuclear photosynthetic genes. The latter involves regulation at the transcriptional level during the early phases of adaptation to changes in light quality changes, but this is subsequently succeeded by mechanisms that control protein translation or degradation, or the efficiency and specificity of protein import (Fig. 1).
State transitions and LHCII phosphorylation have been extensively studied in Chlamydomonas reinhardtii.3,4,7,30 In this green alga the magnitude of state transitions is much larger than in flowering plants. Thus, it has been reported that up to 85% of the LHCII antenna can be displaced from PSII in State 2. In comparison, the mobile fraction in green plants comprises only 20–30% of the total LHCII. State transitions in green algae are not only involved in rebalancing excitation energy between the photosystems, but also represent a unique adaptive mechanism that allows the organism to switch between linear (State 1) and cyclic (State 2) electron flow through PSI.31 Therefore, from a metabolic point of view, state transitions in green algae can be considered to be a regulatory mechanism that raises levels of ATP when its intracellular concentration is low. As a matter of fact, C. reinhardtii mutants that cannot undergo state transitions, such as stt7, exhibit altered photosynthetic performance and a marked decrease in growth rate.32,33 On the other hand, in light of the fact that plant development and fitness are only marginally affected in A. thaliana mutants impaired in state transitions,10 even under fluctuating light15,19 or under field conditions34 when the ability to adapt to illumination changes should become crucial, the physiological significance of state transitions in flowering plants has been a matter of intense debate. To clarify whether state transitions contribute significantly to acclimating photosynthetic performance to perturbations in photosynthetic electron flow, a genetic approach has been employed.26 Mutations that affect linear electron transport and are associated with an increased pool of reduced PQ, including psad1-1 and psae1-3, were introduced into the stn7-1 genetic background, which is impaired both in state transitions and in LTR. In all cases, the double mutants exhibited a marked decrease in growth rate relative to the parental single mutants, together with a consistent drop in the effective quantum yield of PSII and an increase in the reduction state of the PQ pool. Similar results were obtained when the mutations causing perturbations in linear electron flow were introduced into the psal-1 mutant,10 which is affected in state transitions but not in LTR, implying that state transitions become critical for plant performance when linear electron flow is perturbed. Further spectroscopic analyses performed on the different genotypes led to the conclusion that, in flowering plants, as in green algae, state transitions play an important role in balancing energy distribution between photosystems. However, in contrast to C. reinhardtii, impairment of the ability to undergo state transitions did not affect cyclic electron flow in Arabidopsis, indicating that, in flowering plants, the induction of State 2 does not promote the switch from linear to cyclic electron transport.
More recently, wild-type and stn7 plants were grown under PSI and PSII light interrupted by dark periods of increasing length, with a view to estimating the physiological importance of LTR for plant performance.17 This strategy was chosen to test whether the absence of metabolic reprogramming, including the different levels of starch accumulation observed during LTR, might have an impact on mutant plants that cannot mount an LTR. A short night period (8 h) did not alter the growth rate of wild-type or stn7 plants; however, a longer night period (16 h) resulted in a fall in growth rate under PSI light, which was much more marked in the stn7 mutant than in wild-type plants. Because no state transitions occur under PSI light, the marked reduction in growth rate of stn7 plants can be only ascribed to the suppression of LTR and the associated metabolic reprogramming, implying that LTR is indeed essential for optimal acclimation of higher plants to environmental conditions.17
Short- and long-term photosynthetic acclimation responses are triggered by changes in the redox state of the PQ pool and require the modulated activity of the kinase STN7. It seems likely that STN7 is positioned at the top of a phosphorylation cascade that induces state transitions by phosphorylating LHCII and initiates the LTR process via the phosphorylation of as yet unknown chloroplast proteins. Beyond this point, the LTR signaling pathway is divided into two main branches: one is responsible for transcriptionally regulating chloroplast gene expression, while the other regulates the expression of nuclear photosynthesis-related genes at transcriptional and post-transcriptional levels. Ultimately, this complex regulatory network appears to define two main physiological states that permit the plant to optimize productivity under the different light conditions.17,26 During the last few years, the molecular details of state transitions and their physiological relevance have been clarified both in algae and flowering plants.15,19,26,32 Moreover, intense efforts are currently underway to identify the redox-independent, constitutively active protein phosphatase that dephosphorylates LHCII during the transition from State 2 to 1. The importance of LTR for plant performance has become clear only recently;17 however, the components of the LTR signaling transduction pathway remain unknown. The chloroplast sensor kinase (CSK),35 which has been reported to couple the redox state of the PQ pool to the expression of chloroplast photosynthesis-related genes, might be involved in the LTR-chloroplast signaling branch. However, nothing is known about how the LTR signal exits the chloroplast and modulates the accumulation of photosynthesis-related proteins. Because the LTR process apparently involves the regulation of nuclear transcripts shortly after its activation, most of the transcriptomic studies performed to date provide only a limited contribution to our understanding of signaling pathways. In this context, further analyses of transcript profiles at different time points after the primary LTR stimulus are needed. Proteomic and metabolomic studies might also provide routes toward the identification of components of the LTR cascade. In particular, studies of the phosphoproteome promise to identify novel proteins that, upon phosphorylation, might convey or relay to the chloroplast transcription machinery and to other regions of the cell. At present, transcriptomic, proteomic and metabolomic analyses are rarely employed together. Tighter integration of these techniques should give us a more comprehensive view of the molecular events that occur in photosynthetic cells during acclimation to changes in incident light.
Previously published online: www.landesbioscience.com/journals/psb/article/10198