Dynamics of photosynthetic membrane composition and function

Arabidopsis STN7 kinase provides a link between short- and long-term photosynthetic acclimation

Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of FV/FM on excitation wavelength

The F(0) and F(M) level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F(V)/F(M) (=(F(M) - F(0))/F (M)) have been derived: within each of the three spectra of F(V)/F(M), statistically meaningful variations were detected. Also, at distinct wavelength regions, the (V)/F(M) differed significantly between leaf types. From spectra of F(V)/F (M), excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F(V)/F(M) between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F(V)/F(M) at these wavelengths. All three leaves exhibited low values of F(V)/F(M) around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550-650 nm region, the F(V)/F(M) in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C(4) species, than in barley, a C(3) species. Finally, low values of F(V)/F(M) at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F(V)/F(M) ratio to derive information on spectral absorption properties of PS I and PS II is discussed.

Oxidative stress and antioxidant defenses in two green microalgae exposed to copper

The aim of this work was to assess the effects of 1 week copper exposure (6.2, 108, 210 and 414microM) on Scenedesmus vacuolatus and Chlorella kessleri. The strains showed different susceptibility to copper. Copper content was determined in both strains by total X-ray reflection fluorescence analysis (TXRF). In S. vacuolatus, the increase of medium copper concentration induced an augmentation of protein and MDA content, and a significant decrease in the chlorophyll a/chlorophyll b ratio. S. vacuolatus showed a significant increase of catalase activity in 210 and 414microM of copper, and a significant increment of SOD activity and GSH content only in 414microM of copper. On the contrary, C. kessleri did not show significant differences in these parameters between 6.2 and 108microM of copper. Increased copper in the environment evokes oxidative stress and an increase in the antioxidant defenses of S. vacuolatus.

Functional complementation of the Arabidopsis thaliana psbo1 mutant phenotype with an N-terminally His6-tagged PsbO-1 protein in photosystem II

The Arabidopsis thaliana mutant psbo1 has recently been described and characterized. Loss of expression of the PsbO-1 protein leads to a variety of functional perturbations including elevated levels of the PsbO-2 protein and defects on both the oxidizing- and reducing-sides of Photosystem II. In this communication, two plant lines were produced using the psbo1 mutant as transgenic host, which contained an N-terminally histidine(6)-tagged PsbO-1 protein. This protein was expressed and correctly targeted into the thylakoid lumen. Immunological analysis indicated that different levels of expression of the modified PsbO-1 protein were obtained in different transgenic plant lines and that the level of expression in each line was stable over several generations. Examination of the Photosystem II closure kinetics demonstrated that the defective double reduction of Q(B) and the delayed exchange of Q(B)H(2) with the plastoquinone pool which were observed during the characterization of the psbo1 mutant were effectively restored to wild-type levels by the His(6)-tagged PsbO-1 protein. Flash fluorescence induction and decay were also examined. Our results indicated that high expression of the modified PsbO-1 was required to increase the ratio of PS II(alpha)/PS II(beta) reaction centers to wild-type levels. Fluorescence decay kinetics in the absence of DCMU indicated that the expression of the His(6)-tagged PsbO-1 protein restored efficient electron transfer to Q(B), while in the presence of DCMU, charge recombination between Q(A)(-) and the S(2) state of the oxygen-evolving complex occurred at near wild-type rates. Our results indicate that high expression of the His(6)-tagged PsbO-1 protein efficiently complements nearly all of the photochemical defects observed in the psbo1 mutant. Additionally, this study establishes a platform on which the in vivo consequences of site-directed mutagenesis of the PsbO-1 protein can be examined.

Mechanism of downregulation of photosystem I content under high-light conditions in the cyanobacterium Synechocystis sp. PCC 6803

Downregulation of photosystem I (PSI) content is an essential process for cyanobacteria to grow under high-light (HL) conditions. In a pmgA (sll1968) mutant of Synechocystis sp. PCC 6803, the levels of PSI content, chlorophyll and transcripts of the psaAB genes encoding reaction-centre subunits of PSI could not be maintained low during HL incubation, although the causal relationship among these phenotypes remains unknown. In this study, we modulated the activity of psaAB transcription or that of chlorophyll synthesis to estimate their contribution to the regulation of PSI content under HL conditions. Analysis of the psaAB-OX strain, in which the psaAB genes were overexpressed under HL conditions, revealed that the amount of psaAB transcript could not affect PSI content by itself. Suppression of chlorophyll synthesis by an inhibitor, laevulinic acid, in the pmgA mutant revealed that chlorophyll availability could be a determinant of PSI content under HL. It was also suggested that chlorophyll content under HL conditions is mainly regulated at the level of 5-aminolaevulinic acid synthesis. We conclude that, upon the shift to HL conditions, activities of psaAB transcription and of 5-aminolaevulinic acid synthesis are strictly downregulated by regulatory mechanism(s) independent of PmgA during the first 6 h, and then a PmgA-mediated regulatory mechanism becomes active after 6 h onward of HL incubation to maintain these activities at a low level.

The dynamics of photosynthesis

Despite recent elucidation of the three-dimensional structure of major photosynthetic complexes, our understanding of light energy conversion in plant chloroplasts and microalgae under physiological conditions requires exploring the dynamics of photosynthesis. The photosynthetic apparatus is a flexible molecular machine that can acclimate to metabolic and light fluctuations in a matter of seconds and minutes. On a longer time scale, changes in environmental cues trigger acclimation responses that elicit intracellular signaling between the nucleo-cytosol and chloroplast resulting in modification of the biogenesis of the photosynthetic machinery. Here we attempt to integrate well-established knowledge on the functional flexibility of light-harvesting and electron transfer processes, which has greatly benefited from genetic approaches, with data derived from the wealth of recent transcriptomic and proteomic studies of acclimation responses in photosynthetic eukaroytes.

Sensing and responding to excess light

Plants and algae often absorb too much light-more than they can actually use in photosynthesis. To prevent photo-oxidative damage and to acclimate to changes in their environment, photosynthetic organisms have evolved direct and indirect mechanisms for sensing and responding to excess light. Photoreceptors such as phototropin, neochrome, and cryptochrome can sense excess light directly and relay signals for chloroplast movement and gene expression responses. Indirect sensing of excess light through biochemical and metabolic signals can be transduced into local responses within chloroplasts, into changes in nuclear gene expression via retrograde signaling pathways, or even into systemic responses, all of which are associated with photoacclimation.

Optical properties of microalgae for enhanced biofuels production

Research seeks to alter the optical characteristics of microalgae in order to improve solar-to-biofuels energy conversion efficiency in mass culture under bright sunlight conditions. This objective is achieved by genetically truncating the size of the light-harvesting chlorophyll arrays that serve to absorb sunlight in the photosynthetic apparatus.

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of Arabidopsis thaliana under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.

Assembly and disassembly of the photosystem II manganese cluster reversibly alters the coupling of the reaction center with the light-harvesting phycobilisome

The light-driven oxidative assembly of Mn (2+) ions into the H 2O oxidation complex (WOC) of the photosystem II (PSII) reaction center is termed photoactivation. The fluorescence yield characteristics of Synechocystis sp. PCC6803 cells undergoing photoactivation showed that basal fluorescence, F 0, exhibited a characteristic decline when red, but not blue, measuring light was employed. This result was traced to a progressive increase in the coupling of the phycobilisome (PBS) to the PSII reaction center as determined by observing the changes in room temperature and 77 K fluorescence emission spectra that accompany photoactivation. The results support the hypothesis that strong energetic coupling of the PBS to the PSII reaction center depends upon the formation of an active WOC, which presumably diminishes the likelihood of photodamage to reaction centers that have either lost an intact Mn cluster or are in the process of assembling an active WOC.

Photosynthetic redox regulation of the plastocyanin promoter in tobacco

Persistent light quality gradients in dense plant populations induce imbalances in the distribution of excitation energy between the photosystems. Plants counteract such conditions by re-adjusting the stoichiometry of photosystems, which involves control of photosynthesis gene expression both in chloroplasts and in the nucleus. Decisive control parameters are redox signals from the photosynthetic electron transport chain, one prominent is the plastoquinone (PQ) pool. In a recent study, a plastocyanin (PC)-promoter::beta-glucuronidase reporter gene construct in tobacco demonstrated reversible redox regulation in response to varying light qualities. Here, northern and Western analyses demonstrate that this promoter regulation also accounts for the accumulation of the endogenous tobacco PetE gene transcripts and the protein amounts of the encoded PC. Hence, the reporter gene construct reflects the natural regulation of this nuclear gene in tobacco. In kinetic experiments, the response of the construct to either oxidation or reduction of the PQ pool was tested by defined light quality shifts. The construct displayed upregulation in response to a reduction signal and downregulation in response to an oxidation signal, both with a half-time of about 24 h. The response was finished after 48 h. DCMU application abolished the upregulation in response to the reduction signal, indicating the dependence on thylakoid membrane electron transport. To study the redox-responsive promoter region in more detail, several promoter deletion constructs were tested for their responsiveness. All constructs displayed a reversible response to light-induced oxidation and reduction signals; however, a minimal promoter region localised between -168 to -79 bp upstream of the transcription start site was sufficient to confer this redox regulation. This indicates that photosynthetic redox signals act on distinct regions in the PC promoter in a manner independent from photoreceptors and upstream cis elements conferring high basic expression in the light.

Photosynthetic acclimation: state transitions and adjustment of photosystem stoichiometry--functional relationships between short-term and long-term light quality acclimation in plants

In dense plant populations, individuals shade each other resulting in a low-light habitat that is enriched in far-red light. This light quality gradient decreases the efficiency of the photosynthetic light reaction as a result of imbalanced excitation of the two photosystems. Plants counteract such conditions by performing acclimation reactions. Two major mechanisms are known to assure efficient photosynthesis: state transitions, which act on a short-term timescale; and a long-term response, which enables the plant to re-adjust photosystem stoichiometry in favour of the rate-limiting photosystem. Both processes start with the perception of the imbalanced photosystem excitation via reduction/oxidation (redox) signals from the photosynthetic electron transport chain. Recent data in Arabidopsis indicate that initialization of the molecular processes in both cases involve the activity of the thylakoid membrane-associated kinase, STN7. Thus, redox-controlled phosphorylation events may not only adjust photosystem antenna structure but may also affect plastid, as well as nuclear, gene expression. Both state transitions and the long-term response have been described mainly in molecular terms, while the physiological relevance concerning plant survival and reproduction has been poorly investigated. Recent studies have shed more light on this topic. Here, we give an overview on the long-term response, its physiological effects, possible mechanisms and its relationship to state transitions as well as to nonphotochemical quenching, another important short-term mechanism that mediates high-light acclimation. Special emphasis is given to the functional roles and potential interactions between the different light acclimation strategies. A working model displays the various responses as an integrated molecular system that helps plants to acclimate to the changing light environment.

Chlorophyll fluorescence emission spectrum inside a leaf

Chlorophyll a fluorescence can be used as an early stress indicator. Fluorescence is also connected to photosynthesis so it can be proposed for global monitoring of vegetation status from a satellite platform. Nevertheless, the correct interpretation of fluorescence requires accurate physical models. The spectral shape of the leaf fluorescence free of any re-absorption effect plays a key role in the models and is difficult to measure. We present a vegetation fluorescence emission spectrum free of re-absorption based on a combination of measurements and modelling. The suggested spectrum takes into account the photosystem I and II spectra and their relative contribution to fluorescence. This emission spectrum is applicable to describe vegetation fluorescence in biospectroscopy and remote sensing.

Cool temperatures interfere with D1 synthesis in tomato by causing ribosomal pausing

Photodamage occurs when leaves are exposed to light in excess of what can be used for photosynthesis and in excess of the capacity of ancillary photoprotective as well as repair mechanisms. An important site of photodamage is the chloroplast encoded D1 protein, a component of the photosystem II (PSII) reaction center. Even under optimal growth irradiance, D1 is photodamaged necessitating rapid turnover to prevent the accumulation of photodamaged PSII reaction centers and consequent inhibition of photosynthesis. However, this on-going process of D1 turnover and replacement was impeded in the chilling-sensitive tomato (Solanum lycopersicum) plants when exposed to high-growth light at cool temperature. The decrease in D1 turnover and replacement was found not to be due to changes in the steady-state level of the psbA message. While the recruitment of ribosomes to psbA transcript, initiation of D1 translation, and the association of polysomes with the thylakoid membrane occurred normally, chilling temperatures caused ribosomal pausing during D1 peptide elongation in tomato. The pause locations were non-randomly located on the D1 transcript. The interference with translation caused by ribosomal pausing allowed photodamaged PSII centers to accumulate leading to the consequent inhibition of photosynthesis.

Functional analysis of N-terminal domains of Arabidopsis chlorophyllide a oxygenase

Higher plants acclimate to various light environments by changing the antenna size of a light-harvesting photosystem. The antenna size of a photosystem is partly determined by the amount of chlorophyll b in the light-harvesting complexes. Chlorophyllide a oxygenase (CAO) converts chlorophyll a to chlorophyll b in a two-step oxygenation reaction. In our previous study, we demonstrated that the cellular level of the CAO protein controls accumulation of chlorophyll b. We found that the amino acids sequences of CAO in higher plants consist of three domains (A, B, and C domains). The C domain exhibits a catalytic function, and we demonstrated that the combination of the A and B domains regulates the cellular level of CAO. However, the individual function of each of A and B domain has not been determined yet. Therefore, in the present study we constructed a series of deleted CAO sequences that were fused with green fluorescent protein and overexpressed in a chlorophyll b-less mutant of Arabidopsis thaliana, ch1-1, to further dissect functions of A and B domains. Subsequent comparative analyses of the transgenic plants overexpressing B domain containing proteins and those lacking the B domain determined that there was no significant difference in CAO protein levels. These results indicate that the B domain is not involved in the regulation of the CAO protein levels. Taken together, we concluded that the A domain alone is involved in the regulatory mechanism of the CAO protein levels.

Photosynthetic antenna size in higher plants is controlled by the plastoquinone redox state at the post-transcriptional rather than transcriptional level

We analyze the effect of the plastoquinone redox state on the regulation of the light-harvesting antenna size at transcriptional and post-transcriptional levels. This was approached by studying transcription and accumulation of light-harvesting complexes in wild type versus the barley mutant viridis zb63, which is depleted in photosystem I and where plastoquinone is constitutively reduced. We show that the mRNA level of genes encoding antenna proteins is almost unaffected in the mutant; this stability of messenger level is not a peculiarity of antenna-encoding genes, but it extends to all photosynthesis-related genes. In contrast, analysis of protein accumulation by two-dimensional PAGE shows that the mutant undergoes strong reduction of its antenna size, with individual gene products having different levels of accumulation. We conclude that the plastoquinone redox state plays an important role in the long term regulation of chloroplast protein expression. However, its modulation is active at the post-transcriptional rather than transcriptional level.

The PsbP protein is required for photosystem II complex assembly/stability and photoautotrophy in Arabidopsis thaliana

Interfering RNA was used to suppress the expression of the genes At1g06680 and At2g30790 in Arabidopsis thaliana, which encode the PsbP-1 and PsbP-2 proteins, respectively, of photosystem II (PS II). A phenotypic series of transgenic plants was recovered that expressed intermediate and low amounts of PsbP. Chlorophyll fluorescence induction and Q(A)(-) decay kinetics analyses were performed. Decreasing amounts of expressed PsbP protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F(V)/F(M)). This was primarily due to the loss of the J to I transition. Analysis of the fast fluorescence rise kinetics indicated no significant change in the number of PS II(beta) centers present in the mutants. Analysis of Q(A)(-) decay kinetics in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea indicated a defect in electron transfer from Q(A)(-) to Q(B), whereas experiments performed in the presence of this herbicide indicated that charge recombination between Q(A)(-) and the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the PsbP protein. These results demonstrate that the amount of functional PS II reaction centers is compromised in the plants that exhibited intermediate and low amounts of the PsbP protein. Plants that lacked detectable PsbP were unable to survive in the absence of sucrose, indicating that the PsbP protein is required for photoautotrophy. Immunological analysis of the PS II protein complement indicated that significant losses of the CP47 and D2 proteins, and intermediate losses of the CP43 and D1 proteins, occurred in the absence of the PsbP protein. This demonstrates that the extrinsic protein PsbP is required for PS II core assembly/stability.

The stoichiometry of the two photosystems in higher plants revisited

The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions.

Functional analysis of photosystem II in a PsbO-1-deficient mutant in Arabidopsis thaliana

The Arabidopsis thaliana mutant psbo1 (formerly the mutant LE18-30), which contains a point mutation in the psbO-1 gene leading to defective expression of the PsbO-1 protein, has recently been described [Murakami, R. et al. (2002) FEBS Lett. 523, 138-142]. This mutant completely lacks the PsbO-1 protein and overexpresses the PsbO-2 protein. To further study the effect of PsbO-1 deficiency on the function of photosystem II, the polyphasic chlorophyll a fluorescence rise and flash fluorescence induction and decay of the relative fluorescence quantum yield were measured in whole leaves from wild type and the psbo1 mutant. Additionally, flash oxygen yield experiments were performed on thylakoid membranes isolated from wild type and the psbo1 mutant. The results obtained indicate that during fluorescence induction the psbo1 gene exhibited an enhanced O to P transition. Additionally, while the J to I transition in wild type accounted for more than 30% of the total fluorescence yield, in the mutant it accounted for less than 2% rise in the total. Analysis of the flash-induced fluorescence rise in the presence of DCMU indicated that in wild type the ratio of PS IIalpha to PS IIbeta reaction centers was approximately 1.2 while in the mutant the ratio was approximately 0.3. Fluorescence decay kinetics in the absence of DCMU indicated that electron transfer to QB was significantly altered in the mutant. Fluorescence decay kinetics in the presence of DCMU indicated that the charge recombination between QA- and the S2 state of the oxygen-evolving complex was retarded. Furthermore, flash oxygen yield analysis indicated that both the S2 and S3 states exhibited significantly longer lifetimes in the psbo1 mutant than in wild type. Our data indicate that while PsbO-1-deficient plants can grow photoautotrophically (although at a reduced growth rate) the photochemistry of PS II is significantly altered.

Structure, function and regulation of plant photosystem I

Photosystem I (PSI) is a multisubunit protein complex located in the thylakoid membranes of green plants and algae, where it initiates one of the first steps of solar energy conversion by light-driven electron transport. In this review, we discuss recent progress on several topics related to the functioning of the PSI complex, like the protein composition of the complex in the plant Arabidopsis thaliana, the function of these subunits and the mechanism by which nuclear-encoded subunits can be inserted into or transported through the thylakoid membrane. Furthermore, the structure of the native PSI complex in several oxygenic photosynthetic organisms and the role of the chlorophylls and carotenoids in the antenna complexes in light harvesting and photoprotection are reviewed. The special role of the 'red' chlorophylls (chlorophyll molecules that absorb at longer wavelength than the primary electron donor P700) is assessed. The physiology and mechanism of the association of the major light-harvesting complex of photosystem II (LHCII) with PSI during short term adaptation to changes in light quality and quantity is discussed in functional and structural terms. The mechanism of excitation energy transfer between the chlorophylls and the mechanism of primary charge separation is outlined and discussed. Finally, a number of regulatory processes like acclimatory responses and retrograde signalling is reviewed with respect to function of the thylakoid membrane. We finish this review by shortly discussing the perspectives for future research on PSI.

The essential role of phosphatidylglycerol in photosynthesis

Since the first identification of phosphatidylglycerol in Scenedesmus by Benson and Maruo in 1958, researchers have studied many biological functions of this phospholipid. Genetic, biochemical, and structural studies of photosynthetic organisms have revealed that phosphatidylglycerol is crucial to the photosynthetic transport of electrons, the development of chloroplasts, and tolerance to chilling. In this review, we summarize our present understanding of the biochemical and physiological functions of phosphatidylglycerol in cyanobacteria and higher plants.

Coordinated high-light response of genes encoding subunits of photosystem I is achieved by AT-rich upstream sequences in the cyanobacterium Synechocystis sp. strain PCC 6803

Genes encoding subunits of photosystem I (PSI genes) in the cyanobacterium Synechocystis sp. strain PCC 6803 are actively transcribed under low-light conditions, whereas their transcription is coordinately and rapidly down-regulated upon the shift to high-light conditions. In order to identify the molecular mechanism of the coordinated high-light response, we searched for common light-responsive elements in the promoter region of PSI genes. First, the precise architecture of the psaD promoter was determined and compared with the previously identified structure of the psaAB promoter. One of two promoters of the psaAB genes (P1) and of the psaD gene (P2) possessed an AT-rich light-responsive element located just upstream of the basal promoter region. These sequences enhanced the basal promoter activity under low-light conditions, and their activity was transiently suppressed upon the shift to high-light conditions. Subsequent analysis of psaC, psaE, psaK1, and psaLI promoters revealed that their light response was also achieved by AT-rich sequences located at the -70 to -46 region. These results clearly show that AT-rich upstream elements are responsible for the coordinated high-light response of PSI genes dispersed throughout Synechocystis genome.

Development of the light-harvesting chlorophyll antenna in the green alga Chlamydomonas reinhardtii is regulated by the novel Tla1 gene

The Chlamydomonas reinhardtii tla1 (truncated light-harvesting chlorophyll antenna size) mutant was generated upon DNA insertional mutagenesis and shown to specifically possess a smaller than wild type (WT) chlorophyll antenna size in both photosystems. Molecular and genetic analysis revealed that the exogenous plasmid DNA was inserted at the end of the 5' UTR and just prior to the ATG start codon of a hitherto unknown nuclear gene (termed Tla1), which encodes a protein of 213 amino acids. The Tla1 gene in the mutant is transcribed with a new 5' UTR sequence, derived from the 3' end of the transforming plasmid. This replacement of the native 5' UTR and promoter regions resulted in enhanced transcription of the tla1 gene in the mutant but inhibition in the translation of the respective tla1 mRNA. Transformation of the tla1 mutant with WT Tla1 genomic DNA successfully rescued the mutant. These results are evidence that polymorphism in the 5' UTR of the Tla1 transcripts resulted in the tla1 phenotype and that expression of the Tla1 gene is a prerequisite for the development/assembly of the Chl antenna in C. reinhardtii. A blast search with the Tla1 deduced amino acid sequence

Contrasting Behavior of Higher Plant Photosystem I and II Antenna Systems during Acclimation

In this work we analyzed the photosynthetic apparatus in Arabidopsis thaliana plants acclimated to different light intensity and temperature conditions. Plants showed the ability to acclimate into different environments and avoid photoinhibition. When grown in high light, plants had a faster activation rate for energy dissipation (qE). This ability was correlated to higher accumulation levels of a specific photosystem II subunit, PsbS. The photosystem II antenna size was also regulated according to light exposure; smaller antenna size was observed in high light-acclimated plants with respect to low light plants. Different antenna polypeptides did not behave similarly, and Lhcb1, Lchb2, and Lhcb6 (CP24) are shown to undergo major levels of regulation, whereas Lhcb4 and Lhcb5 (CP29 and CP26) maintained their stoichiometry with respect to the reaction center in all growth conditions. The effect of acclimation on photosystem I antenna was different; in fact, the stoichiometry of any Lhca antenna proteins with respect to photosystem I core complex was not affected by growth conditions. Despite this stability in antenna stoichiometry, photosystem I light harvesting function was shown to be regulated through different mechanisms like the control of photosystem I to photosystem II ratio and the association or dissociation of Lhcb polypeptides to photosystem I.

Psychrophily is associated with differential energy partitioning, photosystem stoichiometry and polypeptide phosphorylation in Chlamydomonas raudensis

Chlamydomonas raudensis UWO 241 and SAG 49.72 represent the psychrophilic and mesophilic strains of this green algal species. This novel discovery was exploited to assess the role of psychrophily in photoacclimation to growth temperature and growth irradiance. At their optimal growth temperatures of 8 degrees C and 28 degrees C respectively, UWO 241 and SAG 49.72 maintained comparable photostasis, that is energy balance, as measured by PSII excitation pressure. Although UWO 241 exhibited higher excitation pressure, measured as 1-qL, at all growth light intensities, the relative changes in 1-qL were similar to that of SAG 49.72 in response to growth light. In response to suboptimal temperatures and increased growth irradiance, SAG 49.72 favoured energy partitioning of excess excitation energy through inducible, down regulatory processes (Phi(NPQ)) associated with the xanthophyll cycle and antenna quenching, while UWO 241 favoured xanthophyll cycle-independent energy partitioning through constitutive processes involved in energy dissipation (Phi(NO)). In contrast to SAG 49.72, an elevation in growth temperature induced an increase in PSI/PSII stoichiometry in UWO 241. Furthermore, SAG 49.72 showed typical threonine-phosphorylation of LHCII, whereas UWO 241 exhibited phosphorylation of polypeptides of comparable molecular mass to PSI reaction centres but the absence of LHCII phosphorylation. Thus, although both strains maintain an energy balance irrespective of their differences in optimal growth temperatures, the mechanisms used to maintain photostasis were distinct. We conclude that psychrophily in C. raudensis is complex and appears to involve differential energy partitioning, photosystem stoichiometry and polypeptide phosphorylation.

Stimulation of chlororespiration by heat and high light intensity in oat plants

High irradiance and moderate heat inhibit the activity of the photosynthetic apparatus of oat (Avena sativa L.) leaves. The incubation of oat leaves under high light intensity in conjunction with high temperatures strongly decreased the maximal quantum yield of photosystem (PS) II, indicating the close synergistic effect of both stress factors on PS II inhibition and the subsequent irreversible damage to the photosynthetic apparatus. The PS I A/B protein levels remained similar to control values in leaves incubated under high light intensity or moderate heat, and decreased only when both stress factors were simultaneously applied. Immunoblot analysis of thylakoid membranes using specific antibodies raised against the NDH-K subunit of the thylakoidal NADH dehydrogenase complex (NADH DH) and against plastid terminal oxidase (PTOX) revealed an increase in the amount of both proteins in response to high light intensity and/or heat treatments. In addition, these stress treatments were seen to stimulate the activity of electron donation by NADPH and ferredoxin to plastoquinone, the PTOX activity in plastoquinone oxidation and the NADH DH activity in thylakoid membranes. Incubation with n-propyl gallate (an inhibitor of PTOX) inhibited the increase of NDH-K and PTOX levels under high light intensity and heat, and slightly stimulated the activity of electron donation by NADPH and ferredoxin to plastoquinone. Antimycin A (an inhibitor of cyclic electron flow) increased the NADH DH activity and preserved the levels of NDH-K and PTOX in thylakoid membranes from leaves incubated under high light intensity and heat. The up-regulation of the PTOX and the thylakoidal NADH DH complex under these stress conditions supports a role for chlororespiration in the protection against high irradiance and moderate heat.

Characterization of High-light-responsive Promoters of the psaAB Genes in Synechocystis sp. PCC 6803

In cyanobacteria, transcription of genes encoding subunits of PSI is tightly repressed under high-light conditions. To elucidate the molecular mechanism, we examined the promoter architecture of the psaAB genes encoding reaction center subunits of PSI in a cyanobacterium Synechocystis sp. PCC 6803. Primer extension analysis showed the existence of two promoters, P1 and P2, both of which are responsible for the light intensity-dependent transcription of the psaAB genes. Deletion analysis of the upstream region of psaAB fused to bacterial luciferase reporter genes (luxAB) indicated that the light response of these promoters is achieved in a totally different manner. The cis-element required for the light response of P1, designated as PE1, was located just upstream of the -35 element of P1 and was comprised of AT-rich sequence showing significant homology to the upstream promoter (UP)-element often found in strong bacterial promoters. PE1 activated P1 under low-light conditions, and the down-regulation of P1 was achieved by rapid inactivation of PE1 upon the shift to high-light conditions. On the other hand, the cis-element required for the light response of P2, designated as HNE2, was located upstream of the P1 region, far from the basal promoter of P2. The down-regulation of P2 seemed to be attained through the negative regulation by HNE2 activated only under high-light conditions. DNA gel mobility shift assays showed that at least five regions in psaAB promoters were responsible for the binding of putative regulatory protein factors.

Characterization of chlorophyll-protein complexes isolated from two marine green algae, Bryopsis maxima and Ulva pertusa, growing in the intertidal zone

Three Chl-protein complexes were isolated from thylakoid membranes of Bryopsis maxima and Ulva pertusa, marine green algae that inhabit the intertidal zone of the Pacific Ocean off the eastern coast of Japan by dodecyl-beta-D-maltoside polyacrylamide gel electrophoresis. The slowest-moving fractions showed low Chl a/b and Chl/P-700 ratios, indicating that this fraction corresponds to complexes in PS I, which is large in both algae. The intermediate and fastest-moving fractions showed the traits of PS II complexes, with some associated Chl a/b-protein complexes and LHC II, respectively. The spectral properties of the separated Chl-proteins were also determined. The absorption spectra showed a shallow shoulder at 540 nm derived from siphonaxanthin in Bryopsis maxima, but not in Ulva pertusa. The 77 K emission spectra showed a single peak in Bryopsis maxima and two peaks in Ulva pertusa. Besides the excitation spectra indicated that the excitation energy transfer to the PS I complexes differed quite a lot higher plants. This suggested that the mechanisms of energy transfer in both of these algae differ from those of higher plants. Considering the light environment of this coastal area, the large size of the antennae of PS I complexes implies that the antennae are arranged so as to balance light absorption between the two photosystems. In addition, we discuss the relationships among the photosystem stoichiometry, the energy transfer, and the distribution between the two photosystems.

Isolation and characterization of a xanthophyll aberrant mutant of the green alga Nannochloropsis oculata

Novel mutants (xan1 and xan2) of the unicellular green alga Nannochloropsis oculata are impaired in xanthophyll biosynthesis, thereby producing aberrant levels of xanthophylls. High-performance liquid chromatography (HPLC) analysis revealed that the xan1 and xan2 mutants have double the violaxanthin (V) content, but have significantly decreased lutein content in their cells compared to the wild type. Furthermore, these mutants contain two to three times more zeaxanthin than the wild type under low light (LL) growth conditions. However, this xanthophyll aberration in N. oculata did not affect the normal growth and the major cellular chemical composition of the xan1 strain. The xanthophyll pool size of the LL-grown mutant was 1.8-fold greater than that of the wild type. Under high light (HL) growth conditions, V content was substantially decreased in both the mutant and wild types because of the epoxidation state of the xanthophylls. Under LL growth conditions, the deepoxidation states of the xanthophyll pool sizes were 0.1 and 1.2 in the wild type and the mutant, respectively. However, the deepoxidation states of the xanthophyll pool sizes were 0.78 in the wild type and 0.87 in the mutant under HL growth conditions. We observed that the level of one of the commercially important xanthophylls, zeaxanthin, was higher in the mutant than in the wild type under all culture conditions. This mutant is discussed in terms of its commercial value and potential utilization by the algal biotechnology industry for the production of zeaxanthin.

Compensation for PSII Photoinactivation by Regulated Non-photochemical Dissipation Influences the Impact of Photoinactivation on Electron Transport and CO2 Assimilation

The extent to which PSII photoinactivation affects electron transport (PhiPSII) and CO2 assimilation remains controversial, in part because it frequently occurs alongside inactivation of other components of photosynthesis, such as PSI. By manipulating conditions (darkness versus low light) after a high light/low temperature treatment, we examined the influence of different levels of PSII inactivation at the same level of PSI inactivation on PhiPSII and CO2 assimilation for Arabidopsis. Furthermore, we compared PhiPSII at high light and optimum temperature for wild-type Arabidopsis and a mutant (npq4-1) with impaired capacities for energy dissipation. Levels of PSII inactivation typical of natural conditions (< 50%) were not associated with decreases in PhiPSII and CO2 assimilation at photon flux densities (PFDs) above 150 micromol m(-2) s(-1). At higher PFDs, the light energy being absorbed was in excess of the energy that could be utilized by downstream processes. Arabidopsis plants downregulate PSII activity to dissipate such excess in accordance with the level of PSII photoinactivation that also serves to dissipate absorbed energy. Therefore, the overall levels of non-photochemical dissipation and the efficiency of photochemistry were not affected by PSII inactivation at high PFD. Under low PFD conditions, such compensation is not necessary, because the amount of light energy absorbed is not in excess of that needed for photochemistry, and inactive PSII complexes are dissipating energy. We conclude that moderate photoinactivation of PSII complexes will only affect plant performance when periods of high PFD are followed by periods of low PFD.

Up-regulation of photoprotection and PSII-repair gene expression by irradiance in the unicellular green alga Dunaliella salina

The unicellular green alga Dunaliella salina is an attractive model organism for studying photoacclimation responses and the photosystem II (PSII) damage and repair process in the photosynthetic apparatus. Irradiance during cell growth defines both the photoacclimation and the PSII repair status of the cells. To identify genes specific to these processes, a cDNA library was created from irradiance-stressed D. salina. From the cDNA library, 1112 randomly selected expressed sequence tags (ESTs) were analyzed. Because ESTs constitute the expressed part of the genome, the strategy of randomly sequencing cDNA clones at their 5'-ends allowed us to obtain information about the transcript level of numerous genes in light-stressed D. salina. The results of a BLASTX search performed on the obtained total set of ESTs showed that approximately 1% of the ESTs could be assigned to genes coding for proteins that are known to be up-regulated in response to high-light stress. Specifically, after 48 h of high-light exposure of the cells, an increase in the expression level of antioxidant genes, such as Fe-SOD and APX, was observed, as well as elevated levels of the Cbr transcript, a light-harvesting Chl-protein homolog. Further, the ATP-dependent Clp protease gene was also up-regulated in D. salina cells after 48 h of exposure to high light. The results provide initial insight into the global gene regulation process in response to irradiance.

Protein composition and native state of pigments of thylakoid membrane of wheat genotypes differently tolerant to water stress

Protein composition and native state of chlorophylls were analyzed in two wheat (Triticum durum L.) genotypes with different tolerance to drought, Barakatli-95 (drought-tolerant) and Garagylchyg-2 (drought-sensitive), during water deficit. It is shown that the plants subjected to water deficit appear to have a slight increase in alpha- and beta-subunits of CF1 ATP-synthase complex (57.5 and 55 kD, respectively) in Barakatli-95 and their lower content in Garagylchyg-2. Steady-state levels of the core antenna of PS II (CP47 and CP43) and light-harvesting Chl a/b-apoproteins (LHC) II in the 29.5-24 kD region remained more or less unchanged in both wheat genotypes. The synthesis of 36 kD protein and content of low-molecular-weight polypeptides (21.5, 16.5, and 14 kD) were noticeably increased in the tolerant genotype Barakatli-95. Drought caused significant changes in the carotenoid region of the spectrum (400-500 nm) in drought-sensitive genotype Garagylchyg-2 (especially in the content of pigments of the violaxanthin cycle). A shift of the main band from 740-742 to 738 nm is observed in the fluorescence spectra (77 K) of chloroplasts from both genotypes under water deficiency, and there is a stimulation of the ratio of fluorescence band intensity F687/F740.

Photosystem II Heterogeneity of in hospite Zooxanthellae in Scleractinian Corals Exposed to Bleaching Conditions

Increased ocean temperatures are thought to be triggering mass coral bleaching events around the world. The intracellular symbiotic zooxanthellae (genus Symbiodinium) are expelled from the coral host, which is believed to be a response to photosynthetic damage within these symbionts. Several sites of impact have been proposed, and here we probe the functional heterogeneity of Photosystem II (PSII) in three coral species exposed to bleaching conditions. As length of exposure to bleaching conditions (32 degrees C and 350 micromol photons m(-2) s(-1)) increased, the QA- reoxidation kinetics showed a rise in the proportion of inactive PSII centers (PSIIx), where QB was unable to accept electrons. PSIIx contributed up to 20% of the total PSII centers in Pocillopora damicornis, 35% in Acropora nobilis and 14% in Cyphastrea serailia. Changes in Fv/Fm and amplitude of the J step along fast induction curves were found to be highly dependent upon the proportion of PSIIx centers within the total pool of PSII reaction centers. Determination of PSII antenna size revealed that under control conditions in the three coral species up to 60% of PSII centers were lacking peripheral light-harvesting complexes (PSIIbeta). In P. damicornis, the proportion of PSIIbeta increased under bleaching conditions and this could be a photoprotective mechanism in response to excess light. The rapid increases in PSIIx and PSIIbeta observed in these corals under bleaching conditions indicates these physiological processes are involved in the initial photochemical damage to zooxanthellae.

Chlorophyll a fluorescence induction kinetics in leaves predicted from a model describing each discrete step of excitation energy and electron transfer associated with Photosystem II

Chlorophyll a fluorescence induction (FI) is widely used as a probe for studying photosynthesis. On illumination, fluorescence emission rises from an initial level O to a maximum P through transient steps, termed J and I. FI kinetics reflect the overall performance of photosystem II (PSII). Although FI kinetics are commonly and easily measured, there is a lack of consensus as to what controls the characteristic series of transients, partially because most of the current models of FI focus on subsets of reactions of PSII, but not the whole. Here we present a model of fluorescence induction, which includes all discrete energy and electron transfer steps in and around PSII, avoiding any assumptions about what is critical to obtaining O J I P kinetics. This model successfully simulates the observed kinetics of fluorescence induction including O J I P transients. The fluorescence emission in this model was calculated directly from the amount of excited singlet-state chlorophyll in the core and peripheral antennae of PSII. Electron and energy transfer were simulated by a series of linked differential equations. A variable step numerical integration procedure (ode15s) from MATLAB provided a computationally efficient method of solving these linked equations. This in silico representation of the complete molecular system provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the O J I P kinetics and fluorescence emission at these points. Simulations based on this model showed that J corresponds to the peak concentrations of Q(-)AQB (QA and QB are the first and second quinone electron acceptor of PSII respectively) and Q(-)AQ(-)B and I to the first shoulder in the increase in concentration of Q(-)AQ(2-)B. The P peak coincides with maximum concentrations of both Q(-)AQ(2-)B and PQH2. In addition, simulations using this model suggest that different ratios of the peripheral antenna and core antenna lead to differences in fluorescence emission at O without affecting fluorescence emission at J, I and P. An increase in the concentration of QB-nonreducing PSII centers leads to higher fluorescence emission at O and correspondingly decreases the variable to maximum fluorescence ratio (F v/F m).

Photosystem II core phosphorylation and photosynthetic acclimation require two different protein kinases

Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover, whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems. In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry. In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively. Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.

Effects of chlorophyllide a oxygenase overexpression on light acclimation in Arabidopsis thaliana

Land plants change the compositions of light-harvesting complexes (LHC) and chlorophyll (Chl) a/b ratios in response to the variable light environments which they encounter. In this study, we attempted to determine the mechanism which regulates Chl a/b ratios and whether the changes in Chl a/b ratios are essential in regulation of LHC accumulation during light acclimation. We hypothesized that changes in the mRNA levels for chlorophyll a oxygenase (CAO) involved in Chl b biosynthesis are an essential part of light response of Chl a/b ratios and LHC accumulation. We also examined the light-intensity dependent response of CAO-overexpression and wild-type Arabidopsis thaliana plants. When wild-type plants were acclimated from low-light (LL) to high-light (HL) conditions, CAO mRNA levels decreased and the Chl a/b ratio increased. In transgenic plants overexpressing CAO, the Chl a/b ratio remained low under HL conditions; thereby suggesting that changes in the CAO mRNA levels are necessary for those in Chl a/b ratios upon light acclimation. Under HL conditions, the accumulation of Lhcb1, Lhcb3 and Lhcb6 was enhanced in plants overexpressing CAO. On the contrary, in a CAO-deficient mutant, chlorina 1-1, theaccumulation of Lhcb1, Lhcb2, Lhcb3, Lhcb6 and Lhca4 was reduced. In comparison to wild-type, beta-carotene levels were reduced in CAO-overexpressing plants, while they were elevated in chlorina 1-1 mutants. These results imply that the transcriptional control of CAO is a part of the regulatory mechanism for the accumulation of a distinct set of LHC proteins upon light acclimation.

The stoichiometry and antenna size of the two photosystems in marine green algae, Bryopsis maxima and Ulva pertusa, in relation to the light environment of their natural habitat

The stoichiometry and antenna sizes of the two photosystems in two marine green algae, Bryopsis maxima and Ulva pertusa, were investigated to examine whether the photosynthetic apparatus of the algae can be related to the light environment of their natural habitat. Bryopsis maxima and Ulva pertusa had chlorophyll (Chl) a/b ratios of 1.5 and 1.8, respectively, indicating large levels of Chl b, which absorbs blue-green light, relative to Chl a. The level of photosystem (PS) II was equivalent to that of PS I in Bryopsis maxima but lower than that of PS I in Ulva pertusa. Analysis of Q(A) photoreduction and P-700 photo-oxidation with green light revealed that >50% of PS II centres are non-functional in electron transport. Thus, the ratio of the functional PS II to PS I is only 0.46 in Bryopsis maxima and 0.35 in Ulva pertusa. Light-response curves of electron transport also provided evidence that PS I had a larger light-harvesting capacity than did the functional PS II. Thus, there was a large imbalance in the light absorption between the two photosystems, with PS I showing a larger total light-harvesting capacity than PS II. Furthermore, as judged from the measurements of low temperature fluorescence spectra, the light energy absorbed by Chl b was efficiently transferred to PS I in both algae. Based on the above results, it is hypothesized that marine green algae require a higher ATP:NADPH ratio than do terrestrial plants to grow and survive under a coastal environment.

Photosynthetic redox control of nuclear gene expression

Chloroplasts contain 3000-4000 different proteins but only a small subset of them is encoded in the plastid genome while the majority is encoded in the nucleus. Expression of these genes therefore requires a high degree of co-ordination between nucleus and chloroplast. This is achieved by a bilateral information exchange between both compartments including nucleus-to-plastid (anterograde) and plastid-to-nucleus (retrograde) signals. The latter represent a functional feedback control which couples the expression of nuclear encoded plastid proteins to the actual functional state of the organelle. The efficiency of photosynthesis is a very important parameter in this context since it is influenced by many environmental conditions and therefore represents a sensor for the residing environment. Components of the photosynthetic electron transport chain exhibit significant changes in their reduction/oxidation (redox) state depending on the photosynthetic electron flow and therefore serve as signalling parameters which report environmental influences on photosynthesis. Such redox signals control chloroplast and nuclear gene expression events and play an important role in the co-ordination of both genetic compartments. It is discussed here which photosynthetic parameters are known to control nuclear gene expression, how these signals are transduced toward the nucleus, and how they interact with other plastid retrograde signals and cytosolic light perception systems.

A cell-based model for the photoacclimation and CO(2)-acclimation of the photosynthetic apparatus

We have developed a mathematical model based on the underlying mechanisms concerning the responses of the photosynthetic apparatus of a microalga cell which grows under constant incident light intensity and ambient CO(2) concentration. Photosynthesis involves light and carbon-fixation reactions which are mutually dependent and affect each other, but existing models for photosynthesis don't account for both reactions at once. Our modeling approach allows us to derive distinct equations for the rates of oxygen production, NADPH production, carbon dioxide fixation, carbohydrate production, and rejected energy, which are generally different. The production rates of the photosynthesis products are hyperbolic functions of light and CO(2) concentration. The model predicts that in the absence of photoinhibition, CO(2)-inhibition, photorespiration, and chlororespiration, a cell acclimated to high light and/or CO(2) concentration has higher photosynthetic capacity and lower photosynthetic efficiency than does a cell acclimated to low conditions. This results in crossing between the two curves which represent the oxygen production rates and carbon fixation rates in low and high conditions. Finally, in the absence of photoinhibition and CO(2)-inhibition, the model predicts the carbohydrate production rate in terms of both light intensity and CO(2) concentration.

Light-dependent regulation of chlorophyll b biosynthesis in chlorophyllide a oxygenase overexpressing tobacco plants

Chlorophyllide a oxygenase (CAO) that converts chlorophyllide a to chlorophyllide b was overexpressed in tobacco to increase chlorophyll (Chl) b biosynthesis and alter the Chl a/b ratio. Transgenic plants along with their wild-type cultivars were grown in low and high light intensities. In low light there was 20% increase in chlorophyll b contents in transgenic plants, which resulted in 16% reduction in the Chl a/b ratio. In high light, total Chl contents were 31% higher in transgenic plants than those of wild type. The increase in Chl a was 19% and that of Chl b was 72% leading to 31% decline of Chl a/b ratio. The increase in Chl b contents was accompanied by enhanced CAO expression that was highly pronounced in low light. As compared to low light, in high light Lhcb1 and Chl a/b transcripts abundance was significantly increased in transgenic plants suggesting a close relationship between Chl b synthesis and cab gene expression. However, there was a small increase in expression of LHCII proteins, which did not correspond to 72% increase in Chl b content in transgenic line, implying that LHCPII has the ability to bind more Chl b molecules.