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PubMed2000

Homeostasis of adenylate status during photosynthesis in a fluctuating environment.

This review describes and assesses pathways likely to influence and stabilize the ATP/reductant balance during whole cell photosynthesis. The sole reductive step of the Calvin cycle occurs during the conversion of 3-phosphoglycerate to triose phosphate. Photophosphorylation linked to this reaction can undoubtedly supply most of the ATP required by the Calvin cycle and other chloroplastic reactions. Small but crucial contributions must come from several other pathways, some of which involve co-operation between the chloroplast and the rest of the cell. Extrachloroplastic compartments can contribute to chloroplastic ATP requirements by supplying ATP directly or, probably more significantly, by accepting reducing equivalents and so supporting ATP synthesis within the chloroplast.

Adenosine Triphosphate↗
PubMed2000

Prospects for crop improvement through the genetic manipulation of photosynthesis: morphological and biochemical aspects of light capture.

The prospects for genetic manipulation of photosynthesis are assessed with an emphasis on the biochemical and morphological aspects of light capture. The connection between different parts of the photosynthetic process is considered together with the influence of environmental factors, development and acclimation, and metabolic regulation. The sites of real and potential photosynthetic losses are identified, using tropical rice as a case study. The important interaction between photosynthetic capacity, acclimation to the light environment, nitrogen accumulation and canopy architecture are discussed. The possibility of genetic intervention to increase both biomass accumulation and improve nitrogen economy simultaneously are considered. Finally, the numerous procedures for genetic manipulation of light harvesting are also discussed, with a view of improving radiation-use efficiency in crops.

Biomass↗
PubMed2002

Diurnal variation of photosynthesis and photoinhibition in tea: effects of irradiance and nitrogen supply during growth in the field.

Diurnal changes in the rate of photosynthesis (A) of mature tea (Camellia sinensis (L.) O. Kuntze) bushes grown at high elevation in the field in Sri Lanka, were related to environmental conditions. Bushes were either unshaded, receiving 100% of incident photosynthetically active radiation (PAR), moderately shaded, (65% PAR) or heavily shaded (30% PAR). These treatments were combined with nitrogen fertilizer applications of 0, 360 and 720 kg ha(-1) year(-1). When recently fully expanded leaves were measured under the growing conditions on bright, clear days from dawn to dusk, A was greatest in the morning with increasing radiation between approximately 8 h and 10 h. Stomatal conductances (g(s)) and substomatal carbon dioxide concentrations (C(i)) were then large, leaf temperatures (T(L)) cool, and saturated water vapour deficits (VPD) small. However, as the irradiance, T(L) and VPD increased towards midday, A, g(s), photochemical quenching, and C(i) decreased, and non-photochemical quenching increased. In the late afternoon, irradiance, T(L) and VPD fell, but despite the relatively large increase in g(s) and C(i), A remained low; however, it recovered overnight. The zero-N treatment decreased total-N content of leaves by 50% and A by c. 20% (not significant). Leaves of unshaded plants receiving least N had significantly (P<0.05) smaller A and greater total sugar content than shaded but with abundant N, A and sugars did not differ between shade treatments. Analysis of the responses of A to environment in the morning compared to the afternoon, and of chlorophyll fluorescence, suggests that A was photoinhibited as a consequence of greatly increased PAR, whilst decreasing g(s) (related to changes in PAR, VPD and T(L)) caused C(i) to fall. End-product inhibition of A is not consistent with decreased C(i). Inhibition of A as a result of photoinhibition was minimized, but not eliminated, by abundant N. Interactions between factors regulating A in tea are discussed.

Agriculture↗
PubMed2002

Overexpression of C(4)-cycle enzymes in transgenic C(3) plants: a biotechnological approach to improve C(3)-photosynthesis.

The process of photorespiration diminishes the efficiency of CO(2) assimilation and yield of C(3)-crops such as wheat, rice, soybean or potato, which are important for feeding the growing world population. Photorespiration starts with the competitive inhibition of CO(2) fixation by O(2) at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and can result in a loss of up to 50% of the CO(2) fixed in ambient air. By contrast, C(4) plants, such as maize, sugar cane and Sorghum, possess a CO(2) concentrating mechanism, by which atmospheric CO(2) is bound to C(4)-carbon compounds and shuttled from the mesophyll cells where the prefixation of bicarbonate occurs via phosphoenolpyruvate carboxylase (PEPC) into the gas-tight bundle-sheath cells, where the bound carbon is released again as CO(2) and enters the Calvin cycle. However, the anatomical division into mesophyll and bundle-sheaths cells ("Kranz"-anatomy) appears not to be a prerequisite for the operation of a CO(2) concentrating mechanism. Submerged aquatic macrophytes, for instance, can induce a C(4)-like CO(2) concentrating mechanism in only one cell type when CO(2) becomes limiting. A single cell C(4)-mechanism has also been reported recently for a terrestrial chenopod. For over 10 years researchers in laboratories around the world have attempted to improve photosynthesis and crop yield by introducing a single cell C(4)-cycle in C(3) plants by a transgenic approach. In the meantime, there has been substantial progress in overexpressing the key enzymes of the C(4) cycle in rice, potato, and tobacco. In this review there will be a focus on biochemical and physiological consequences of the overexpression of C(4)-cycle genes in C(3) plants. Bearing in mind that C(4)-cycle enzymes are also present in C(3) plants, the pitfalls encountered when C(3) metabolism is perturbed by the overexpression of individual C(4) genes will also be discussed.

Carbon Dioxide↗
PubMed2002

Are there associations between grain-filling rate and photosynthesis in the flag leaves of field-grown rice?

Rate of grain filling in terms of dry mass accumulated per panicle per day was measured in field-grown rice in the dry season in the Philippines and compared to rates of light-saturated photosynthesis per unit leaf area (P(max)) measured at 350 micro l l(-1) CO(2) for 21 d after flowering. Five new plant type (tropical japonica) varieties (NPT) and one indica variety (IR72) were used and these gave some variation in rates and patterns of grain filling. A rapid grain-filling phase (RGFP) occurred approximately 10 d after flowering in most varieties. There was no consistent relationship in any variety between the rate of grain-filling and P(max) and chlorophyll content, both of which remained mostly unchanged throughout grain filling. Significant declines in the amount of total leaf protein and ribulose bisphosphate carboxylase-oxygenase (Rubisco) occurred, but these did not occur at the same time as the RGFP in all varieties. A decrease in the ratio of chlorophyll a/b preceded these changes and a transient rise in chlorophyll content was also observed in four varieties at this time. There was no significant change in leaf non-structural carbohydrate content during or following the RGFP. It is concluded that the decline in Rubisco and protein content in NPT was not reflected in photosynthetic activity. Hence in these field experiments Rubisco accumulated to a level in excess of photosynthetic requirements, serving as a store of nitrogen for grain filling.

Carbon Dioxide↗
PubMed2003

Elevated sucrose-phosphate synthase activity in transgenic tobacco sustains photosynthesis in older leaves and alters development.

Constitutive over-expression of a maize sucrose-phosphate synthase (SPS) gene in tobacco (Nicotiana tabacum) had major effects on leaf carbohydrate budgets with consequences for whole plant development. Transgenic tobacco plants flowered earlier and had greater flower numbers than wild-type plants. These changes were not linked to modified source leaf carbon assimilation or carbon export, although sucrose to starch ratios were significantly higher in leaves expressing the transgene. The youngest and oldest leaves of plants over-expressing SPS had up to 10-fold wild-type maximal extractable SPS activity, but source leaf SPS activities were only 2-3 times greater in these lines than in the wild type. In the oldest leaves, where the expression of the transgene led to the most marked enhancement in SPS activity, photosynthesis was also increased. It was concluded that these increases in the capacity for sucrose synthesis and carbon assimilation, particularly in older leaves, accelerate the whole plant development and increase the abundance of flowers without substantial changes in the overall shoot biomass.

Aging↗
PubMed2003

Development of C4 photosynthesis in sorghum leaves grown under free-air CO2 enrichment (FACE).

The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.

Air↗
PubMed2004

Effect of fruit load and girdling on leaf photosynthesis in Mangifera indica L.

Leaf nitrogen concentration (Nm), mass-to-area ratio, amount of nitrogen per unit leaf area (Na), non-structural carbohydrate concentration (TNCa), maximal rate of carboxylation (Vcmax), light-saturated rate of photosynthetic electron transport (Jmax), dark respiration (Rd), net photosynthetic assimilation (Anet), quantum yield of photosystem II (PhiPSII), and intercellular CO2 concentration (Ci) were measured in Mangifera indica L. leaves on three types of fruit-bearing branches (non-girdled, NG; girdled with high (HFL) and low (LFL) fruit load), experiencing similar light exposure. TNCa, Vcmax/Na, Jmax/Na, Rd/Na, Ci, and the initial quantum yield of photosynthetic electron transport (alpha) were similar in both HFL and NG treatments, but Nm, Na, and photosynthetic capacity parameters (Vcmax and Jmax) were lower in the HFL than in the NG treatment. The strong depressing effect of girdling on leaf nitrogen concentration cannot therefore be attributed to a change in TNCa. By contrast, Na and TNCa were lower and higher, respectively, in the LFL than in the HFL treatment, suggesting that carbohydrate content may become the driving force behind photosynthetic acclimation to changing source-sink relationships, like the ones resulting from the presence of developing fruits. Vcmax/Na and Jmax/Na were lower in the LFL than in the HFL treatment, while Rd/Na, Ci, and alpha were not affected by fruit load. It is concluded that girdling and high fruit load affect photosynthesis permanently by decreasing and increasing, respectively, leaf nitrogen concentration. Fruit load, moreover, may have an additional effect on photosynthetic capacity by affecting the relationship between Vcmax and Jmax, and Na.

Chlorophyll↗
PubMed2004

Influence of leaf-to-air vapour pressure deficit (VPD) on the biochemistry and physiology of photosynthesis in Prosopis juliflora.

The effect of leaf-to-air vapour pressure deficit (VPD) was studied in well-watered, potted, 1-2-year-old plants of the leguminous tree P. juliflora grown outside in northern India. The long-term responses to VPD were analysed from diurnal and seasonal variations in gas exchange parameters measured in two cohorts of leaves produced in February and July, respectively. In general, inhibitory effects of high VPD were visible only when the VPD level exceeded a threshold of >3 kPa. There was a substantial decline in net photosynthesis rate and stomatal conductance at high VPD >4 kPa and transpiration showed a decrease in steady-state rate or feedforward response to VPD. The feedforward responses were visible in all seasons, although the plants were exposed to a wide range of VPD during the year and leaf relative water content was constant. The maximum quantum efficiency of PSII measured predawn was constant (around 0.8) in all seasons except summer. Short-term experiments showed that, although gas exchange was severely affected by high VPD in the leaves of both cohorts, the plant maintained a constant, water use efficiency in different seasons. High VPD also caused reductions in Rubisco activity, affecting carboxylation efficiency, and reductions in sucrose and starch content due to a decrease in the activity of sucrose-phosphate synthase. However, the relative quantum yield of PSII and electron transport rates measured at 1500 micromol m(-2) s(-1) were unaffected by increasing VPD, indicating the presence of a large alternative sink possibly, photorespiration. The overall results showed that P. juliflora can withstand high VPD by reducing metabolic activity and by effective adjustments in the partitioning of electron flow between assimilation and non-assimilation processes, which, in turn, imposed a strong limitation on the potential carbon gain.

Air↗
PubMed2004

Singlet oxygen production in photosynthesis.

A photosynthetic organism is subjected to photo-oxidative stress when more light energy is absorbed than is used in photosynthesis. In the light, highly reactive singlet oxygen can be produced via triplet chlorophyll formation in the reaction centre of photosystem II and in the antenna system. In the antenna, triplet chlorophyll is produced directly by excited singlet chlorophyll, while in the reaction centre it is formed via charge recombination of the light-induced charge pair. Changes of the mid-point potential of the primary quinone acceptor in photosystem II modulate the pathway of charge recombination in photosystem II and influence the yield of singlet oxygen production. Singlet oxygen can be quenched by beta-carotene, alpha-tocopherol or can react with the D1 protein of photosystem II as target. If not completely quenched, it can specifically trigger the up-regulation of the expression of genes which are involved in the molecular defence response of plants against photo-oxidative stress.

Chlorophyll↗
PubMed2004

Plasticity in light reactions of photosynthesis for energy production and photoprotection.

Plant photosynthesis channels some of the most highly reactive intermediates in biology, in a way that captures a large fraction of their energy to power the plant. A viable photosynthetic apparatus must not only be efficient and robust machinery, but also well integrated into the plant's biochemical and physiological networks. This requires flexibility in its responses to the dramatically changing environmental conditions and biochemical demands. First, the output of the energy-storing light reactions must match the demands of plant metabolism. Second, regulation of the antenna must be flexible to allow responses to diverse challenges that could result in excess light capture and subsequent photoinhibition. Evidence is presented for the interplay of two types of mechanistic flexibility, one that modulates the relative sensitivity of antenna down-regulation to electron flow, and the other, which primarily modulates the output ratio of ATP/NADPH, but also contributes to down-regulation.

Adaptation, Physiological↗
PubMed2004

Molecular design of the photosystem II light-harvesting antenna: photosynthesis and photoprotection.

The photosystem II (PSII) light-harvesting system carries out two essential functions, the efficient collection of light energy for photosynthesis, and the regulated dissipation of excitation energy in excess of that which can be used. This dual function requires structural and functional flexibility, in which light-harvesting proteins respond to an external signal, the thylakoid DeltapH, to induce feedback control. This process, referred to as non-photochemical quenching (NPQ) depends upon the xanthophyll cycle and the PsbS protein. In nature, NPQ is heterogeneous in terms of kinetics and capacity, and this adapts photosynthetic systems to the specific dynamic features of the light environment. The molecular features of the thylakoid membrane which may enable this flexibility and plasticity are discussed.

Adaptation, Physiological↗
PubMed2005

Redox regulation in the chloroplast thylakoid lumen: a new frontier in photosynthesis research.

Initially linked to photosynthesis, regulation by change in the redox state of thiol groups (S-S<-- -->2SH) is now known to occur throughout biology. Thus, in addition to serving important structural and catalytic functions, it is recognized that, in many cases, disulphide bonds can be broken and reformed for regulation. Several systems, each linking a hydrogen donor to an intermediary disulphide protein, act to effect changes that alter the activity of target proteins by change in the thiol redox state. Pertinent to the present discussion is the chloroplast ferredoxin/thioredoxin system, comprised of photoreduced ferredoxin, a thioredoxin, and the enzyme ferredoxin-thioredoxin reductase, that occur in the stroma. In this system, thioredoxin links the activity of enzymes to light: those enzymes functional in biosynthesis are reductively activated by light via thioredoxin (S-S-->2SH), whereas counterparts acting in degradation are deactivated under illumination conditions and are oxidatively activated in the dark (2SH-->S-S). Recent research has uncovered a new paradigm in which an immunophilin, FKBP13, and potentially other enzymes of the chloroplast thylakoid lumen are oxidatively activated in the light (2SH-->S-S). The present review provides a perspective on this recent work.

Gene Expression Regulation, Plant↗
PubMed2005

Imaging photosynthesis in wounded leaves of Arabidopsis thaliana.

Chlorophyll fluorescence imaging provides a non-invasive and non-destructive means with which to measure photosynthesis. This technique has been used, in combination with 14CO2 feeding, to study the spatial and temporal changes in source-sink relationships which occur in mechanically wounded leaves of Arabidopsis thaliana. Twenty-four hours after wounding, cells proximal to the wound margin showed a rapid induction of PhiII upon illumination (a measure of the efficiency of photosystem II photochemistry) whilst cells more distal to the wound margin exhibited a much slower induction of PhiII and a large, transient increase in NPQ (a measure of the rate constant for non-photochemical energy dissipation within the light-harvesting antenna). These results are indicative of an increase in sink strength in the vicinity of the wound and this was confirmed by the retention of 14C photosynthate in this region. It has been hypothesized that wound-induced cell wall (apoplastic) invertase (cwINV) activity plays a central role in generating localized increases in sink strength in stressed plant tissue and that hexose sugars generated by the sucrolytic activity of cwINV may act as a signal regulating gene expression. Enzyme activity measurements, quantitative RT-PCR, and T-DNA insertional mutagenesis have been used to determine that expression of AtcwINV1 is responsible for all induced cwINV activity in mechanically wounded leaves. Whilst inactivation of this gene abolished wound-induced cwINV activity, it did not affect localized alterations in source-sink relationships of wounded leaves or wound-regulated gene expression. The signals that may regulate source-sink relationships and signalling in wounded leaves are discussed.

Arabidopsis↗
PubMed2006

Differential adaptation of two varieties of common bean to abiotic stress: II. Acclimation of photosynthesis.

The photosynthetic characteristics of two contrasting varieties of common bean (Phaseolus vulgaris) have been determined. These varieties, Arroz and Orfeo, differ in their productivity under stress conditions, resistance to drought stress, and have distinctly different stomatal behaviour. When grown under conditions of high irradiance and high temperature, both varieties displayed evidence of photosynthetic acclimation at the chloroplast level-there was an increase in chlorophyll a/b ratio, a decreased content of Lhcb proteins, and an increased xanthophyll cycle pool size. Both varieties also showed reduced chlorophyll content on a leaf area basis and a decrease in leaf area. Both varieties showed an increase in leaf thickness but only Arroz showed the characteristic elongated palisade cells in the high light-grown plants; Orfeo instead had a larger number of smaller, rounded cells. Differences were found in stomatal development: whereas Arroz showed very little change in stomatal density, Orfeo exhibited a large increase, particularly on the upper leaf surface. It is suggested that these differences in leaf cell structure and stomatal density give rise to altered rates of photosynthesis and stomatal conductance. Whereas, Arroz had the same photosynthetic rate in plants grown at both low and high irradiance, Orfeo showed a higher photosynthetic capacity at high irradiance. It is suggested that the higher yield of Orfeo compared with Arroz under stress conditions can be explained, in part, by these cellular differences.

Acclimatization↗
PubMed1989

Maize phosphoenolpyruvate carboxylase involved in C4 photosynthesis: nucleotide sequence analysis of the 5' flanking region of the gene.

To clone the genomic DNA fragment containing the putative promoter region of the gene for phosphoenolpyruvate carboxylase [EC 4.1.1.31] involved in C4 photosynthesis (C4-type PEPC), maize genomic libraries were screened. On probing with a 384-bp fragment from the N-terminal coding region of the maize cDNA for C4-type PEPC, four EcoRI-fragments differing in the restriction map were cloned, reflecting the presence of a small gene family. Southern blot analyses were carried out on the genomic DNA and the cloned DNA fragments using several segments of the cDNA for C4-type PEPC as probes. The results indicated that the C4-type PEPC is encoded by a single gene and the cloned 7.0-kb fragment was derived from this gene. The transcription start site, as determined in a primer extension experiment, was located at about 4 kb downstream of the 5' end of the cloned fragment. The nucleotide sequence was determined for the region which extended about 1 kb upstream from the transcription start site and possible signal sequences related to gene expression were found, including four classes of direct repeats. The sequences of the corresponding regions of the other two cloned fragments (8.9 and 12.9 kb) which strongly hybridized with the 384-bp probe were very similar to each other, but they were quite different in the 5' upstream region from the sequence of the gene for C4-type PEPC.

Base Sequence↗
PubMed1992

Molecular evolution of phosphoenolpyruvate carboxylase for C4 photosynthesis in maize: comparison of its cDNA sequence with a newly isolated cDNA encoding an isozyme involved in the anaplerotic function.

Molecular events associated with the evolution of an enzyme for C4 photosynthesis were investigated. In maize, at least three isozymes of phosphoenolpyruvate carboxylase [EC 4.1.1.31] are known: the C4-form, the C2-form and the root-form, being named according to their physiological roles and pattern of tissue distribution [Ting, I.P. & Osmond, C.B. (1973) Plant Physiol. 51, 448-453]. A cDNA clone which presumably encodes the C3-form isozyme was newly isolated and analyzed. Comparison of the sequences of the C3-form and C4-form isozymes revealed that (i) the homologies in the nucleotide and deduced amino acid sequences were 71 and 77%, respectively, and (ii) the gene for the C4-form isozyme evolved under strong G/C pressure. The genes for these isozymes were found to be located apart on different chromosomes. A phylogenetic tree was constructed using 8 amino acid sequences of phosphoenolpyruvate carboxylases from various sources. The topology of the tree indicated that, at least in monocots such as maize and sorghum, the genes for the C4-form and C3-form isozymes diverged from their common ancestral gene earlier than the monocot-dicot divergence (about 2 x 10(8) yr ago), though the divergence of maize (C4 plant) from wheat (C3 plant) is supposed to have occurred much later (6 x 10(7) yr ago).

Amino Acid Sequence↗
PubMed2001

Accelerated evolution of functional plastid rRNA and elongation factor genes due to reduced protein synthetic load after the loss of photosynthesis in the chlorophyte alga Polytoma.

Polytoma obtusum and Polytoma uvella are members of a clade of nonphotosynthetic chlorophyte algae closely related to Chlamydomonas humicola and other photosynthetic members of the Chlamydomonadaceae. Descended from a nonphotosynthetic mutant, these obligate heterotrophs retain a plastid (leucoplast) with a functional protein synthetic system, and a plastid genome (lpDNA) with functional genes encoding proteins required for transcription and translation. Comparative studies of the evolution of genes in chloroplasts and leucoplasts can identify modes of selection acting on the plastid genome. Two plastid genes--rrn16, encoding the plastid small-subunit rRNA, and tufA, encoding elongation factor Tu--retain their functions in protein synthesis after the loss of photosynthesis in two nonphotosynthetic Polytoma clades but show a substantially accelerated rate of base substitution in the P. uvella clade. The accelerated evolution of tufA is due, at least partly, to relaxed codon bias favoring codons that can be read without wobble, mainly in three amino acids. Selection for these codons may be relaxed because leucoplasts are required to synthesize fewer protein molecules per unit time than are chloroplasts (reduced protein synthetic load) and thus require a lower rate of synthesis of elongation factor Tu. Relaxed selection due to a lower protein synthetic load is also a plausible explanation for the accelerated rate of evolution of rrn16, but the available data are insufficient to test the hypothesis for this gene. The tufA and rrn16 genes in Polytoma oviforme, the sole member of a second nonphotosynthetic clade, are also functional but show no sign of relaxed selection.

Algal Proteins↗