2'-carboxy-D-arabitinol 1-phosphate protects ribulose 1, 5-bisphosphate carboxylase/oxygenase against proteolytic breakdown

Dynamics of Rubisco regulation by sugar phosphate derivatives and their phosphatases

Regulating the central CO2-fixing enzyme Rubisco is as complex as its ancient reaction mechanism and involves interaction with a series of cofactors and auxiliary proteins that activate catalytic sites and maintain activity. A key component among the regulatory mechanisms is the binding of sugar phosphate derivatives that inhibit activity. Removal of inhibitors via the action of Rubisco activase is required to restore catalytic competency. In addition, specific phosphatases dephosphorylate newly released inhibitors, rendering them incapable of binding to Rubisco catalytic sites. The best studied inhibitor is 2-carboxy-d-arabinitol 1-phosphate (CA1P), a naturally occurring nocturnal inhibitor that accumulates in most species during darkness and low light, progressively binding to Rubisco. As light increases, Rubisco activase removes CA1P from Rubisco, and the specific phosphatase CA1Pase dephosphorylates CA1P to CA, which cannot bind Rubisco. Misfire products of Rubisco's complex reaction chemistry can also act as inhibitors. One example is xylulose-1,5-bisphosphate (XuBP), which is dephosphorylated by XuBPase. Here we revisit key findings related to sugar phosphate derivatives and their specific phosphatases, highlighting outstanding questions and how further consideration of these inhibitors and their role is important for better understanding the regulation of carbon assimilation.

The discovery of rubisco

Rubisco is possibly the most important enzyme on Earth, certainly in terms of amount. This review describes the initial reports of ribulose 1,5-bisphosphate carboxylating activity. Discoveries of core concepts are described, including its quaternary structure, the requirement for post-translational modification, and its role as an oxygenase as well as a carboxylase. Finally, the requirement for numerous chaperonins for assembly of rubisco in plants is described.

Cyanobacterial carboxysomes contain an unique rubisco-activase-like protein

In plants, rubisco activase (Rca) regulates rubisco by removing inhibitory molecules such as ribulose-1,5-bisphosphate (RuBP). In cyanobacteria, a homologous protein (activase-like cyanobacterial protein, ALC), contains a distinctive C-terminal fusion resembling the small-subunit of rubisco. Although cyanobacterial rubisco is believed to be less sensitive to RuBP inhibition, the ALC is widely distributed among diverse cyanobacteria. Using microscopy, biochemistry and molecular biology, the cellular localization of the ALC, its effect on carboxysome/cell ultrastructure in Fremyella diplosiphon, and its function in vitro were studied. Bioinformatic analysis uncovered evolutionary relationships between the ALC and rubisco. ALC localizes to carboxysomes and exhibits ATPase activity. Furthermore, the ALC induces rubisco aggregation in a manner similar to that of another carboxysomal protein, M35, and this activity is affected by ATP. An alc deletion mutant showed modified cell morphology when grown under enriched CO₂ and impaired regulation of carboxysome biogenesis, without affecting growth rate. Carbamylation of Fremyella recombinant rubisco was inhibited by RuBP, but this inhibition was not relieved by the ALC. The ALC does not appear to function like a canonical Rca; instead, it exerts an effect on the response to CO₂ availability at the level of a metabolic module, the carboxysome, through rubisco network formation, and carboxysome organization.

Discovery and Functional Characterization of a Yeast Sugar Alcohol Phosphatase

Sugar alcohols (polyols) exist widely in nature. While some specific sugar alcohol phosphatases are known, there is no known phosphatase for some important sugar alcohols (e.g., sorbitol-6-phosphate). Using liquid chromatography-mass spectrometry-based metabolomics, we screened yeast strains with putative phosphatases of unknown function deleted. We show that the yeast gene YNL010W, which has close homologues in all fungi species and some plants, encodes a sugar alcohol phosphatase. We term this enzyme, which hydrolyzes sorbitol-6-phosphate, ribitol-5-phosphate, and (d)-glycerol-3-phosphate, polyol phosphatase 1 or PYP1. Polyol phosphates are structural analogs of the enediol intermediate of phosphoglucose isomerase (Pgi). We find that sorbitol-6-phosphate and ribitol-5-phosphate inhibit Pgi and that Pyp1 activity is important for yeast to maintain Pgi activity in the presence of environmental sugar alcohols. Pyp1 expression is strongly positively correlated with yeast growth rate, presumably because faster growth requires greater glycolytic and accordingly Pgi flux. Thus, yeast express the previously uncharacterized enzyme Pyp1 to prevent inhibition of glycolysis by sugar alcohol phosphates. Pyp1 may be useful for engineering sugar alcohol production.

Increasing metabolic potential: C-fixation

Due to the growing world population, crop yields must increase to meet the rising demand. Crop plants also require adaptation to optimize performance in the changing environments caused by climate change. Improving photosynthetic carbon fixation is a promising, albeit technically challenging, strategy whose potential has only just begun to be considered in breeding programmes. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), a fundamental enzyme of carbon fixation, is extremely inefficient and many strategies to improve photosynthesis focus on overcoming the limitations of this enzyme, either by improving Rubisco activity and regulation or by improving the supply of substrates. Although progress is being made, the need to tailor solutions for each crop and their respective environments has been highlighted. Even so, continuing research will be required to achieve these objectives and to grow crops more sustainably in the future.

Engineering chloroplasts to improve Rubisco catalysis: prospects for translating improvements into food and fiber crops

494 I. 495 II. 496 III. 496 IV. 499 V. 499 VI. 501 VII. 501 VIII. 502 IX. 505 X. 506 507 References 507 SUMMARY: The uncertainty of future climate change is placing pressure on cropping systems to continue to provide stable increases in productive yields. To mitigate future climates and the increasing threats against global food security, new solutions to manipulate photosynthesis are required. This review explores the current efforts available to improve carbon assimilation within plant chloroplasts by engineering Rubisco, which catalyzes the rate-limiting step of CO₂ fixation. Fixation of CO₂ and subsequent cycling of 3-phosphoglycerate through the Calvin cycle provides the necessary carbohydrate building blocks for maintaining plant growth and yield, but has to compete with Rubisco oxygenation, which results in photorespiration that is energetically wasteful for plants. Engineering improvements in Rubisco is a complex challenge and requires an understanding of chloroplast gene regulatory pathways, and the intricate nature of Rubisco catalysis and biogenesis, to transplant more efficient forms of Rubisco into crops. In recent times, major advances in Rubisco engineering have been achieved through improvement of our knowledge of Rubisco synthesis and assembly, and identifying amino acid catalytic switches in the L-subunit responsible for improvements in catalysis. Improving the capacity of CO₂ fixation in crops such as rice will require further advances in chloroplast bioengineering and Rubisco biogenesis.

Salt stress-induced modulations in the shoot proteome of Brassica juncea genotypes

Indian mustard [Brassica juncea (L.) Czern and Coss] is cultivated mainly in the northwestern agroclimatic region of India and suffers huge losses in productivity due to salinization. In an effort to figure out adaptation strategies of Indian mustard to salt stress, we conducted a comparative proteome analysis of shoots of its two genotypes, with contrasting sensitivity to salt stress. Differential expression of 21 proteins was observed during the two-dimensional electrophoresis (2DE). The identified salt-stress-responsive proteins were associated with different functional processes including osmoregulation, photosynthesis, carbohydrate metabolism, ion homeostasis, protein synthesis and stabilization, energy metabolism, and antioxidant defense system. Salt-tolerant genotype (CS-52) showed a relatively higher expression of proteins involved in turgor regulation, stabilization of photosystems and proteins, and salt compartmentalization, as compared to salt-sensitive genotype (Pusa Varuna). Our results suggest that modulating the expression of salt-responsive proteins can pave the way for developing salt tolerance in the Indian mustard plants.

Optimizing Rubisco and its regulation for greater resource use efficiency

Rubisco catalyses the carboxylation of ribulose-1,5-bisphosphate (RuBP), enabling net CO2 assimilation in photosynthesis. The properties and regulation of Rubisco are not optimal for biomass production in current and projected future environments. Rubisco is relatively inefficient, and large amounts of the enzyme are needed to support photosynthesis, requiring large investments in nitrogen. The competing oxygenation of RuBP by Rubisco decreases photosynthetic efficiency. Additionally, Rubisco is inhibited by some sugar phosphates and depends upon interaction with Rubisco activase (Rca) to be reactivated. Rca activity is modulated by the chloroplast redox status and ADP/ATP ratios, thereby mediating Rubisco activation and photosynthetic induction in response to irradiance. The extreme thermal sensitivity of Rca compromises net CO2 assimilation at moderately high temperatures. Given its central role in carbon assimilation, the improvement of Rubisco function and regulation is tightly linked with irradiance, nitrogen and water use efficiencies. Although past attempts have had limited success, novel technologies and an expanding knowledge base make the challenge of improving Rubisco activity in crops an achievable goal. Strategies to optimize Rubisco and its regulation are addressed in relation to their potential to improve crop resource use efficiency and climate resilience of photosynthesis.

Maintaining photosynthetic CO2 fixation via protein remodelling: the Rubisco activases

The key photosynthetic, CO2-fixing enzyme Rubisco forms inactivated complexes with its substrate ribulose 1,5-bisphosphate (RuBP) and other sugar phosphate inhibitors. The independently evolved AAA+ proteins Rubisco activase and CbbX harness energy from ATP hydrolysis to remodel Rubisco complexes, facilitating release of these inhibitors. Here, we discuss recent structural and mechanistic advances towards the understanding of protein-mediated Rubisco activation. Both activating proteins appear to form ring-shaped hexameric arrangements typical for AAA+ ATPases in their functional form, but display very different regulatory and biochemical properties. Considering the thermolability of the plant enzyme, an improved understanding of the mechanism for Rubisco activation may help in developing heat-resistant plants adapted to the challenge of global warming.

Rubisco activity and regulation as targets for crop improvement

Rubisco (ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase) enables net carbon fixation through the carboxylation of RuBP. However, some characteristics of Rubisco make it surprisingly inefficient and compromise photosynthetic productivity. For example, Rubisco catalyses a wasteful reaction with oxygen that leads to the release of previously fixed CO(2) and NH(3) and the consumption of energy during photorespiration. Furthermore, Rubisco is slow and large amounts are needed to support adequate photosynthetic rates. Consequently, Rubisco has been studied intensively as a prime target for manipulations to 'supercharge' photosynthesis and improve both productivity and resource use efficiency. The catalytic properties of Rubiscos from diverse sources vary considerably, suggesting that changes in turnover rate, affinity, or specificity for CO(2) can be introduced to improve Rubisco performance in specific crops and environments. While attempts to manipulate plant Rubisco by nuclear transformation have had limited success, modifying its catalysis by targeted changes to its catalytic large subunit via chloroplast transformation have been much more successful. However, this technique is still in need of development for most major food crops including maize, wheat, and rice. Other bioengineering approaches for improving Rubisco performance include improving the activity of its ancillary protein, Rubisco activase, in addition to modulating the synthesis and degradation of Rubisco's inhibitory sugar phosphate ligands. As the rate-limiting step in carbon assimilation, even modest improvements in the overall performance of Rubisco pose a viable pathway for obtaining significant gains in plant yield, particularly under stressful environmental conditions.

Overexpression of rubisco activase decreases the photosynthetic CO2 assimilation rate by reducing rubisco content in rice leaves

The effects of overexpression of Rubisco activase on photosynthesis were studied in transgenic rice expressing barley or maize Rubisco activase. Immunoblot and SDS-PAGE analyses showed that transgenic lines from both gene constructs expressed the foreign Rubisco activase at high levels. The activation state of Rubisco in transgenic lines was slightly higher than that in non-transgenic plants (NT). In addition, light activation of Rubisco was significantly more rapid in transgenic lines compared with NT. These findings indicate that the overexpression of Rubisco activase can enhance Rubisco activation. However, despite enhanced activation of Rubisco in these transgenic plants, the CO(2) assimilation rate at ambient CO(2) conditions was decreased. This decrease in CO(2) assimilation rate was observed in both young developing and mature leaves independent of nitrogen nutrition. The contents of nitrogen and Chl did not differ significantly between transformants and NT; however, Rubisco content was substantially decreased in transgenic lines. There was no evidence for reduced transcription of RbcS or RbcL in these transgenic lines; in fact, transcript levels were marginally increased compared with NT. These results indicate that the overexpression of Rubisco activase leads to a decrease in Rubisco content, possibly due to post-transcriptional mechanisms.

Rubisco activities, properties, and regulation in three different C4 grasses under drought

In C4 plants, water deficit may decrease photosynthetic CO2 assimilation independently of changes in stomatal conductance, suggesting decreased turnover by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The activity and biochemistry of Rubisco was studied in three different C4 grasses: Paspalum dilatatum, Cynodon dactylon, and Zoysia japonica. The objectives were to characterize the C4 Rubisco in these species and to identify factors associated with decreased photosynthetic rates caused by drought. Rubisco isolated from each of the three C4 grasses was characterized by smaller specificity factors (SC/O), larger Michaelis-Menten constants for CO2 (Kc) and O2 (Ko), and larger maximum carboxylation velocities (Vc) than Rubisco from wheat, which can be rationalized in terms of the CO2-rich environment of C4 Rubisco in the bundle sheath. During leaf dehydration the quantity and maximum activity of Rubisco remained unchanged but the initial and total activities declined slightly, possibly due to increased inhibition. Tight-binding inhibitors were present in the light but were more abundant in the dark, especially in Z. japonica, and increased in quantity with drought stress. The inhibitor from darkened leaves of Z. japonica was identified as 2-carboxyarabinitol-1-phosphate (CA1P). Consistent with the presence of CA1P, the total activity of Rubisco was decreased after 12 h darkness in Z. japonica. Ribulose-1,5-bisphosphate (RuBP) in the leaves decreased with drought stress, to quantities approximating those of Rubisco catalytic sites. The magnitude of the decrease in RuBP suggested that, at least in C. dactylon and Z. japonica, it could contribute to the drought-induced decrease in photosynthesis.

Biochemical changes associated with in vivo RbcL fragmentation by reactive oxygen species under chilling-light conditions

During physiological stress, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) degradation is accelerated, which is considered to be one of the key factors responsible for photosynthetic decline. A recent study has shown that the large subunit (RbcL) of Rubisco is directly fragmented by hydroxyl radicals in Cucumis sativus leaves under chilling-light conditions. In the present study, we investigated biochemical aspects associated with this in vivo RbcL fragmentation by reactive oxygen species. RbcL fragmentation was observed in C. sativus and Phaseolus vulgaris, but not in Solanum lycopersicum, Glycine max, Oryza sativa, Triticum aestivum, Spinacia oleracea or Arabidopsis thaliana. In C. sativus and P. vulgaris, RbcL fragmentation followed the fragmentation of PsaB, while in the other species, PsaB fragmentation did not occur. In C. sativus and P. vulgaris, the activities of antioxidant enzymes decreased dramatically under chilling-light conditions, and the proportion of uncarbamylated Rubisco increased. These data suggest that in vivo RbcL fragmentation under chilling-light conditions is associated with a combination of events, namely, inactivation of antioxidant enzymes, destruction of photosystem I and an increase of uncarbamylated Rubisco, which can produce hydroxyl radicals via the Fenton reaction at the catalytic site of RbcL.

Binding of 2CA1P (nocturnal inhibitor) to the active site of RubisCO using genetic algorithm (GA)

Ribulose-1, 5- Bisphosphate carboxylase/ oxygenase (RubisCO) catalyzes the first step in net photosynthetic assimilation and photorespiratory carbon oxidation. The activity of this enzyme is modulated in response to changes in light intensity as suggested in a number of early reports. Several studies found that the natural inhibitor 2CA1P is involved in the inhibition of the enzyme under reduced light intensity in rice (Oryza sativa). Due to the lack of studies and information on the interaction between this inhibitor and the active site of the enzyme, we attempted to predict the interaction between the amino acids in the active site and the inhibitor using both Hyperchem7.5 and GOLD software. After the docking; three possibilities having the highest fitness score were found (65.71, 64.72, 62.04), in these possibilities the inhibitor was bound to the enzyme, the phosphate and carboxylate groups in the same positions with a clear difference in the position of OH. In order to confirm the accuracy of the genetic algorithm, the artificial inhibitor 2CABP was docked back in the active site of the enzyme using the same parameters used in the case of the 2CA1P and the algorithm's predictions were compared with the experimentally observed binding mode. The results showed that the difference in the active sites before and after the docking was in the range of 0.93 A which indicated that the results were very accurate. Depending on this result it was concluded that the results obtained in the case of the 2CA1P were close to the experimental results.

Raising yield potential in wheat

Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco's catalytic rate or adopting C(4) metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Gene expression, cellular localisation and function of glutamine synthetase isozymes in wheat (Triticum aestivum L.)

We present the first cloning and study of glutamine synthetase (GS) genes in wheat (Triticum aestivum L.). Based on sequence analysis, phylogenetic studies and mapping data, ten GS sequences were classified into four sub-families: GS2 (a, b and c), GS1 (a, b and c), GSr (1 and 2) and GSe (1 and 2). Phylogenetic analysis showed that the wheat GS sub-families together with the GS genes from other monocotyledonous species form four distinct clades. Immunolocalisation studies in leaves, stems and rachis in plants at flowering showed GS protein to be present in parenchyma, phloem companion and perifascicular sheath cells. In situ localisation confirmed that GS1 transcripts were present in the perifascicular sheath cells whilst those for GSr were confined to the vascular cells. Studies of the expression and protein profiles showed that all GS sub-families were differentially expressed in the leaves, peduncle, glumes and roots. Expression of GS genes in leaves was developmentally regulated, with both GS2 and GS1 assimilating or recycling ammonia in leaves during the period of grain development and filling. During leaf senescence the cytosolic isozymes, GS1 and GSr, were the predominant forms, suggesting major roles in assimilating ammonia during the critical phases of remobilisation of nitrogen to the grain. A preliminary analysis of three different wheat genotypes showed that the ratio of leaf GS2 protein to GS1 protein was variable. Use of this genetic variation should inform future efforts to modulate this enzyme for pre-breeding efforts to improve nitrogen use in wheat.

Cysteine proteinases regulate chloroplast protein content and composition in tobacco leaves: a model for dynamic interactions with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) vesicular bodies

The roles of cysteine proteinases (CP) in leaf protein accumulation and composition were investigated in transgenic tobacco (Nicotiana tabacum L.) plants expressing the rice cystatin, OC-1. The OC-1 protein was present in the cytosol, chloroplasts, and vacuole of the leaves of OC-1 expressing (OCE) plants. Changes in leaf protein composition and turnover caused by OC-1-dependent inhibition of CP activity were assessed in 8-week-old plants using proteomic analysis. Seven hundred and sixty-five soluble proteins were detected in the controls compared to 860 proteins in the OCE leaves. A cyclophilin, a histone, a peptidyl-prolyl cis-trans isomerase, and two ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase isoforms were markedly altered in abundance in the OCE leaves. The senescence-related decline in photosynthesis and Rubisco activity was delayed in the OCE leaves. Similarly, OCE leaves maintained higher leaf Rubisco activities and protein than controls following dark chilling. Immunogold labelling studies with specific antibodies showed that Rubisco was present in Rubisco vesicular bodies (RVB) as well as in the chloroplasts of leaves from 8-week-old control and OCE plants. Western blot analysis of plants at 14 weeks after both genotypes had flowered revealed large increases in the amount of Rubisco protein in the OCE leaves compared to controls. These results demonstrate that CPs are involved in Rubisco turnover in leaves under optimal and stress conditions and that extra-plastidic RVB bodies are present even in young source leaves. Furthermore, these data form the basis for a new model of Rubisco protein turnover involving CPs and RVBs.

Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase): a historical perspective

Historic discoveries and key observations related to Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase), from 1947 to 2006, are presented. Currently, around 200 papers describing Rubisco research are published each year and the literature contains more than 5000 manuscripts on the subject. While trying to ensure that all the major events over this period are recorded, this analysis will inevitably be incomplete and will reflect the areas of particular interest to the authors.

A dynamic model of Rubisco turnover in cereal leaves

A simple, improved model of Rubisco synthesis and degradation in cereal leaves is developed using data obtained over the leaf lifespan to return maximum likelihood values for Rubisco proteolysis and biosynthesis. It assumes that the time course of leaf Rubisco content can be described using a log-normal curve, and degradation of the Rubisco pool occurs exponentially. Curve parameters give an insight into how Rubisco dynamics differ among treatments or genotypes; also, statistical analyses can be performed more easily, requiring fewer data and allowing more flexibility in sampling than previous studies. Predicted patterns of synthesis correlate well with independent rbc gene transcript data. Rubisco degradation is a first-order decay process, and biosynthesis correlates with leaf elongation rates. As Rubisco degradation takes place according to first-order kinetics, control of leaf Rubisco concentration must be exerted by adjustment of biosynthetic, rather than proteolytic, rates.

Modification of the proteolytic fragmentation pattern upon oxidation of cysteines from ribulose 1,5-bisphosphate carboxylase/oxygenase

The proteolytic susceptibility of the native CO(2)-fixing photosynthetic enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39, Rubisco) has been shown to increase in vitro after oxidative treatments that affect cysteine thiols. A limited incubation of oxidized (pretreated with the disulfide cystamine) Rubisco from Chlamydomonas reinhardtii with subtilisin or proteinase K generated fragments of molecular mass about 53 kDa (band I in SDS-PAGE) and 47 kDa (band II) derived from the large subunit (55 kDa) of the enzyme. In contrast, proteolysis of the reduced Rubisco (pretreated with the free thiol cysteamine) produced only the 53 kDa band. The same fragmentation pattern was reproduced with Rubiscos from other algae and higher plants, as well as with other chemical modifications of protein cysteines. N-terminal sequencing of the fragments showed that band I arised from clipping the unstructured N-terminal stretch of the large subunit up to Lys18. Band II was generated by a cleavage close to Val69. The increased susceptibility of the oxidized form resulted from proteases gaining access to a loop (from Ser61 to Thr68) located between stretches of secondary structure that form the N-terminal domain. Native electrophoresis and kinetic analysis of fragment accumulation during subtilisin digestion demonstrated that subunit dissociation was induced by the proteolytic processing at the Ser61-Thr68 loop, which is characteristic of the oxidized Rubisco. Holoenzyme dissasembly was readily followed by the full degradation of the released subunits. In contrast, the limited processing to band I observed with the reduced enzyme did not compromise the quaternary structure of the Rubisco hexadecamer, thus preventing further proteolysis.

Genetic modification of photosynthesis with E. coli genes for trehalose synthesis

Improvement in photosynthesis per unit leaf area has been difficult to alter by breeding or genetic modification. We report large changes in photosynthesis in Nicotiana tabacum transformed with E. coli genes for the trehalose pathway. Significantly, photosynthetic capacity (CO2 assimilation at varying light and CO2, and quantum yield of PSII electron transport) per unit leaf area and per leaf dry weight were increased in lines of N. tabacum transformed with the E. coli gene otsA, which encodes trehalose phosphate synthase. In contrast, transformation with otsB, which encodes trehalose phosphate phosphatase or Trec, encoding trehalose phosphate hydrolase, produced the opposite effect. Changes in CO2 assimilation per unit leaf area were closely related to the amount and activity of Rubisco, but not to the maximum activities of other Calvin cycle enzymes. Alterations in photosynthesis were associated with trehalose 6-phosphate content rather than trehalose. When growth parameters were determined, a greater photosynthetic capacity did not translate into greater relative growth rate or biomass. This was because photosynthetic capacity was negatively related to leaf area and leaf area ratio. In contrast, relative growth rate and biomass were positively related to leaf area. These results demonstrate a novel means of modifying Rubisco content and photosynthesis, and the complexities of regulation of photosynthesis at the whole plant level, with potential benefits to biomass production through improved leaf area.

Elucidating the biosynthesis of 2-carboxyarabinitol 1-phosphate through reduced expression of chloroplastic fructose 1,6-bisphosphate phosphatase and radiotracer studies with 14CO2

2-carboxyarabinitol 1-phosphate limits photosynthetic CO2 assimilation at low light because it is a potent, naturally occurring inhibitor of ribulose 1,5-bisphosphate carboxylase/oxygenase. Evidence is presented that this inhibitor is derived from chloroplastic fructose 1,6-bisphosphate. First, transgenic plants containing decreased amounts of chloroplastic fructose 1,6-bisphosphate phosphatase contained increased amounts of fructose 1,6-bisphosphate and 2-carboxyarabinitol 1-phosphate and greatly increased amounts of the putative intermediates hamamelose and 2-carboxyarabinitol, which in some cases were as abundant as sucrose. Second, French bean leaves in the light were shown to incorporate 14C from 14CO2 sequentially into fructose 1,6-bisphosphate, hamamelose bisphosphate, hamamelose monophosphate, hamamelose, and 2-carboxyarabinitol. As shown previously, 14C assimilated by photosynthesis was also incorporated into 2-carboxyarabinitol 1-phosphate during subsequent darkness.

Nitrogen metabolism and remobilization during senescence

Senescence is a highly organized and well-regulated process. As much as 75% of total cellular nitrogen may be located in mesophyll chloroplasts of C(3)-plants. Proteolysis of chloroplast proteins begins in an early phase of senescence and the liberated amino acids can be exported to growing parts of the plant (e.g. maturing fruits). Rubisco and other stromal enzymes can be degraded in isolated chloroplasts, implying the involvement of plastidial peptide hydrolases. Whether or not ATP is required and if stromal proteins are modified (e.g. by reactive oxygen species) prior to their degradation are questions still under debate. Several proteins, in particular cysteine proteases, have been demonstrated to be specifically expressed during senescence. Their contribution to the general degradation of chloroplast proteins is unclear. The accumulation in intact cells of peptide fragments and inhibitor studies suggest that multiple degradation pathways may exist for stromal proteins and that vacuolar endopeptidases might also be involved under certain conditions. The breakdown of chlorophyll-binding proteins associated with the thylakoid membrane is less well investigated. The degradation of these proteins requires the simultaneous catabolism of chlorophylls. The breakdown of chlorophylls has been elucidated during the last decade. Interestingly, nitrogen present in chlorophyll is not exported from senescencing leaves, but remains within the cells in the form of linear tetrapyrrolic catabolites that accumulate in the vacuole. The degradation pathways for chlorophylls and chloroplast proteins are partially interconnected.