Ribulose-1,5-bisphosphate carboxylase/oxygenase activase deficiency delays senescence of ribulose-1,5-bisphosphate carboxylase/oxygenase but progressively impairs its catalysis during tobacco leaf development

Molecular characterization and expression pattern of Rubisco activase gene GhRCAβ2 in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Rubisco activase (RCA) is a pivotal enzyme that can catalyse the activation of Rubisco in carbon assimilation pathway. Many studies have shown that RCA may be a potential target for genetic manipulation aimed at enhancing photosynthetic efficiency and crop yield.

OBJECTIVE

To understand the biological function of the GhRCAβ2 gene in upland cotton, we cloned the coding sequence (CDS) of the GhRCAβ2 gene and investigated its sequence features, evolutionary relationship, subcellular localization, promoter sequence and expression pattern.

METHODS

The bioinformatics tools were used to analyze the sequence features of GhRCAβ2 protein. Transient transformation of Arabidopsis mesophyll protoplasts was performed to determine the subcellular localization of the GhRCAβ2 protein. The expression pattern of the GhRCAβ2 gene was examined by analyzing transcriptome data and using the quantitative real-time PCR (qRT-PCR).

RESULTS

The full-length CDS of GhRCAβ2 was 1317 bp, and it encoded a protein with a chloroplast transit peptide. The GhRCAβ2 had two conserved ATP-binding domains, and did not have the C-terminal extension (CTE) domain that was unique to the RCA α-isoform in plants. Evolutionarily, GhRCAβ2 was clustered in Group A, and had a close evolutionary relationship with the soybean RCA. Western blot analysis demonstrated that GhRCAβ2 was immunoreactive to the RCA antibody displaying a molecular weight similar to that of the RCA β-isoform. The GhRCAβ2 protein was found in chloroplast, aligning with its role as a vital enzyme in the process of photosynthesis. The GhRCAβ2 gene had a leaf tissue-specific expression pattern, and the yellow-green leaf mutant exhibited a decreased expression of GhRCAβ2 in comparison to the wild-type cotton plants. The GhRCAβ2 promoter contained several cis-acting elements that respond to light, phytohormones and stress, suggesting that the expression of GhRCAβ2 may be regulated by these factors. An additional examination of stress response indicated that GhRCAβ2 expression was influenced by cold, heat, salt, and drought stress. Notably, diverse expression pattern was observed across different stress conditions. Additionally, low phosphorus and low potassium stress may result in a notable reduction in the expression of GhRCAβ2 gene.

CONCLUSION

Our findings will establish a basis for further understanding the function of the GhRCAβ2 gene, as well as providing valuable genetic knowledge to improve cotton photosynthetic efficiency and yield under challenging environmental circumstances.

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.

Removal of redox-sensitive Rubisco Activase does not alter Rubisco regulation in soybean

Rubisco activase (Rca) facilitates the catalytic repair of Rubisco, the CO₂-fixing enzyme of photosynthesis, following periods of darkness, low to high light transitions or stress. Removal of the redox-regulated isoform of Rubisco activase, Rca-α, enhances photosynthetic induction in Arabidopsis and has been suggested as a strategy for the improvement of crops, which may experience frequent light transitions in the field; however, this has never been tested in a crop species. Therefore, we used RNAi to reduce the Rca-α content of soybean (Glycine max cv. Williams 82) below detectable levels and then characterized the growth, photosynthesis, and Rubisco activity of the resulting transgenics, in both growth chamber and field conditions. Under a 16 h sine wave photoperiod, the reduction of Rca-α contents had no impact on morphological characteristics, leaf expansion rate, or total biomass. Photosynthetic induction rates were unaltered in both chamber-grown and field-grown plants. Plants with reduced Rca-α content maintained the ability to regulate Rubisco activity in low light just as in control plants. This result suggests that in soybean, Rca-α is not as centrally involved in the regulation of Rca oligomer activity as it is in Arabidopsis. The isoform stoichiometry supports this conclusion, as Rca-α comprises only ~ 10% of the Rubisco activase content of soybean, compared to ~ 50% in Arabidopsis. This is likely to hold true in other species that contain a low ratio of Rca-α to Rca-ß isoforms.

Enhancing crop yield by using Rubisco activase to improve photosynthesis under elevated temperatures

With the rapid growth of world population, it is essential to increase agricultural productivity to feed the growing population. Over the past decades, many methods have been used to increase crop yields. Despite the success in boosting the crop yield through these methods, global food production still needs to be increased to be on par with the increasing population and its dynamic consumption patterns. Additionally, given the prevailing environmental conditions pertaining to the global temperature increase, heat stress will likely be a critical factor that negatively affects plant biomass and crop yield. One of the key elements hindering photosynthesis and plant productivity under heat stress is the thermo-sensitivity of the Rubisco activase (RCA), a molecular chaperone that converts Rubisco back to active form after it becomes inactive. It would be an attractive and practical strategy to maintain photosynthetic activity under elevated temperatures by enhancing the thermo-stability of RCA. In this context, this review discusses the need to improve the thermo-tolerance of RCA under current climatic conditions and to further study RCA structure and regulation, and its limitations at elevated temperatures. This review summarizes successful results and provides a perspective on RCA research and its implication in improving crop yield under elevated temperature conditions in the future.

Rubisco carboxylase/oxygenase: From the enzyme to the globe: A gas exchange perspective

Rubisco is the primary carboxylase of the photosynthetic process, the most abundant enzyme in the biosphere, and also one of the best-characterized enzymes. Rubisco also functions as an oxygenase, a discovery made 50 years ago by Bill Ogren. Carboxylation of ribulose bisphosphate (RuBP) is the first step of the photosynthetic carbon reduction cycle and leads to the assimilation of CO₂, whereas the oxygenase activity necessitates the recycling of phosphoglycolate through the photorespiratory carbon oxidation cycle with concomitant loss of CO₂. Since the discovery of Rubisco's dual function, the biochemical properties of Rubisco have underpinned the mechanistic mathematical models of photosynthetic CO₂ fixation which link Rubisco kinetic properties to gas exchange of leaves. This has allowed assessments of global CO₂ exchange and predictions of how Rubisco has and will shape the environmental responses of crop and global photosynthesis in future climates. Rubisco's biochemical properties, including its slow catalytic turnover and poor affinity for CO₂, constrain crop growth and therefore improving its activity and regulation and minimising photorespiration are key targets for crop improvement.

Expression level of Rubisco activase negatively correlates with Rubisco content in transgenic rice

The relationship between ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase (Rca) levels was studied using transgenic rice overexpressing maize Rca (OX-mRca) and knockdown transgenic rice expressing antisense Rca (KD-Rca). The ratio of Rubisco to total soluble protein was lower in OX-mRca, whereas it was higher in KD-Rca than in WT, indicating that Rca expression was negatively correlated with Rubisco content. The expressions of other Calvin-Benson-Bassham cycle enzymes such as sedoheptulose-1,7-bisphosphatase and phosphoribulokinase analyzed by immunoblotting did not show such a negative correlation with Rca, suggesting that the effect of Rca on protein expression may be specific for Rubisco. Although Rubisco content was decreased in OX-mRca, the transcript levels of the Rubisco large subunit (OsRbcL) and the Rubisco small subunit mostly increased in OX-mRca as well as in KD-Rca. Additionally, polysome loading of OsRbcL was slightly higher in OX-mRca than it was in WT, suggesting that the OsRbcL translation activity was likely stimulated by overexpression of Rca. ³⁵S-methionine labeling experiments demonstrated that there was no significant difference in the stability of newly synthesized Rubisco among genotypes. However, ³⁵S-methionine-labeled Rubisco was marginally decreased in OX-mRca and increased in KD-Rca compared to the WT. These results suggest that Rca negatively affects the Rubisco content, possibly in the synthesis step.

An improved Escherichia coli screen for Rubisco identifies a protein-protein interface that can enhance CO2-fixation kinetics

An overarching goal of photosynthesis research is to identify how components of the process can be improved to benefit crop productivity, global food security, and renewable energy storage. Improving carbon fixation has mostly focused on enhancing the CO2 fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This grand challenge has mostly proved ineffective because of catalytic mechanism constraints and required chaperone complementarity that hinder Rubisco biogenesis in alternative hosts. Here we refashion Escherichia coli metabolism by expressing a phosphoribulokinase-neomycin phosphotransferase fusion protein to produce a high-fidelity, high-throughput Rubisco-directed evolution (RDE2) screen that negates false-positive selection. Successive evolution rounds using the plant-like Te-Rubisco from the cyanobacterium Thermosynechococcus elongatus BP1 identified two large subunit and six small subunit mutations that improved carboxylation rate, efficiency, and specificity. Structural analysis revealed the amino acids clustered in an unexplored subunit interface of the holoenzyme. To study its effect on plant growth, the Te-Rubisco was transformed into tobacco by chloroplast transformation. As previously seen for Synechocccus PCC6301 Rubisco, the specialized folding and assembly requirements of Te-Rubisco hinder its heterologous expression in leaf chloroplasts. Our findings suggest that the ongoing efforts to improve crop photosynthesis by integrating components of a cyanobacteria CO2-concentrating mechanism will necessitate co-introduction of the ancillary molecular components required for Rubisco biogenesis.

Identification of Putative RuBisCo Activase (TaRca1)-The Catalytic Chaperone Regulating Carbon Assimilatory Pathway in Wheat (Triticum aestivum) under the Heat Stress

RuBisCo activase (Rca) is a catalytic chaperone involved in modulating the activity of RuBisCo (key enzyme of photosynthetic pathway). Here, we identified eight novel transcripts from wheat through data mining predicted to be Rca and cloned a transcript of 1.4 kb from cv. HD2985, named as TaRca1 (GenBank acc. no. KC776912). Single copy number of TaRca1 was observed in wheat genome. Expression analysis in diverse wheat genotypes (HD2985, Halna, PBW621, and HD2329) showed very high relative expression of TaRca1 in Halna under control and HS-treated, as compared to other cultivars at different stages of growth. TaRca1 protein was predicted to be chloroplast-localized with numerous potential phosphorylation sites. Northern blot analysis showed maximum accumulation of TaRca1 transcript in thermotolerant cv. during mealy-ripe stage, as compared to thermosusceptible. Decrease in the photosynthetic parameters was observed in all the cultivars, except PBW621 in response to HS. We observed significant increase in the Rca activity in all the cultivars under HS at different stages of growth. HS causes decrease in the RuBisCo activity; maximum reduction was observed during pollination stage in thermosusceptible cvs. as validated through immunoblotting. We observed uniform carbon distribution in different tissues of thermotolerant cvs., as compared to thermosusceptible. Similarly, tolerance level of leaf was observed maximum in Halna having high Rca activity under HS. A positive correlation was observed between the transcript and activity of TaRca1 in HS-treated Halna. Similarly, TaRca1 enzyme showed positive correlation with the activity of RuBisCo. There is, however, need to manipulate the thermal stability of TaRca1 enzyme through protein engineering for sustaining the photosynthetic rate under HS-a novel approach toward development of "climate-smart" crop.

Differential Expression of Genes of the Calvin-Benson Cycle and its Related Genes During Leaf Development in Rice

To understand how the machinery for photosynthetic carbon assimilation is formed and maintained during leaf development, changes in the mRNA levels of the Calvin-Benson cycle enzymes, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase and two key enzymes for sucrose synthesis were determined in rice (Oryza sativa L.). According to the patterns of changes in the mRNA levels, these genes were categorized into three groups. Group 1 included most of the genes involved in the carboxylation and reduction phases of the Calvin-Benson cycle, as well as three genes in the regeneration phase. The mRNA levels increased and reached maxima during leaf expansion and then rapidly declined, although there were some variations in the residual mRNA levels in senescent leaves. Group 2 included a number of genes involved in the regeneration phase, one gene in the reduction phase of the Calvin-Benson cycle and one gene in sucrose synthesis. The mRNA levels increased and almost reached maxima before full expansion and then gradually declined. Group 3 included Rubisco activase, one gene involved in the regeneration phase and one gene in sucrose synthesis. The overall pattern was similar to that in group 2 genes except that the mRNA levels reached maxima after the stage of full expansion. Thus, genes of the Calvin-Benson cycle and its related genes were differentially expressed during leaf development in rice, suggesting that such differential gene expression is necessary for formation and maintenance of the machinery of photosynthetic carbon assimilation.

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.

Changes in Rubisco activase gene expression and polypeptide content in Brachypodium distachyon

Regulation of Rubisco (D-ribulose-1,5-bisphosphate carboxylase/oxygenase activase (RCA) gene expression and polypeptide content were determined in Brachypodium distachyon leaves, stems and ear elements at different developmental stages under optimal growth conditions as well as under drought and salt stress conditions. B. distachyon leaf contains a much greater amount of Rubisco activase small (RCAS) isoform than the large one (RCAL) under optimal growth conditions. Increased levels of the RCAL isoform compared with the RCAS isoform were found in leaves and in green stems under salt and drought stress, respectively. Transcriptional levels of RCA are almost identical in different leaf positions. Short-term drought and salt stresses did not cause the impairment of RCA gene expression in early seedlings. But gradually increasing drought stress significantly decreased gene expression in early seedling samples. Amounts of the RCAS isoform were found to be more in different leaves of the plant compared with the RCAL isoform and their ratio was constant under normal condition. In green stems gene expression of RCA decreased under salt and drought stresses, although as it was in green leaves protein amounts of RCAL isoform increased compared with the RCAS isoform. All of the above described results clearly indicate that the accumulation of each RCA isoform is differentially regulated by developmental and environmental cues.

RuBPCase activase (RCA) mediates growth-defense trade-offs: silencing RCA redirects jasmonic acid (JA) flux from JA-isoleucine to methyl jasmonate (MeJA) to attenuate induced defense responses in Nicotiana attenuata

• RuBPCase activase (RCA), an abundant photosynthetic protein, is strongly down-regulated in response to Manduca sexta's oral secretion (OS) in Nicotiana attenuata. RCA-silenced plants are impaired not only in photosynthetic capacity and growth, but also in jasmonic acid-isoleucine (JA-Ile) signaling, and herbivore resistance mediated by JA-Ile-dependent defense traits. These responses are consistent with a resource-based growth-defense trade-off. • As JA + Ile supplementation of OS restored wild-type (WT) levels of JA-Ile, defenses and resistance to M. sexta, but OS supplemented individually with JA or Ile did not, the JA-Ile deficiency of RCA-silenced plants could not be attributed to lower JA or Ile pools or JAR4/6 conjugating activity. Similar levels of JA-Ile derivatives after OS elicitation indicated unaltered JA-Ile turnover, and lower levels of other JA conjugates ruled out competition from other conjugation reactions. • RCA-silenced plants accumulated more methyl jasmonate (MeJA) after OS elicitation, which corresponded to increased jasmonate methyltransferase (JMT) activity. RCA silencing phenocopies JMT overexpression, wherein elevated JMT activity redirects OS-elicited JA flux towards inactive MeJA, creating a JA sink which depletes JA-Ile and its associated defense responses. • Hence, RCA plays an additional non-photosynthetic role in attenuating JA-mediated defenses and their associated costs, potentially allowing plants to anticipate resource-based constraints on growth before they actually occur.

The coordination of C4 photosynthesis and the CO2-concentrating mechanism in maize and Miscanthus x giganteus in response to transient changes in light quality

Unequal absorption of photons between photosystems I and II, and between bundle-sheath and mesophyll cells, are likely to affect the efficiency of the CO2-concentrating mechanism in C4 plants. Under steady-state conditions, it is expected that the biochemical distribution of energy (ATP and NADPH) and photosynthetic metabolite concentrations will adjust to maintain the efficiency of C4 photosynthesis through the coordination of the C3 (Calvin-Benson-Bassham) and C4 (CO2 pump) cycles. However, under transient conditions, changes in light quality will likely alter the coordination of the C3 and C4 cycles, influencing rates of CO2 assimilation and decreasing the efficiency of the CO2-concentrating mechanism. To test these hypotheses, we measured leaf gas exchange, leaf discrimination, chlorophyll fluorescence, electrochromatic shift, photosynthetic metabolite pools, and chloroplast movement in maize (Zea mays) and Miscanthus × giganteus following transitional changes in light quality. In both species, the rate of net CO2 assimilation responded quickly to changes in light treatments, with lower rates of net CO2 assimilation under blue light compared with red, green, and blue light, red light, and green light. Under steady state, the efficiency of CO2-concentrating mechanisms was similar; however, transient changes affected the coordination of C3 and C4 cycles in M. giganteus but to a lesser extent in maize. The species differences in the ability to coordinate the activities of C3 and C4 cycles appear to be related to differences in the response of cyclic electron flux around photosystem I and potentially chloroplast rearrangement in response to changes in light quality.

Photosynthetic characterization of Rubisco transplantomic lines reveals alterations on photochemistry and mesophyll conductance

Improving Rubisco catalysis is considered a promising way to enhance C3-photosynthesis and photosynthetic water use efficiency (WUE) provided the introduced changes have little or no impact on other processes affecting photosynthesis such as leaf photochemistry or leaf CO2 diffusion conductances. However, the extent to which the factors affecting photosynthetic capacity are co-regulated is unclear. The aim of the present study was to characterize the photochemistry and CO2 transport processes in the leaves of three transplantomic tobacco genotypes expressing hybrid Rubisco isoforms comprising different Flaveria L-subunits that show variations in catalysis and differing trade-offs between the amount of Rubisco and its activation state. Stomatal conductance (g s) in each transplantomic tobacco line matched wild-type, while their photochemistry showed co-regulation with the variations in Rubisco catalysis. A tight co-regulation was observed between Rubisco activity and mesophyll conductance (g m) that was independent of g s thus producing plants with varying g m/g s ratios. Since the g m/g s ratio has been shown to positively correlate with intrinsic WUE, the present results suggest that altering photosynthesis by modifying Rubisco catalysis may also be useful for targeting WUE.

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.

Determination of ¹⁵N-incorporation into plant proteins and their absolute quantitation: a new tool to study nitrogen flux dynamics and protein pool sizes elicited by plant-herbivore interactions

Herbivory leads to changes in the allocation of nitrogen among different pools and tissues; however, a detailed quantitative analysis of these changes has been lacking. Here, we demonstrate that a mass spectrometric data-independent acquisition approach known as LC-MS(E), combined with a novel algorithm to quantify heavy atom enrichment in peptides, is able to quantify elicited changes in protein amounts and (15)N flux in a high throughput manner. The reliable identification/quantitation of rabbit phosphorylase b protein spiked into leaf protein extract was achieved. The linear dynamic range, reproducibility of technical and biological replicates, and differences between measured and expected (15)N-incorporation into the small (SSU) and large (LSU) subunits of ribulose-1,5-bisphosphate-carboxylase/oxygenase (RuBisCO) and RuBisCO activase 2 (RCA2) of Nicotiana attenuata plants grown in hydroponic culture at different known concentrations of (15)N-labeled nitrate were used to further evaluate the procedure. The utility of the method for whole-plant studies in ecologically realistic contexts was demonstrated by using (15)N-pulse protocols on plants growing in soil under unknown (15)N-incorporation levels. Additionally, we quantified the amounts of lipoxygenase 2 (LOX2) protein, an enzyme important in antiherbivore defense responses, demonstrating that the approach allows for in-depth quantitative proteomics and (15)N flux analyses of the metabolic dynamics elicited during plant-herbivore interactions.

Rubisco activase is a key regulator of non-steady-state photosynthesis at any leaf temperature and, to a lesser extent, of steady-state photosynthesis at high temperature

The role of Rubisco activase in steady-state and non-steady-state photosynthesis was analyzed in wild-type (Oryza sativa) and transgenic rice that expressed different amounts of Rubisco activase. Below 25°C, the Rubisco activation state and steady-state photosynthesis were only affected when Rubisco activase was reduced by more than 70%. However, at 40°C, smaller reductions in Rubisco activase content were linked to a reduced Rubisco activation state and steady-state photosynthesis. As a result, overexpression of maize Rubisco activase in rice did not lead to an increase of the Rubisco activation state, nor to an increase in photosynthetic rate below 25°C, but had a small stimulatory effect at 40°C. On the other hand, the rate at which photosynthesis approached the steady state following an increase in light intensity was rapid in Rubisco activase-overexpressing plants, intermediate in the wild-type, and slowest in antisense plants at any leaf temperature. In Rubisco activase-overexpressing plants, Rubisco activation state at low light was maintained at higher levels than in the wild-type. Thus, rapid regulation by Rubisco activase following an increase in light intensity and/or maintenance of a high Rubisco activation state at low light would result in a rapid increase in Rubisco activation state and photosynthetic rate following an increase in light intensity. It is concluded that Rubisco activase plays an important role in the regulation of non-steady-state photosynthesis at any leaf temperature and, to a lesser extent, of steady-state photosynthesis at high temperature.

The absence of alternative oxidase AOX1A results in altered response of photosynthetic carbon assimilation to increasing CO(2) in Arabidopsis thaliana

In higher plants, the mitochondrial electron transport chain has non-phosphorylating alternative pathways that include the alternative terminal oxidase (AOX). This alternative pathway has been suggested to act as a sink for dissipating excess reducing power, minimizing oxidative stress and possibly optimizing photosynthesis in response to changing conditions. The expression patterns of the AOX genes have been well characterized under different growth conditions, particularly in response to light and temperature stress. Additionally, it has been suggested that mitochondrial electron transport is important for avoiding chloroplast over-reduction and balancing energy partitioning among photosynthesis, photorespiration and respiration. Nonetheless, the role AOX plays in optimizing photosynthetic carbon metabolism is unclear. Therefore, the response of photosynthesis to the disruption of AOX was investigated in the Arabidopsis thaliana T-DNA mutant aox1a (SALK_084897). Gas exchange analysis revealed a lower net CO(2) assimilation rate (A) at high CO(2) concentrations in the aox1a mutant compared to wild type. This decrease in A was accompanied by a lower maximum electron transport rate and quantum yield of PSII, and higher excitation pressure on PSII and non-photochemical quenching. The aox1a mutant also exhibited a lower estimated rate of ribulose 1,5-bisphosphate regeneration, and the ribulose 1,5-bisphosphate content was lower at high CO(2) concentrations, suggesting an ATP limitation of the Calvin-Benson cycle. Additionally, the activity of the malate-oxaloacetate shuttle was lower in the mutant compared to wild type. These results indicate that AOX is important for optimizing rates of photosynthetic CO(2) assimilation in response to rising CO(2) concentration by balancing the NAD(P)H/ATP ratio and rates of ribulose 1,5-bisphosphate regeneration within the chloroplast.

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.

The influence of light quality on C4 photosynthesis under steady-state conditions in Zea mays and Miscanthus×giganteus: changes in rates of photosynthesis but not the efficiency of the CO2 concentrating mechanism

Differences in light quality penetration within a leaf and absorption by the photosystems alter rates of CO(2) assimilation in C(3) plants. It is also expected that light quality will have a profound impact on C(4) photosynthesis due to disrupted coordination of the C(4) and C(3) cycles. To test this hypothesis, we measured leaf gas exchange, (13) CO(2) discrimination (Δ(13) C), photosynthetic metabolite pools and Rubisco activation state in Zea mays and Miscanthus × giganteus under steady-state red, green, blue and white light. Photosynthetic rates, quantum yield of CO(2) assimilation, and maximum phosphoenolpyruvate carboxylase activity were significantly lower under blue light than white, red and green light in both species. However, similar leakiness under all light treatments suggests the C(4) and C(3) cycles were coordinated to maintain the photosynthetic efficiency. Measurements of photosynthetic metabolite pools also suggest coordination of C(4) and C(3) cycles across light treatments. The energy limitation under blue light affected both C(4) and C(3) cycles, as we observed a reduction in C(4) pumping of CO(2) into bundle-sheath cells and a limitation in the conversion of C(3) metabolite phosphoglycerate to triose phosphate. Overall, light quality affects rates of CO(2) assimilation, but not the efficiency of CO(2) concentrating mechanism.

SDS-dependent proteases induced by ABA and its relation to Rubisco and Rubisco activase contents in rice leaves

Protease activities and its relation to the contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase were investigated in detached leaves of rice (Oryza sativa L.) floated on the solutions containing abscisic acid (ABA) or benzyladenine (BA). Rubisco and Rubisco activase contents were decreased during the time course and the decreases were enhanced by ABA and suppressed by BA. The decrease in Rubisco activase was faster than that in Rubisco. SDS-dependent protease activities at 50-70 kDa (rice SDS-dependent protease: RSP) analyzed by the gelatin containing PAGE were significantly enhanced by ABA. RSPs were also increased in attached leaves during senescence. RSPs had the pH optimum of 5.5, suggesting that RSPs are vacuolar protease. Both decrease in Rubisco and Rubisco activase contents and increase in RSPs activities were suppressed by cycloheximide. These findings indicate that the activities of RSPs are well correlated with the decrease in these protein contents. Immunoblotting analysis showed that Rubisco in the leaf extracts was completely degraded by 5h at pH 5.5 with SDS where it was optimal condition for RSPs. However, the degradation of Rubisco did not proceed at pH 7.5 without SDS where it is near physiological condition for stromal proteins. Rubisco activase was degraded at similar rate under both conditions. These results suggest that RSPs can functions in a senescence related degradation system of chloroplast protein in rice leaves. Rubisco activase would be more susceptible to proteolysis than Rubisco under physiological condition and this could affect the contents of these proteins in leaves.

Structural changes associated with the acute thermal instability of Rubisco activase

Inhibition of photosynthesis by heat has been linked to the instability of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) chaperone, Rubisco activase. Examination of the recombinant enzyme showed that ADP and ATP protected against inactivation, whereas Mg(2+) promoted inactivation. Heating caused aggregation of Rubisco activase characterized by disruption of secondary structure content and formation of insoluble protein. In contrast, incubation at room temperature without nucleotide caused the active approximately 660 kDa protein to form a soluble, but inactive aggregate of > 2 x 10(6) Da. Circular dichroism (CD) spectroscopy and fluorescence established that structural perturbations in the aggregate did not reduce alpha-helical content significantly. Differences in the thermal stability between wild type and mutant Rubisco activase were observed for the recombinant proteins and when the proteins were expressed in transgenic Arabidopsis. That the sensitivity of these plants to heat differs indicates that the thermal instability of Rubisco activase is a main determinant of the temperature-sensitivity of photosynthesis.

Expression quantitative trait loci analysis of two genes encoding rubisco activase in soybean

Rubisco activase (RCA) catalyzes the activation of Rubisco in vivo and plays a crucial role in photosynthesis. However, until now, little was known about the molecular genetics of RCA in soybean (Glycine max), one of the most important legume crops. Here, we cloned and characterized two genes encoding the longer alpha -isoform and the shorter beta -isoform of soybean RCA (GmRCA alpha and GmRCA beta, respectively). The two corresponding cDNAs are divergent in both the translated and 3 ' untranslated regions. Analysis of genomic DNA sequences suggested that the corresponding mRNAs are transcripts of two different genes and not the products of a single alternatively splicing pre-mRNA. Two additional possible alpha -form RCA-encoding genes, GmRCA03 and GmRCA14, and one additional beta -form RCA-encoding gene, GmRCA11, were also isolated. To examine the function and modulation of RCA genes in soybean, we determined the expression levels of GmRCA alpha and GmRCA beta, Rubisco initial activity, photosynthetic rate, and seed yield in 184 soybean recombinant inbred lines. Correlation of gene expression levels with three other traits indicates that RCA genes could play an important role in regulating soybean photosynthetic capacity and seed yield. Expression quantitative trait loci mapping revealed four trans-expression quantitative trait loci for GmRCA alpha and GmRCA beta. These results could provide a new approach for the modulation of RCA genes to improve photosynthetic rate and plant growth in soybean and other plants.

Effect of Rubisco activase deficiency on the temperature response of CO2 assimilation rate and Rubisco activation state: insights from transgenic tobacco with reduced amounts of Rubisco activase

The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. To elucidate its role in maintaining CO(2) assimilation rate at high temperature, we examined the temperature response of CO(2) assimilation rate at 380 microL L(-1) CO(2) concentration (A(380)) and Rubisco activation state in wild-type and transgenic tobacco (Nicotiana tabacum) with reduced Rubisco activase content grown at either 20 degrees C or 30 degrees C. Analyses of gas exchange and chlorophyll fluorescence showed that in the wild type, A(380) was limited by ribulose 1,5-bisphosphate regeneration at lower temperatures, whereas at higher temperatures, A(380) was limited by ribulose 1,5-bisphosphate carboxylation irrespective of growth temperatures. Growth temperature induced modest differences in Rubisco activation state that declined with measuring temperature, from mean values of 76% at 15 degrees C to 63% at 40 degrees C in wild-type plants. At measuring temperatures of 25 degrees C and below, an 80% reduction in Rubisco activase content was required before Rubisco activation state was decreased. Above 35 degrees C, Rubisco activation state decreased slightly with more modest decreases in Rubisco activase content, but the extent of the reductions in Rubisco activation state were small, such that a 55% reduction in Rubisco activase content did not alter the temperature sensitivity of Rubisco activation and had no effect on in vivo catalytic turnover rates of Rubisco. There was a strong correlation between Rubisco activase content and Rubisco activation state once Rubisco activase content was less that 20% of wild type at all measuring temperatures. We conclude that reduction in Rubisco activase content does not lead to an increase in the temperature sensitivity of Rubisco activation state in tobacco.

Rubisco in planta kcat is regulated in balance with photosynthetic electron transport

Site turnover rate (k(cat)) of Rubisco was measured in intact leaves of different plants. Potato (Solanum tuberosum L.) and birch (Betula pendula Roth.) leaves were taken from field-growing plants. Sunflower (Helianthus annuus L.), wild type (wt), Rubisco-deficient (-RBC), FNR-deficient (-FNR), and Cyt b(6)f deficient (-CBF) transgenic tobacco (Nicotiana tabacum L.) were grown in a growth chamber. Rubisco protein was measured with quantitative SDS-PAGE and FNR protein content with quantitative immunoblotting. The Cyt b(6)f level was measured in planta by maximum electron transport rate and the photosystem I (PSI) content was assessed by titration with far-red light. The CO(2) response of Rubisco was measured in planta with a fast-response gas exchange system at maximum ribulose 1,5-bisphosphate concentration. Reaction site k(cat) was calculated from V(m) and Rubisco content. Biological variation of k(cat) was significant, ranging from 1.5 to 4 s(-1) in wt, but was >6 s(-1) at 23 degrees C in -RBC leaves. The lowest k(cat) of 0.5 s(-1) was measured in -FNR and -CBF plants containing sufficient Rubisco but having slow electron transport rates. Plotting k(cat) against PSI per Rubisco site resulted in a hyperbolic relationship where wt plants are on the initial slope. A model is suggested in which Rubisco Activase is converted into an active ATP-form on thylakoid membranes with the help of a factor related to electron transport. The activation of Rubisco is accompanied by the conversion of the ATP-form into an inactive ADP-form. The ATP and ADP forms of Activase shuttle between thylakoid membranes and stromally-located Rubisco. In normal wt plants the electron transport-related activation of Activase is rate-limiting, maintaining 50-70% Rubisco sites in the inactive state.

Independently silencing two photosynthetic proteins in Nicotiana attenuata has different effects on herbivore resistance

Insect attack frequently down-regulates photosynthetic proteins. To understand how this influences the plant-insect interaction, we transformed Nicotiana attenuata to independently silence ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) activase (RCA) and RuBPCase and selected lines whose photosynthetic capacity was similarly reduced. Decreases in plant growth mirrored the decreases in photosynthesis, but the effects on herbivore performance differed. Both generalist (Spodoptera littoralis) and specialist (Manduca sexta) larvae grew larger on RCA-silenced plants, which was consistent with decreased levels of trypsin protease inhibitors and diterpene glycosides and increased levels of RuBPCase, the larvae's main dietary protein. RCA-silenced plants were impaired in their attack-elicited jasmonate (JA)-isoleucine (Ile)/leucine levels, but RuBPCase-silenced plants were not, a deficiency that could not be restored by supplementation with Ile or attributed to lower transcript levels of JAR4/6, the key enzyme for JA-Ile conjugation. From these results, we infer that JA-Ile/leucine signaling and the herbivore resistance traits elicited by JA-Ile are influenced by adenylate charge, or more generally, carbon availability in RCA- but not RuBPCase-silenced plants. Growth of generalist larvae on RuBPCase-silenced plants did not differ from growth on empty vector controls, but the specialist larvae grew faster on RuBPCase-silenced plants, which suggests that the specialist can better tolerate the protein deficiency resulting from RuBPCase silencing than the generalist can. We conclude that the plant-herbivore interaction is more influenced by the particular mechanisms that reduce photosynthetic capacity after herbivore attack than by the magnitude of the decrease, which highlights the value of understanding defense mechanisms in evaluating growth-defense tradeoffs.

Phosphorylation of phosphoenolpyruvate carboxylase is not essential for high photosynthetic rates in the C4 species Flaveria bidentis

Phosphoenolpyruvate carboxylase (PEPC; EC4.1.1.31) plays a key role during C(4) photosynthesis. The enzyme is activated by metabolites such as glucose-6-phosphate and inhibited by malate. This metabolite sensitivity is modulated by the reversible phosphorylation of a conserved serine residue near the N terminus in response to light. The phosphorylation of PEPC is modulated by a protein kinase specific to PEPC (PEPC-PK). To explore the role PEPC-PK plays in the regulation of C(4) photosynthetic CO(2) fixation, we have transformed Flaveria bidentis (a C(4) dicot) with antisense or RNA interference constructs targeted at the mRNA of this PEPC-PK. We generated several independent transgenic lines where PEPC is not phosphorylated in the light, demonstrating that this PEPC-PK is essential for the phosphorylation of PEPC in vivo. Malate sensitivity of PEPC extracted from these transgenic lines in the light was similar to the malate sensitivity of PEPC extracted from darkened wild-type leaves but greater than the malate sensitivity observed in PEPC extracted from wild-type leaves in the light, confirming the link between PEPC phosphorylation and the degree of malate inhibition. There were, however, no differences in the CO(2) and light response of CO(2) assimilation rates between wild-type plants and transgenic plants with low PEPC phosphorylation, showing that phosphorylation of PEPC in the light is not essential for efficient C(4) photosynthesis for plants grown under standard glasshouse conditions. This raises the intriguing question of what role this complexly regulated reversible phosphorylation of PEPC plays in C(4) photosynthesis.

Low stomatal and internal conductance to CO2 versus Rubisco deactivation as determinants of the photosynthetic decline of ageing evergreen leaves

A novel A-Ci curve (net CO2 assimilation rate of a leaf -An- as a function of its intercellular CO2 concentration -Ci) analysis method (Plant, Cell & Environment 27, 137-153, 2004) was used to estimate the CO2 transfer conductance (gi) and the maximal carboxylation (Vcmax) and electron transport (Jmax) potentials of ageing, non-senescing Pseudotsuga menziesii leaves in relation to their nitrogen (N) content and protein and pigment composition. Both gi and the stomatal conductance (gsc) of leaves were closely coupled to Vcmax, Jmax and An with all variables decreasing with increasing leaf age. Consequently, both Ci and Cc (chloroplastic CO2 concentration) remained largely conserved through successive growing seasons. The N content of leaves, as well as the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other sodium dodecyl sulfate-soluble proteins, increased during the first three growing seasons, then stabilized or decreased only slightly afterwards. Thus, the age-related photosynthetic nitrogen use efficiency (PNUE) decline of leaves was not a consequence of decreased allocation of N towards Rubisco and other proteins involved in bioenergetics and light harvesting. Rather, loss of photosynthetic capacity was the result of the decreased activation state of Rubisco and proportional down-regulation of electron transport towards the photosynthetic carbon reduction (PCR) and photorespiratory (PCO) cycles in response to a reduction of CO2 supply to the chloroplasts' stroma. This study emphasizes the regulatory potential and homeostaticity of Cc- rather than photosynthetic metabolites or Ci- in relation to the commonly observed correlation between photosynthesis and gsc.

Antisense inhibition of Rubisco activase increases Rubisco content and alters the proportion of Rubisco activase in stroma and thylakoids in chloroplasts of rice leaves

BACKGROUND AND AIMS

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase (RCA) is a nuclear-encoded chloroplast protein that modifies the conformation of Rubisco, releases inhibitors from active sites, and increases enzymatic activity. It appears to have other functions, e.g. in gibberellin signalling and as a molecular chaperone, which are related to its distribution within the chloroplast. The aim of this research was to resolve uncertainty about the localization of RCA, and to determine whether the distributions of Rubisco and RCA were altered when RCA content was reduced. The monocotyledon, Oryza sativa was used as a model species.

METHODS

Gas exchange and Rubisco were measured, and the sub-cellular locations of Rubisco and RCA were determined using immunogold-labelling electron microscopy, in wild-type and antisense rca rice plants.

KEY RESULTS

In antisense rca plants, net photosynthetic rate and the initial Rubisco activity decreased much less than RCA content. Immunocytolocalization showed that Rubisco in wild-type and antisense plants was localized in the stroma of chloroplasts. However, the amount of Rubisco in the antisense rca plants was greater than in the wild-type plants. RCA was detected in both the chloroplast stroma and in the thylakoid membranes of wild-type plants. The percentage of RCA labelling in the thylakoid membrane was shown to be substantially decreased, while the fraction in the stroma was increased, by the antisense rca treatment.

CONCLUSIONS

From the changes in RCA distribution and alterations in Rubisco activity, RCA in the stroma of the chloroplast probably contributes to the activation of Rubisco, and RCA in thylakoids compensates for the reduction of RCA in the stroma, allowing steady-state photosynthesis to be maintained when RCA is depleted. RCA may also have a second role in protecting membranes against environmental stresses as a chaperone.

Reductions of Rubisco activase by antisense RNA in the C4 plant Flaveria bidentis reduces Rubisco carbamylation and leaf photosynthesis

To function, the catalytic sites of Rubisco (EC 4.1.1.39) need to be activated by the reversible carbamylation of a lysine residue within the sites followed by rapid binding of magnesium. The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. This enzyme is thought to aid the release of sugar phosphate inhibitors from Rubisco's catalytic sites, thereby influencing carbamylation. In C3 species, Rubisco operates in a low CO2 environment, which is suboptimal for both catalysis and carbamylation. In C4 plants, Rubisco is located in the bundle sheath cells and operates in a high CO2 atmosphere close to saturation. To explore the role of Rubisco activase in C4 photosynthesis, activase levels were reduced in Flaveria bidentis, a C4 dicot, by transformation with an antisense gene directed against the mRNA for Rubisco activase. Four primary transformants with very low activase levels were recovered. These plants and several of their segregating T1 progeny required high CO2 (>1 kPa) for growth. They had very low CO2 assimilation rates at high light and ambient CO2, and only 10% to 15% of Rubisco sites were carbamylated at both ambient and very high CO2. The amount of Rubisco was similar to that of wild-type plants. Experiments with the T1 progeny of these four primary transformants showed that CO2 assimilation rate and Rubisco carbamylation were severely reduced in plants with less than 30% of wild-type levels of activase. We conclude that activase activity is essential for the operation of the C4 photosynthetic pathway.

Low temperature effects on grapevine photosynthesis: the role of inorganic phosphate

The photosynthetic response of grapevine leaves (Vitis vinifera L. cv. Riesling) to low temperature was studied to determine the role of end-product limitation and orthophosphate (P_i) recycling to the chloroplast under these conditions. As reported previously, the response of photosynthesis in air to stomatal conductance declined at temperatures below 15°C, suggesting that at low temperatures inhibition of photosynthesis in grapevine has a strong non-stomatal component. Stimulation of carbon assimilation at ambient CO₂ by reducing O₂ from 21 to 2 kPa, O₂ declined to zero below 15°C, a phenomenon often associated with a restriction in photosynthesis due to end-product-synthesis limitation. This stimulation could be restored by feeding P_i. Photosynthesis in leaf disks at both high and low irradiances in non-photorespiratory conditions (1% CO₂) was highly sensitive to reductions in temperature. Below 15°C, feeding Pi caused a large stimulation of photosynthetic O₂ evolution. Metabolite measurements indicated that despite a decline in Rubisco carbamylation state, ribulose 1,5-bisphosphate (RuBP) levels dropped at low temperature and the ratio of 3-phosphoglycerate (3-PGA) to triose phosphate (TP) remained largely unchanged. These results suggest that grapevine-leaf photosynthesis is severely restricted at low temperature by non-stomatal mechanisms. The return of P_i to the chloroplast plays an important role in this limitation but a coordinated set of regulatory processes maintain a homeostasis of phosphorylated sugar levels.

Proteome reference maps of vegetative tissues in pea. An investigation of nitrogen mobilization from leaves during seed filling

A proteomic approach was used to analyze protein changes during nitrogen mobilization (N mobilization) from leaves to filling seeds in pea (Pisum sativum). First, proteome reference maps were established for mature leaves and stems. They displayed around 190 Coomassie Blue-stained spots with pIs from 4 to 7. A total of 130 spots were identified by mass spectrometry as corresponding to 80 different proteins implicated in a variety of cellular functions. Although the leaf proteome map contained more abundant spots, corresponding to proteins involved in energy/carbon metabolism, than the stem map, their comparison revealed a highly similar protein profile. Second, the leaf proteome map was used to analyze quantitative variations in leaf proteins during N mobilization. Forty percent of the spots showed significant changes in their relative abundance in the total protein extract. The results confirmed the importance of Rubisco as a source of mobilizable nitrogen, and suggested that in pea leaves the rate of degradation of Rubisco may vary throughout N mobilization. Correlated with the loss of Rubisco was an increase in relative abundance of chloroplastic protease regulatory subunits. Concomitantly, the relative abundance of some proteins related to the photosynthetic apparatus (Rubisco activase, Rubisco-binding proteins) and of several chaperones increased. A role for these proteins in the maintenance of a Rubisco activation state and in the PSII repair during the intense proteolytic activity within the chloroplasts was proposed. Finally, two 14-3-3-like proteins, with a potential regulatory role, displayed differential expression patterns during the massive remobilization of nitrogen.

Photosynthesis and growth of tobacco with a substituted bacterial Rubisco mirror the properties of the introduced enzyme

Complete replacement, by biolistic plastid transformation, of the hexadecameric ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of tobacco (Nicotiana tabacum) with the dimeric version from the bacterium, Rhodospirillum rubrum, resulted in fully autotrophic and reproductive tobacco plants that required high CO(2) concentrations to grow (Whitney SM, Andrews TJ [2001] Proc Natl Acad Sci USA 98: 14738-14743). Growth and photosynthesis of these plants was compared with that of nontransformed tobacco and other controls where the rbcL gene for the large subunit of tobacco Rubisco was linked to the aadA selectable-marker gene, simulating the gene arrangement of the transformants with R. rubrum Rubisco. An arrangement of the rbcL and aadA genes that gave rise to an abundant monocistronic rbcL transcript and a one-fifth as abundant bicistronic rbcL-aadA transcript had Rubisco levels and photosynthetic properties similar to those of nontransformed tobacco. Direct linkage of the rbcL and aadA genes, resulting in exclusive production of a bicistronic mRNA transcript analogous to that of the transformants with R. rubrum Rubisco, reduced transcript abundance and tobacco Rubisco content. The analogous transcript with the R. rubrum rbcM gene substituted for rbcL was not only reduced in abundance, but was also translated less efficiently. The photosynthetic rates of the transformants and controls were measured at high CO(2) concentrations, using a mass spectrometric method. The rates and their responses to atmospheric CO(2) concentration mirrored the amounts and the kinetic properties of the Rubiscos present. The contents of total nitrogen, carbohydrates, and photosynthetic metabolites of the leaves were also consistent with the content and type of Rubisco.

Rubisco activase - Rubisco's catalytic chaperone

The current status of research on the structure, regulation, mechanism and importance of Rubisco activase is reviewed. The activase is now recognized to be a member of the AAA(+) family, whose members participate in macromolecular complexes that perform diverse chaperone-like functions. The conserved nucleotide-binding domain of AAA(+) family members appears to have a common fold that when applied to the activase is generally consistent with previous site-directed mutagenesis studies of the activase. Regulation of the activase in species containing both isoforms can occur via redox changes in the carboxy-terminus of the larger isoform, mediated by thioredoxin-f, which alters the response of activase to the ratio of ADP to ATP in the stroma. Studies of Rubisco activation in transgenic Arabidopsis plants demonstrated that light modulation is dependent on redox regulation of the larger isoform, providing a model for the regulation in other species. Further insights into the mechanism of the activase have emerged from an analysis of the crystal structures of Rubisco conformational variants and the identification of Rubisco residues that confer specificity in its interaction with the activase. The physiological importance of the activase is reinforced by recent studies indicating that it plays a vital role in the response of photosynthesis to temperature. Rubisco activase is one of a new type of chaperone, which in this case functions to promote and maintain the catalytic activity of Rubisco.

Carbon metabolite feedback regulation of leaf photosynthesis and development

Photosynthesis is regulated as a two-way process. Light regulates the expression of genes for photosynthesis and the activity of the gene products (feedforward control). Rate of end-product use down-stream of the Calvin cycle, determined largely by nutrition and temperature, also affects photosynthetic activity and photosynthetic gene expression (feedback control). Whereas feedforward control ensures efficient light use, feedback mechanisms ensure that carbon flow is balanced through the pathways that produce and consume carbon, so that inorganic phosphate is recycled and nitrogen is distributed optimally to different processes to ensure growth and survival. Actual mechanisms are sketchy and complex, but carbon to nitrogen balance rather than carbon status per se is central to understanding carbon metabolite feedback control of photosynthesis. In addition to determining the activity of the metabolic machinery, carbon metabolite feedback mechanisms also regulate photosynthesis at the leaf level through the regulation of leaf development. This review summarizes the current sketchy, but growing, knowledge of the mechanisms through which carbon metabolite feedback mechanisms regulate leaf photosynthesis.

Plastome-encoded bacterial ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) supports photosynthesis and growth in tobacco

The efficiency with which crop plants use their resources of light, water, and fertilizer nitrogen could be enhanced by replacing their CO(2)-fixing enzyme, d-ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO), with more efficient forms, such as those found in some algae, for example. This important challenge has been frustrated by failure of all previous attempts to substitute a fully functional, foreign RubisCO (efficient or inefficient) into higher plants. This failure could be caused by incompatibility between the plastid-encoded large subunits and the nucleus-encoded small subunits or by inability of the foreign RubisCO subunits to fold or assemble efficiently in the plastid. Mismatch between the regulatory requirements of the foreign RubisCO and conditions in the chloroplast also might render the substituted enzyme inactive but, previously, it has not been possible to test this. To answer the general question of whether a foreign RubisCO can support photosynthesis in a plant, we used plastid transformation to replace RubisCO in tobacco with the simple homodimeric form of the enzyme from the alpha-proteobacterium, Rhodospirillum rubrum, which has no small subunits and no special assembly requirements. The transplastomic plants so obtained are fully autotrophic and reproductive but require CO(2) supplementation, consistent with the kinetic properties of the bacterial RubisCO. This establishes that the activity of a RubisCO from a very different phylogeny can be integrated into chloroplast photosynthetic metabolism without prohibitive problems.

Increased heat sensitivity of photosynthesis in tobacco plants with reduced Rubisco activase

High temperature inhibits photosynthesis by several mechanisms including deactivation of Rubisco. The inhibition of photosynthesis by high temperature and its relationship to Rubisco deactivation was studied using tobacco (Nicotiana tabaccum L. cv W38) transformed with a Rubisco activase gene inserted in the antisense orientation and untransformed controls. High temperature (42 degrees C) reduced photosynthesis in both lines of plants. However, photosynthesis recovered nearly completely in wild-type plants and very little in plants lacking Rubisco activase. The F(0)' level of chlorophyll fluorescence decreased and q(N) increased in the control plants during heating. In the antisense plants, q(N) was always high and F(0)' increased slightly during heat stress. NADP-malate dehydrogenase activation was unaffected by heat stress in control plants but was increased in the transgenic plants, consistent with a high redox status in the chloroplast. In wild-type plants, the inhibition of photosynthesis could be explained by a reversible decarbamylation of Rubisco and an acceptor-side limitation imposed on photosynthetic electron transport. However, in the anti-activase plants, carbamylation was low and constant and could not explain how photosynthesis was reduced at high temperature. Because ribulose bisphosphate was saturating at high temperature, the reduction in photosynthesis must have been caused by some impairment of Rubisco function not reflected in measurements of activation state or carbamylation status. This in vivo Rubisco impairment was not relieved upon return to lower temperature. We speculate that the reversible decarbamylation of Rubisco at moderately high temperature may be a protective mechanism by which the plant avoids more serious effects on Rubisco and the rest of the photosynthetic apparatus.

Differential regulation of plastidial and cytosolic isoforms of peptide methionine sulfoxide reductase in Arabidopsis

We report the characterization of two members of a gene family from Arabidopsis that encode, respectively, cytosolic (cPMSR) and plastid-targeted (pPMSR) isoforms of the oxidative-stress-repair enzyme peptide methionine sulfoxide reductase. Overexpression of these proteins in Escherichia coli confirmed that each had PMSR enzyme activity with a synthetic substrate, N-acetyl-[(3)H]-methionine sulfoxide, or a biological substrate, alpha-1 proteinase inhibitor. The pPMSR was imported into intact chloroplasts in vitro with concomitant cleavage of its approximately 5-kD N-terminal signal peptide. The two PMSR isoforms exhibited divergent pH optima, tissue localization, and responses to developmental and environmental effects. Analysis of the Arabidopsis database indicated that there are probably at least two p-pmsr-like genes and three c-pmsr-like genes in the Arabidopsis genome. Expression of the p-pmsr genes and their protein products was restricted to photosynthetic tissues and was strongly induced following illumination of etiolated seedlings. In contrast, the c-pmsr genes were expressed at moderate levels in all tissues and were only weakly affected by light. Exposure to a variety of biotic and abiotic stresses showed relatively little effect on pmsr gene expression, with the exception of leaves subjected to a long-term exposure to the cauliflower mosaic virus. These leaves showed a strong induction of the c-pmsr gene after 2 to 3 weeks of chronic pathogen infection. These data suggest novel roles for PMSR in photosynthetic tissues and in pathogen defense responses in plants.

The role of chloroplast electron transport and metabolites in modulating Rubisco activity in tobacco. Insights from transgenic plants with reduced amounts of cytochrome b/f complex or glyceraldehyde 3-phosphate dehydrogenase

Leaf metabolites, adenylates, and Rubisco activation were studied in two transgenic tobacco (Nicotiana tabacum L. cv W38) types. Plants with reduced amounts of cytochrome b/f complex (anti-b/f) have impaired electron transport and a low transthylakoid pH gradient that restrict ATP and NADPH synthesis. Plants with reduced glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH) have a decreased capacity to use ATP and NADPH in carbon assimilation. The activation of the chloroplast NADP-malate dehydrogenase decreased in anti-b/f plants, indicating a low NADPH/NADP(+) ratio. The whole-leaf ATP/ADP in anti-b/f plants was similar to wild type, while it increased in anti-GAPDH plants. In both plant types, the CO(2) assimilation rates decreased with decreasing ribulose 1, 5-bisphosphate concentrations. In anti-b/f plants, CO(2) assimilation was further compromised by reduced carbamylation of Rubisco, whereas in anti-GAPDH plants the carbamylation remained high even at subsaturating ribulose 1,5-bisphosphate concentrations. We propose that the low carbamylation in anti-b/f plants is due to reduced activity of Rubisco activase. The results suggest that light modulation of activase is not directly mediated via the electron transport rate or stromal ATP/ADP, but some other manifestation of the balance between electron transport and the consumption of its products. Possibilities include the transthylakoid pH gradient and the reduction state of the acceptor side of photosystem I and/or the degree of reduction of the thioredoxin pathway.

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

Trypsin-catalysed cleavage of purified ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the resultant irreversible loss of carboxylase activity were prevented by prior incubation with the naturally occurring nocturnal Rubisco inhibitor 2'-carboxy-D-arabitinol 1-phosphate (CA1P), as well as with ribulose 1,5-bisphosphate (RuBP), Mg2+ and CO2. CA1P also protected Rubisco from loss of activity caused by carboxypeptidase A. When similar experiments were carried out using soluble chloroplast proteases, CA1P was again able to protect Rubisco against proteolytic degradation and the consequent irreversible loss of catalytic activity. Thus, CA1P prevents the proteolytic breakdown of Rubisco by endogenous and exogenous proteases. In this way, CA1P may affect the amounts of Rubisco protein available for photosynthetic CO2 assimilation. Rubisco turnover (in the presence of RuBP, Mg2+ and CO2) may confer similar protection against proteases in the light.

Directed mutation of the Rubisco large subunit of tobacco influences photorespiration and growth

The gene for the large subunit of Rubisco was specifically mutated by transforming the chloroplast genome of tobacco (Nicotiana tabacum). Codon 335 was altered to encode valine instead of leucine. The resulting mutant plants could not grow without atmospheric CO2 enrichment. In 0.3% (v/v) CO2, the mutant and wild-type plants produced similar amounts of Rubisco but the extent of carbamylation was nearly twice as great in the mutants. The mutant enzyme's substrate-saturated CO2-fixing rate and its ability to distinguish between CO2 and O2 as substrates were both reduced to 25% of the wild type's values. Estimates of these parameters obtained from kinetic assays with the purified mutant enzyme were the same as those inferred from measurements of photosynthetic gas exchange with leaves of mutant plants. The Michaelis constants for CO2, O2, and ribulose-1,5-bisphosphate were reduced and the mutation enhanced oxygenase activity at limiting O2 concentrations. Consistent with the reduced CO2 fixation rate at saturating CO2, the mutant plants grew slower than the wild type but they eventually flowered and reproduced apparently normally. The mutation and its associated phenotype were inherited maternally. The chloroplast-transformation strategy surmounts previous obstacles to mutagenesis of higher-plant Rubisco and allows the consequences for leaf photosynthesis to be assessed.

Patterns and mechanisms of growth of fifth-instar Manduca sexta caterpillars following exposure to low- or high-protein food during early instars

For many insect herbivores, variation in protein availability is a pervasive part of the environment. I explore how variable protein availability affects growth rates of fifth-instar Manduca sexta caterpillars and how growth is related to behavior and physiology. Groups of larvae were reared on low- or high-protein artificial diets (5.9% and 17.7% casein by dry weight, respectively) and then transferred in the fifth instar to the same or opposite diet. During or after the 24-h period following transfer, I measured growth rate, consumption rate, growth efficiency, midgut proteolytic activity, and masses of midgut contents and tissues. Fifth-instar caterpillars reared in earlier instars on high-protein diet grew about 20% more rapidly over 24 h than did caterpillars reared on low-protein diet. This growth pattern appears to be caused by differences in consumption and growth efficiency: caterpillars reared on high protein consumed more food, and used it more efficiently, than did caterpillars reared on low-protein diet. Over the short term (24 h), in contrast, fifth instars that received low-protein diet grew as rapidly as caterpillars that received high-protein diet. Increased (compensatory) consumption appears to be the primary mechanism by which caterpillars consuming low-protein food maintained growth rates.

Inhibition and acclimation of photosynthesis to heat stress is closely correlated with activation of ribulose-1,5-bisphosphate Carboxylase/Oxygenase

Increasing the leaf temperature of intact cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) plants caused a progressive decline in the light-saturated CO2-exchange rate (CER). CER was more sensitive to increased leaf temperature in wheat than in cotton, and both species demonstrated photosynthetic acclimation when leaf temperature was increased gradually. Inhibition of CER was not a consequence of stomatal closure, as indicated by a positive relationship between leaf temperature and transpiration. The activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is regulated by Rubisco activase, was closely correlated with temperature-induced changes in CER. Nonphotochemical chlorophyll fluorescence quenching increased with leaf temperature in a manner consistent with inhibited CER and Rubisco activation. Both nonphotochemical fluorescence quenching and Rubisco activation were more sensitive to heat stress than the maximum quantum yield of photochemistry of photosystem II. Heat stress led to decreased 3-phosphoglyceric acid content and increased ribulose-1, 5-bisphosphate content, which is indicative of inhibited metabolite flow through Rubisco. We conclude that heat stress inhibited CER primarily by decreasing the activation state of Rubisco via inhibition of Rubisco activase. Although Rubisco activation was more closely correlated with CER than the maximum quantum yield of photochemistry of photosystem II, both processes could be acclimated to heat stress by gradually increasing the leaf temperature.