Effects of Ambient CO2 Concentration on Growth and Nitrogen Use in Tobacco (Nicotiana tabacum) Plants Transformed with an Antisense Gene to the Small Subunit of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

Decreased Photosynthetic Efficiency in Nicotiana tabacum L. under Transient Heat Stress

Heat stress is an abiotic factor that affects the photosynthetic parameters of plants. In this study, we examined the photosynthetic mechanisms underlying the rapid response of tobacco plants to heat stress in a controlled environment. To evaluate transient heat stress conditions, changes in photochemical, carboxylative, and fluorescence efficiencies were measured using an infrared gas analyser (IRGA Licor 6800) coupled with chlorophyll a fluorescence measurements. Our findings indicated that significant disruptions in the photosynthetic machinery occurred at 45 °C for 6 h following transient heat treatment, as explained by 76.2% in the principal component analysis. The photosynthetic mechanism analysis revealed that the dark respiration rate (Rd and Rd*CO2) increased, indicating a reduced potential for carbon fixation during plant growth and development. When the light compensation point (LCP) increased as the light saturation point (LSP) decreased, this indicated potential damage to the photosystem membrane of the thylakoids. Other photosynthetic parameters, such as AMAX, VCMAX, JMAX, and ΦCO2, also decreased, compromising both photochemical and carboxylative efficiencies in the Calvin-Benson cycle. The energy dissipation mechanism, as indicated by the NPQ, qN, and thermal values, suggested that a photoprotective strategy may have been employed. However, the observed transitory damage was a result of disruption of the electron transport rate (ETR) between the PSII and PSI photosystems, which was initially caused by high temperatures. Our study highlights the impact of rapid temperature changes on plant physiology and the potential acclimatisation mechanisms under rapid heat stress. Future research should focus on exploring the adaptive mechanisms involved in distinguishing mutants to improve crop resilience against environmental stressors.

CRISPR-Cas9-Mediated Mutagenesis of the Rubisco Small Subunit Family in Nicotiana tabacum

Engineering the small subunit of the key CO2-fixing enzyme Rubisco (SSU, encoded by rbcS) in plants currently poses a significant challenge, as many plants have polyploid genomes and SSUs are encoded by large multigene families. Here, we used CRISPR-Cas9-mediated genome editing approach to simultaneously knock-out multiple rbcS homologs in the model tetraploid crop tobacco (Nicotiana tabacum cv. Petit Havana). The three rbcS homologs rbcS_S1a, rbcS_S1b and rbcS_T1 account for at least 80% of total rbcS expression in tobacco. In this study, two multiplexing guide RNAs (gRNAs) were designed to target homologous regions in these three genes. We generated tobacco mutant lines with indel mutations in all three genes, including one line with a 670 bp deletion in rbcS-T1. The Rubisco content of three selected mutant lines in the T1 generation was reduced by ca. 93% and mutant plants accumulated only 10% of the total biomass of wild-type plants. As a second goal, we developed a proof-of-principle approach to simultaneously introduce a non-native rbcS gene while generating the triple SSU knockout by co-transformation into a wild-type tobacco background. Our results show that CRISPR-Cas9 is a viable tool for the targeted mutagenesis of rbcS families in polyploid species and will contribute to efforts aimed at improving photosynthetic efficiency through expression of superior non-native Rubisco enzymes in plants.

High-throughput field phenotyping using hyperspectral reflectance and partial least squares regression (PLSR) reveals genetic modifications to photosynthetic capacity

Spectroscopy is becoming an increasingly powerful tool to alleviate the challenges of traditional measurements of key plant traits at the leaf, canopy, and ecosystem scales. Spectroscopic methods often rely on statistical approaches to reduce data redundancy and enhance useful prediction of physiological traits. Given the mechanistic uncertainty of spectroscopic techniques, genetic modification of plant biochemical pathways may affect reflectance spectra causing predictive models to lose power. The objectives of this research were to assess over two separate years, whether a predictive model can represent natural and imposed variation in leaf photosynthetic potential for different crop cultivars and genetically modified plants, to assess the interannual capabilities of a partial least square regression (PLSR) model, and to determine whether leaf N is a dominant driver of photosynthesis in PLSR models. In 2016, a PLSR analysis of reflectance spectra coupled with gas exchange data was used to build predictive models for photosynthetic parameters including maximum carboxylation rate of Rubisco (V _c,max ), maximum electron transport rate (J _max ) and percentage leaf nitrogen ([N]). The model was developed for wild type and genetically modified plants that represent a wide range of photosynthetic capacities. Results show that hyperspectral reflectance accurately predicted V _c,max, J _max and [N] for all plants measured in 2016. Applying these PLSR models to plants grown in 2017 resulted in a strong predictive ability relative to gas exchange measurements for V _c,max, but not for J _max, and not for genotypes unique to 2017. Building a new model including data collected in 2017 resulted in more robust predictions, with R² increases of 17% for V _c,max . and 13% J _max . Plants generally have a positive correlation between leaf nitrogen and photosynthesis, however, tobacco with reduced Rubisco (SSuD) had significantly higher [N] despite much lower V _c,max. The PLSR model was able to accurately predict both lower V _c,max and higher leaf [N] for this genotype suggesting that the spectral based estimates of V _c,max and leaf nitrogen [N] are independent. These results suggest that the PLSR model can be applied across years, but only to genotypes used to build the model and that the actual mechanism measured with the PLSR technique is not directly related to leaf [N]. The success of the leaf-scale analysis suggests that similar approaches may be successful at the canopy and ecosystem scales but to use these methods across years and between genotypes at any scale, application of accurately populated physical based models based on radiative transfer principles may be required.

Synthetic biology approaches for improving photosynthesis

The phenomenal increase in agricultural yields that we have witnessed in the last century has slowed down as we approach the limits of selective breeding and optimization of cultivation techniques. To support the yield increase required to feed an ever-growing population, we will have to identify new ways to boost the efficiency with which plants convert light into biomass. This challenge could potentially be tackled using state-of-the-art synthetic biology techniques to rewrite plant carbon fixation. In this review, we use recent studies to discuss and demonstrate different approaches for enhancing carbon fixation, including engineering Rubisco for higher activity, specificity, and activation; changing the expression level of enzymes within the Calvin cycle to avoid kinetic bottlenecks; introducing carbon-concentrating mechanisms such as inorganic carbon transporters, carboxysomes, and C4 metabolism; and rewiring photorespiration towards more energetically efficient routes or pathways that do not release CO2. We conclude by noting the importance of prioritizing and combining different approaches towards continuous and sustainable increase of plant productivities.

One crop breeding cycle from starvation? How engineering crop photosynthesis for rising CO2 and temperature could be one important route to alleviation

Global climate change is likely to severely impact human food production. This comes at a time when predicted demand for primary foodstuffs by a growing human population and changing global diets is already outpacing a stagnating annual rate of increase in crop productivity. Additionally, the time required by crop breeding and bioengineering to release improved varieties to farmers is substantial, meaning that any crop improvements needed to mitigate food shortages in the 2040s would need to start now. In this perspective, the rationale for improvements in photosynthetic efficiency as a breeding objective for higher yields is outlined. Subsequently, using simple simulation models it is shown how predicted changes in temperature and atmospheric [CO2] affect leaf photosynthetic rates. The chloroplast accounts for the majority of leaf nitrogen in crops. Within the chloroplast about 25% of nitrogen is invested in the carboxylase, Rubisco, which catalyses the first step of CO2 assimilation. Most of the remaining nitrogen is invested in the apparatus to drive carbohydrate synthesis and regenerate ribulose-1:5-bisphosphate (RuBP), the CO2-acceptor molecule at Rubisco. At preindustrial [CO2], investment in these two aspects may have been balanced resulting in co-limitation. At today's [CO2], there appears to be over-investment in Rubisco, and despite the counter-active effects of rising temperature and [CO2], this imbalance is predicted to worsen with global climate change. By breeding or engineering restored optimality under future conditions increased productivity could be achieved in both tropical and temperate environments without additional nitrogen fertilizer. Given the magnitude of the potential shortfall, better storage conditions, improved crop management and better crop varieties will all be needed. With the short time-scale at which food demand is expected to outpace supplies, all available technologies to improve crop varieties, from classical crop breeding to crop genetic engineering should be employed. This will require vastly increased public and private investment to support translation of first discovery in laboratories to replicated field trials, and an urgent re-evaluation of regulation of crop genetic engineering.

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.

Whole-plant growth and N utilization in transgenic rice plants with increased or decreased Rubisco content under different CO2 partial pressures

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) strongly limits photosynthesis at lower CO2 concentration [CO2] whereas [corrected] Rubisco limitation is cancelled by elevated [CO2]. Therefore, increase or reduction in Rubisco content by transformation with a sense or an antisense RBCS construct are expected to alter the biomass production under different CO2 levels. RBCS-sense (125% Rubisco of wild-type) and -antisense (35% Rubisco of wild-type) rice (Oryza sativa L.) plants were grown for 63 days at three different CO2 levels: low [CO2] (28 Pa), normal [CO2] (40 Pa) and elevated [CO2] (120 Pa). The biomass of RBCS-sense plants was 32% and 15% greater at low [CO2] and normal [CO2] than that of the wild-type plants, respectively, but did not differ at elevated [CO2]. Conversely, the biomass of RBCS-antisense plants was the smallest at low [CO2]. Thus, overproduction of Rubisco was effective for biomass production at low [CO2]. Greater biomass production at low [CO2] in RBCS-sense plants was caused by an increase in the net assimilation rate, and associated with an increase in the amount of N uptake. Furthermore, Rubisco overproduction in RBCS-sense plants was also promoted at low [CO2]. Although it seems that low [CO2]-growth additionally stimulates the effect of RBCS overexpression, such a phenomenon observed at low [CO2] was mediated through an increase in total leaf N content. Thus, the dependence of the growth improvement in RBCS-sense rice on growth [CO2] was closely related to the degree of Rubisco overproduction which was accompanied not only by leaf N content but also by whole plant N content.

The challenges of commercializing second-generation transgenic crop traits necessitate the development of international public sector research infrastructure

It has been 30 years since the first transformation of a gene into a plant species, and since that time a number of biotechnology products have been developed, with the most important being insect- and herbicide-resistant crops. The development of second-generation products, including nutrient use efficiency and tolerance to important environmental stressors such as drought, has, up to this time, been less successful. This is in part due to the inherent complexities of these traits and in part due to limitations in research infrastructure necessary for public sector researchers to test their best ideas. Here we discuss lessons from previous work in the generation of the first-generation traits, as well as work from our labs and others on identifying genes for nitrogen use efficiency. We then describe some of the issues that have impeded rapid progress in this area. Finally, we propose the type of public sector organization that we feel is necessary to make advances in important second-generation traits such as nitrogen use efficiency.

Temperature response of in vivo Rubisco kinetics and mesophyll conductance in Arabidopsis thaliana: comparisons to Nicotiana tabacum

Biochemical models are used to predict and understand the response of photosynthesis to rising temperatures and CO2 partial pressures. These models require the temperature dependency of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and mesophyll conductance to CO2 (g(m)). However, it is not known how the temperature response of Rubisco kinetics differs between species, and comprehensive in vivo Rubisco kinetics that include gm have only been determined in the warm-adapted Nicotiana tabacum. Here, we measured the temperature response of Rubisco kinetics and gm in N. tabacum and the cold-adapted Arabidopsis thaliana using gas exchange and (13)CO2 isotopic discrimination on plants with genetically reduced levels of Rubisco. While the individual Rubisco kinetic parameters in N. tabacum and A. thaliana were similar across temperatures, they collectively resulted in significantly different modelled rates of photosynthesis. Additionally, gm increased with temperature in N. tabacum but not in A. thaliana. These findings highlight the importance of considering species-dependent differences in Rubisco kinetics and gm when modelling the temperature response of photosynthesis.

Engineering nitrogen use efficient crop plants: the current status

In the last 40 years the amount of synthetic nitrogen (N) applied to crops has risen drastically, resulting in significant increases in yield but with considerable impacts on the environment. A requirement for crops that require decreased N fertilizer levels has been recognized in the call for a 'Second Green Revolution' and research in the field of nitrogen use efficiency (NUE) has continued to grow. This has prompted a search to identify genes that improve the NUE of crop plants, with candidate NUE genes existing in pathways relating to N uptake, assimilation, amino acid biosynthesis, C/N storage and metabolism, signalling and regulation of N metabolism and translocation, remobilization and senescence. Herein is a review of the approaches taken to determine possible NUE candidate genes, an overview of experimental study of these genes as effectors of NUE in both cereal and non-cereal plants and the processes of commercialization of enhanced NUE crop plants. Patents issued regarding increased NUE in plants as well as gene pyramiding studies are also discussed as well as future directions of NUE research.

Metabolome analysis of photosynthesis and the related primary metabolites in the leaves of transgenic rice plants with increased or decreased Rubisco content

Because the comprehensive effects on metabolism by genetic manipulation of leaf Rubisco content are unknown, metabolome analysis was carried out on transgenic rice plants with increased or decreased Rubisco content using the capillary electrophoresis-time-of-flight mass spectrometry (CE-TOFMS) technique. In RBCS-sense plants, an increase in Rubisco content did not improve light-saturated photosynthesis. Glyceraldehyde 3-phosphate and sedoheputulose 7-phosphate levels increased, but ribulose bisphosphate (RuBP), ATP and ADP levels were not affected. It is considered from these results that RuBP regeneration independent of ATP supply became a bottleneck for photosynthesis. In RBCS-antisense plants, a decline in Rubisco content decreased photosynthesis with a substantial accumulation of RuBP. ATP and ADP levels also increased and were associated with increases in the diphosphate and triphosphate compounds of other nucleosides. These results imply that a decline in Rubisco content slowed down the Calvin cycle and that the resultant excess energy of ATP was transferred to other nucleoside diphosphates and triphosphates. The levels of amino acids tended to decline in RBCS-sense plants and increase in RBCS-antisense plants, probably reflecting the demand for Rubisco synthesis. Starch and carbohydrate levels decreased only in RBCS-antisense plants. Thus, genetic manipulation of Rubisco contents widely affected C and N metabolism in rice.

A critical review on the improvement of photosynthetic carbon assimilation in C3 plants using genetic engineering

Global warming is one of the most serious challenges facing us today. It may be linked to the increase in atmospheric CO2 and other greenhouse gases (GHGs), leading to a rise in sea level, notable shifts in ecosystems, and in the frequency and intensity of wild fires. There is a strong interest in stabilizing the atmospheric concentration of CO2 and other GHGs by decreasing carbon emission and/or increasing carbon sequestration. Biotic sequestration is an important and effective strategy to mitigate the effects of rising atmospheric CO2 concentrations by increasing carbon sequestration and storage capacity of ecosystems using plant photosynthesis and by decreasing carbon emission using biofuel rather than fossil fuel. Improvement of photosynthetic carbon assimilation, using transgenic engineering, potentially provides a set of available and effective tools for enhancing plant carbon sequestration. In this review, firstly different biological methods of CO2 assimilation in C3, C4 and CAM plants are introduced and three types of C4 pathways which have high photosynthetic performance and have evolved as CO2 pumps are briefly summarized. Then (i) the improvement of photosynthetic carbon assimilation of C3 plants by transgenic engineering using non-C4 genes, and (ii) the overexpression of individual or multiple C4 cycle photosynthetic genes (PEPC, PPDK, PCK, NADP-ME and NADP-MDH) in transgenic C3 plants (e.g. tobacco, potato, rice and Arabidopsis) are highlighted. Some transgenic C3 plants (e.g. tobacco, rice and Arabidopsis) overexpressing the FBP/SBPase, ictB and cytochrome c6 genes showed positive effects on photosynthetic efficiency and growth characteristics. However, over the last 28 years, efforts to overexpress individual, double or multiple C4 enzymes in C3 plants like tobacco, potato, rice, and Arabidopsis have produced mixed results that do not confirm or eliminate the possibility of improving photosynthesis of C3 plants by this approach. Finally, a prospect is provided on the challenges of enhancing carbon assimilation of C3 plants using transgenic engineering in the face of global warming, and the trends of the most promising approaches to improving the photosynthetic performance of C3 plants.

Rising atmospheric carbon dioxide on grain quality in crop plants

There is a general concern that changes in plant productivity and composition caused by increase in atmospheric CO2 concentration will alter the chemical composition of the grain. This review describes the impact of rising atmospheric CO2 on the grain characteristics in wheat, rice, brassica, mungbean and soybean, which are significantly responsive to the elevated CO2 for their growth, physiology and biochemical processes. The synthesis of the CO2 induced changes in the chemical composition and nutritional qualities of their grains has been discussed. It was demonstrated that the rise in atmospheric CO2 affects the nutritional and industrial application properties of the grains of crop plants. The grain proteins and other nutritionally important constituents significantly reduced, adversely affecting the nutritional and bread making quality in wheat. However, there are evidences suggesting the sustenance of the bread making properties by fertilizer application. Similarly, the CO2 induced changes in the composition of starch in rice grains, result into easy gelatinization and higher viscosity on cooking. These grains bring firmness due to increase in amylose content. Adequately larger size of grains was the outcome of the elevated CO2 effects, in Brassica species. It increased the oil content due to greater acetyl Co A enzyme activity and also help in regulating fatty acid biosynthesis. Some of the nutritionally undesirable fatty acids were significantly reduced in this process, making this oil less harmful for heart patients. The adequate use of fertilizer application and selection pressure of breeders may significantly contribute in developing cultivars, which will counter the adverse effect of rising atmospheric CO2 on grain quality.

Photosynthetic characteristics in genetically modified sense-RbcS silver birch lines

Transgenic silver birch lines carrying extra copies of endogenous small subunit of Rubisco (RbcS)-gene under 35S CaMV promoter were used to study the carbon use efficiency of silver birch (Betula pendula Roth). A five week greenhouse experiment was carried out with four transgenic lines, R3.2, R7.2, E5 and E25, and their corresponding wild types (wt). The first fully developed leaves were used for analyses. Three of the produced lines, R3.2, E5 and E25, differed from the wt lines. Line R3.2 showed an altered growth rhythm; its chlorophyll content, Rubisco amount and activity as well as photosynthetic characteristics were reduced at the beginning of the experiment, which resulted in decreased biomass and growth. In lines E25 and E5, the biomass accumulation was shifted to roots, and in line E25, the total biomass was also reduced. In line E25, the differences were particularly marked in the dry mass, indicating a difference in water use, seen as increased transpiration. Introduction of sense RbcS decreased the Rubisco amount in birch leaves to 80% of wt at times during the tree development, but the lower amount of Rubisco was usually not seen in photosynthesis. The accumulation and distribution of biomass within the plants was altered.

Rubisco content and photosynthesis of leaves at different positions in transgenic rice with an overexpression of RBCS

As ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) activity limits light-saturated photosynthesis under present atmospheric condition, the effects of an overexpression of RBCSon Rubisco content and photosynthesis were examined in the leaves at different positions in rice (Oryza sativa L.). Rubisco content in the transformant was significantly greater in the uppermost, fully expanded leaves but decreased to levels similar to those in wild-type plants in the lower leaves. The mRNA levels of total RBCS and rbcL in these leaves were much less than those in the expanding leaves, where Rubisco synthesis is active, suggesting commensurately low level of synthesis. Although the activation state of Rubisco was lower in the uppermost, fully expanded leaves of the transformant, it recovered to its full level in the lower leaves. As a result, the photosynthetic rate did not differ in leaves at the same position between the transformant and the wild type. Similarly, whole plant biomass did not differ between these genotypes. Thus, we conclude that although the overexpression of RBCS led to an enhancement of Rubisco protein content in the uppermost, fully expanded leaves, it does not result in increased photosynthetic rates or plant biomass, because of an apparent down-regulation in its activation state.

Causes and consequences of variation in leaf mass per area (LMA): a meta-analysis

Here, we analysed a wide range of literature data on the leaf dry mass per unit area (LMA). In nature, LMA varies more than 100-fold among species. Part of this variation (c. 35%) can be ascribed to differences between functional groups, with evergreen species having the highest LMA, but most of the variation is within groups or biomes. When grown in the same controlled environment, leaf succulents and woody evergreen, perennial or slow-growing species have inherently high LMA. Within most of the functional groups studied, high-LMA species show higher leaf tissue densities. However, differences between evergreen and deciduous species result from larger volumes per area (thickness). Response curves constructed from experiments under controlled conditions showed that LMA varied strongly with light, temperature and submergence, moderately with CO2 concentration and nutrient and water stress, and marginally under most other conditions. Functional groups differed in the plasticity of LMA to these gradients. The physiological regulation is still unclear, but the consequences of variation in LMA and the suite of traits interconnected with it are strong. This trait complex is an important factor determining the fitness of species in their environment and affects various ecosystem processes.

The contribution of photosynthesis to the red light response of stomatal conductance

To determine the contribution of photosynthesis on stomatal conductance, we contrasted the stomatal red light response of wild-type tobacco (Nicotiana tabacum 'W38') with that of plants impaired in photosynthesis by antisense reductions in the content of either cytochrome b(6)f complex (anti-b/f plants) or Rubisco (anti-SSU plants). Both transgenic genotypes showed a lowered content of the antisense target proteins in guard cells as well as in the mesophyll. In the anti-b/f plants, CO(2) assimilation rates were proportional to leaf cytochrome b(6)f content, but there was little effect on stomatal conductance and the rate of stomatal opening. To compare the relationship between photosynthesis and stomatal conductance, wild-type plants and anti-SSU plants were grown at 30 and 300 micromol photon m(-2) s(-1) irradiance (low light and medium light [ML], respectively). Growth in ML increased CO(2) assimilation rates and stomatal conductance in both genotypes. Despite the significantly lower CO(2) assimilation rate in the anti-SSU plants, the differences in stomatal conductance between the genotypes were nonsignificant at either growth irradiance. Irrespective of plant genotype, stomatal density in the two leaf surfaces was 2-fold higher in ML-grown plants than in low-light-grown plants and conductance normalized to stomatal density was unaffected by growth irradiance. We conclude that the red light response of stomatal conductance is independent of the concurrent photosynthetic rate of the guard cells or of that of the underlying mesophyll. Furthermore, we suggest that the correlation of photosynthetic capacity and stomatal conductance observed under different light environments is caused by signals largely independent of photosynthesis.

Molecular targets of elevated [CO2] in leaves and stems of Populus deltoides: implications for future tree growth and carbon sequestration

We report the first comprehensive analysis of the effects of elevated [CO₂] on gene expression in source leaf and stem sink tissues in woody plants. We have taken advantage of coppiced Populus deltoides (Bartr.) stands grown for 3 years under three different and constant elevated [CO₂] in the agriforest mesocosms of Biosphere 2. Leaf area per tree was doubled by elevated [CO₂] but although growth at 800 v. 400 µmol mol^-1 CO₂ resulted in a significant increase in stem biomass, growth was not stimulated at 1200 µmol mol^-1 CO₂. Growth under elevated [CO₂] also resulted in significant increases in stem wood density. Analysis of expression data for the 13 490 clones present on POP1 microarrays revealed 95 and 277 [CO₂]-responsive clones in leaves and stems respectively, with the response being stronger at 1200 µmol mol^-1. When these [CO₂]-responsive genes were assigned to functional categories, metabolism-related genes were the most responsive to elevated [CO₂]. However within this category, expression of genes relating to bioenergetic processes was unchanged in leaves whereas the expression of genes for storage proteins and of those involved in control of wall expansion was enhanced. In contrast to leaves, the genes up-regulated in stems under elevated [CO₂] were primarily enzymes responsible for lignin formation and polymerisation or ethylene response factors, also known to induce lignin biosynthesis. Concomitant with this enhancement of lignin biosynthesis in stems, there was a pronounced repression of genes related to cell wall formation and cell growth. These changes in gene expression have clear consequences for long-term carbon sequestration, reducing the carbon-fertilisation effect, and the potential for increased lignification may negatively impact on future wood quality for timber and paper production.

Transgenic approaches to manipulate the environmental responses of the C3 carbon fixation cycle

The limitation to photosynthetic CO2 assimilation in C3 plants in hot, dry environments is dominated by ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) because CO2 availability is restricted and photorespiration is stimulated. Using a combination of genetic engineering and transgenic technology, three approaches to reduce photorespiration have been taken; two of these focused on increasing the carboxylation efficiency of Rubisco either by reducing the oxygenase reaction directly or by manipulating the Rubisco enzyme by concentrating CO2 in the region of Rubisco through the introduction of enzymes of the C4 pathway. The third approach attempted to reduce photorespiration directly by manipulation of enzymes in this pathway. The progress in each of these areas is discussed, and the most promising approaches are highlighted. Under saturating CO2 conditions, Rubisco did not limit photosynthesis, and limitation shifted to ribulose bisphosphate (RuBP) regeneration capacity of the C3 cycle. Transgenic analysis was used to identify the specific enzymes that may be targets for improving carbon fixation, and the way this may be exploited in the high CO2 future is considered.

Decreased SBPase activity alters growth and development in transgenic tobacco plants

The effects of reduced SBPase activity on growth and development were examined in a set of transgenic tobacco plants produced using an antisense construct driven by the ribulose bisphosphate carboxylase, small subunit promoter. Photosynthetic carbon assimilation rates and carbohydrate levels in source leaves were decreased in the antisense plants. Growth rate and total shoot biomass were reduced in the SBPase antisense plants, even in plants where SBPase activity was reduced by only 25%. Floral biomass also decreased in response to reductions in SBPase activity and the onset of flowering was delayed by 5-10 d. This is the first demonstration of a link between reproductive biomass and reductions in Calvin cycle enzyme activity using antisense plants. Furthermore, unexpected changes in the growth and development of the antisense plants were evident. Small reductions in SBPase activity (above 50% wild type) resulted in shorter plants with only a small decrease in stem biomass and specific leaf area. In contrast, plants with larger reductions in SBPase activity had an increase in specific leaf area and attained heights similar to that of the wild-type plants but with a much reduced stem biomass, largely due to a decrease in xylem tissue. This bi-modal response of growth to reductions in SBPase activity has similarities to changes in leaf and stem anatomy and morphology that accompany light acclimation.

The ERECTA gene regulates plant transpiration efficiency in Arabidopsis

Assimilation of carbon by plants incurs water costs. In the many parts of the world where water is in short supply, plant transpiration efficiency, the ratio of carbon fixation to water loss, is critical to plant survival, crop yield and vegetation dynamics. When challenged by variations in their environment, plants often seem to coordinate photosynthesis and transpiration, but significant genetic variation in transpiration efficiency has been identified both between and within species. This has allowed plant breeders to develop effective selection programmes for the improved transpiration efficiency of crops, after it was demonstrated that carbon isotopic discrimination, Delta, of plant matter was a reliable and sensitive marker negatively related to variation in transpiration efficiency. However, little is known of the genetic controls of transpiration efficiency. Here we report the isolation of a gene that regulates transpiration efficiency, ERECTA. We show that ERECTA, a putative leucine-rich repeat receptor-like kinase (LRR-RLK) known for its effects on inflorescence development, is a major contributor to a locus for Delta on Arabidopsis chromosome 2. Mechanisms include, but are not limited to, effects on stomatal density, epidermal cell expansion, mesophyll cell proliferation and cell-cell contact.

Photosynthetic Acclimation in Rice Leaves to Free-air CO2 Enrichment Related to Both Ribulose-1,5-bisphosphate Carboxylation Limitation and Ribulose-1,5-bisphosphate Regeneration Limitation

Net photosynthetic rates (Pns) in leaves were compared between rice plants grown in ambient air control and free-air CO2 enrichment (FACE, about 200 micromol mol(-1) above ambient) treatment rings. When measured at the same CO2 concentration, the Pn of FACE leaves decreased significantly, indicating that photosynthetic acclimation to high CO2 occurs. Although stomatal conductance (Gs) in FACE leaves was markedly decreased, intercellular CO2 concentrations (Ci) were almost the same in FACE and ambient leaves, indicating that the photosynthetic acclimation is not caused by the decreased Gs. Furthermore, carboxylation efficiency and maximal Pn, both light and CO2-saturated Pn, were decreased in FACE leaves, as shown by the Pn-Ci curves. In addition, the soluble protein, Rubisco (ribulose-1,5-bisphosphate caboxylase/oxygenase), and its activase contents as well as the sucrose-phosphate synthase activity decreased significantly, while some soluble sugar, inorganic phosphate, chlorophyll and light-harvesting complex II (LHC II) contents increased in FACE leaves. It appears that the photosynthetic acclimation in rice leaves is related to both ribulose-1,5-bisphosphate (RuBP) carboxylation limitation and RuBP regeneration limitation.

The Calvin cycle revisited

The sequence of reactions in the Calvin cycle, and the biochemical characteristics of the enzymes involved, have been known for some time. However, the extent to which any individual enzyme controls the rate of carbon fixation has been a long standing question. Over the last 10 years, antisense transgenic plants have been used as tools to address this and have revealed some unexpected findings about the Calvin cycle. It was shown that under a range of environmental conditions, the level of Rubisco protein had little impact on the control of carbon fixation. In addition, three of the four thioredoxin regulated enzymes, FBPase, PRKase and GAPDH, had negligible control of the cycle. Unexpectedly, non-regulated enzymes catalysing reversible reactions, aldolase and transketolase, both exerted significant control over carbon flux. Furthermore, under a range of growth conditions SBPase was shown to have a significant level of control over the Calvin cycle. These data led to the hypothesis that increasing the amounts of these enzymes may lead to an increase in photosynthetic carbon assimilation. Remarkably, photosynthetic capacity and growth were increased in tobacco plants expressing a bifunctional SBPase/FBPase enzyme. Future work is discussed which will further our understanding of this complex and important pathway, particularly in relation to the mechanisms that regulate and co-ordinate enzyme activity.

The Calvin cycle revisited

The sequence of reactions in the Calvin cycle, and the biochemical characteristics of the enzymes involved, have been known for some time. However, the extent to which any individual enzyme controls the rate of carbon fixation has been a long standing question. Over the last 10 years, antisense transgenic plants have been used as tools to address this and have revealed some unexpected findings about the Calvin cycle. It was shown that under a range of environmental conditions, the level of Rubisco protein had little impact on the control of carbon fixation. In addition, three of the four thioredoxin regulated enzymes, FBPase, PRKase and GAPDH, had negligible control of the cycle. Unexpectedly, non-regulated enzymes catalysing reversible reactions, aldolase and transketolase, both exerted significant control over carbon flux. Furthermore, under a range of growth conditions SBPase was shown to have a significant level of control over the Calvin cycle. These data led to the hypothesis that increasing the amounts of these enzymes may lead to an increase in photosynthetic carbon assimilation. Remarkably, photosynthetic capacity and growth were increased in tobacco plants expressing a bifunctional SBPase/FBPase enzyme. Future work is discussed which will further our understanding of this complex and important pathway, particularly in relation to the mechanisms that regulate and co-ordinate enzyme activity.

The effects of elevated CO(2) concentrations on cell division rates, growth patterns, and blade anatomy in young wheat plants are modulated by factors related to leaf position, vernalization, and genotype

This study demonstrates that elevated [CO(2)] has profound effects on cell division and expansion in developing wheat (Triticum aestivum L.) leaves and on the quantitative integration of these processes in whole-leaf growth kinetics, anatomy, and carbon content. The expression of these effects, however, is modified by intrinsic factors related to genetic makeup and leaf position, and also by exposure to low vernalizing temperatures at germination. Beyond these interactions, leaf developmental responses to elevated [CO(2)] in wheat share several remarkable features that were conserved across all leaves examined. Most significantly: (a) the contribution of [CO(2)] effects on meristem size and activity in driving differences in whole-blade growth kinetics and final dimensions; (b) an anisotropy in cellular growth responses to elevated [CO(2)], with final cell length and expansion in the paradermal plane being highly conserved, even when the rates and duration of cell elongation were modified, while cell cross-sectional areas were increased; (c) tissue-specific effects of elevated [CO(2)], with significant modifications of mesophyll anatomy, including an increased extension of intercellular air spaces and the formation of, on average, one extra cell layer, while epidermal anatomy was mostly unaltered. Our results indicate complex developmental regulations of sugar effects in expanding leaves that are subjected to genetic variation and influenced by environmental cues important in the promotion of floral initiation. They also provide insights into apparently contradictory and inconsistent conclusions of published CO(2) enrichment studies in wheat.

Photosynthesis, plant growth and N allocation in transgenic rice plants with decreased Rubisco under CO2 enrichment

Ribulose-1,5-bisphosphate carboxylase (Rubisco) efficiency for CO2-saturated photosynthesis was examined in leaves of rice (Oryza sativa L.). The amount of Rubisco in a leaf was calculated to be 30-55% in excess for the light-saturated rate of photosynthesis at 100 Pa CO2. Long-term exposure to CO2 enrichment decreased the amount of Rubisco protein. However, N was not reallocated from decreased Rubisco to other components limiting photosynthesis, and the decrease in Rubisco was simply due to a decrease in total leaf-N content by CO2 enrichment. Thus, rice plants did not optimize N allocation into Rubisco at elevated CO2. Transgenic rice plants with decreased Rubisco were obtained by transformation with the rbcS antisense gene. The transformant with 65% wild-type Rubisco was selected as a plant with optimal Rubisco content for CO2-saturated photosynthesis at the level of a single leaf. This selected transgenic plant had 20% lower rates of photosynthesis at normal CO2 (36 Pa), but 5-15% higher rates of photosynthesis at elevated CO2 (100 Pa) for a given leaf N content. However, such transgenic plants did not necessarily show greater production of biomass even under conditions of CO2 enrichment. Although they had a higher N-use efficiency for plant growth under such conditions during the middle stage of growth, the growth rate was lower during the early stage of growth. Thus, improvement of N-use efficiency by a single leaf did not necessarily lead to greater production of biomass by the whole plant.

Is there scope for improving balance between RuBP-regeneration and carboxylation capacities in wheat at elevated CO2?

Carboxylation and RuBP-regeneration capacities, which determine light-saturated photosynthetic rate, were analysed in leaves of spring wheat (Triticum aestivum L. cv. Minaret) grown under different atmospheric CO2 partial pressure (pCa) and N supply regimes. Capacities were estimated from a large number of gas exchange, Rubisco and ATP-synthase content measurements, and from these, the pCa at which the two capacities are equal was derived, to allow direct comparison with growth pCa. Acclimation of the balance between the two capacities to growth at elevated pCa in wheat was only partial and appears to occur mostly in older flag leaves and at low N. However, in contrast to conclusions drawn from previous analyses of these data, there was evidence of a specific effect of growth at 70 Pa pCa, where carboxylation capacity is reduced more than RuBP-regeneration capacity for a given leaf N content. A model was used to estimate the effects of fluctuations in PPFD and temperature in the growth environment on the optimal balance between these capacities. This showed that the observed balance between carboxylation and RuBP-regeneration capacities in young wheat leaves could be consistent with adaptation to the current, or even the preindustrial pCa.

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.

A moderate decrease of plastid aldolase activity inhibits photosynthesis, alters the levels of sugars and starch, and inhibits growth of potato plants

Antisense expression of a full length cDNA encoding plastid aldolase led to decreased expression of aldolase at the transcript and protein level in several 'antisense' potato transformants. To quantify the inhibition, activity was compared in corresponding leaves down a plant and in plants of different ages. Aldolase activity was decreased by 32-43%, 56-71%, 79-83% and 91-97% in A-70, A-3, A-51 and A-2. Separation on a Q-Sepharose-FF column showed the decrease was due to inhibition of plastid aldolase. The transformants showed a small increase of Rubisco activity, a small decrease of phosphoribulokinase activity, and larger but subproportional decreases of sedoheptulose-1,7-biphosphatase and plastid fructose-1,6-bisphosphatase activity. Ambient photosynthesis was inhibited by 10%, 40%, 66% and 85% in A-70, A-3, A-51 and A-2. The transformants contained increased triose phosphates, and very low ribulose-1,5-bisphosphate and glycerate-3-phosphate. Chlorophyll fluorescence indicated that photosystem II was more reduced and thylakoid energization was increased. Starch synthesis was decreased by 16% and 36% in A-70 and A-3, whereas sucrose synthesis was less strongly inhibited. Plant growth was not significantly altered in A-70, was decreased by 41% in A-3, and was severely inhibited in plants with under 20% of wild-type aldolase activity. Although plastid aldolase catalyses a readily reversible reaction, possesses no known regulatory properties, and would appear irrelevant for the control of metabolism and growth, small changes in its activity have marked consequences for photosynthesis, carbon partitioning and growth.

Source Strength Regulates an Early Phase Transition of Tobacco Shoot Morphogenesis

We have taken advantage of specific reductions in the ribulose-1,5-bisphosphate carboxylase/oxygenase concentration in rbcS antisense mutants of tobacco (Nicotiana tabacum L.) to assess the contribution of source strength (carbohydrate production) to the control of shoot development. Wild-type and antisense plants undergo distinct phases of shoot development that can be distinguished from one another on the basis of differences in stem elongation rates, internode distances, plastochron indices, leaf sizes, and leaf morphologies. An early phase of shoot morphogenesis is markedly prolonged in the antisense plants, and an increased number of leaves emerge during this phase in the mutants. This delay is specific, inasmuch as the duration and expression of traits characteristic of later phases of shoot development proceed normally. In addition to altered shoot developmental patterns, the antisense mutants have enhanced shoot/root ratios and markedly increased leaf longevities. It is likely that these are adaptations that enhance photosynthetic rates. Consistent with this proposal, the total leaf areas and dry weights of the mutant and wild type are similar at flowering. Collectively, our results indicate that source strength regulates the duration of an early phase of tobacco shoot development and the transition to a later phase. We suggest that this phase change may occur in response to the attainment of a threshold source strength, which is delayed in the mutant plants.

Transgenic Tobacco Plants Expressing Pea Chloroplast Nmdh cDNA in Sense and Antisense Orientation (Effects on NADP-Malate Dehydrogenase Level, Stability of Transformants, and Plant Growth)

A full-length cDNA encoding light-activated chloroplast NADP-malate dehydrogenase (NADP-MDH) (EC 1.1.1.82) from pea (Pisum sativum L.) was introduced in the sense and antisense orientation into tobacco (Nicotiana tabacum L.). Transgenic plants with decreased or increased expression levels were obtained. Because of substantial age-dependent differences in individual leaves of a single plant, standardization of NADP-MDH levels was required first. Then, extent and stability of over- or under-expression of Nmdh, the gene encoding NADP-MDH, was characterized in the various transformants. Frequently, cosuppression effects were observed, indicating sufficient homology between the endogenous tobacco and the heterologous pea gene. Analysis of the T1 and T2 progeny of a series of independent transgenic lines revealed that NADP-MDH capacity ranged between 10% and [greater than or equal to]10-fold compared with the wild type. Under ambient conditions whole-plant development, growth period, and fertility were unaffected by NADP-MDH reduction to 20% of the wild-type level; below this threshold plant growth was retarded. A positive growth effect was registered in young plants with stably enhanced NADP-MDH levels within a defined developmental window.

Growth and N Allocation in Rice Plants under CO2 Enrichment

The effects of CO2 enrichment on growth and N allocation of rice (Oryza sativa L.) were examined. The plants were grown hydroponically in growth chambers with a 14-h photoperiod (1000 [mu]mol quanta m-2 s-1) and a day/night temperature of 25/20[deg]C. From the 28th to 70th d after germination, the plants were exposed to two CO2 partial pressures, namely 36 and 100 Pa. The CO2 enrichment increased the final biomass, but this was caused by a stimulation of the growth rate during the first week of the exposure to elevated CO2 partial pressures. The disappearance of the initial stimulation of the growth rate was associated with a decreased leaf area ratio. Furthermore, CO2 enrichment decreased the investment of N in the leaf blades, whereas the N allocation into the leaf sheaths and roots increased. Thus, the decrease in leaf N content by CO2 enrichment was not due to dilution of N caused by a relative increase in the plant biomass but was due to the change in N allocation at the whole-plant level. We conclude that the growth responses of rice to CO2 enrichment are mainly controlled by leaf area expansion and N allocation into leaf blades at the whole-plant level.

Accelerated Early Growth of Rice at Elevated CO2 (Is It Related to Developmental Changes in the Shoot Apex?)

The influence of elevated CO2 on the development of the shoot apex and on subsequent vegetative growth and grain yield was investigated using rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 [mu]L CO2 L-1. At 8 d after planting (DAP), elevated CO2 increased the height and diameter of the apical dome and lengths of leaf primordia and tiller buds but had no effect on their numbers. By 16 DAP, there were five tiller buds in the apex at 700 [mu]L CO2 L-1 compared with only three tiller buds at 350 [mu]L CO2 L-1. These changes in development of the shoot apex at high CO2 were forerunners to faster development of the vegetative shoot at elevated CO2 between 11 and 26 DAP as evidenced by increases in the relative growth rates of the shoot and tillers. Accelerated development at high CO2 was responsible for the 42% increase in tiller number at the maximum tillering stage and the 57% enhancement of grain yield at the final harvest. The link between high CO2 effects on development during the first 15 DAP and final tiller number and grain yield was demonstrated by delaying exposure of plants to high CO2 for 15 d. The delay totally inhibited the tillering response to high CO2, and the increase in grain yield of 20% arose from a greater number of grains per panicle. Consequently, it can be concluded that accelerated development in the shoot apex early in development is crucial for obtaining maximum increases in grain yield at elevated atmospheric CO2 concentrations.

Does Decrease in Ribulose-1,5-Bisphosphate Carboxylase by Antisense RbcS Lead to a Higher N-Use Efficiency of Photosynthesis under Conditions of Saturating CO2 and Light in Rice Plants?

Rice (Oryza sativa L.) plants with decreased ribulose-1,5-bisphosphate carboxylase (Rubisco) were obtained by transformation with the rice rbcS antisense gene under the control of the rice rbcS promoter. The primary transformants were screened for the Rubisco to leaf N ratio, and the transformant with 65% wild-type Rubisco was selected as a plant set with optimal Rubisco content at saturating CO2 partial pressures for photosynthesis under conditions of high irradiance and 25[deg]C. This optimal Rubisco content was estimated from the amounts and kinetic constants of Rubisco and the gas-exchange data. The R1 selfed progeny of the selected transformant were grown hydroponically with different N concentrations. Rubisco content in the R1 population was distributed into two groups: 56 plants had about 65% wild-type Rubisco, whereas 23 plants were very similar to the wild type. Although the plants with decreased Rubisco showed 20% lower rates of light-saturated photosynthesis in normal air (36 Pa CO2), they had 5 to 15% higher rates of photosynthesis in elevated partial pressures of CO2, (100-115 Pa CO2) than the wild-type plants for a given leaf N content. We conclude that the rice plants with 65% wild-type Rubisco show a higher N-use efficiency of photosynthesis under conditions of saturating CO2 and high irradiance.

MORE EFFICIENT PLANTS: A Consequence of Rising Atmospheric CO2?

The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil-water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.

Growth and photosynthesis under high and low irradiance of Arabidopsis thaliana antisense mutants with reduced ribulose-1,5-bisphosphate carboxylase/oxygenase activase content

Photosynthesis and growth to maturity of antisense ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase Arabidopsis thaliana with reduced concentrations of activase relative to wild-type (Wt) plants were measured under low (200 mumol m-2 s-1) and high (600 mumol m-2 s-1) photosynthetic photon flux density growing conditions. Both growth and photosynthesis were significantly reduced in an Arabidopsis clone (R100) with 30 to 40% Wt activase, an effect that was more pronounced in high light. The aboveground biomass of the antisense clone R100 reached 80% of Wt under low light and 65% of Wt under high light. Decreased growth in the antisense plants was attributed to reduced relative rates of growth and leaf area expansion early in development; all plants attained similar values of relative rates of growth and leaf elongation by 21 d after planting. Reductions in photosynthesis were attributed to decreased Rubisco activation in the antisense plants. Rubisco constituted about 40% of total soluble protein in both Wt and clone R100 under both light regimes. Activase content was 5% and 1.4% of total soluble protein in Wt and clone R100, respectively, and also was unaffected by growth irradiance. The stoichiometry of Rubisco to activase was estimated at 20 Rubisco active sites per activase tetramer in Wt Arabidopsis and 60 to 80 in the transgenic clone R100. We conclude that Wt Arabidopsis does not contain Rubisco activase in great excess of the amount required for optimal growth.

Diurnal Regulation of Leaf Blade Elongation in Rice by CO2 (Is it Related to Sucrose-Phosphate Synthase Activity?)

The relationship between leaf blade elongation rates (LER) and sucrose-phosphate synthase (SPS) activity was investigated at different times during ontogeny of rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 [mu]L CO2 L-1. High CO2 concentrations increased LER of expanding blades and in vivo activity (Vlimiting) SPS activity of expanded blades during the early vegetative stage (21 d after planting [DAP]), when tiller number was small and growing blades were strong carbohydrate sinks. Despite a constant light environment, there was a distinct diurnal pattern in LER, Vlimiting SPS activity, and concentration of soluble sugars, with an increase in the early part of the light period and a decrease later in the light period. The strong correlation (r = 0.65) between LER and Vlimiting SPS activity over the diurnal cycle indicated that SPS activity played an important role in controlling blade growth. The higher Vlimiting SPS activity at elevated CO2 at 21 DAP was caused by an increase in the activation state of the enzyme rather than an increase in Vmax. Fructose and glucose accumulated to a greater extent than sucrose at high CO2 and may have been utilized for synthesis of cell-wall components, contributing to higher specific leaf weight. By the mid-tillering stage (42 DAP), CO2 enrichment enhanced Vlimiting and Vmax activities of source blades. Nevertheless, LER was depressed by high CO2, probably because tillers were stronger carbohydrate sinks than growing blades.

Regulation of Photosynthesis during Leaf Development in RbcS Antisense DNA Mutants of Tobacco

We have previously characterized RbcS antisense DNA mutants of tobacco that have drastic reductions in their content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; S.R. Rodermel, M.S. Abbott, L. Bogorad [1988] Cell 55: 673-681). In this report we examine the impact of Rubisco loss on photosynthesis during tobacco (Nicotiana tabacum) leaf development. Photosynthetic capacities are depressed in the antisense leaves, but the patterns of change in photosynthetic rates during the development of these leaves are similar to those in wild-type plants: after attaining a maximum in young leaves, photosynthetic capacities undergo a prolonged senescence decline in older leaves. The alterations in photosynthetic capacities in both the wild type and mutant are closely correlated with changes in Rubisco activity and content. During wild-type leaf development, Rubisco accumulation is regulated by coordinate changes in RbcS and rbcL transcript accumulation, whereas in the antisense leaves, Rubisco content is a function of RbcS, but not rbcL, transcript abundance. This indicates that large subunit protein production is controlled posttranscriptionally in the mutants. The antisense leaves accumulate near-normal levels of chlorophyll and representative photosynthetic proteins throughout development, suggesting that photosynthetic gene expression is not feedback regulated by Rubisco abundance. Considered together, the data in this paper indicate that leaf developmental programs are generally insensitive to sharp reductions in Rubisco content and emphasize the metabolic plasticity of plant cells in achieving optimal photosynthetic rates.

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase, Electron Transport Components, and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice, and Their Relationships to Photosynthesis

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.