Response of two pea hybrids to CO2 enrichment: a test of the energy overflow hypothesis for alternative respiration

Enhancing crop yields through improvements in the efficiency of photosynthesis and respiration

The rate with which crop yields per hectare increase each year is plateauing at the same time that human population growth and other factors increase food demand. Increasing yield potential (Y p ) of crops is vital to address these challenges. In this review, we explore a component ofY p that has yet to be optimised - that being improvements in the efficiency with which light energy is converted into biomass (ε c ) via modifications to CO2 fixed per unit quantum of light (α), efficiency of respiratory ATP production (ε prod ) and efficiency of ATP use (ε use ). For α, targets include changes in photoprotective machinery, ribulose bisphosphate carboxylase/oxygenase kinetics and photorespiratory pathways. There is also potential forε prod to be increased via targeted changes to the expression of the alternative oxidase and mitochondrial uncoupling pathways. Similarly, there are possibilities to improveε use via changes to the ATP costs of phloem loading, nutrient uptake, futile cycles and/or protein/membrane turnover. Recently developed high-throughput measurements of respiration can serve as a proxy for the cumulative energy cost of these processes. There are thus exciting opportunities to use our growing knowledge of factors influencing the efficiency of photosynthesis and respiration to create a step-change in yield potential of globally important crops.

A meta-analysis of responses of C3 plants to atmospheric CO2 : dose-response curves for 85 traits ranging from the molecular to the whole-plant level

Generalised dose-response curves are essential to understand how plants acclimate to atmospheric CO₂ . We carried out a meta-analysis of 630 experiments in which C₃ plants were experimentally grown at different [CO₂ ] under relatively benign conditions, and derived dose-response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200-1200 µmol mol^-1 CO₂ , some traits more than doubled (e.g. area-based photosynthesis; intrinsic water-use efficiency), whereas others more than halved (area-based transpiration). At current atmospheric [CO₂ ], 64% of the total stimulation in biomass over the 200-1200 µmol mol^-1 range has already been realised. We also mapped the trait responses of plants to [CO₂ ] against those we have quantified before for light intensity. For most traits, CO₂ and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO₂ ], and some traits (such as area-based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO₂ ] at different integration levels and offers the quantitative dose-response curves that can be used to improve global change simulation models.

Gold/alpha-lactalbumin nanoprobes for the imaging and treatment of breast cancer

Theranostic agents should ideally be renally cleared and biodegradable. Here, we report the synthesis, characterization and theranostic applications of fluorescent ultrasmall gold quantum clusters that are stabilized by the milk metalloprotein alpha-lactalbumin. We synthesized three types of these nanoprobes that together display fluorescence across the visible and near-infrared spectra when excited at a single wavelength through optical colour coding. In live tumour-bearing mice, the near-infrared nanoprobe generates contrast for fluorescence, X-ray computed tomography and magnetic resonance imaging, and exhibits long circulation times, low accumulation in the reticuloendothelial system, sustained tumour retention, insignificant toxicity and renal clearance. An intravenously administrated near-infrared nanoprobe with a large Stokes shift facilitated the detection and image-guided resection of breast tumours in vivo using a smartphone with modified optics. Moreover, the partially unfolded structure of alpha-lactalbumin in the nanoprobe helps with the formation of an anti-cancer lipoprotein complex with oleic acid that triggers the inhibition of the MAPK and PI3K-AKT pathways, immunogenic cell death and the recruitment of infiltrating macrophages. The biodegradability and safety profile of the nanoprobes make them suitable for the systemic detection and localized treatment of cancer.

Effects of elevated CO2 and temperature on seed quality

Successful crop production depends initially on the availability of high-quality seed. By 2050 global climate change will have influenced crop yields, but will these changes affect seed quality? The present review examines the effects of elevated carbon dioxide (CO₂) and temperature during seed production on three seed quality components: seed mass, germination and seed vigour. In response to elevated CO₂, seed mass has been reported to both increase and decrease in C₃ plants, but not change in C₄ plants. Increases are greater in legumes than non-legumes, and there is considerable variation among species. Seed mass increases may result in a decrease of seed nitrogen (N) concentration in non-legumes. Increasing temperature may decrease seed mass because of an accelerated growth rate and reduced seed filling duration, but lower seed mass does not necessarily reduce seed germination or vigour. Like seed mass, reported seed germination responses to elevated CO₂ have been variable. The reported changes in seed C/N ratio can decrease seed protein content which may eventually lead to reduced viability. Conversely, increased ethylene production may stimulate germination in some species. High-temperature stress before developing seeds reach physiological maturity (PM) can reduce germination by inhibiting the ability of the plant to supply the assimilates necessary to synthesize the storage compounds required for germination. Nothing is known concerning the effects of elevated CO₂ on seed vigour. However, seed vigour can be reduced by high-temperature stress both before and after PM. High temperatures induce or increase the physiological deterioration of seeds. Limited evidence suggests that only short periods of high-temperature stress at critical seed development stages are required to reduce seed vigour, but further research is required. The predicted environmental changes will lead to losses of seed quality, particularly for seed vigour and possibly germination. The seed industry will need to consider management changes to minimize the risk of this occurring.

Identification of the alternative terminal oxidase of higher plant mitochondria

In addition to cytochrome oxidase, plant mitochondria have a second terminal oxidase called the alternative oxidase. The alternative oxidase is of great interest in that energy is not conserved when electrons flow through it. The potential energy of the system is thus lost as heat, and, in plants with high levels of the alternative oxidase, this results in thermogenesis. We have purified the alternative oxidase from mitochondria of the thermogenic spadix of Sauromatum guttatum and have identified its polypeptide constituents by using polyclonal antibodies. A 166-fold purification was achieved through a combination of cation-exchange (carboxymethyl-Sepharose) and hydrophobic-interaction (phenyl-Sepharose) chromatography. Polyclonal antibodies raised to the CM-Sepharose fractions readily immunoprecipitated alternative oxidase activity and immunoprecipitated four of the proteins that copurify with the activity. These proteins have apparent molecular masses of 37, 36, 35.5, and 35 kDa. Polyclonal antibodies raised individually to the 37-, 36-, and 35.5- plus 35-kDa proteins cross-reacted with all of these proteins, indicating the presence of common antigenic sites. The 37-kDa protein appears to be constitutive in Sauromatum, whereas expression of the 36- and 35-kDa proteins was correlated with presence of alternative pathway activity. The 35.5-kDa protein appears with loss of alternative pathway activity during senescence, indicating that this protein may be a degradation product of the 36-kDa protein. Binding of anti-36-kDa protein antibodies to total mitochondrial protein blots of five plant species indicated that similar proteins were always present when alternative pathway activity was observed.

Phenology, growth and physiological adjustments of oil palm (Elaeis guineensis) to sink limitation induced by fruit pruning

BACKGROUND AND AIMS

Despite its simple architecture and small phenotypic plasticity, oil palm has complex phenology and source-sink interactions. Phytomers appear in regular succession but their development takes years, involving long lag periods between environmental influences and their effects on sinks. Plant adjustments to resulting source-sink imbalances are poorly understood. This study investigated oil palm adjustments to imbalances caused by severe fruit pruning.

METHODS

An experiment with two treatments (control and complete fruit pruning) during 22 months in 2006-2008) and six replications per treatment was conducted in Indonesia. Phenology, growth of above-ground vegetative and reproductive organs, leaf morphology, inflorescence sex differentiation, dynamics of non-structural carbohydrate reserves and light-saturated net photosynthesis (A(max)) were monitored.

KEY RESULTS

Artificial sink limitation by complete fruit pruning accelerated development rate, resulting in higher phytomer, leaf and inflorescence numbers. Leaf size and morphology remained unchanged. Complete fruit pruning also suppressed the abortion of male inflorescences, estimated to be triggered at about 16 months before bunch maturity. The number of female inflorescences increased after an estimated lag of 24-26 months, corresponding to time from sex differentiation to bunch maturity. The most important adjustment process was increased assimilate storage in the stem, attaining nearly 50 % of dry weight in the stem top, mainly as starch, whereas glucose, which in controls was the most abundant non-structural carbohydrate stored in oil palm, decreased.

CONCLUSIONS

The development rate of oil palm is in part controlled by source-sink relationships. Although increased rate of development and proportion of female inflorescences constituted observed adjustments to sink limitation, the low plasticity of plant architecture (constant leaf size, absence of branching) limited compensatory growth. Non-structural carbohydrate storage was thus the main adjustment process.

Flowering time and elevated atmospheric CO2

Flowering is a critical milestone in the life cycle of plants, and changes in the timing of flowering may alter processes at the species, community and ecosystem levels. Therefore understanding flowering-time responses to global change drivers, such as elevated atmospheric carbon dioxide concentrations, [CO(2)], is necessary to predict the impacts of global change on natural and agricultural ecosystems. Here we summarize the results of 60 studies reporting flowering-time responses (defined as the time to first visible flower) of both crop and wild species at elevated [CO(2)]. These studies suggest that elevated [CO(2)] will influence flowering time in the future. In addition, interactions between elevated [CO(2)] and other global change factors may further complicate our ability to predict changes in flowering time. One approach to overcoming this problem is to elucidate the primary mechanisms that control flowering-time responses to elevated [CO(2)]. Unfortunately, the mechanisms controlling these responses are not known. However, past work has indicated that carbon metabolism exerts partial control on flowering time, and therefore may be involved in elevated [CO(2)]-induced changes in flowering time. This review also indicates the need for more studies addressing the effects of global change drivers on developmental processes in plants.

The sites of interaction of triphenyltetrazolium chloride with mitochondrial respiratory chains

The inability of cells and microorganisms to reduce the colourless electron acceptor triphenyltetrazolium chloride (TTC) to a red formazan precipitate is commonly used as a means of screening for cells that have a dysfunctional respiratory chain. The site of reduction of TTC is often stated to be at the level of cytochrome c oxidase where it is assumed to compete with oxygen for reducing equivalents. However, we show here that TTC is reduced not by cytochrome c oxidase but instead by dehydrogenases, particularly complex I, probably by accepting electrons directly from low potential cofactors. The reduction rate is fastest in coupled membranes because of accumulation in the matrix of the positively charged TTC+ cation. However, the initial product of TTC reduction is rapidly reoxidised by molecular oxygen, so that generation of the stable red formazan product from this intermediate occurs only under strictly anaerobic conditions. Colonies of mutants defective in cytochrome oxidase do not generate sufficiently anaerobic conditions to allow the intermediate to form the stable red formazan. This revision of the mode of interaction of TTC with respiratory chains has implications for the types of respiratory-defective mutants that might be detected by TTC screening.

ALTERNATIVE OXIDASE: From Gene to Function

Plants, some fungi, and protists contain a cyanide-resistant, alternative mitochondrial respiratory pathway. This pathway branches at the ubiquinone pool and consists of an alternative oxidase encoded by the nuclear gene Aox1. Alternative pathway respiration is only linked to proton translocation at Complex 1 (NADH dehydrogenase). Alternative oxidase expression is influenced by stress stimuli-cold, oxidative stress, pathogen attack-and by factors constricting electron flow through the cytochrome pathway of respiration. Control is exerted at the levels of gene expression and in response to the availability of carbon and reducing potential. Posttranslational control involves reversible covalent modification of the alternative oxidase and activation by specific carbon metabolites. This dynamic system of coarse and fine control may function to balance upstream respiratory carbon metabolism and downstream electron transport when these coupled processes become imbalanced as a result of changes in the supply of, or demand for, carbon, reducing power, and ATP.

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.

ATP/ADP antiporter is involved in uncoupling of plant mitochondria induced by low concentrations of palmitate

Carboxyatractyloside partially restored the transmembrane electrical potential difference (delta psi) dissipated by low concentrations of palmitate in pea stem mitochondria. This effect was more marked when mitochondria from sunflower were assayed. It is suggested that the ATP/ADP translocator is involved in the free fatty acid-induced uncoupling of oxidative phosphorylation in plant mitochondria, only when its level is sufficiently high and the concentration of the fatty acid is low to collapse only partially the delta psi.

ATP/ADP antiporter is involved in uncoupling of plant mitochondria induced by low concentrations of palmitate

Carboxyatractyloside partially restored the transmembrane electrical potential difference (delta psi) dissipated by low concentrations of palmitate in pea stem mitochondria. This effect was more marked when mitochondria from sunflower were assayed. It is suggested that the ATP/ADP translocator is involved in the free fatty acid-induced uncoupling of oxidative phosphorylation in plant mitochondria, only when its level is sufficiently high and the concentration of the fatty acid is low to collapse only partially delta psi.

Effects of Nitrogen Nutrition on Nitrogen Partitioning between Chloroplasts and Mitochondria in Pea and Wheat

Nitrogen partitioning among proteins in chloroplasts and mitochondria was examined in pea (Pisum sativum L.) and wheat (Triticum aestivum L.) grown hydroponically with different nitrogen concentrations. In pea leaves, chloroplast nitrogen accounted for 75 to 80% of total leaf nitrogen. We routinely found that 8% of total ribulose-1,5-bisphosphate carboxylase/oxygenase adhered to thylakoids during preparation and could be removed with Triton X-100. With this precaution, the ratio of stroma nitrogen increased from 53 to 61% of total leaf nitrogen in response to the nitrogen supply, but thylakoid nitrogen remained almost constant around 20% of total. The changes in the activities of the stromal enzymes and electron transport in response to the nitrogen supply reflected the nitrogen partitioning into stroma and thylakoids. On the other hand, nitrogen partitioning into mitochondria was appreciably smaller than that in chloroplasts, and the ratio of nitrogen allocated to mitochondria decreased with increasing leaf-nitrogen content, ranging from 7 to 4% of total leaf nitrogen. The ratio of mitochondrial respiratory enzyme activities to leaf-nitrogen content also decreased with increasing leaf-nitrogen content. These differences in nitrogen partitioning between chloroplasts and mitochondria were reflected in differences in the rates of photosynthesis and dark respiration in wheat leaves measured with an open gas-exchange system. The response of photosynthesis to nitrogen supply was much greater than that of dark respiration, and the CO(2) compensation point decreased with increasing leaf-nitrogen content.

Response of Two Wheat Cultivars to CO(2) Enrichment under Subambient Oxygen Conditions

Two cultivars of wheat (Triticum aestivum L. cvs Sonoita and Yecora Rojo) were grown to maturity in a growth chamber within four sub-chambers under two CO(2) levels (350 or 1000 microliters per liter) at either ambient (21%) or low O(2) (5%). Growth analysis was used to characterize changes in plant carbon budgets imposed by the gas regimes. Large increases in leaf areas were seen in the low O(2) treatments, due primarily to a stimulation of tillering. Roots developed normally at 5% O(2). Seed development was inhibited by the subambient O(2) treatment, but this effect was overcome by CO(2) enrichment at 1000 microliters per liter. Dry matter accumulation and seed number responded differently to the gas treatments. The greatest dry matter production occurred in the low O(2), high CO(2) treatment, while the greatest seed production occurred in the ambient O(2), high CO(2) treatment. Growth and assimilation were stimulated more by either CO(2) enrichment or low O(2) in cv Yecora Rojo than in Sonoita. These experiments are the first to explore the effect of whole plant low O(2) treatments on growth and reproduction. The finding that CO(2) enrichment overcomes low O(2)-induced sterility may help elucidate the nature of this effect.