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

Metabolite-mediated adaptation of crops to drought and the acquisition of tolerance

Drought is one of the major and growing threats to agriculture productivity and food security. Metabolites are involved in the regulation of plant responses to various environmental stresses, including drought stress. The complex drought tolerance can be ascribed to several simple metabolic traits. These traits could then be used for detecting the genetic architecture of drought tolerance. Plant metabolomes show dynamic differences when drought occurs during different developmental stages or upon different levels of drought stress. Here, we reviewed the major and most recent findings regarding the metabolite-mediated plant drought response. Recent progress in the development of drought-tolerant agents is also discussed. We provide an updated schematic overview of metabolome-driven solutions for increasing crop drought tolerance and thereby addressing an impending agricultural challenge.

Optimization of irrigation and fertilization of apples under magnetoelectric water irrigation in extremely arid areas

Apple (Malus pumila Mill.) is one of the important economic crops in the arid areas of Xinjiang, China. For a long time, there has been a problem of high consumption but low yield in water and fertilizer management, prevent improvements in apple quality and yield. In this study, 5-year-old 'Royal Gala' apple trees in extremely arid areas of Xinjiang were used as experimental materials to carry out field experiments. considering 5 irrigation levels (W1, 30 mm; W2, 425 mm; W3, 550 mm; W4, 675 mm; W5, 800 mm) and 5 fertilization levels (F1, 280 kg·ha-1; F2, 360 kg·ha-1; F3, 440 kg·ha-1; F4, 520 kg·ha-1; F5, 600 kg·ha-1) under magnetoelectric water irrigation conditions. The results demonstrated that magnetoelectric water combined with the application of 675 mm irrigation amount and 520 kg·ha-1 fertilization amount was the most effective combination. These results occurred by increasing net photosynthetic rate of apple leaves, improved the quality of apples, increased apple yield, and promoted the improvement of water and fertilizer use efficiency. Additionally, the quadratic regression model was used to fit the response process of yield, IWUE and PFP to irrigation amount and fertilization amount, and the accuracy was greater than 0.8, indicating good fitting effects. The synergistic effect of water and fertilizer has a positive effect on optimizing apple water and fertilizer management. Principal component analysis showed that the magnetoelectric treatment combined water and fertilizer mainly affected apple yield, water and fertilizer use efficiency and vitamin C content related to quality. This study provides valuable guidance for improving water and fertilizer productivity, crop yield and quality in extreme arid areas of Xinjiang by using Magnetoelectric water irrigation.

Extraction of Soluble Proteins from Leaves

Protein biochemistry can provide valuable answers to better understand plant performance and responses to the surrounding environment. In this chapter, we describe the process of extracting proteins from plant leaf samples. We highlight the key aspects to take into consideration to preserve protein integrity, from sample collection to extraction and preparation or storage for subsequent analysis of protein abundance and/or enzymatic activities.

The impact of growth at elevated [CO2] on stomatal anatomy and behavior differs between wheat species and cultivars

The ability of plants to respond to changes in the environment is crucial to their survival and reproductive success. The impact of increasing the atmospheric CO2 concentration (a[CO2]), mediated by behavioral and developmental responses of stomata, on crop performance remains a concern under all climate change scenarios, with potential impacts on future food security. To identify possible beneficial traits that could be exploited for future breeding, phenotypic variation in morphological traits including stomatal size and density, as well as physiological responses and, critically, the effect of growth [CO2] on these traits, was assessed in six wheat relative accessions (including Aegilops tauschii, Triticum turgidum ssp. Dicoccoides, and T. turgidum ssp. dicoccon) and five elite bread wheat T. aestivum cultivars. Exploiting a range of different species and ploidy, we identified key differences in photosynthetic capacity between elite hexaploid wheat and wheat relatives. We also report differences in the speed of stomatal responses which were found to be faster in wheat relatives than in elite cultivars, a trait that could be useful for enhanced photosynthetic carbon gain and water use efficiency. Furthermore, these traits do not all appear to be influenced by elevated [CO2], and determining the underlying genetics will be critical for future breeding programmes.

Red Rubiscos and opportunities for engineering green plants

Nature's vital, but notoriously inefficient, CO2-fixing enzyme Rubisco often limits the growth of photosynthetic organisms including crop species. Form I Rubiscos comprise eight catalytic large subunits and eight auxiliary small subunits and can be classified into two distinct lineages-'red' and 'green'. While red-type Rubiscos (Form IC and ID) are found in rhodophytes, their secondary symbionts, and certain proteobacteria, green-type Rubiscos (Form IA and IB) exist in terrestrial plants, chlorophytes, cyanobacteria, and other proteobacteria. Eukaryotic red-type Rubiscos exhibit desirable kinetic properties, namely high specificity and high catalytic efficiency, with certain isoforms outperforming green-type Rubiscos. However, it is not yet possible to functionally express a high-performing red-type Rubisco in chloroplasts to boost photosynthetic carbon assimilation in green plants. Understanding the molecular and evolutionary basis for divergence between red- and green-type Rubiscos could help us to harness the superior CO2-fixing power of red-type Rubiscos. Here we review our current understanding about red-type Rubisco distribution, biogenesis, and sequence-structure, and present opportunities and challenges for utilizing red-type Rubisco kinetics towards crop improvements.

Stomata on the abaxial and adaxial leaf surfaces contribute differently to leaf gas exchange and photosynthesis in wheat

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces. Using a combination of differential illuminations and CO₂ concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density). When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses. Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange.

Targeted metabolite profiling as a top-down approach to uncover interspecies diversity and identify key conserved operational features in the Calvin-Benson cycle

Improving photosynthesis is a promising avenue to increase crop yield. This will be aided by better understanding of natural variance in photosynthesis. Profiling of Calvin-Benson cycle (CBC) metabolites provides a top-down strategy to uncover interspecies diversity in CBC operation. In a study of four C4 and five C3 species, principal components analysis separated C4 species from C3 species and also separated different C4 species. These separations were driven by metabolites that reflect known species differences in their biochemistry and pathways. Unexpectedly, there was also considerable diversity between the C3 species. Falling atmospheric CO2 and changing temperature, nitrogen, and water availability have driven evolution of C4 photosynthesis in multiple lineages. We propose that analogous selective pressures drove lineage-dependent evolution of the CBC in C3 species. Examples of species-dependent variation include differences in the balance between the CBC and the light reactions, and in the balance between regulated steps in the CBC. Metabolite profiles also reveal conserved features including inactivation of enzymes in low irradiance, and maintenance of CBC metabolites at relatively high levels in the absence of net CO2 fixation. These features may be important for photosynthetic efficiency in low light, fluctuating irradiance, and when stomata close due to low water availability.

Photoprotection and optimization of sucrose usage contribute to faster recovery of photosynthesis after water deficit at high temperatures in wheat

Plants are increasingly exposed to events of elevated temperature and water deficit, which threaten crop productivity. Understanding the ability to rapidly recover from abiotic stress, restoring carbon assimilation and biomass production, is important to unravel crop climate resilience. This study compared the photosynthetic performance of two Triticum aestivum L. cultivars, Sokoll and Paragon, adapted to the climate of Mexico and UK, respectively, exposed to 1-week water deficit and high temperatures, in isolation or combination. Measurements included photosynthetic assimilation rate, stomatal conductance, in vitro activities of Rubisco (EC 4.1.1.39) and invertase (INV, EC 3.2.1.26), antioxidant capacity and chlorophyll a fluorescence. In both genotypes, under elevated temperatures and water deficit (WD38°C), the photosynthetic limitations were mainly due to stomatal restrictions and to a decrease in the electron transport rate. Chlorophyll a fluorescence parameters clearly indicate differences between the two genotypes in the photoprotection when subjected to WD38°C and showed faster recovery of Paragon after stress relief. The activity of the cytosolic invertase (CytINV) under these stress conditions was strongly related to the fast photosynthesis recovery of Paragon. Taken together, the results suggest that optimal sucrose export/utilization and increased photoprotection of the electron transport machinery are important components to limit yield fluctuations due to water shortage and elevated temperatures.

Root endophytic fungus Serendipita indica modulates barley leaf blade proteome by increasing the abundance of photosynthetic proteins in response to salinity

AIMS

The present study aimed at analysing the proteome pattern of the leaf blade of barley (Hordeum vulgare L.) in Serendipita indica-colonised plants to decipher the molecular mechanism of S. indica-mediated salt stress. This work is aligned with our previous research on barley leaf sheath to study proteomic pattern variability in leaf blade and sheath of barley plant in response to salinity and S. indica inoculation.

METHODS AND RESULTS

The experiment was conducted using a completely randomised factorial design with four replications and two treatments: salinity (0 and 300 mmol l^-1 NaCl) and fungus (noninoculated and S. indica-inoculated). The leaf blades of the salt-treated S. indica-colonised and noninoculated plants were harvested after 2 weeks of salt treatment for the physiological and proteomic analyses. After exposure to 300 mmol l^-1 NaCl, shoot dry matter production in noninoculated control plants decreased 84% which was about twofold higher than inoculated plants with S. indica. However, the accumulation of sodium in the shoot of S. indica-inoculated plants was significantly lower than the control plants. Analysis of the proteome profile revealed a high number of significantly up-regulated proteins involved in photosynthesis (26 out of 42 identified proteins).

CONCLUSIONS

The results demonstrated how the enhanced plant growth and salt stress resistance induced by S. indica was positively associated with the up-regulation of several proteins involved in photosynthesis and carbohydrate metabolism. In fact, S. indica improved photosynthesis in order to reach the best possible performance of the host plant under salt stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

Current research provides new insight into the mechanism applied by S. indica in reducing the negative impacts of salt stress in barley at physiological and molecular levels.

Ozone tolerant maize hybrids maintain Rubisco content and activity during long-term exposure in the field

Ozone pollution is a damaging air pollutant that reduces maize yields equivalently to nutrient deficiency, heat, and aridity stress. Therefore, understanding the physiological and biochemical responses of maize to ozone pollution and identifying traits predictive of ozone tolerance is important. In this study, we examined the physiological, biochemical and yield responses of six maize hybrids to elevated ozone in the field using Free Air Ozone Enrichment. Elevated ozone stress reduced photosynthetic capacity, in vivo and in vitro, decreasing Rubisco content, but not activation state. Contrary to our hypotheses, variation in maize hybrid responses to ozone was not associated with stomatal limitation or antioxidant pools in maize. Rather, tolerance to ozone stress in the hybrid B73 × Mo17 was correlated with maintenance of leaf N content. Sensitive lines showed greater ozone-induced senescence and loss of photosynthetic capacity compared to the tolerant line.

Diurnal Regulation of In Vivo Localization and CO2-Fixing Activity of Carboxysomes in Synechococcus elongatus PCC 7942

Carboxysomes are the specific CO2-fixing microcompartments in all cyanobacteria. Although it is known that the organization and subcellular localization of carboxysomes are dependent on external light conditions and are highly relevant to their functions, how carboxysome organization and function are actively orchestrated in natural diurnal cycles has remained elusive. Here, we explore the dynamic regulation of carboxysome positioning and carbon fixation in the model cyanobacterium Synechococcus elongatus PCC 7942 in response to diurnal light-dark cycles, using live-cell confocal imaging and Rubisco assays. We found that carboxysomes are prone to locate close to the central line along the short axis of the cell and exhibit a greater preference of polar distribution in the dark phase, coupled with a reduction in carbon fixation. Moreover, we show that deleting the gene encoding the circadian clock protein KaiA could lead to an increase in carboxysome numbers per cell and reduced portions of pole-located carboxysomes. Our study provides insight into the diurnal regulation of carbon fixation in cyanobacteria and the general cellular strategies of cyanobacteria living in natural habitat for environmental acclimation.

Effects of Water Stress on Photosynthesis, Yield, and Water Use Efficiency in Winter Wheat

Drought has become one of the major constraints to agricultural development, particularly in areas that lack water. Studying the effects of different water stresses on the photosynthesis, growth, yield, water use efficiency (WUE) and irrigation water productivity (IWP) of winter wheat will provide data for the development of scientific irrigation strategies for water-saving agricultural methods. According to the size of the field water capacity, four different water stress levels were set, i.e., 30–40% (severe stress), 40–50% (moderate stress), 50–60% (mild stress) and 60–80% (well-watered) of field water capacity, controlling the amount of irrigation through an automatic irrigation system. The results showed that the seasonal changes in photosynthetic parameters, such as net photosynthetic rate (Pn), intercellular carbon concentration (Ci), stomatal conductance (Gs) and transpiration (E), significantly decreased under moderate and severe stress. As a result, the height, biomass and grain size of winter wheat decreased significantly, which led to low WUE and IWP. The Pn of the mild stress group only slightly decreased compared to that of the well-watered group, and was actually higher during the flowering and grain-filling stages, resulting in increases in dry biomass and 1000 grain weight of 2.07% and 1.95%, respectively. Higher WUE and IWP were attributed to higher yields and less water use. Thus, mild stress (60–80% field water capacity) resulted in the optimal use of water resources without a significant reduction in yield in the North China Plain (NCP). Therefore, mild stress can be considered a suitable environment for winter wheat growth in arid areas.

Photochemistry and proteomics of ginger (Zingiber officinale Roscoe) under drought and shading

Drought has become an increasingly serious ecological problem that limits crop production. However, little is known about the response of ginger (Zingiber officinale Roscoe) to drought and shading, especially with respect to photosynthetic electron transport. Here, differential proteomics was used to study the response of ginger to four experimental treatments: control, drought, 50% shading, and the combination of 50% shading and drought. Proteomic analysis suggested that ginger increased cyclic electron flow under drought stress by enhancing the expression of proteins related to photosystem I and cytochrome b6f. Shading significantly increased the expression of proteins related to the light harvesting complex, even under drought stress. In addition, shading increased the expression of proteins related to the oxygen evolution complex, plastocyanin, and ferredoxin-NADP+ reductase (FNR), thereby enhancing the efficiency of photosynthetic electron utilization. The shading and drought combination treatment appeared to enhance ginger's drought tolerance by reducing the expression of FNR and enhancing cyclic electron flow. Photosynthetic and fluorescence parameters showed that drought stress caused non-stomatal limitation of photosynthesis in ginger leaves. Drought stress also significantly reduced the quantum efficiency of photosystem II (Fv/Fm), the non-cyclic electron transfer efficiency of photosystem II (ϕPSII), and photochemical quenching (qP), while simultaneously increasing nonphotochemical quenching (NPQ). The addition of shading improved photosynthetic efficiency under drought. These results provide important baseline information on the photosynthetic mechanisms by which ginger responds to drought and shading. In addition, they provide a theoretical basis for the study of shade cultivation during the arid season.

Overexpression of ca1pase Decreases Rubisco Abundance and Grain Yield in Wheat

Rubisco catalyzes the fixation of CO2 into organic compounds that are used for plant growth and the production of agricultural products, and specific sugar-phosphate derivatives bind tightly to the active sites of Rubisco, locking the enzyme in a catalytically inactive conformation. 2-carboxy-d-arabinitol-1-phosphate phosphatase (CA1Pase) dephosphorylates such tight-binding inhibitors, contributing to the maintenance of Rubisco activity. Here, we investigated the hypothesis that overexpressing ca1pase would decrease the abundance of Rubisco inhibitors, thereby increasing the activity of Rubisco and enhancing photosynthetic performance and productivity in wheat (Triticum aestivum). Plants of four independent wheat transgenic lines overexpressing ca1pase showed up to 30-fold increases in ca1pase expression compared to the wild type. Plants overexpressing ca1pase had lower numbers of Rubisco tight-binding inhibitors and higher Rubisco activation state than the wild type; however, there were 17% to 60% fewer Rubisco active sites in the four transgenic lines than in the wild type. The lower Rubisco content in plants overexpressing ca1pase resulted in lower initial and total carboxylating activities measured in flag leaves at the end of the vegetative stage and lower aboveground biomass and grain yield measured in fully mature plants. Hence, contrary to what would be expected, ca1pase overexpression decreased Rubisco content and compromised wheat grain yields. These results support a possible role for Rubisco inhibitors in protecting the enzyme and maintaining an adequate number of Rubisco active sites to support carboxylation rates in planta.

Metabolite profiles reveal interspecific variation in operation of the Calvin-Benson cycle in both C4 and C3 plants

Low atmospheric CO2 in recent geological time led to the evolution of carbon-concentrating mechanisms (CCMs) such as C4 photosynthesis in >65 terrestrial plant lineages. We know little about the impact of low CO2 on the Calvin-Benson cycle (CBC) in C3 species that did not evolve CCMs, representing >90% of terrestrial plant species. Metabolite profiling provides a top-down strategy to investigate the operational balance in a pathway. We profiled CBC intermediates in a panel of C4 (Zea mays, Setaria viridis, Flaveria bidentis, and F. trinervia) and C3 species (Oryza sativa, Triticium aestivum, Arabidopsis thaliana, Nicotiana tabacum, and Manihot esculenta). Principal component analysis revealed differences between C4 and C3 species that were driven by many metabolites, including lower ribulose 1,5-bisphosphate in C4 species. Strikingly, there was also considerable variation between C3 species. This was partly due to different chlorophyll and protein contents, but mainly to differences in relative levels of metabolites. Correlation analysis indicated that one contributory factor was the balance between fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, and Rubisco. Our results point to the CBC having experienced different evolutionary trajectories in C3 species since the ancestors of modern plant lineages diverged. They underline the need to understand CBC operation in a wide range of species.

Spectrophotometric Determination of RuBisCO Activity and Activation State in Leaf Extracts

RuBisCO plays a central role in photosynthesis and, due to its catalytic inefficiencies, frequently limits CO₂ assimilation in fully illuminated leaves at the top of unstressed crop canopies. The CO₂-fixing enzyme is heavily regulated and not all the enzyme present in the leaf is active at any given moment. In this chapter, a spectrophotometric assay is described for measuring RuBisCO activity and activation state in leaf extracts. Most of the assay components are available commercially and others can be produced by established protocols, making adoption of the assay achievable by most plant biochemistry laboratories. Its relative high-throughput capacity enables large-scale experiments aimed at screening germplasm for improved RuBisCO function.

Proteomic Analysis of the Resistance Mechanisms in Sugarcane during Sporisorium scitamineum Infection

Smut disease is caused by Sporisorium scitamineum, an important sugarcane fungal pathogen causing an extensive loss in yield and sugar quality. The available literature suggests that there are two types of smut resistance mechanisms: external resistance by physical or chemical barriers and intrinsic internal resistance mechanisms operating at host–pathogen interaction at cellular and molecular levels. The nature of smut resistance mechanisms, however, remains largely unknown. The present study investigated the changes in proteome occurring in two sugarcane varieties with contrasting susceptibility to smut—F134 and NCo310—at whip development stage after S. scitamineum infection. Total proteins from pathogen inoculated and uninoculated (control) leaves were separated by two-dimensional gel electrophoresis (2D-PAGE). Protein identification was performed using BLASTp and tBLASTn against NCBI nonredundant protein databases and EST databases, respectively. A total of thirty proteins spots representing differentially expressed proteins (DEPs), 16 from F134 and 14 from NCo310, were identified and analyzed by MALDI-TOF/TOF MS. In F134, 4 DEPs were upregulated and nine were downregulated, while, nine were upregulated and three were downregulated in NCo310. The DEPs were associated with DNA binding, metabolic processes, defense, stress response, photorespiration, protein refolding, chloroplast, nucleus and plasma membrane. Finally, the expression of CAT, SOD, and PAL with recognized roles in S. scitamineum infection in both sugarcane verities were analyzed by real-time quantitative PCR (RT-qPCR) technique. Identification of genes critical for smut resistance in sugarcane will increase our knowledge of S. scitamineum-sugarcane interaction and help to develop molecular and conventional breeding strategies for variety improvement.

Structural and functional analyses of Rubisco from arctic diatom species reveal unusual posttranslational modifications

The catalytic performance of the major CO₂-assimilating enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), restricts photosynthetic productivity. Natural diversity in the catalytic properties of Rubisco indicates possibilities for improvement. Oceanic phytoplankton contain some of the most efficient Rubisco enzymes, and diatoms in particular are responsible for a significant proportion of total marine primary production as well as being a major source of CO₂ sequestration in polar cold waters. Until now, the biochemical properties and three-dimensional structures of Rubisco from diatoms were unknown. Here, diatoms from arctic waters were collected, cultivated, and analyzed for their CO₂-fixing capability. We characterized the kinetic properties of five and determined the crystal structures of four Rubiscos selected for their high CO₂-fixing efficiency. The DNA sequences of the rbcL and rbcS genes of the selected diatoms were similar, reflecting their close phylogenetic relationship. The V_max and K_m for the oxygenase and carboxylase activities at 25 °C and the specificity factors (S_c/o) at 15, 25, and 35 °C were determined. The S_c/o values were high, approaching those of mono- and dicot plants, thus exhibiting good selectivity for CO₂ relative to O₂ Structurally, diatom Rubiscos belong to form I C/D, containing small subunits characterized by a short βA-βB loop and a C-terminal extension that forms a β-hairpin structure (βE-βF loop). Of note, the diatom Rubiscos featured a number of posttranslational modifications of the large subunit, including 4-hydroxyproline, β-hydroxyleucine, hydroxylated and nitrosylated cysteine, mono- and dihydroxylated lysine, and trimethylated lysine. Our studies suggest adaptation toward achieving efficient CO₂ fixation in arctic diatom Rubiscos.

Suboptimal Acclimation of Photosynthesis to Light in Wheat Canopies

Photosynthetic acclimation (photoacclimation) is the process whereby leaves alter their morphology and/or biochemistry to optimize photosynthetic efficiency and productivity according to long-term changes in the light environment. The three-dimensional architecture of plant canopies imposes complex light dynamics, but the drivers for photoacclimation in such fluctuating environments are poorly understood. A technique for high-resolution three-dimensional reconstruction was combined with ray tracing to simulate a daily time course of radiation profiles for architecturally contrasting field-grown wheat (Triticum aestivum) canopies. An empirical model of photoacclimation was adapted to predict the optimal distribution of photosynthesis according to the fluctuating light patterns throughout the canopies. While the photoacclimation model output showed good correlation with field-measured gas-exchange data at the top of the canopy, it predicted a lower optimal light-saturated rate of photosynthesis at the base. Leaf Rubisco and protein contents were consistent with the measured optimal light-saturated rate of photosynthesis. We conclude that, although the photosynthetic capacity of leaves is high enough to exploit brief periods of high light within the canopy (particularly toward the base), the frequency and duration of such sunflecks are too small to make acclimation a viable strategy in terms of carbon gain. This suboptimal acclimation renders a large portion of residual photosynthetic capacity unused and reduces photosynthetic nitrogen use efficiency at the canopy level, with further implications for photosynthetic productivity. It is argued that (1) this represents an untapped source of photosynthetic potential and (2) canopy nitrogen could be lowered with no detriment to carbon gain or grain protein content.

Directions for Optimization of Photosynthetic Carbon Fixation: RuBisCO's Efficiency May Not Be So Constrained After All

The ubiquitous enzyme Ribulose 1,5-bisphosphate carboxylase-oxygenase (RuBisCO) fixes atmospheric carbon dioxide within the Calvin-Benson cycle that is utilized by most photosynthetic organisms. Despite this central role, RuBisCO's efficiency surprisingly struggles, with both a very slow turnover rate to products and also impaired substrate specificity, features that have long been an enigma as it would be assumed that its efficiency was under strong evolutionary pressure. RuBisCO's substrate specificity is compromised as it catalyzes a side-fixation reaction with atmospheric oxygen; empirical kinetic results show a trend to tradeoff between relative specificity and low catalytic turnover rate. Although the dominant hypothesis has been that the active-site chemistry constrains the enzyme's evolution, a more recent study on RuBisCO stability and adaptability has implicated competing selection pressures. Elucidating these constraints is crucial for directing future research on improving photosynthesis, as the current literature casts doubt on the potential effectiveness of site-directed mutagenesis to improve RuBisCO's efficiency. Here we use regression analysis to quantify the relationships between kinetic parameters obtained from empirical data sets spanning a wide evolutionary range of RuBisCOs. Most significantly we found that the rate constant for dissociation of CO₂ from the enzyme complex was much higher than previous estimates and comparable with the corresponding catalytic rate constant. Observed trends between relative specificity and turnover rate can be expressed as the product of negative and positive correlation factors. This provides an explanation in simple kinetic terms of both the natural variation of relative specificity as well as that obtained by reported site-directed mutagenesis results. We demonstrate that the kinetic behaviour shows a lesser rather than more constrained RuBisCO, consistent with growing empirical evidence of higher variability in relative specificity. In summary our analysis supports an explanation for the origin of the tradeoff between specificity and turnover as due to competition between protein stability and activity, rather than constraints between rate constants imposed by the underlying chemistry. Our analysis suggests that simultaneous improvement in both specificity and turnover rate of RuBisCO is possible.

Positively selected amino acid replacements within the RuBisCO enzyme of oak trees are associated with ecological adaptations

Phylogenetic analysis by maximum likelihood (PAML) has become the standard approach to study positive selection at the molecular level, but other methods may provide complementary ways to identify amino acid replacements associated with particular conditions. Here, we compare results of the decision tree (DT) model method with ones of PAML using the key photosynthetic enzyme RuBisCO as a model system to study molecular adaptation to particular ecological conditions in oaks (Quercus). We sequenced the chloroplast rbcL gene encoding RuBisCO large subunit in 158 Quercus species, covering about a third of the global genus diversity. It has been hypothesized that RuBisCO has evolved differentially depending on the environmental conditions and leaf traits governing internal gas diffusion patterns. Here, we show, using PAML, that amino acid replacements at the residue positions 95, 145, 251, 262 and 328 of the RuBisCO large subunit have been the subject of positive selection along particular Quercus lineages associated with the leaf traits and climate characteristics. In parallel, the DT model identified amino acid replacements at sites 95, 219, 262 and 328 being associated with the leaf traits and climate characteristics, exhibiting partial overlap with the results obtained using PAML.

Biochemical model of C3 photosynthesis applied to wheat at different temperatures

We examined the effects of leaf temperature on the estimation of maximal Rubisco capacity (V_cmax ) from gas exchange measurements of wheat leaves using a C₃ photosynthesis model. Cultivars of spring wheat (Triticum aestivum (L)) and triticale (X Triticosecale Wittmack) were grown in a greenhouse or in the field and measured at a range of temperatures under controlled conditions in a growth cabinet (2 and 21% O₂ ) or in the field using natural diurnal variation in temperature, respectively. Published Rubisco kinetic constants for tobacco did not describe the observed CO₂ response curves well as temperature varied. By assuming values for the Rubisco Michaelis-Menten constants for CO₂ (K_c ) and O₂ (K_o ) at 25 °C derived from tobacco and the activation energies of V_cmax from wheat and respiration in the light, R_d , from tobacco, we derived activation energies for K_c and K_o (93.7 and 33.6 kJ mol^-1 , respectively) that considerably improved the fit of the model to observed data. We confirmed that temperature dependence of dark respiration for wheat was well described by the activation energy for R_d from tobacco. The new parameters improved the estimation of V_cmax under field conditions, where temperatures increased through the day.

Phenotyping of field-grown wheat in the UK highlights contribution of light response of photosynthesis and flag leaf longevity to grain yield

Improving photosynthesis is a major target for increasing crop yields and ensuring food security. Phenotyping of photosynthesis in the field is critical to understand the limits to crop performance in agricultural settings. Yet, detailed phenotyping of photosynthetic traits is relatively scarce in field-grown wheat, with previous studies focusing on narrow germplasm selections. Flag leaf photosynthetic traits, crop development, and yield traits were compared in 64 field-grown wheat cultivars in the UK. Pre-anthesis and post-anthesis photosynthetic traits correlated significantly and positively with grain yield and harvest index (HI). These traits included net CO2 assimilation measured at ambient CO2 concentrations and a range of photosynthetic photon flux densities, and traits associated with the light response of photosynthesis. In most cultivars, photosynthesis decreased post-anthesis compared with pre-anthesis, and this was associated with decreased Rubisco activity and abundance. Heritability of photosynthetic traits suggests that phenotypic variation can be used to inform breeding programmes. Specific cultivars were identified with traits relevant to breeding for increased crop yields in the UK: pre-anthesis photosynthesis, post-anthesis photosynthesis, light response of photosynthesis, and Rubisco amounts. The results indicate that flag leaf longevity and operating photosynthetic activity in the canopy can be further exploited to maximize grain filling in UK bread wheat.

Temperature responses of Rubisco from Paniceae grasses provide opportunities for improving C3 photosynthesis

Enhancing the catalytic properties of the CO₂-fixing enzyme Rubisco is a target for improving agricultural crop productivity. Here, we reveal extensive diversity in the kinetic response between 10 and 37 °C by Rubisco from C₃ and C₄ species within the grass tribe Paniceae. The CO₂ fixation rate (kcatc) for Rubisco from the C₄ grasses with nicotinamide adenine dinucleotide (NAD) phosphate malic enzyme (NADP-ME) and phosphoenolpyruvate carboxykinase (PCK) photosynthetic pathways was twofold greater than the kcatc of Rubisco from NAD-ME species across all temperatures. The declining response of CO₂/O₂ specificity with increasing temperature was less pronounced for PCK and NADP-ME Rubisco, which would be advantageous in warmer climates relative to the NAD-ME grasses. Modelled variation in the temperature kinetics of Paniceae C₃ Rubisco and PCK Rubisco differentially stimulated C₃ photosynthesis relative to tobacco above and below 25 °C under current and elevated CO₂. Amino acid substitutions in the large subunit that could account for the catalytic variation among Paniceae Rubisco are identified; however, incompatibilities with Paniceae Rubisco biogenesis in tobacco hindered their mutagenic testing by chloroplast transformation. Circumventing these bioengineering limitations is critical to tailoring the properties of crop Rubisco to suit future climates.

Acclimation of Biochemical and Diffusive Components of Photosynthesis in Rice, Wheat, and Maize to Heat and Water Deficit: Implications for Modeling Photosynthesis

The impact of the combined effects of heat stress, increased vapor pressure deficit (VPD) and water deficit on the physiology of major crops needs to be better understood to help identifying the expected negative consequences of climate change and heat waves on global agricultural productivity. To address this issue, rice, wheat, and maize plants were grown under control temperature (CT, 25°C, VPD 1.8 kPa), and a high temperature (HT, 38°C, VPD 3.5 kPa), both under well-watered (WW) and water deficit (WD) conditions. Gas-exchange measurements showed that, in general, WD conditions affected the leaf conductance to CO2, while growth at HT had a more marked effect on the biochemistry of photosynthesis. When combined, HT and WD had an additive effect in limiting photosynthesis. The negative impacts of the imposed treatments on the processes governing leaf gas-exchange were species-dependent. Wheat presented a higher sensitivity while rice and maize showed a higher acclimation potential to increased temperature. Rubisco and PEPC kinetic constants determined in vitro at 25°C and 38°C were used to estimate Vcmax, Jmax, and Vpmax in the modeling of C3 and C4 photosynthesis. The results here obtained reiterate the need to use species-specific and temperature-specific values for Rubisco and PEPC kinetic constants for a precise parameterization of the photosynthetic response to changing environmental conditions in different crop species.

Contrasting Proteomic and Metabolomic Responses of Bermudagrass to Drought and Salt Stresses

Bermudagrass (Cynodon dactylon) is one of tolerant grass species to drought and salt. The comparative analyses of bermudagrass in response to drought and salt stresses at the physiological, proteomic, and metabolomic levels were performed in this study. The physiological results indicated that osmolytes accumulation, ROS level and antioxidant enzyme activities were extensively changed by drought and salt stresses. Through comparative proteomic analyses, we successfully identified a total of 77 proteins involved in photosynthesis, oxidative pentose phosphate, glycolysis, and redox metabolic pathways when exposed to drought and salt stresses. Among them, 36 proteins were commonly regulated by both treatments, while other 40 and 13 proteins were specifically regulated by drought and salt, respectively. Totally 15 proteins were involved in carbon metabolic pathway. Moreover, contents of 37 metabolites including amino acids, organic acids, sugars, and sugar alcohols were regulated by drought and salt treatments. Among them, 18 commonly modulated metabolites were involved in carbon and amino acid metabolic pathways. Drought treatment for 21 days caused less accumulation of sugars and sugar alcohols and increased ROS level in bermudagrass which led to relatively more severe cell membrane reflected by high EL-value and lower survival rate when compared to 400 mM salt treatment for 21 days. These results suggested that drought and 400 mM NaCl stresses for 21 days treatment affected common and specific changes in bermudagrass, which would provide new insights to understand the underlying molecular mechanisms and metabolic homeostasis of bermudagrass in responses to abiotic stresses.

Towards engineering carboxysomes into C3 plants

Photosynthesis in C3 plants is limited by features of the carbon-fixing enzyme Rubisco, which exhibits a low turnover rate and can react with O2 instead of CO2 , leading to photorespiration. In cyanobacteria, bacterial microcompartments, known as carboxysomes, improve the efficiency of photosynthesis by concentrating CO2 near the enzyme Rubisco. Cyanobacterial Rubisco enzymes are faster than those of C3 plants, though they have lower specificity toward CO2 than the land plant enzyme. Replacement of land plant Rubisco by faster bacterial variants with lower CO2 specificity will improve photosynthesis only if a microcompartment capable of concentrating CO2 can also be installed into the chloroplast. We review current information about cyanobacterial microcompartments and carbon-concentrating mechanisms, plant transformation strategies, replacement of Rubisco in a model C3 plant with cyanobacterial Rubisco and progress toward synthesizing a carboxysome in chloroplasts.

Prospects for improving CO2 fixation in C3-crops through understanding C4-Rubisco biogenesis and catalytic diversity

By operating a CO2 concentrating mechanism, C4-photosynthesis offers highly successful solutions to remedy the inefficiency of the CO2-fixing enzyme Rubisco. C4-plant Rubisco has characteristically evolved faster carboxylation rates with low CO2 affinity. Owing to high CO2 concentrations in bundle sheath chloroplasts, faster Rubisco enhances resource use efficiency in C4 plants by reducing the energy and carbon costs associated with photorespiration and lowering the nitrogen investment in Rubisco. Here, we show that C4-Rubisco from some NADP-ME species, such as maize, are also of potential benefit to C3-photosynthesis under current and future atmospheric CO2 pressures. Realizing this bioengineering endeavour necessitates improved understanding of the biogenesis requirements and catalytic variability of C4-Rubisco, as well as the development of transformation capabilities to engineer Rubisco in a wider variety of food and fibre crops.

Improved analysis of C4 and C3 photosynthesis via refined in vitro assays of their carbon fixation biochemistry

Plants operating C3 and C4 photosynthetic pathways exhibit differences in leaf anatomy and photosynthetic carbon fixation biochemistry. Fully understanding this underpinning biochemical variation is requisite to identifying solutions for improving photosynthetic efficiency and growth. Here we refine assay methods for accurately measuring the carboxylase and decarboxylase activities in C3 and C4 plant soluble protein. We show that differences in plant extract preparation and assay conditions are required to measure NADP-malic enzyme and phosphoenolpyruvate carboxylase (pH 8, Mg(2+), 22 °C) and phosphoenolpyruvate carboxykinase (pH 7, >2mM Mn(2+), no Mg(2+)) maximal activities accurately. We validate how the omission of MgCl2 during leaf protein extraction, lengthy (>1min) centrifugation times, and the use of non-pure ribulose-1,5-bisphosphate (RuBP) significantly underestimate Rubisco activation status. We show how Rubisco activation status varies with leaf ontogeny and is generally lower in mature C4 monocot leaves (45-60% activation) relative to C3 monocots (55-90% activation). Consistent with their >3-fold lower Rubisco contents, full Rubisco activation in soluble protein from C4 leaves (<5min) was faster than in C3 plant samples (<10min), with addition of Rubisco activase not required for full activation. We conclude that Rubisco inactivation in illuminated leaves primarily stems from RuBP binding to non-carbamylated enzyme, a state readily reversible by dilution during cellular protein extraction.

Rubisco catalytic properties of wild and domesticated relatives provide scope for improving wheat photosynthesis

Rubisco is a major target for improving crop photosynthesis and yield, yet natural diversity in catalytic properties of this enzyme is poorly understood. Rubisco from 25 genotypes of the Triticeae tribe, including wild relatives of bread wheat (Triticum aestivum), were surveyed to identify superior enzymes for improving photosynthesis in this crop. In vitro Rubisco carboxylation velocity (V c), Michaelis-Menten constants for CO2 (K c) and O2 (K o) and specificity factor (S c/o) were measured at 25 and 35 °C. V c and K c correlated positively, while V c and S c/o were inversely related. Rubisco large subunit genes (rbcL) were sequenced, and predicted corresponding amino acid differences analysed in relation to the corresponding catalytic properties. The effect of replacing native wheat Rubisco with counterparts from closely related species was analysed by modelling the response of photosynthesis to varying CO2 concentrations. The model predicted that two Rubisco enzymes would increase photosynthetic performance at 25 °C while only one of these also increased photosynthesis at 35 °C. Thus, under otherwise identical conditions, catalytic variation in the Rubiscos analysed is predicted to improve photosynthetic rates at physiological CO2 concentrations. Naturally occurring Rubiscos with superior properties amongst the Triticeae tribe can be exploited to improve wheat photosynthesis and crop productivity.

Biochemical characterization of predicted Precambrian RuBisCO

The antiquity and global abundance of the enzyme, RuBisCO, attests to the crucial and longstanding role it has played in the biogeochemical cycles of Earth over billions of years. The counterproductive oxygenase activity of RuBisCO has persisted over billions of years of evolution, despite its competition with the carboxylase activity necessary for carbon fixation, yet hypotheses regarding the selective pressures governing RuBisCO evolution have been limited to speculation. Here we report the resurrection and biochemical characterization of ancestral RuBisCOs, dating back to over one billion years ago (Gyr ago). Our findings provide an ancient point of reference revealing divergent evolutionary paths taken by eukaryotic homologues towards improved specificity for CO2, versus the evolutionary emphasis on increased rates of carboxylation observed in bacterial homologues. Consistent with these distinctions, in vivo analysis reveals the propensity of ancestral RuBisCO to be encapsulated into modern-day carboxysomes, bacterial organelles central to the cyanobacterial CO2 concentrating mechanism.

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.

Improved abiotic stress tolerance of bermudagrass by exogenous small molecules

As a widely used warm-season turfgrass in landscapes and golf courses, bermudagrass encounters multiple abiotic stresses during the growth and development. Physiology analysis indicated that abiotic stresses induced the accumulation of ROS and decline of photosynthesis, resulting in increased cell damage and inhibited growth. Proteomic and metabolomic approaches showed that antioxidant enzymes and osmoprotectant contents (sugar, sucrose, dehydrin, proline) were extensively changed under abiotic stress conditions. Exogenous application of small molecules, such as ABA, NO, CaCl2, H2S, polyamine and melatonin, could effectively alleviate damages caused by multiple abiotic stresses, including drought, salt, heat and cold. Based on high through-put RNA seq analysis, genes involved in ROS, transcription factors, hormones, and carbohydrate metabolisms were largely enriched. The data indicated that small molecules induced the accumulation of osmoprotectants and antioxidants, kept cell membrane integrity, increased photosynthesis and kept ion homeostasis, which protected bermudagrass from damages caused by abiotic stresses.

Rubisco catalytic properties optimized for present and future climatic conditions

Because of its catalytic inefficiencies, Rubisco is the most obvious target for improvement to enhance the photosynthetic capacity of plants. Two hypotheses are tested in the present work: (1) existing Rubiscos have optimal kinetic properties to maximize photosynthetic carbon assimilation in existing higher plants; (2) current knowledge allows proposal of changes to kinetic properties to make Rubiscos more suited to changed conditions in chloroplasts that are likely to occur with climate change. The catalytic mechanism of Rubisco results in higher catalytic rates of carboxylation being associated with decreased affinity for CO2, so that selection for different environments involves a trade-off between these two properties. The simulations performed in this study confirm that the optimality of Rubisco kinetics depends on the species and the environmental conditions. In particular, environmental drivers affecting the CO2 availability for carboxylation (Cc) or directly shifting the photosynthetic limitations between Rubisco and RuBP regeneration determine to what extend Rubisco kinetics are optimally suited to maximize CO2 assimilation rate. In general, modeled values for optimal kinetic reflect the predominant environmental conditions currently encountered by the species in the field. Under future climatic conditions, photosynthetic CO2 assimilation will be limited by RuBP-regeneration, especially in the absence of water stress, the largest rise in [CO2] and the lowest increases in temperature. Under these conditions, the model predicts that optimal Rubisco should have high Sc/o and low kcat(c).

Natural variation in photosynthetic capacity, growth, and yield in 64 field-grown wheat genotypes

Increasing photosynthesis in wheat has been identified as an approach to enhance crop yield, with manipulation of key genes involved in electron transport and the Calvin cycle as one avenue currently being explored. However, natural variation in photosynthetic capacity is a currently unexploited genetic resource for potential crop improvement. Using gas-exchange analysis and protein analysis, the existing natural variation in photosynthetic capacity in a diverse panel of 64 elite wheat cultivars grown in the field was examined relative to growth traits, including biomass and harvest index. Significant variations in photosynthetic capacity, biomass, and yield were observed, although no consistent correlation was found between photosynthetic capacity of the flag leaf and grain yield when all cultivars were compared. The majority of the variation in photosynthesis could be explained by components related to maximum capacity and operational rates of CO2 assimilation, and to CO2 diffusion. Cluster analysis revealed that cultivars may have been bred unintentionally for desirable traits at the expense of photosynthetic capacity. These findings suggest that there is significant underutilized photosynthetic capacity among existing wheat varieties. Our observations are discussed in the context of exploiting existing natural variation in physiological processes for the improvement of photosynthesis in wheat.

Expanding knowledge of the Rubisco kinetics variability in plant species: environmental and evolutionary trends

The present study characterizes the kinetic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from 28 terrestrial plant species, representing different phylogenetic lineages, environmental adaptations and photosynthetic mechanisms. Our findings confirm that past atmospheric CO(2)/O(2) ratio changes and present environmental pressures have influenced Rubisco kinetics. One evolutionary adaptation to a decreasing atmospheric CO(2)/O(2) ratio has been an increase in the affinity of Rubisco for CO(2) (Kc falling), and a consequent decrease in the velocity of carboxylation (kcat (c)), which in turn has been ameliorated by an increase in the proportion of leaf protein accounted by Rubisco. The trade-off between K(c) and k(cat)(c) was not universal among the species studied and deviations from this relationship occur in extant forms of Rubisco. In species adapted to particular environments, including carnivorous plants, crassulacean acid metabolism species and C(3) plants from aquatic and arid habitats, Rubisco has evolved towards increased efficiency, as demonstrated by a higher k(cat)(c)/K(c) ratio. This variability in kinetics was related to the amino acid sequence of the Rubisco large subunit. Phylogenetic analysis identified 13 residues under positive selection during evolution towards specific Rubisco kinetic parameters. This crucial information provides candidate amino acid replacements, which could be implemented to optimize crop photosynthesis under a range of environmental conditions.

Reference gene selection for quantitative real-time PCR normalization in Reaumuria soongorica

Despite its superiority for evaluating gene expression, real-time quantitative polymerase chain reaction (qPCR) results can be significantly biased by the use of inappropriate reference genes under different experimental conditions. Reaumuria soongorica is a dominant species of desert ecosystems in arid central Asia. Given the increasing interest in ecological engineering and potential genetic resources for arid agronomy, it is important to analyze gene function. However, systematic evaluation of stable reference genes should be performed prior to such analyses. In this study, the stabilities of 10 candidate reference genes were analyzed under 4 kinds of abiotic stresses (drought, salt, dark, and heat) within 4 accessions (HG010, HG020, XGG030, and XGG040) from 2 different habitats using 3 algorithms (geNorm, NormFinder, and BestKeeper). After validation of the ribulose-1,5-bisphosphate carboxylase/oxygenase large unite (rbcL) expression pattern, our data suggested that histone H2A (H2A) and eukaryotic initiation factor 4A-2 (EIF4A2) were the most stable reference genes, cyclophilin (CYCL) was moderate, and elongation factor 1α (EF1α) was the worst choice. This first systematic analysis for stably expressed genes will facilitate future functional analyses and deep mining of genetic resources in R. soongorica and other species of the Reaumuria genus.

Variation in Rubisco content and activity under variable climatic factors

The main objective of the present review is to provide a compilation of published data of the effects of several climatic conditions on Rubisco, particularly its activity, state of activation, and concentration, and its influence on leaf gas exchange and photosynthesis. The environmental conditions analyzed include drought, salinity, heavy metals, growth temperature, and elevated [O3], [CO2], and ultraviolet-B irradiance. The results show conclusive evidence for a major negative effect on activity of Rubisco with increasing intensity of a range of abiotic stress factors. This decrease in the activity of Rubisco is associated with down-regulation of the activation state of the enzyme (e.g., by de-carbamylation and/or binding of inhibitory sugar phosphates) in response to drought or high temperature. On the contrary, the negative effects of low temperature, heavy metal stress (cadmium), ozone, and UV-B stress on Rubisco activity are associated with changes in the concentration of Rubisco. Notably, in response to all environmental factors, the regulation of in vivo CO2 assimilation rate was related to Rubisco in vitro parameters, either concentration and/or carboxylation, depending on the particular stress. The importance of the loss of Rubisco activity and its repercussion on plant photosynthesis are discussed in the context of climate change. It is suggested that decreased Rubisco activity will be a major effect induced by climate change, which will need to be considered in any prediction model on plant productivity in the near future.

Origins and diversity of eukaryotic CO2-concentrating mechanisms: lessons for the future

The importance of the eukaryotic algal CO(2)-concentrating mechanism (CCM) is considered in terms of global productivity as well as molecular phylogeny and diversity. The three major constituents comprising the CCM in the majority of eukaryotes are described. These include: (i) likely plasma- and chloroplast-membrane inorganic carbon transporters; (ii) a suite of carbonic anhydrase enzymes in strategic locations; and usually (iii) a microcompartment in which most Rubisco aggregates (the chloroplast pyrenoid). The molecular diversity of known CCM components are set against the current green algal model for their probable operation. The review then focuses on the kinetic and cystallographic interactions of Rubisco, which permit pyrenoid formation and CCM function. Firstly, we consider observations that surface residues of the Rubisco small subunit directly condition Rubisco aggregation and pyrenoid formation. Secondly, we reanalyse the phylogenetic progression in green Rubisco kinetic properties, and suggest that Rubisco substrate selectivity (the specificity factor, S(rel), and affinity for CO(2), K(c)) demonstrate a systematic relaxation, which directly relates to the origins and effectiveness of a CCM. Finally, we consider the implications of eukaryotic CCM regulation and minimum components needed for introduction into higher plants as a possible means to enhance crop productivity in the future.

An integrated approach to crop genetic improvement

The balance between the supply and demand of the major food crops is fragile, fueling concerns for long-term global food security. The rising population, increasing wealth and a proliferation of non-food uses (e.g. bioenergy) has led to growing demands on agriculture, while increased production is limited by greater urbanization, and the degradation of land. Furthermore, global climate change with increasing temperatures and lower, more erratic rainfall is projected to decrease agricultural yields. There is a predicted need to increase food production by at least 70% by 2050 and therefore an urgent need to develop novel and integrated approaches, incorporating high-throughput phenotyping that will both increase production per unit area and simultaneously improve the resource use efficiency of crops. Yield potential, yield stability, nutrient and water use are all complex multigenic traits and while there is genetic variability, their complexity makes such traits difficult to breed for directly. Nevertheless molecular plant breeding has the potential to deliver substantial improvements, once the component traits and the genes underlying these traits have been identified. In addition, interactions between the individual traits must also be taken into account, a demand that is difficult to fulfill with traditional screening approaches. Identified traits will be incorporated into new cultivars using conventional or biotechnological tools. In order to better understand the relationship between genotype, component traits, and environment over time, a multidisciplinary approach must be adopted to both understand the underlying processes and identify candidate genes, QTLs and traits that can be used to develop improved crops.

Raising yield potential of wheat. II. Increasing photosynthetic capacity and efficiency

Past increases in yield potential of wheat have largely resulted from improvements in harvest index rather than increased biomass. Further large increases in harvest index are unlikely, but an opportunity exists for increasing productive biomass and harvestable grain. Photosynthetic capacity and efficiency are bottlenecks to raising productivity and there is strong evidence that increasing photosynthesis will increase crop yields provided that other constraints do not become limiting. Even small increases in the rate of net photosynthesis can translate into large increases in biomass and hence yield, since carbon assimilation is integrated over the entire growing season and crop canopy. This review discusses the strategies to increase photosynthesis that are being proposed by the wheat yield consortium in order to increase wheat yields. These include: selection for photosynthetic capacity and efficiency, increasing ear photosynthesis, optimizing canopy photosynthesis, introducing chloroplast CO(2) pumps, increasing RuBP regeneration, improving the thermal stability of Rubisco activase, and replacing wheat Rubisco with that from other species with different kinetic properties.