The activity of Rubisco's molecular chaperone, Rubisco activase, in leaf extracts

Improving Crop Yield through Increasing Carbon Gain and Reducing Carbon Loss

Photosynthesis is a process where solar energy is utilized to convert atmospheric CO2 into carbohydrates, which forms the basis for plant productivity. The increasing demand for food has created a global urge to enhance yield. Earlier, the plant breeding program was targeting the yield and yield-associated traits to enhance the crop yield. However, the yield cannot be further improved without improving the leaf photosynthetic rate. Hence, in this review, various strategies to enhance leaf photosynthesis were presented. The most promising strategies were the optimization of Rubisco carboxylation efficiency, the introduction of a CO2 concentrating mechanism in C3 plants, and the manipulation of photorespiratory bypasses in C3 plants, which are discussed in detail. Improving Rubisco's carboxylation efficiency is possible by engineering targets such as Rubisco subunits, chaperones, and Rubisco activase enzyme activity. Carbon-concentrating mechanisms can be introduced in C3 plants by the adoption of pyrenoid and carboxysomes, which can increase the CO2 concentration around the Rubisco enzyme. Photorespiration is the process by which the fixed carbon is lost through an oxidative process. Different approaches to reduce carbon and nitrogen loss were discussed. Overall, the potential approaches to improve the photosynthetic process and the way forward were discussed in detail.

Rubisco and sucrose synthesis and translocation are involved in the regulation of photosynthesis in wheat with different source-sink relationships

Source-sink relationships influence photosynthesis. So far, the limiting factors for photosynthesis of wheat cultivars with different source-sink relationships have not been determined. We aimed to determine the variation patterns of photosynthetic characteristics of wheat cultivars with different source-sink relationships. In this study, two wheat cultivars with different source-sink relationships were selected for photosynthetic physiological analyses. The results showed that YM25 (source-limited cultivar) had higher photosynthetic efficiency compared to YM1 (sink-limited cultivar). This is mainly due to a stronger photochemical efficiency, electron transfer capacity, and Rubisco carboxylation capacity of YM25. YM25 accumulated less soluble carbohydrates in flag leaves than YM1. This is mainly due to the stronger sucrose synthesis and transport capacity of YM25 by presenting higher sucrose-related enzyme activities and gene expression. A PCA analysis showed that Rubisco was the main factor limiting the photosynthetic capacity of YM25. The soluble sugar accumulation in flag leaves and sink limitation decreased the photosynthetic activity of YM1. Increased N application improved source-sink relationships and increased grain yield and source leaf photosynthetic capacity in both two wheat cultivars. Taken together, our findings suggest that Rubisco and sucrose synthesis and translocation are involved in the regulation of photosynthesis of wheat cultivars with different source-sink relationships and that source and sink limitation effects should be considered in photosynthesis.

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.

Photosynthetic plasticity aggravates the susceptibility of magnesium-deficient leaf to high light in rapeseed plants: the importance of Rubisco and mesophyll conductance

Plants grown under low magnesium (Mg) soils are highly susceptible to encountering light intensities that exceed the capacity of photosynthesis (A), leading to a depression of photosynthetic efficiency and eventually to photooxidation (i.e., leaf chlorosis). Yet, it remains unclear which processes play a key role in limiting the photosynthetic energy utilization of Mg-deficient leaves, and whether the plasticity of A in acclimation to irradiance could have cross-talk with Mg, hence accelerating or mitigating the photodamage. We investigated the light acclimation responses of rapeseed (Brassica napus) grown under low- and adequate-Mg conditions. Magnesium deficiency considerably decreased rapeseed growth and leaf A, to a greater extent under high than under low light, which is associated with higher level of superoxide anion radical and more severe leaf chlorosis. This difference was mainly attributable to a greater depression in dark reaction under high light, with a higher Rubisco fallover and a more limited mesophyll conductance to CO2 (gm ). Plants grown under high irradiance enhanced the content and activity of Rubisco and gm to optimally utilize more light energy absorbed. However, Mg deficiency could not fulfill the need to activate the higher level of Rubisco and Rubisco activase in leaves of high-light-grown plants, leading to lower Rubisco activation and carboxylation rate. Additionally, Mg-deficient leaves under high light invested more carbon per leaf area to construct a compact leaf structure with smaller intercellular airspaces, lower surface area of chloroplast exposed to intercellular airspaces, and CO2 diffusion conductance through cytosol. These caused a more severe decrease in within-leaf CO2 diffusion rate and substrate availability. Taken together, plant plasticity helps to improve photosynthetic energy utilization under high light but aggravates the photooxidative damage once the Mg nutrition becomes insufficient.

Improved chloroplast Pi allocation helps sustain electron transfer to enhance photosynthetic low-phosphorus tolerance of wheat

Phosphorus (P) deficit limits high wheat (Triticum aestivum L.) yields. Breeding low-P-tolerant cultivars is vital for sustainable agriculture and food security, but the low-P adaptation mechanisms are largely not understood. Two wheat cultivars, ND2419 (low-P-tolerant) and ZM366 (low-P-sensitive) were used in this study. They were grown under hydroponic conditions with low-P (0.015 mM) or normal-P (1 mM). Low-P suppressed biomass accumulation and net photosynthetic rate (A) in both cultivars, whereas ND2419 was relatively less suppressed. Intercellular CO2 concentration did not decrease with the decline of stomatal conductance. Additionally, maximum electron transfer rate (Jmax) decreased sooner than maximum carboxylation rate (Vcmax). Results indicate that impeded electron transfer is directly responsible for decreased A. Under low-P, ND2419 exhibited greater PSII functionality (potential activity (Fv/Fo), maximum quantum efficiency (Fv/Fm), photochemical quenching (qL) and non-photochemical quenching (NPQ) required for electron transfer than ZM366, resulting more ATP for Rubisco activation. Furthermore, ND2419 maintained higher chloroplast Pi concentrations by enhancing chloroplast Pi allocation, compared with ZM366. Overall, the low-P-tolerant cultivar sustained electron transfer under low-P by enhancing chloroplast Pi allocation, allowing more ATP synthesis for Rubisco activation, ultimately presenting stronger photosynthesis capacities. The improved chloroplasts Pi allocation may provide new insights into improve low-P tolerance.

Improving plant heat tolerance through modification of Rubisco activase in C3 plants to secure crop yield and food security in a future warming world

The world's population may reach 10 billion by 2050, but 10% still suffer from food shortages. At the same time, global warming threatens food security by decreasing crop yields, so it is necessary to develop crops with enhanced resistance to high temperatures in order to secure the food supply. In this review, the role of Rubisco activase as an important factor in plant heat tolerance is summarized, based on the conclusions of recent findings. Rubisco activase is a molecular chaperone determining the activation of Rubisco, whose heat sensitivity causes reductions of photosynthesis at high temperatures. Thus, the thermostability of Rubisco activase is considered to be critical for improving plant heat tolerance. It has been shown that the introduction of thermostable Rubisco activase through gene editing into Arabidopsis thaliana and from heat-adapted wild Oryza species or C4Zea mays into Oryza sativa improves Rubisco activation, photosynthesis, and plant growth at high temperatures. We propose that developing a universal thermostable Rubisco activase could be a promising direction for further studies.

Short-term elevated temperature and CO2 promote photosynthetic induction in the C3 plant Glycine max, but not in the C4 plant Amaranthus tricolor

The continuous increases of atmospheric temperature and CO2 concentration will impact global photosynthesis. However, there are few studies considering the interaction of elevated temperature (eT) and elevated CO2 (eCO2 ) on dynamic photosynthesis, particularly for C4 species. We examine dynamic photosynthesis under four different temperature and [CO2 ] treatments: (1) 400ppm×28°C (CT); (2) 400ppm×33°C (CT+); (3) 800ppm×28°C (C+T); and (4) 800ppm×33°C (C+T+). In Glycine max L., the time required to reach 50% (T 50%A ) and 90% (T 90%A ) of full photosynthetic induction was smaller under the CT+, C+T, and C+T+ treatments than those under the CT treatment. In Amaranthus tricolor L., however, neither T 50%A nor T 90%A was not significantly affected by eT or eCO2 . In comparison with the CT treatment, the achieved carbon gain was increased by 58.3% (CT+), 112% (C+T), and 136.6% (C+T+) in G. max and was increased by 17.1% (CT+), 2.6% (C+T) and 56.9% (C+T+) in A. tricolor . The increases of achieved carbon gain in G. max were attributable to both improved photosynthetic induction efficiency (IE) and enhanced steady-state photosynthesis, whereas those in A. tricolor were attributable to enhanced steady-state photosynthesis.

Variation in photosynthetic induction between super hybrid rice and inbred super rice

It is well documented that yield superiority of super hybrid rice is linked with its improved photosynthetic capacity and/or efficiency. In natural environments, the amounts of CO₂ assimilated by plants was also impacted by the rapidity of leaf photosynthesis response to fluctuations of light. However, it remains unknow whether the high yield of super hybrid rice was associated with photosynthetic traits under dynamic state. Here, photosynthetic traits under steady-and dynamic state in two super hybrid rice varieties (Ylinagyou 3218 and Yliangyou 5867) with high yield and two inbred super rice varieties (Zhendao 11 and Nanjing 9108) with lower yield. Under steady state, the net photosynthetic rate (A*) in super hybrid rice was 25.3% larger compared with inbred super rice. During photosynthetic induction, there was no obvious association of the rapidity of net photosynthesis rate (A) to sunflecks with rice subpopulations. Stomatal conductance (g_s) of super hybrid rice increased slower than that of inbred super rice. The cumulative CO₂ fixation (CCF) during photosynthetic induction was 25.2% larger in super hybrid rice than that in inbred super rice. The primary limitation during induction was biochemical limitation rather than stomatal limitation. There was a significantly positive relationship between A* and CCF, while A* was not related with the induction response rate of A. Overall, A* and CCF in super hybrid rice have been improved together, which contributed to its yield superiority, whereas its yield potential still can be improved by increasing induction rate of A under fluctuations of irradiance.

Regulation of Calvin-Benson cycle enzymes under high temperature stress

The Calvin-Benson cycle (CBC) consists of three critical processes, including fixation of CO₂ by Rubisco, reduction of 3-phosphoglycerate (3PGA) to triose phosphate (triose-P) with NADPH and ATP generated by the light reactions, and regeneration of ribulose 1,5-bisphosphate (RuBP) from triose-P. The activities of photosynthesis-related proteins, mainly from the CBC, were found more significantly affected and regulated in plants challenged with high temperature stress, including Rubisco, Rubisco activase (RCA) and the enzymes involved in RuBP regeneration, such as sedoheptulose-1,7-bisphosphatase (SBPase). Over the past years, the regulatory mechanism of CBC, especially for redox-regulation, has attracted major interest, because balancing flux at the various enzymatic reactions and maintaining metabolite levels in a range are of critical importance for the optimal operation of CBC under high temperature stress, providing insights into the genetic manipulation of photosynthesis. Here, we summarize recent progress regarding the identification of various layers of regulation point to the key enzymes of CBC for acclimation to environmental temperature changes along with open questions are also discussed.

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

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

Overexpression of both Rubisco and Rubisco activase rescues rice photosynthesis and biomass under heat stress

Global warming threatens food security by decreasing crop yields through damage to photosynthetic systems, especially Rubisco activation. We examined whether co-overexpression of Rubisco and Rubisco activase improves the photosynthetic and growth performance of rice under high temperatures. We grew three rice lines-the wild-type (WT), a Rubisco activase-overexpressing line (oxRCA) and a Rubisco- and Rubisco activase-co-overexpressing line (oxRCA-RBCS)-and analysed photosynthesis and biomass at 25 and 40°C. Compared with the WT, the Rubisco activase content was 153% higher in oxRCA and 138% higher in oxRCA-RBCS, and the Rubisco content was 27% lower in oxRCA and similar in oxRCA-RBCS. The CO₂ assimilation rate (A) of WT was lower at 40°C than at 25°C, attributable to Rubisco deactivation by heat. On the other hand, that of oxRCA and oxRCA-RBCS was maintained at 40°C, resulting in higher A than WT. Notably, the dry weight of oxRCA-RBCS was 26% higher than that of WT at 40°C. These results show that increasing the Rubisco activase content without the reduction of Rubisco content could improve yield and sustainability in rice at high temperature.

Magnesium Application Promotes Rubisco Activation and Contributes to High-Temperature Stress Alleviation in Wheat During the Grain Filling

Inhibited photosynthesis caused by post-anthesis high-temperature stress (HTS) leads to decreased wheat grain yield. Magnesium (Mg) plays critical roles in photosynthesis; however, its function under HTS during wheat grain filling remains poorly understood. Therefore, in this study, we investigated the effects of Mg on the impact of HTS on photosynthesis during wheat grain filling by conducting pot experiments in controlled-climate chambers. Plants were subjected to a day/night temperature cycle of 32°C/22°C for 5 days during post-anthesis; the control temperature was set at 26°C/16°C. Mg was applied at the booting stage, with untreated plants used as a control. HTS reduced the yield and net photosynthetic rate (P _n ) of wheat plants. The maximum carboxylation rate (V _Cmax ), which is limited by Rubisco activity, decreased earlier than the light-saturated potential electron transport rate. This decrease in V _Cmax was caused by decreased Rubisco activation state under HTS. Mg application reduced yield loss by stabilizing P _n . Rubisco activation was enhanced by increasing Rubisco activase activity following Mg application, thereby stabilizing P _n . We conclude that Mg maintains Rubisco activation, thereby helping to stabilize P _n under HTS.

Heat-induced changes in the abundance of wheat Rubisco activase isoforms

The Triticum aestivum (wheat) genome encodes three isoforms of Rubisco activase (Rca) differing in thermostability, which could be exploited to improve the resilience of this crop to global warming. We hypothesized that elevated temperatures would cause an increase in the relative abundance of heat-stable Rca1β. Wheat plants were grown at 25° C : 18°C (day : night) and exposed to heat stress (38° C : 22°C) for up to 5 d at pre-anthesis. Carbon (C) assimilation, Rubisco activity, CA1Pase activity, transcripts of Rca1β, Rca2β, and Rca2α, and the quantities of the corresponding protein products were measured during and after heat stress. The transcript of Rca1β increased 40-fold in 4 h at elevated temperatures and returned to the original level after 4 h upon return of plants to control temperatures. Rca1β comprised up to 2% of the total Rca protein in unstressed leaves but increased three-fold in leaves exposed to elevated temperatures for 5 d and remained high at 4 h after heat stress. These results show that elevated temperatures cause rapid changes in Rca gene expression and adaptive changes in Rca isoform abundance. The improved understanding of the regulation of C assimilation under heat stress will inform efforts to improve wheat productivity and climate resilience.

Efficient Regulation of CO2 Assimilation Enables Greater Resilience to High Temperature and Drought in Maize

Increasing temperatures and extended drought episodes are among the major constraints affecting food production. Maize has a relatively high temperature optimum for photosynthesis compared to C₃ crops, however, the response of this important C₄ crop to the combination of heat and drought stress is poorly understood. Here, we hypothesized that resilience to high temperature combined with water deficit (WD) would require efficient regulation of the photosynthetic traits of maize, including the C₄-CO₂ concentrating mechanism (CCM). Two genotypes of maize with contrasting levels of drought and heat tolerance, B73 and P0023, were acclimatized at high temperature (38°C versus 25°C) under well-watered (WW) or WD conditions. The photosynthetic performance was evaluated by gas exchange and chlorophyll a fluorescence, and in vitro activities of key enzymes for carboxylation (phosphoenolpyruvate carboxylase), decarboxylation (NADP-malic enzyme), and carbon fixation (Rubisco). Both genotypes successfully acclimatized to the high temperature, although with different mechanisms: while B73 maintained the photosynthetic rates by increasing stomatal conductance (gs), P0023 maintained gs and showed limited transpiration. When WD was experienced in combination with high temperatures, limited transpiration allowed water-savings and acted as a drought stress avoidance mechanism. The photosynthetic efficiency in P0023 was sustained by higher phosphorylated PEPC and electron transport rate (ETR) near vascular tissues, supplying chemical energy for an effective CCM. These results suggest that the key traits for drought and heat tolerance in maize are limited transpiration rate, allied with a synchronized regulation of the carbon assimilation metabolism. These findings can be exploited in future breeding efforts aimed at improving maize resilience to climate change.

Arabidopsis Plant Natriuretic Peptide Is a Novel Interactor of Rubisco Activase

Plant natriuretic peptides (PNPs) are a group of systemically acting peptidic hormones affecting solute and solvent homeostasis and responses to biotrophic pathogens. Although an increasing body of evidence suggests PNPs modulate plant responses to biotic and abiotic stress, which could lead to their potential biotechnological application by conferring increased stress tolerance to plants, the exact mode of PNPs action is still elusive. In order to gain insight into PNP-dependent signalling, we set out to identify interactors of PNP present in the model plant Arabidopsis thaliana, termed AtPNP-A. Here, we report identification of rubisco activase (RCA), a central regulator of photosynthesis converting Rubisco catalytic sites from a closed to an open conformation, as an interactor of AtPNP-A through affinity isolation followed by mass spectrometric identification. Surface plasmon resonance (SPR) analyses reveals that the full-length recombinant AtPNP-A and the biologically active fragment of AtPNP-A bind specifically to RCA, whereas a biologically inactive scrambled peptide fails to bind. These results are considered in the light of known functions of PNPs, PNP-like proteins, and RCA in biotic and abiotic stress responses.

Measuring Rubisco activity: challenges and opportunities of NADH-linked microtiter plate-based and 14C-based assays

Rubisco is central to carbon assimilation, and efforts to improve the efficiency and sustainability of crop production have spurred interest in phenotyping Rubisco activity. We tested the hypothesis that microtiter plate-based methods provide comparable results to those obtained with the radiometric assay that measures the incorporation of 14CO2 into 3-phosphoglycerate (3-PGA). Three NADH-linked assays were tested that use alternative coupling enzymes: glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and glycerolphosphate dehydrogenase (GlyPDH); phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH); and pyruvate kinase (PK) and lactate dehydrogenase (LDH). To date there has been no thorough evaluation of their reliability by comparison with the 14C-based method. The three NADH-linked assays were used in parallel to estimate (i) the 3-PGA concentration-response curve of NADH oxidation, (ii) the Michaelis-Menten constant for ribulose-1,5-bisphosphate, (iii) fully active and inhibited Rubisco activities, and (iv) Rubisco initial and total activities in fully illuminated and shaded leaves. All three methods correlated strongly with the 14C-based method, and the PK-LDH method showed a strong correlation and was the cheapest method. PEPC-MDH would be a suitable option for situations in which ADP/ATP might interfere with the assay. GAPDH-GlyPDH proved more laborious than the other methods. Thus, we recommend the PK-LDH method as a reliable, cheaper, and higher throughput method to phenotype Rubisco activity for crop improvement efforts.

Improving drought-, salinity-, and heat-tolerance in transgenic plants by co-overexpressing Arabidopsis vacuolar pyrophosphatase gene AVP1 and Larrea Rubisco activase gene RCA

The severity and frequency of many abiotic stresses such as drought, salinity and heat, cause substantial crop losses worldwide, which poses a serious challenge in food security. To increase crop production, new approaches are needed. Previous research has shown that overexpression of the tonoplast H⁺ pyrophosphatase gene AVP1 leads to improved drought and salt tolerance in transgenic plants. Other research showed that overexpression of thermotolerant ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase gene could maintain photosynthesis at higher temperatures, which contributes to higher heat tolerance in transgenic plants. In nature, abiotic stresses rarely come alone, instead these stresses often occur in various combinations. Therefore, it is desirable to make crops more tolerant to multiple stresses, which will likely lead to higher crop yield under various stress conditions. It is shown here that co-overexpression of the Arabidopsis gene AVP1 and the Larrea Rubisco activase gene RCA significantly increases drought, salinity and heat tolerance, resulting in higher biomass and seed yield than wild-type plants. AVP1/RCA co-overexpressing plants are as more drought- and salt-tolerant as AVP1-overexpressing plants, and as more heat-tolerant as RCA-overexpressing plants. More importantly, they produce higher seed yields than AVP1-overexpressing, RCA-overexpressing, and wild-type plants under combined drought and heat conditions.

An isoleucine residue acts as a thermal and regulatory switch in wheat Rubisco activase

The regulation of Rubisco, the gatekeeper of carbon fixation into the biosphere, by its molecular chaperone Rubisco activase (Rca) is essential for photosynthesis and plant growth. Using energy from ATP hydrolysis, Rca promotes the release of inhibitors and restores catalytic competence to Rubisco-active sites. Rca is sensitive to moderate heat stress, however, and becomes progressively inhibited as the temperature increases above the optimum for photosynthesis. Here, we identify a single amino acid substitution (M159I) that fundamentally alters the thermal and regulatory properties of Rca in bread wheat (Triticum aestivum L.). Using site-directed mutagenesis, we demonstrate that the M159I substitution extends the temperature optimum of the most abundant Rca isoform by 5°C in vitro, while maintaining the efficiency of Rubisco activation by Rca. The results suggest that this single amino acid substitution acts as a thermal and regulatory switch in wheat Rca that can be exploited to improve the climate resilience and efficiency of carbon assimilation of this cereal crop as temperatures become warmer and more volatile.

MdATG18a overexpression improves basal thermotolerance in transgenic apple by decreasing damage to chloroplasts

High temperature is an abiotic stress factor that threatens plant growth and development. Autophagy in response to heat stress involves the selective removal of heat-induced protein complexes. Previously, we showed that a crucial autophagy protein from apple, MdATG18a, has a positive effect on drought tolerance. In the present study, we treated transgenic apple (Malus domestica) plants overexpressing MdATG18a with high temperature and found that autophagy protected them from heat stress. Overexpression of MdATG18a in apple enhanced antioxidase activity and contributed to the production of increased beneficial antioxidants under heat stress. Transgenic apple plants exhibited higher photosynthetic capacity, as shown by the rate of CO2 assimilation, the maximum photochemical efficiency of photosystem II (PSII), the effective quantum yield, and the electron transport rates in photosystems I and II (PSI and PSII, respectively). We also detected elevated autophagic activity and reduced damage to chloroplasts in transgenic plants compared to WT plants. In addition, the transcriptional activities of several HSP genes were increased in transgenic apple plants. In summary, we propose that autophagy plays a critical role in basal thermotolerance in apple, primarily through a combination of enhanced antioxidant activity and reduced chloroplast damage.

How do vascular plants perform photosynthesis in extreme environments? An integrative ecophysiological and biochemical story

In this work, we review the physiological and molecular mechanisms that allow vascular plants to perform photosynthesis in extreme environments, such as deserts, polar and alpine ecosystems. Specifically, we discuss the morpho/anatomical, photochemical and metabolic adaptive processes that enable a positive carbon balance in photosynthetic tissues under extreme temperatures and/or severe water-limiting conditions in C₃ species. Nevertheless, only a few studies have described the in situ functioning of photoprotection in plants from extreme environments, given the intrinsic difficulties of fieldwork in remote places. However, they cover a substantial geographical and functional range, which allowed us to describe some general trends. In general, photoprotection relies on the same mechanisms as those operating in the remaining plant species, ranging from enhanced morphological photoprotection to increased scavenging of oxidative products such as reactive oxygen species. Much less information is available about the main physiological and biochemical drivers of photosynthesis: stomatal conductance (g_s ), mesophyll conductance (g_m ) and carbon fixation, mostly driven by RuBisCO carboxylation. Extreme environments shape adaptations in structures, such as cell wall and membrane composition, the concentration and activation state of Calvin-Benson cycle enzymes, and RuBisCO evolution, optimizing kinetic traits to ensure functionality. Altogether, these species display a combination of rearrangements, from the whole-plant level to the molecular scale, to sustain a positive carbon balance in some of the most hostile environments on Earth.

Increase rate of light-induced stomatal conductance is related to stomatal size in the genus Oryza

The rapid response of stomatal conductance (gs) to fluctuating irradiance is of great importance to maximize carbon assimilation while minimizing water loss. Smaller stomata have been proven to have a faster response rate than larger ones, but most of these studies have been conducted with forest trees. In the present study, the effects of stomatal anatomy on the kinetics of gs and photosynthesis were investigated in 16 Oryza genotypes. Light-induced stomatal opening includes an initial time lag (λ) followed by an exponential increase. Smaller stomata had a larger maximum stomatal conductance increase rate (Slmax) during the exponential increase phase, but showed a longer time lag and a lower initial stomatal conductance (gs,initial) at low light. Stomatal size was, surprisingly, negatively correlated with the time required to reach 50% of maximum gs and photosynthesis (T50%gs and T50%A), which was shown to be positively correlated with λ and negatively correlated with gs,initial. With a lower gs,initial and a larger λ, small stomata showed a faster decrease of intercellular CO2 concentration (Ci) during the induction process, which may have led to a slower apparent Rubisco activation rate. Therefore, smaller stomata do not always benefit photosynthesis as reported before; the influence of stomatal size on dynamic photosynthesis is also correlated with λ and gs,initial.

Rubisco activation by wheat Rubisco activase isoform 2β is insensitive to inhibition by ADP

Rubisco activase (Rca) is a catalytic chaperone that remodels the active site, promotes the release of inhibitors and restores catalytic competence to Rubisco. Rca activity and its consequent effect on Rubisco activation and photosynthesis are modulated by changes to the chloroplast environment induced by fluctuations in light levels that reach the leaf, including redox status and adenosine diphosphate (ADP)/adenosine triphosphate (ATP) ratio. The Triticum aestivum (wheat) genome encodes for three Rca protein isoforms: 1β (42.7 kDa), 2β (42.2 kDa) and 2α (46.0 kDa). The regulatory properties of these isoforms were characterised by measuring rates of Rubisco activation and ATP hydrolysis by purified recombinant Rca proteins in the presence of physiological ADP/ATP ratios. ATP hydrolysis by all three isoforms was sensitive to inhibition by increasing amounts of ADP in the assay. In contrast, Rubisco activation activity of Rca 2β was insensitive to ADP inhibition, while Rca 1β and 2α were inhibited. Two double and one quadruple site-directed mutants were designed to elucidate if differences in the amino acid sequences between Rca 1β and 2β could explain the differences in ADP sensitivity. Changing two amino acids in Rca 2β to the corresponding residues in 1β (T358K & Q362E) resulted in significant inhibition of Rubisco activation in presence of ADP. The results show that the wheat Rca isoforms differ in their regulatory properties and that amino acid changes in the C domain influence ADP sensitivity. Advances in the understanding of Rubisco regulation will aid efforts to improve the efficiency of photosynthetic CO₂ assimilation.

Identification of drought responsive proteins and related proteomic QTLs in barley

Drought is a major abiotic stress that negatively influences crop yield. Breeding strategies for improved drought resistance require an improved knowledge of plant drought responses. We therefore applied drought to barley recombinant inbred lines and their parental genotypes shortly before tillering. A large-scale proteomic analysis of leaf and root tissue revealed proteins that respond to drought in a genotype-specific manner. Of these, Rubisco activase in chloroplast, luminal binding protein in endoplasmic reticulum, phosphoglycerate mutase, glutathione S-transferase, heat shock proteins and enzymes involved in phenylpropanoid biosynthesis showed strong genotype×environment interactions. These data were subjected to genetic linkage analysis and the identification of proteomic QTLs that have potential value in marker-assisted breeding programs.

What proteomics can reveal about plant-virus interactions? Photosynthesis-related proteins on the spotlight

Plant viruses are responsible for losses in worldwide production of numerous economically important food and fuel crops. As obligate cellular parasites with very small genomes, viruses rely on their hosts for replication, assembly, intra- and intercellular movement, and attraction of vectors for dispersal. Chloroplasts are photosynthesis and are the site of replication for several viruses. When viruses replicate in chloroplasts, photosynthesis, an essential process in plant physiology, is inhibited. The mechanisms underlying molecular and biochemical changes during compatible and incompatible plants-virus interactions, are only beginning to be elucidated, including changes in proteomic profiles induced by virus infections. In this review, we highlight the importance of proteomic studies to understand plant-virus interactions, especially emphasizing the changes in photosynthesis-related protein accumulation. We focus on: (a) chloroplast proteins that differentially accumulate during viral infection; (b) the significance with respect to chloroplast-virus interaction; and (c) alterations in plant's energetic metabolism and the subsequently the plant defense mechanisms to overcome viral infection.

Enhanced Rubisco activation associated with maintenance of electron transport alleviates inhibition of photosynthesis under low nitrogen conditions in winter wheat seedlings

Studying the response of photosynthesis to low nitrogen (N) and the underlying physiological mechanism can provide a theoretical basis for breeding N-efficient cultivars and optimizing N management. We conducted hydroponic experiments using two wheat (Triticum aestivum) cultivars, Zaoyangmai (low N sensitive) and Yangmai158 (low N tolerant), with either 0.25 mM N as a low N (LN) treatment or 5 mM N as a control. Under LN, a decrease in net photosynthetic rate (Pn) was attributed to reduction in the maximum Rubisco carboxylation rate, which then accelerated a reduction in the maximum ribulose-1,5-bisphosphate regeneration rate, and the reduction in Pn was 5-35% less in Yangmai158 than in Zaoyangmai. Yangmai158 maintained a 10-25% higher Rubisco concentration, especially in the upper leaves, and up-regulated Rubisco activase activity compared with Zaoyangmai to increase the Rubisco activation to sustain Rubisco carboxylation under LN conditions. In addition, Yangmai158 increased electron flux to the photorespiratory carbon oxidation cycle and alternative electron flux to maintain a faster electron transport rate and avoid photodamage. In conclusion, the LN-tolerant cultivar showed enhanced Rubisco activation and sustained electron transport to maintain a greater photosynthetic capacity under LN conditions.

Improved leaf nitrogen reutilisation and Rubisco activation under short-term nitrogen-deficient conditions promotes photosynthesis in winter wheat (Triticum aestivum L.) at the seedling stage

Excess N input results in low N use efficiency and environmental crisis, so nitrogenous fertiliser applications must be reduced. However, this can lead to low-N stress. Previous studies on low N have not explored the unique adjustment strategy to N deficiency in the short term, which is important for developing long-term N deficiency tolerance. In this case, two wheat (Triticum aestivum L.) cultivars with different tolerances to low N, Zaoyangmai (sensitive) and Yangmai158 (tolerant), were exposed to 0.25mM N as a N-deficient condition with 5.0mM N as a control. Under long-term N-deficient conditions, a significant decrease in Rubisco content resulted in decreased Rubisco activity and net photosynthetic rate (Pn) in both cultivars. However, the NO3-:soluble protein ratio decreased, and nitrate reductase and glutamine synthetase activity increased under short-term N deficiency, especially in Yangmai158. As a result, Rubisco content was not decreased in Yangmai158, while total N content decreased significantly. Moreover, increased Rubisco activase activity promoted Rubisco activation under short-term N deficiency. In sequence, Rubisco activity and Pn improved under short-term N deficiency. In conclusion, N deficiency-tolerant cultivars can efficiently assimilate N to Rubisco and enhance Rubisco activation to improve photosynthetic capabilities under short-term N deficiency conditions.

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.

Dynamic modelling of limitations on improving leaf CO2 assimilation under fluctuating irradiance

A dynamic model of leaf CO₂ assimilation was developed as an extension of the canonical steady-state model, by adding the effects of energy-dependent non-photochemical quenching (qE), chloroplast movement, photoinhibition, regulation of enzyme activity in the Calvin cycle, metabolite concentrations, and dynamic CO₂ diffusion. The model was calibrated and tested successfully using published measurements of gas exchange and chlorophyll fluorescence on Arabidopsis thaliana ecotype Col-0 and several photosynthetic mutants and transformants affecting the regulation of Rubisco activity (rca-2 and rwt43), non-photochemical quenching (npq4-1 and npq1-2), and sucrose synthesis (spsa1). The potential improvements on CO₂ assimilation under fluctuating irradiance that can be achieved by removing the kinetic limitations on the regulation of enzyme activities, electron transport, and stomatal conductance were calculated in silico for different scenarios. The model predicted that the rates of activation of enzymes in the Calvin cycle and stomatal opening were the most limiting (up to 17% improvement) and that effects varied with the frequency of fluctuations. On the other hand, relaxation of qE and chloroplast movement had a strong effect on average low-irradiance CO₂ assimilation (up to 10% improvement). Strong synergies among processes were found, such that removing all kinetic limitations simultaneously resulted in improvements of up to 32%.

A Thermotolerant Variant of Rubisco Activase From a Wild Relative Improves Growth and Seed Yield in Rice Under Heat Stress

Genes encoding thermostable variants of the photosynthesis heat-labile protein Rubisco activase (Rca) from a wild relative Oryza australiensis were overexpressed in domesticated rice (Oryza sativa). Proteomics was used to quantify the abundance of O. australiensis Rca (Rca-Oa) in the resulting plants. Plants were grown to maturity in growth rooms and from early tillering until immediately prior to anthesis, they were exposed to daytime maximum temperatures of 28, 40, and 45°C and constant night temperatures of 22°C. Non-destructive measurements of leaf elongation and photosynthesis were used to compare the null segregant with a transfected line in which 19% of its total Rca content was the recombinant O. australiensis Rca (T-Oa-19). Height, fresh mass, panicle number, seed set, and seed number were measured at final harvest. Traits at maturity after heat stress at 45°C correlated strongly with recombinant protein abundance. Seed number was far the most responsive trait to an increase in Rca-Oa abundance, improving by up to 150%. Leaf elongation rates (LER) and tiller number were significantly greater in the transformed plants in the first two weeks of exposure to 45°C but tiller numbers later became equal in the two genotypes. Gas exchange measurements showed that T-Oa-19 had faster light induction of photosynthesis but not significantly higher CO₂ assimilation rates, indicating that the carbon gain that resulted in large yield improvement after growth at 45°C was not strongly correlated with an instantaneous measurement of steady-state photosynthesis. When plants were grown at 40°C daytime maximum, there was no improvement in the final biomass, panicle or seed number when compared with 28°C, indicating that the threshold for heat damage and beneficial effects of the thermostable Rca recombinant protein was between 40 and 45°C, which corresponded to leaf temperatures in the range 38-42°C. The results suggest that the thermotolerant form of Rca from O. australiensis was sufficient to enhance carbohydrate accumulation and storage by rice over the life of the plant, dramatically improving yields after exposure to heat throughout the vegetative phase.

Strong thermal acclimation of photosynthesis in tropical and temperate wet-forest tree species: the importance of altered Rubisco content

Understanding of the extent of acclimation of light-saturated net photosynthesis (A_n ) to temperature (T), and associated underlying mechanisms, remains limited. This is a key knowledge gap given the importance of thermal acclimation for plant functioning, both under current and future higher temperatures, limiting the accuracy and realism of Earth system model (ESM) predictions. Given this, we analysed and modelled T-dependent changes in photosynthetic capacity in 10 wet-forest tree species: six from temperate forests and four from tropical forests. Temperate and tropical species were each acclimated to three daytime growth temperatures (T_growth ): temperate - 15, 20 and 25 °C; tropical - 25, 30 and 35 °C. CO₂ response curves of A_n were used to model maximal rates of RuBP (ribulose-1,5-bisphosphate) carboxylation (V_cmax ) and electron transport (J_max ) at each treatment's respective T_growth and at a common measurement T (25 °C). SDS-PAGE gels were used to determine abundance of the CO₂ -fixing enzyme, Rubisco. Leaf chlorophyll, nitrogen (N) and mass per unit leaf area (LMA) were also determined. For all species and T_growth , A_n at current atmospheric CO₂ partial pressure was Rubisco-limited. Across all species, LMA decreased with increasing T_growth . Similarly, area-based rates of V_cmax at a measurement T of 25 °C (V_cmax²⁵ ) linearly declined with increasing T_growth , linked to a concomitant decline in total leaf protein per unit leaf area and Rubisco as a percentage of leaf N. The decline in Rubisco constrained V_cmax and A_n for leaves developed at higher T_growth and resulted in poor predictions of photosynthesis by currently widely used models that do not account for T_growth -mediated changes in Rubisco abundance that underpin the thermal acclimation response of photosynthesis in wet-forest tree species. A new model is proposed that accounts for the effect of T_growth -mediated declines in V_cmax²⁵ on A_n , complementing current photosynthetic thermal acclimation models that do not account for T sensitivity of V_cmax²⁵ .

The Diverse AAA+ Machines that Repair Inhibited Rubisco Active Sites

Gaseous carbon dioxide enters the biosphere almost exclusively via the active site of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). This highly conserved catalyst has an almost universal propensity to non-productively interact with its substrate ribulose 1,5-bisphosphate, leading to the formation of dead-end inhibited complexes. In diverse autotrophic organisms this tendency has been counteracted by the recruitment of dedicated AAA+ (ATPases associated with various cellular activities) proteins that all use the energy of ATP hydrolysis to remodel inhibited Rubisco active sites leading to release of the inhibitor. Three evolutionarily distinct classes of these Rubisco activases (Rcas) have been discovered so far. Green and red-type Rca are mostly found in photosynthetic eukaryotes of the green and red plastid lineage respectively, whereas CbbQO is associated with chemoautotrophic bacteria. Ongoing mechanistic studies are elucidating how the various motors are utilizing both similar and contrasting strategies to ultimately perform their common function of cracking the inhibited Rubisco active site. The best studied mechanism utilized by red-type Rca appears to involve transient threading of the Rubisco large subunit C-terminal peptide, reminiscent of the action performed by Clp proteases. As well as providing a fascinating example of convergent molecular evolution, Rca proteins can be considered promising crop-improvement targets. Approaches aiming to replace Rubisco in plants with improved enzymes will need to ensure the presence of a compatible Rca protein. The thermolability of the Rca protein found in crop plants provides an opportunity to fortify photosynthesis against high temperature stress. Photosynthesis also appears to be limited by Rca when light conditions are fluctuating. Synthetic biology strategies aiming to enhance the autotrophic CO₂ fixation machinery will need to take into consideration the requirement for Rubisco activases as well as their properties.

Rubisco and Rubisco Activase Play an Important Role in the Biochemical Limitations of Photosynthesis in Rice, Wheat, and Maize under High Temperature and Water Deficit

To understand the effect of heat and drought on three major cereal crops, the physiological and biochemical (i.e., metabolic) factors affecting photosynthesis were examined in rice, wheat, and maize plants grown under long-term water deficit (WD), high temperature (HT) and the combination of both stresses (HT-WD). Diffusional limitations to photosynthesis prevailed under WD for the C3 species, rice and wheat. Conversely, biochemical limitations prevailed under WD for the C4 species, maize, under HT for all three species, and under HT-WD in rice and maize. These biochemical limitations to photosynthesis were associated with Rubisco activity that was highly impaired at HT and under HT-WD in the three species. Decreases in Rubisco activation were unrelated to the amount of Rubisco and Rubisco activase (Rca), but were probably caused by inhibition of Rca activity, as suggested by the mutual decrease and positive correlation between Rubisco activation state and the rate of electron transport. Decreased Rubisco activation at HT was associated with biochemical limitation of net CO2 assimilation rate (AN). Overall, the results highlight the importance of Rubisco as a target for improving the photosynthetic performance of these C3 (wheat and rice) and C4 (maize) cereal crops under increasingly variable and warmer climates.

Comparative Proteomic and Physiological Analysis Reveals the Variation Mechanisms of Leaf Coloration and Carbon Fixation in a Xantha Mutant of Ginkgo biloba L

Yellow-green leaf mutants are common in higher plants, and these non-lethal chlorophyll-deficient mutants are ideal materials for research on photosynthesis and plant development. A novel xantha mutant of Ginkgo biloba displaying yellow-colour leaves (YL) and green-colour leaves (GL) was identified in this study. The chlorophyll content of YL was remarkably lower than that in GL. The chloroplast ultrastructure revealed that YL had less dense thylakoid lamellae, a looser structure and fewer starch grains than GL. Analysis of the photosynthetic characteristics revealed that YL had decreased photosynthetic activity with significantly high nonphotochemical quenching. To explain these phenomena, we analysed the proteomic differences in leaves and chloroplasts between YL and GL of ginkgo using two-dimensional gel electrophoresis (2-DE) coupled with MALDI-TOF/TOF MS. In total, 89 differential proteins were successfully identified, 82 of which were assigned functions in nine metabolic pathways and cellular processes. Among them, proteins involved in photosynthesis, carbon fixation in photosynthetic organisms, carbohydrate/energy metabolism, amino acid metabolism, and protein metabolism were greatly enriched, indicating a good correlation between differentially accumulated proteins and physiological changes in leaves. The identifications of these differentially accumulated proteins indicates the presence of a specific different metabolic network in YL and suggests that YL possess slower chloroplast development, weaker photosynthesis, and a less abundant energy supply than GL. These studies provide insights into the mechanism of molecular regulation of leaf colour variation in YL mutants.

Heat tolerance in a wild Oryza species is attributed to maintenance of Rubisco activation by a thermally stable Rubisco activase ortholog

The mechanistic basis of tolerance to heat stress was investigated in Oryza sativa and two wild rice species, Oryza meridionalis and Oryza australiensis. The wild relatives are endemic to the hot, arid Australian savannah. Leaf elongation rates and gas exchange were measured during short periods of supra-optimal heat, revealing species differences. The Rubisco activase (RCA) gene from each species was sequenced. Using expressed recombinant RCA and leaf-extracted RCA, the kinetic properties of the two isoforms were studied under high temperatures. Leaf elongation was undiminished at 45°C in O. australiensis. The net photosynthetic rate was almost 50% slower in O. sativa at 45°C than at 28°C, while in O. australiensis it was unaffected. Oryza meridionalis exhibited intermediate heat tolerance. Based on previous reports that RCA is heat-labile, the Rubisco activation state was measured. It correlated positively with leaf elongation rates across all three species and four periods of exposure to 45°C. Sequence analysis revealed numerous polymorphisms in the RCA amino acid sequence from O. australiensis. The O. australiensis RCA enzyme was thermally stable up to 42°C, contrasting with RCA from O. sativa, which was inhibited at 36°C. We attribute heat tolerance in the wild species to thermal stability of RCA, enabling Rubisco to remain active.

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.

Dynamic photosynthesis in different environmental conditions

Incident irradiance on plant leaves often fluctuates, causing dynamic photosynthesis. Whereas steady-state photosynthetic responses to environmental factors have been extensively studied, knowledge of dynamic modulation of photosynthesis remains scarce and scattered. This review addresses this discrepancy by summarizing available data and identifying the research questions necessary to advance our understanding of interactions between environmental factors and dynamic behaviour of photosynthesis using a mechanistic framework. Firstly, dynamic photosynthesis is separated into sub-processes related to proton and electron transport, non-photochemical quenching, control of metabolite flux through the Calvin cycle (activation states of Rubisco and RuBP regeneration, and post-illumination metabolite turnover), and control of CO₂ supply to Rubisco (stomatal and mesophyll conductance changes). Secondly, the modulation of dynamic photosynthesis and its sub-processes by environmental factors is described. Increases in ambient CO₂ concentration and temperature (up to ~35°C) enhance rates of photosynthetic induction and decrease its loss, facilitating more efficient dynamic photosynthesis. Depending on the sensitivity of stomatal conductance, dynamic photosynthesis may additionally be modulated by air humidity. Major knowledge gaps exist regarding environmental modulation of loss of photosynthetic induction, dynamic changes in mesophyll conductance, and the extent of limitations imposed by stomatal conductance for different species and environmental conditions. The study of mutants or genetic transformants for specific processes under various environmental conditions could provide significant progress in understanding the control of dynamic photosynthesis.

Enhancing C3 photosynthesis: an outlook on feasible interventions for crop improvement

Despite the declarations and collective measures taken to eradicate hunger at World Food Summits, food security remains one of the biggest issues that we are faced with. The current scenario could worsen due to the alarming increase in world population, further compounded by adverse climatic conditions, such as increase in atmospheric temperature, unforeseen droughts and decreasing soil moisture, which will decrease crop yield even further. Furthermore, the projected increase in yields of C3 crops as a result of increasing atmospheric CO2 concentrations is much less than anticipated. Thus, there is an urgent need to increase crop productivity beyond existing yield potentials to address the challenge of food security. One of the domains of plant biology that promises hope in overcoming this problem is study of C3 photosynthesis. In this review, we have examined the potential bottlenecks of C3 photosynthesis and the strategies undertaken to overcome them. The targets considered for possible intervention include RuBisCO, RuBisCO activase, Calvin-Benson-Bassham cycle enzymes, CO2 and carbohydrate transport, and light reactions among many others. In addition, other areas which promise scope for improvement of C3 photosynthesis, such as mining natural genetic variations, mathematical modelling for identifying new targets, installing efficient carbon fixation and carbon concentrating mechanisms have been touched upon. Briefly, this review intends to shed light on the recent advances in enhancing C3 photosynthesis for crop improvement.

Evolutionary development of redox regulation in chloroplasts

SIGNIFICANCE

The post-translational modification of thiol groups stands out as a key strategy that cells employ for metabolic regulation and adaptation to changing environmental conditions. Nowhere is this more evident than in chloroplasts-the O2-evolving photosynthetic organelles of plant cells that are fitted with multiple redox systems, including the thioredoxin (Trx) family of oxidoreductases functional in the reversible modification of regulatory thiols of proteins in all types of cells. The best understood member of this family in chloroplasts is the ferredoxin-linked thioredoxin system (FTS) by which proteins are modified via light-dependent disulfide/dithiol (S-S/2SH) transitions.

RECENT ADVANCES

Discovered in the reductive activation of enzymes of the Calvin-Benson cycle in illuminated chloroplast preparations, recent studies have extended the role of the FTS far beyond its original boundaries to include a spectrum of cellular processes. Together with the NADP-linked thioredoxin reductase C-type (NTRC) and glutathione/glutaredoxin systems, the FTS also plays a central role in the response of chloroplasts to different types of stress.

CRITICAL ISSUES

The comparisons of redox regulatory networks functional in chloroplasts of land plants with those of cyanobacteria-prokaryotes considered to be the ancestors of chloroplasts-and different types of algae summarized in this review have provided new insight into the evolutionary development of redox regulation, starting with the simplest O2-evolving organisms.

FUTURE DIRECTIONS

The evolutionary appearance, mode of action, and specificity of the redox regulatory systems functional in chloroplasts, as well as the types of redox modification operating under diverse environmental conditions stand out as areas for future study.

The contribution of dynamic changes in photosynthesis to shade tolerance of two conifer species

Generally 'shade tolerance' refers to the capacity of a plant to exist at low light levels but characteristics of shade can vary and must be taken into account in defining the term. We studied Abies amabilis Dougl. ex J.Forbes and Tsuga heterophylla (Raf.) Sarg. under a forest canopy in the northwest of the Olympic Peninsula, USA, which has low annual sunshine hours and frequent overcast days. Using BF3 sunshine sensors, we surveyed diffuse and total light received by saplings growing under a range of canopy openness up to 30%. We measured variation in photosynthetic capacity over the growing season and within days and estimated photosynthesis induction in relation to ambient light. Three components of shade tolerance are associated with variation in light climate: (i) Total light on the floor of an 88-year stand of naturally regenerated T. heterophylla was greater on overcast than clear days. Light on overcast days varied throughout the day sometimes with a cyclical pattern. (ii) Photosynthetic capacity, Amax, varied both through the growing season and within days. Amax was generally greater in the latter part of the growing season, being limited by temperature and stomatal conductance, gs, at times during the early part. Saplings in more shaded areas had lower Amax and in the latter part of the growing season Amax was found to decline from mid-afternoon. (iii) Two patterns of photosynthesis induction to increased light were found. In a mean ambient light of 139 μmol m(-2) s(-1), induction had a curvilinear response to a step increase in light with a mean time constant, τ, of 112.3 s. In a mean ambient light of 74 μmol m(-2) s(-1), induction had a two-part increase: one with τ1 of 11.3 s and the other with τ2 of 184.0 s. These are the smallest published values of τ to date. (iv) Both variation in photosynthetic capacity and induction are components of shade tolerance where light varies over time. Amax acclimates to seasonal and diurnal changes in light and varies between microenvironments. The rapid induction processes can cause a rapid response of photosynthesis to changes in diffuse or direct light.

Substitutions at the opening of the Rubisco central solvent channel affect holoenzyme stability and CO2/O 2 specificity but not activation by Rubisco activase

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the initial step of carbon metabolism in photosynthesis. The holoenzyme comprises eight large subunits, arranged as a tetramer of dimers around a central solvent channel that defines a fourfold axis of symmetry, and eight small subunits, arranged as two tetramers at the poles of the axis. The phylogenetically divergent small-subunit loops between β-strands A and B form the entrance to the solvent channel. In the green alga Chlamydomonas reinhardtii, Ile-58 from each of the four small-subunit βA-βB loops defines the minimal diameter of the channel opening. To understand the role of the central solvent channel in Rubisco function, directed mutagenesis and transformation of Chlamydomonas were employed to replace Ile-58 with Ala, Lys, Glu, Trp, or three Trp residues (I58W3) to close the entrance to the channel. The I58E, I58K, and I58W substitutions caused only small decreases in photosynthetic growth at 25 and 35 °C, whereas I58W3 had a substantial effect at both temperatures. The mutant enzymes had decreased carboxylation rates, but the I58W3 enzyme had decreases in both carboxylation and CO2/O2 specificity. The I58E, I58W, and I58W3 enzymes were inactivated at lower temperatures than wild-type Rubisco, and were degraded at slower rates under oxidative stress. However, these mutant enzymes were activated by Rubisco activase at normal rates, indicating that the structural transition required for carboxylation is not affected by altering the solvent channel opening. Structural dynamics alone may not be responsible for these distant effects on the Rubisco active site.

Activation of interspecies-hybrid Rubisco enzymes to assess different models for the Rubisco-Rubisco activase interaction

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is prone to inactivation from non-productive binding of sugar-phosphates. Reactivation of Rubisco requires conformational remodeling by a specific chaperone, Rubisco activase. Rubisco activase from tobacco and other plants in the family Solanaceae is an inefficient activator of Rubisco from non-Solanaceae plants and from the green alga Chlamydomonas reinhardtii. To determine if the Rubisco small subunit plays a role in the interaction with Rubisco activase, a hybrid Rubisco (SSNT) composed of tobacco small subunits and Chlamydomonas large subunits was constructed. The SSNT hybrid, like other hybrid Rubiscos containing plant small subunits, supported photoautotrophic growth in Chlamydomonas, but growth in air was much slower than for cells containing wild-type Rubisco. The kinetic properties of the SSNT hybrid Rubisco were similar to the wild-type enzyme, indicating that the poor growth in air was probably caused by disruption of pyrenoid formation and the consequent impairment of the CO2concentrating mechanism. Recombinant Rubisco activase from Arabidopsis activated the SSNT hybrid Rubisco and hybrid Rubiscos containing spinach and Arabidopsis small subunits at rates similar to the rates with wild-type Rubisco. However, none of the hybrid Rubiscos was activated by tobacco Rubisco activase. That replacement of Chlamydomonas small subunits with plant small subunits does not affect the species-specific interaction between Rubisco and Rubisco activase suggests that the association is not dominated by the small subunits that surround the Rubisco central solvent channel. Therefore, the geometry of a side-on binding mode is more consistent with the data than a top-on or ring-stacking binding mode.

Characterization and expression of Rubisco activase genes in Ipomoea batatas

Two-dimensional electrophoresis, coupled with MALDI-TOF-MS, was used to identify differentially expressed proteins between young and mature leaves of sweet potato [Ipomoea batatas (L.) Lam]. The results showed that there were 25 differential proteins between young and mature leaves. The Rubisco activase (RCA) that catalyzes the activation of Rubisco in vivo and plays a crucial role in photosynthesis was among these 25 proteins. So far, little was known about the molecular biology of RCA in sweet potato. Here, this research reports the cloning and characterization of two genes encoding the short isoform and the long isoform of sweet potato RCAs. Analysis of DNA sequences of RCA suggested that the corresponding mRNAs were transcribed from two different genes. To study the roles of these two RCA isoforms in photosynthesis, we investigated the expression patterns of these RCA genes at the mRNA and protein levels every 2 h in a photoperiod and under different temperatures conditions. The results indicated that these two RCA isoforms may play different roles in regulating photosynthesis and they may be regulated by light, heat or both. In addition, there were interactions between Rubisco large subunit (RBCl) and short isoform RCA (RCAs) as well as RCAs and long isoform RCA (RCAl), but no interaction between RBCl and RCAl, implying they might form a sandwich-like structure (RBCl–RCAs–RCAl), at least in yeast cells. These results provided new information on the modulation of RCA genes in sweet potato, which could be useful in improving photosynthesis and plant growth in sweet potato.

The regulatory properties of Rubisco activase differ among species and affect photosynthetic induction during light transitions

Rubisco's catalytic chaperone, Rubisco activase (Rca), uses the energy from ATP hydrolysis to restore catalytic competence to Rubisco. In Arabidopsis (Arabidopsis thaliana), inhibition of Rca activity by ADP is fine tuned by redox regulation of the α-isoform. To elucidate the mechanism for Rca regulation in species containing only the redox-insensitive β-isoform, the response of activity to ADP was characterized for different Rca forms. When assayed in leaf extracts, Rubisco activation was significantly inhibited by physiological ratios of ADP to ATP in species containing both α-Rca and β-Rca (Arabidopsis and camelina [Camelina sativa]) or just the β-Rca (tobacco [Nicotiana tabacum]). However, Rca activity was insensitive to ADP inhibition in an Arabidopsis transformant, rwt43, which expresses only Arabidopsis β-Rca, although not in a transformant of Arabidopsis that expresses a tobacco-like β-Rca. ATP hydrolysis by recombinant Arabidopsis β-Rca was much less sensitive to inhibition by ADP than recombinant tobacco β-Rca. Mutation of 17 amino acids in the tobacco β-Rca to the corresponding Arabidopsis residues reduced ADP sensitivity. In planta, Rubisco deactivated at low irradiance except in the Arabidopsis rwt43 transformant containing an ADP-insensitive Rca. Induction of CO2 assimilation after transition from low to high irradiance was much more rapid in the rwt43 transformant compared with plants containing ADP-sensitive Rca forms. The faster rate of photosynthetic induction and a greater enhancement of growth under a fluctuating light regime by the rwt43 transformant compared with wild-type Arabidopsis suggests that manipulation of Rca regulation might provide a strategy for enhancing photosynthetic performance in certain variable light environments.

Rubisco activity and regulation as targets for crop improvement

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

The temperature response of CO2 assimilation, photochemical activities and Rubisco activation in Camelina sativa, a potential bioenergy crop with limited capacity for acclimation to heat stress

The temperature optimum of photosynthesis coincides with the average daytime temperature in a species' native environment. Moderate heat stress occurs when temperatures exceed the optimum, inhibiting photosynthesis and decreasing productivity. In the present study, the temperature response of photosynthesis and the potential for heat acclimation was evaluated for Camelina sativa, a bioenergy crop. The temperature optimum of net CO(2) assimilation rate (A) under atmospheric conditions was 30-32 °C and was only slightly higher under non-photorespiratory conditions. The activation state of Rubisco was closely correlated with A at supra-optimal temperatures, exhibiting a parallel decrease with increasing leaf temperature. At both control and elevated temperatures, the modeled response of A to intercellular CO(2) concentration was consistent with Rubisco limiting A at ambient CO(2). Rubisco activation and photochemical activities were affected by moderate heat stress at lower temperatures in camelina than in the warm-adapted species cotton and tobacco. Growth under conditions that imposed a daily interval of moderate heat stress caused a 63 % reduction in camelina seed yield. Levels of cpn60 protein were elevated under the higher growth temperature, but acclimation of photosynthesis was minimal. Inactivation of Rubisco in camelina at temperatures above 35 °C was consistent with the temperature response of Rubisco activase activity and indicated that Rubisco activase was a prime target of inhibition by moderate heat stress in camelina. That photosynthesis exhibited no acclimation to moderate heat stress will likely impact the development of camelina and other cool season Brassicaceae as sources of bioenergy in a warmer world.

Protein oligomerization monitored by fluorescence fluctuation spectroscopy: self-assembly of rubisco activase

A methodology is presented to characterize complex protein assembly pathways by fluorescence correlation spectroscopy. We have derived the total autocorrelation function describing the behavior of mixtures of labeled and unlabeled protein under equilibrium conditions. Our modeling approach allows us to quantitatively consider the relevance of any proposed intermediate form, and K(d) values can be estimated even when several oligomeric species coexist. We have tested this method on the AAA+ ATPase Rubisco activase (Rca). Rca self-association regulates the CO(2) fixing activity of the enzyme Rubisco, directly affecting biomass accumulation in higher plants. However, the elucidation of its assembly pathway has remained challenging, precluding a detailed mechanistic investigation. Here, we present the first, to our knowledge, thermodynamic characterization of oligomeric states of cotton β-Rca complexed with Mg·ADP. We find that the monomer is the dominating species below 0.5 micromolar. The most plausible model supports dissociation constants of ∼4, 1, and 1 micromolar for the monomer-dimer, dimer-tetramer, and tetramer-hexamer equilibria, in line with the coexistence of four different oligomeric forms under typical assay conditions. Large aggregates become dominant above 40 micromolar, with continued assembly at even higher concentrations. We propose that under some conditions, ADP-bound Rca self-associates by forming spiral arrangements that grow along the helical axis. Other models such as the stacking of closed hexameric rings are also discussed.

Changes at the 3'-untranslated region stabilize Rubisco activase transcript levels during heat stress in Arabidopsis

Inhibition of photosynthesis by heat stress is accompanied by functional impairment of Rubisco's chaperone, activase (RCA), resulting in deactivation of Rubisco. Since activase is extremely sensitive to thermal denaturation, changes in expression of RCA at the transcript or protein level could provide a mechanism for acclimation of photosynthesis to prolonged heat stress. Using quantitative real-time PCR (qPCR) we show steady-state RCA transcript levels in Arabidopsis thaliana are stabilized during prolonged exposure to moderate heat (35  °C). A survey of RCA transcripts indicates heat stress did not alter the relative abundance of transcripts encoding α and β-isoforms of activase that are produced by alternative splicing of the pre-mRNA. Instead, mRNA stabilization in heat-stressed plants coincided with a significant reduction in the average length of activase 3'-untranslated regions, and was associated with enrichment of an uncharacterized activase mRNA splice variant, AtRCAβ2. Transcript-specific qPCR revealed AtRCAβ2 mRNA was more stable than AtRCAα and AtRCAβ mRNA in heat-stressed plants. Using an inducible transgenic system, we found that RCA transcripts lacking their native 3'-untranslated region were significantly more stable than their full-length counterparts in vivo. Using this system, stability of the RCA protein was examined over 24 h in vivo, in the absence of RCA transcription. At both optimal and elevated temperatures, RCA protein levels remained stable in plants lacking RCA mRNA, but increased when RCA mRNA was present, particularly in heat-stressed plants. This study reveals a possible mechanism, involving post-transcriptional regulation of an important photosynthesis regulatory gene, for acclimation of photosynthesis to heat stress.

Osmosensitive changes of carbohydrate metabolism in response to cellulose biosynthesis inhibition

Cellulose is the most abundant biopolymer in the world, the main load-bearing element in plant cell walls, and represents a major sink for carbon fixed during photosynthesis. Previous work has shown that photosynthetic activity is partially regulated by carbohydrate sinks. However, the coordination of cellulose biosynthesis with carbohydrate metabolism and photosynthesis is not well understood. Here, we demonstrate that cellulose biosynthesis inhibition (CBI) leads to reductions in transcript levels of genes involved in photosynthesis, the Calvin cycle, and starch degradation in Arabidopsis (Arabidopsis thaliana) seedlings. In parallel, we show that CBI induces changes in carbohydrate distribution and influences Rubisco activase levels. We find that the effects of CBI on gene expression and carbohydrate metabolism can be neutralized by osmotic support in a concentration-dependent manner. However, osmotic support does not suppress CBI-induced metabolic changes in seedlings impaired in mechanoperception (mid1 complementing activity1 [mca1]) and osmoperception (cytokinin receptor1 [cre1]) or reactive oxygen species production (respiratory burst oxidase homolog DF [rbohDF]). These results show that carbohydrate metabolism is responsive to changes in cellulose biosynthesis activity and turgor pressure. The data suggest that MCA1, CRE1, and RBOHDF-derived reactive oxygen species are involved in the regulation of osmosensitive metabolic changes. The evidence presented here supports the notion that cellulose and carbohydrate metabolism may be coordinated via an osmosensitive mechanism.