Uncertainties and limitations of using carbon-13 and oxygen-18 leaf isotope exchange to estimate the temperature response of mesophyll CO2 conductance in C3 plants

The internal CO₂ gradient imposed by mesophyll conductance (g_m ) reduces substrate availability for C₃ photosynthesis. With several assumptions, estimates of g_m can be made from coupled leaf gas exchange with isoflux analysis of carbon ∆¹³ C-g_m and oxygen in CO₂ , coupled with transpired water (H₂ O) ∆¹⁸ O-g_m to partition g_m into its biochemical and anatomical components. However, these assumptions require validation under changing leaf temperatures. To test these assumptions, we measured and modeled the temperature response (15-40°C) of ∆¹³ C-g_m and ∆¹⁸ O-g_m along with leaf biochemistry in the C₃ grass Panicum bisulcatum, which has naturally low carbonic anhydrase activity. Our study suggests that assumptions regarding the extent of isotopic equilibrium (θ) between CO₂ and H₂ O at the site of exchange, and that the isotopic composition of the H₂ O at the sites of evaporation ( δw-e18 ) and at the site of exchange ( δw-ce18 ) are similar, may lead to errors in estimating the ∆¹⁸ O-g_m temperature response. The input parameters for ∆¹³ C-g_m appear to be less sensitive to temperature. However, this needs to be tested in species with diverse carbonic anhydrase activity. Additional information on the temperature dependency of cytosolic and chloroplastic pH may clarify uncertainties used for ∆¹⁸ O-g_m under changing leaf temperatures.

Insights from transcriptome profiling on the non-photosynthetic and stomatal signaling response of maize carbonic anhydrase mutants to low CO2

BACKGROUND

Carbonic anhydrase (CA) catalyzes the hydration of CO2 in the first biochemical step of C4 photosynthesis, and has been considered a potentially rate-limiting step when CO2 availability within a leaf is low. Previous work in Zea mays (maize) with a double knockout of the two highest-expressed β-CA genes, CA1 and CA2, reduced total leaf CA activity to less than 3% of wild-type. Surprisingly, this did not limit photosynthesis in maize at ambient or higher CO2concentrations. However, the ca1ca2 mutants exhibited reduced rates of photosynthesis at sub-ambient CO2, and accumulated less biomass when grown under sub-ambient CO2 (9.2 Pa). To further clarify the importance of CA for C4 photosynthesis, we assessed gene expression changes in wild-type, ca1 and ca1ca2 mutants in response to changes in pCO2 from 920 to 9.2 Pa.

RESULTS

Leaf samples from each genotype were collected for RNA-seq analysis at high CO2 and at two time points after the low CO2 transition, in order to identify early and longer-term responses to CO2 deprivation. Despite the existence of multiple isoforms of CA, no other CA genes were upregulated in CA mutants. Although photosynthetic genes were downregulated in response to low CO2, differential expression was not observed between genotypes. However, multiple indicators of carbon starvation were present in the mutants, including amino acid synthesis, carbohydrate metabolism, and sugar signaling. In particular, multiple genes previously implicated in low carbon stress such as asparagine synthetase, amino acid transporters, trehalose-6-phosphate synthase, as well as many transcription factors, were strongly upregulated. Furthermore, genes in the CO2 stomatal signaling pathway were differentially expressed in the CA mutants under low CO2.

CONCLUSIONS

Using a transcriptomic approach, we showed that carbonic anhydrase mutants do not compensate for the lack of CA activity by upregulating other CA or photosynthetic genes, but rather experienced extreme carbon stress when grown under low CO2. Our results also support a role for CA in the CO2 stomatal signaling pathway. This study provides insight into the importance of CA for C4 photosynthesis and its role in stomatal signaling.

A single serine to alanine substitution decreases bicarbonate affinity of phosphoenolpyruvate carboxylase in C4Flaveria trinervia

Phosphoenolpyruvate (PEP) carboxylase (PEPc) catalyzes the first committed step of C4 photosynthesis generating oxaloacetate from bicarbonate (HCO3-) and PEP. It is hypothesized that PEPc affinity for HCO3- has undergone selective pressure for a lower KHCO3 (Km for HCO3-) to increase the carbon flux entering the C4 cycle, particularly during conditions that limit CO2 availability. However, the decrease in KHCO3 has been hypothesized to cause an unavoidable increase in KPEP (Km for PEP). Therefore, the amino acid residue S774 in the C4 enzyme, which has been shown to increase KPEP, should lead to a decrease in KHCO3. Several studies reported the effect S774 has on KPEP; however, the influence of this amino acid substitution on KHCO3 has not been tested. To test these hypotheses, membrane-inlet mass spectrometry (MIMS) was used to measure the KHCO3 of the photosynthetic PEPc from the C4Flaveria trinervia and the non-photosynthetic PEPc from the C3F. pringlei. The cDNAs for these enzymes were overexpressed and purified from the PEPc-less PCR1 Escherichia coli strain. Our work in comparison with previous reports suggests that KHCO3 and KPEP are linked by specific amino acids, such as S774; however, these kinetic parameters respond differently to the tested allosteric regulators, malate and glucose-6-phosphate.

Temperature response of Rubisco kinetics in Arabidopsis thaliana: thermal breakpoints and implications for reaction mechanisms

Enhancement of Rubisco kinetics could improve photosynthetic efficiency, ultimately resulting in increased crop yield. However, imprecise knowledge of the reaction mechanism and the individual rate constants limits our ability to optimize the enzyme. Membrane inlet mass spectrometry (MIMS) may offer benefits over traditional methods for determining individual rate constants of the Rubisco reaction mechanism, as it can directly monitor concentration changes in CO2, O2, and their isotopologs during assays. However, a direct comparison of MIMS with the traditional radiolabel method of determining Rubisco kinetic parameters has not been made. Here, the temperature responses of Rubisco kinetic parameters from Arabidopsis thaliana were measured using radiolabel and MIMS methods. The two methods provided comparable parameters above 25 °C, but temperature responses deviated at low temperature as MIMS-derived catalytic rates of carboxylation, oxygenation, and CO2/O2 specificity showed thermal breakpoints. Here, we discuss the variability and uncertainty surrounding breakpoints in the Rubisco temperature response and the relevance of individual rate constants of the reaction mechanisms to potential breakpoints.

Components of Water Use Efficiency Have Unique Genetic Signatures in the Model C4 Grass Setaria

Plant growth and water use are interrelated processes influenced by genetically controlled morphological and biochemical characteristics. Improving plant water use efficiency (WUE) to sustain growth in different environments is an important breeding objective that can improve crop yields and enhance agricultural sustainability. However, genetic improvement of WUE using traditional methods has proven difficult due to the low throughput and environmental heterogeneity of field settings. To overcome these limitations, this study utilizes a high-throughput phenotyping platform to quantify plant size and water use of an interspecific Setaria italica × Setaria viridis recombinant inbred line population at daily intervals in both well-watered and water-limited conditions. Our findings indicate that measurements of plant size and water use are correlated strongly in this system; therefore, a linear modeling approach was used to partition this relationship into predicted values of plant size given water use and deviations from this relationship at the genotype level. The resulting traits describing plant size, water use, and WUE all were heritable and responsive to soil water availability, allowing for a genetic dissection of the components of plant WUE under different watering treatments. Linkage mapping identified major loci underlying two different pleiotropic components of WUE. This study indicates that alleles controlling WUE derived from both wild and domesticated accessions can be utilized to predictably modulate trait values given a specified precipitation regime in the model C4 genus Setaria.

Diffusion of CO2 across the Mesophyll-Bundle Sheath Cell Interface in a C4 Plant with Genetically Reduced PEP Carboxylase Activity

Phosphoenolpyruvate carboxylase (PEPC), localized to the cytosol of the mesophyll cell, catalyzes the first carboxylation step of the C₄ photosynthetic pathway. Here, we used RNA interference to target the cytosolic photosynthetic PEPC isoform in Setaria viridis and isolated independent transformants with very low PEPC activities. These plants required high ambient CO₂ concentrations for growth, consistent with the essential role of PEPC in C₄ photosynthesis. The combination of estimating direct CO₂ fixation by the bundle sheath using gas-exchange measurements and modeling C₄ photosynthesis with low PEPC activity allowed the calculation of bundle sheath conductance to CO₂ diffusion (g_bs ) in the progeny of these plants. Measurements made at a range of temperatures suggested no or negligible effect of temperature on g_bs depending on the technique used to calculate g_bs Anatomical measurements revealed that plants with reduced PEPC activity had reduced cell wall thickness and increased plasmodesmata (PD) density at the mesophyll-bundle sheath (M-BS) cell interface, whereas we observed little difference in these parameters at the mesophyll-mesophyll cell interface. The increased PD density at the M-BS interface was largely driven by an increase in the number of PD pit fields (cluster of PDs) rather than an increase in PD per pit field or the size of pit fields. The correlation of g_bs with bundle sheath surface area per leaf area and PD area per M-BS area showed that these parameters and cell wall thickness are important determinants of g_bs It is intriguing to speculate that PD development is responsive to changes in C₄ photosynthetic flux.

Carbonic Anhydrase Mutants in Zea mays Have Altered Stomatal Responses to Environmental Signals

Stomata regulate transpirational water loss and CO2 uptake for photosynthesis in response to changing environmental conditions. Research investigating stomatal movement has mostly been conducted in C3 eudicot species, which have very different CO2 requirements for photosynthesis relative to C4 grasses. Carbonic anhydrase (CA) catalyzes the hydration of CO2, and its activity has been linked to stomatal aperture regulation in eudicots. The number of Ca genes and their evolutionary history differ between monocots and dicots, and many questions remain unanswered about potential neofunctionalization and subfunctionalization of grass Ca paralogs and their roles in photosynthesis and stomatal conductance. To investigate the roles of different Ca genes in maize (Zea mays), we examined stomatal responses in ca1 and ca2 single mutants as well as a ca1ca2 double mutant. The ca1 and ca2 single mutants had 10% and 87% of the CA activity exhibited by the wild type, respectively, while ca1ca2 had less than 5% of wild-type CA activity. The ca mutants had higher stomatal conductance than the wild type and slower stomatal closure in response to increases in CO2 partial pressure. Contrary to previous reports in eudicots, ca mutants showed slowed stomatal closure in response to the light-dark transition and did not show differences in stomatal density compared with the wild type. These results implicate CA-mediated signaling in the control of stomatal movement but not stomatal development. Drought experiments with ca1ca2 mutant plants suggest a role for CA in water-use efficiency and reveal that Z. mays is not optimized for water-use efficiency under well-watered conditions.

Cell wall properties in Oryza sativa influence mesophyll CO2 conductance

Diffusion of CO₂ from the leaf intercellular air space to the site of carboxylation (g_m ) is a potential trait for increasing net rates of CO₂ assimilation (A_net ), photosynthetic efficiency, and crop productivity. Leaf anatomy plays a key role in this process; however, there are few investigations into how cell wall properties impact g_m and A_net . Online carbon isotope discrimination was used to determine g_m and A_net in Oryza sativa wild-type (WT) plants and mutants with disruptions in cell wall mixed-linkage glucan (MLG) production (CslF6 knockouts) under high- and low-light growth conditions. Cell wall thickness (T_cw ), surface area of chloroplast exposed to intercellular air spaces (S_c ), leaf dry mass per area (LMA), effective porosity, and other leaf anatomical traits were also analyzed. The g_m of CslF6 mutants decreased by 83% relative to the WT, with c. 28% of the reduction in g_m explained by S_c . Although A_net /LMA and A_net /Chl partially explained differences in A_net between genotypes, the change in cell wall properties influenced the diffusivity and availability of CO₂ . The data presented here indicate that the loss of MLG in CslF6 plants had an impact on g_m and demonstrate the importance of cell wall effective porosity and liquid path length on g_m .

Biochemical and transcriptomic analysis of maize diversity to elucidate drivers of leaf carbon isotope composition

Carbon isotope discrimination is used to study CO2 diffusion, substrate availability for photosynthesis, and leaf biochemistry, but the intraspecific drivers of leaf carbon isotope composition (δ13C) in C4 species are not well understood. In this study, the role of photosynthetic enzymes and post-photosynthetic fractionation on δ13C (‰) was explored across diverse maize inbred lines. A significant 1.3‰ difference in δ13C was observed between lines but δ13C did not correlate with in vitro leaf carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), or ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity. RNA-sequencing was used to identify potential differences in post-photosynthetic metabolism that would influence δ13C; however, no correlations were identified that would indicate significant differences in post-photosynthetic fractionation between lines. Variation in δ13C has been observed between C4 subtypes, but differential expression of NADP-ME and PEP-CK pathways within these lines did not correlate with δ13C. However, co-expression network analysis provided novel evidence for isoforms of C4 enzymes and putative transporters. Together, these data indicate that diversity in maize δ13C cannot be fully explained by variation in CA, PEPC, or Rubisco activity or gene expression. The findings further emphasise the need for future work exploring the influence of stomatal sensitivity and mesophyll conductance on δ13C in maize.

Mesophyll conductance in Zea mays responds transiently to CO2 availability: implications for transpiration efficiency in C4 crops

Mesophyll conductance (g_m ) describes the movement of CO₂ from the intercellular air spaces below the stomata to the site of initial carboxylation in the mesophyll. In contrast with C₃ -g_m , little is currently known about the intraspecific variation in C₄ -g_m or its responsiveness to environmental stimuli. To address these questions, g_m was measured on five maize (Zea mays) lines in response to CO₂ , employing three different estimates of g_m . Each of the methods indicated a significant response of g_m to CO₂ . Estimates of g_m were similar between methods at ambient and higher CO₂ , but diverged significantly at low partial pressures of CO₂ . These differences are probably driven by incomplete chemical and isotopic equilibrium between CO₂ and bicarbonate under these conditions. Carbonic anhydrase and phosphoenolpyruvate carboxylase in vitro activity varied significantly despite similar values of g_m and leaf anatomical traits. These results provide strong support for a CO₂ response of g_m in Z. mays, and indicate that g_m in maize is probably driven by anatomical constraints rather than by biochemical limitations. The CO₂ response of g_m indicates a potential role for facilitated diffusion in C₄ -g_m . These results also suggest that water-use efficiency could be enhanced in C₄ species by targeting g_m .

The response of mesophyll conductance to short-term variation in CO2 in the C4 plants Setaria viridis and Zea mays

Mesophyll conductance (gm) limits rates of C3 photosynthesis but little is known about its role in C4 photosynthesis. If gm were to limit C4 photosynthesis, it would likely be at low CO2 concentrations (pCO2). However, data on C4-gm across ranges of pCO2 are scarce. We describe the response of C4-gm to short-term variation in pCO2, at three temperatures in Setaria viridis, and at 25 °C in Zea mays. Additionally, we quantified the effect of finite gm calculations of leakiness (ϕ) and the potential limitations to photosynthesis imposed by stomata, mesophyll, and carbonic anhydrase (CA) across pCO2. In both species, gm increased with decreasing pCO2. Including a finite gm resulted in either no change or increased ϕ compared with values calculated with infinite gm depending on whether the observed 13C discrimination was high (Setaria) or low (Zea). Post-transitional regulation of the maximal PEP carboxylation rate and PEP regeneration limitation could influence estimates of gm and ϕ. At pCO2 below ambient, the photosynthetic rate was limited by CO2 availability. In this case, the limitation imposed by the mesophyll was similar or slightly lower than stomata limitation. At very low pCO2, CA further constrained photosynthesis. High gm could increase CO2 assimilation at low pCO2 and improve photosynthetic efficiency under situations when CO2 is limited, such as drought.

Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C4 grasses Setaria viridis and Setaria italica

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (gs) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ13CBL) and bulk leaf oxygen enrichment above source water (Δ18OBL) with gas exchange and TE in the model C4 grasses Setaria viridis and S. italica. Leaf δ13C had strong relationships with E, gs, water use, biomass, and TE. Additionally, the consistent difference in δ13CBL between well-watered and water-limited plants suggests that δ13CBL is effective in separating C4 plants with different availability of water. Alternatively, the use of Δ18OBL as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ18OLW), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ18OLW and Δ18OBL with E. Therefore, the data collected here suggest that δ13CBL but not Δ18OBL may be an effective proxy for TE in C4 grasses.

Temperature response of mesophyll conductance in three C4 species calculated with two methods: 18 O discrimination and in vitro Vpmax

Mesophyll conductance (g_m ) is an important factor limiting rates of C₃ photosynthesis. However, its role in C₄ photosynthesis is poorly understood because it has been historically difficult to estimate. We use two methods to derive the temperature responses of g_m in C₄ species. The first (Δ¹⁸ O) combines measurements of gas exchange with models and measurements of ¹⁸ O discrimination. The second method (in vitro V_pmax ) derives g_m by retrofitting models of C₄ photosynthesis and ¹³ C discrimination with gas exchange, kinetic constants and in vitro V_pmax measurements. The two methods produced similar g_m for Setaria viridis and Zea mays. Additionally, we present the first temperature response (10-40°C) of C₄ g_m in S. viridis, Z. mays and Miscanthus × giganteus. Values for g_m at 25°C ranged from 2.90 to 7.85 μmol m^-2  s^-1  Pa^-1 . Our study demonstrated that: the two described methods are suitable to calculate g_m in C₄ species; g_m values in C₄ are similar to high-end values reported for C₃ species; and g_m increases with temperature analogous to reports for C₃ species and the response is species specific. These results improve our mechanistic understanding of C₄ photosynthesis.

Influence of light and nitrogen on the photosynthetic efficiency in the C4 plant Miscanthus × giganteus

There are numerous studies describing how growth conditions influence the efficiency of C₄ photosynthesis. However, it remains unclear how changes in the biochemical capacity versus leaf anatomy drives this acclimation. Therefore, the aim of this study was to determine how growth light and nitrogen availability influence leaf anatomy, biochemistry and the efficiency of the CO₂ concentrating mechanism in Miscanthus × giganteus. There was an increase in the mesophyll cell wall surface area but not cell well thickness in the high-light (HL) compared to the low-light (LL) grown plants suggesting a higher mesophyll conductance in the HL plants, which also had greater photosynthetic capacity. Additionally, the HL plants had greater surface area and thickness of bundle-sheath cell walls compared to LL plants, suggesting limited differences in bundle-sheath CO₂ conductance because the increased area was offset by thicker cell walls. The gas exchange estimates of phosphoenolpyruvate carboxylase (PEPc) activity were significantly less than the in vitro PEPc activity, suggesting limited substrate availability in the leaf due to low mesophyll CO₂ conductance. Finally, leakiness was similar across all growth conditions and generally did not change under the different measurement light conditions. However, differences in the stable isotope composition of leaf material did not correlate with leakiness indicating that dry matter isotope measurements are not a good proxy for leakiness. Taken together, these data suggest that the CO₂ concentrating mechanism in Miscanthus is robust under low-light and limited nitrogen growth conditions, and that the observed changes in leaf anatomy and biochemistry likely help to maintain this efficiency.

Structural insights into the LCIB protein family reveals a new group of β-carbonic anhydrases

Aquatic microalgae have evolved diverse CO2-concentrating mechanisms (CCMs) to saturate the carboxylase with its substrate, to compensate for the slow kinetics and competing oxygenation reaction of the key photosynthetic CO2-fixing enzyme rubisco. The limiting CO2-inducible B protein (LCIB) is known to be essential for CCM function in Chlamydomonas reinhardtii To assign a function to this previously uncharacterized protein family, we purified and characterized a phylogenetically diverse set of LCIB homologs. Three of the six homologs are functional carbonic anhydrases (CAs). We determined the crystal structures of LCIB and limiting CO2-inducible C protein (LCIC) from C. reinhardtii and a CA-functional homolog from Phaeodactylum tricornutum, all of which harbor motifs bearing close resemblance to the active site of canonical β-CAs. Our results identify the LCIB family as a previously unidentified group of β-CAs, and provide a biochemical foundation for their function in the microalgal CCMs.

The draft genome of the C3 panicoid grass species Dichanthelium oligosanthes

BACKGROUND

Comparisons between C3 and C4 grasses often utilize C3 species from the subfamilies Ehrhartoideae or Pooideae and C4 species from the subfamily Panicoideae, two clades that diverged over 50 million years ago. The divergence of the C3 panicoid grass Dichanthelium oligosanthes from the independent C4 lineages represented by Setaria viridis and Sorghum bicolor occurred approximately 15 million years ago, which is significantly more recent than members of the Bambusoideae, Ehrhartoideae, and Pooideae subfamilies. D. oligosanthes is ideally placed within the panicoid clade for comparative studies of C3 and C4 grasses.

RESULTS

We report the assembly of the nuclear and chloroplast genomes of D. oligosanthes, from high-throughput short read sequencing data and a comparative transcriptomics analysis of the developing leaf of D. oligosanthes, S. viridis, and S. bicolor. Physiological and anatomical characterizations verified that D. oligosanthes utilizes the C3 pathway for carbon fixation and lacks Kranz anatomy. Expression profiles of transcription factors along developing leaves of D. oligosanthes and S. viridis were compared with previously published data from S. bicolor, Zea mays, and Oryza sativa to identify a small suite of transcription factors that likely acquired functions specifically related to C4 photosynthesis.

CONCLUSIONS

The phylogenetic location of D. oligosanthes makes it an ideal C3 plant for comparative analysis of C4 evolution in the panicoid grasses. This genome will not only provide a better C3 species for comparisons with C4 panicoid grasses, but also highlights the power of using high-throughput sequencing to address questions in evolutionary biology.

Protection of the photosynthetic apparatus against dehydration stress in the resurrection plant Craterostigma pumilum

The group of homoiochlorophyllous resurrection plants evolved the unique capability to survive severe drought stress without dismantling the photosynthetic machinery. This implies that they developed efficient strategies to protect the leaves from reactive oxygen species (ROS) generated by photosynthetic side reactions. These strategies, however, are poorly understood. Here, we performed a detailed study of the photosynthetic machinery in the homoiochlorophyllous resurrection plant Craterostigma pumilum during dehydration and upon recovery from desiccation. During dehydration and rehydration, C. pumilum deactivates and activates partial components of the photosynthetic machinery in a specific order, allowing for coordinated shutdown and subsequent reinstatement of photosynthesis. Early responses to dehydration are the closure of stomata and activation of electron transfer to oxygen accompanied by inactivation of the cytochrome b6 f complex leading to attenuation of the photosynthetic linear electron flux (LEF). The decline in LEF is paralleled by a gradual increase in cyclic electron transport to maintain ATP production. At low water contents, inactivation and supramolecular reorganization of photosystem II becomes apparent, accompanied by functional detachment of light-harvesting complexes and interrupted access to plastoquinone. This well-ordered sequence of alterations in the photosynthetic thylakoid membranes helps prepare the plant for the desiccated state and minimize ROS production.

Carbon isotopes and water use efficiency in C4 plants

Drought is a major agricultural problem worldwide. Therefore, selection for increased water use efficiency (WUE) in food and biofuel crop species will be an important trait in plant breeding programs. The leaf carbon isotopic composition (δ(13)Cleaf) has been suggested to serve as a rapid and effective high throughput phenotyping method for WUE in both C3 and C4 species. This is because WUE, leaf carbon discrimination (Δ(13)Cleaf), and δ(13)Cleaf are correlated through their relationships with intercellular to ambient CO2 partial pressures (Ci/Ca). However, in C4 plants, changing environmental conditions may influence photosynthetic efficiency (bundle-sheath leakiness) and post-photosynthetic fractionation that will potentially alter the relationship between δ(13)Cleaf and Ci/Ca. Here we discuss how these factors influence the relationship between δ(13)Cleaf and WUE, and the potential of using δ(13)Cleaf as a meaningful proxy for WUE.

Targeted Knockdown of GDCH in Rice Leads to a Photorespiratory-Deficient Phenotype Useful as a Building Block for C4 Rice

The glycine decarboxylase complex (GDC) plays a critical role in the photorespiratory C2 cycle of C3 species by recovering carbon following the oxygenation reaction of ribulose-1,5-bisphosphate carboxylase/oxygenase. Loss of GDC from mesophyll cells (MCs) is considered a key early step in the evolution of C4 photosynthesis. To assess the impact of preferentially reducing GDC in rice MCs, we decreased the abundance of OsGDCH (Os10g37180) using an artificial microRNA (amiRNA) driven by a promoter that preferentially drives expression in MCs. GDC H- and P-proteins were undetectable in leaves of gdch lines. Plants exhibited a photorespiratory-deficient phenotype with stunted growth, accelerated leaf senescence, reduced chlorophyll, soluble protein and sugars, and increased glycine accumulation in leaves. Gas exchange measurements indicated an impaired ability to regenerate ribulose 1,5-bisphosphate in photorespiratory conditions. In addition, MCs of gdch lines exhibited a significant reduction in chloroplast area and coverage of the cell wall when grown in air, traits that occur during the later stages of C4 evolution. The presence of these two traits important for C4 photosynthesis and the non-lethal, down-regulation of the photorespiratory C2 cycle positively contribute to efforts to produce a C4 rice prototype.

The unique structural and biochemical development of single cell C4 photosynthesis along longitudinal leaf gradients in Bienertia sinuspersici and Suaeda aralocaspica (Chenopodiaceae)

Temporal and spatial patterns of photosynthetic enzyme expression and structural maturation of chlorenchyma cells along longitudinal developmental gradients were characterized in young leaves of two single cell C4 species, Bienertia sinuspersici and Suaeda aralocaspica Both species partition photosynthetic functions between distinct intracellular domains. In the C4-C domain, C4 acids are formed in the C4 cycle during capture of atmospheric CO2 by phosphoenolpyruvate carboxylase. In the C4-D domain, CO2 released in the C4 cycle via mitochondrial NAD-malic enzyme is refixed by Rubisco. Despite striking differences in origin and intracellular positioning of domains, these species show strong convergence in C4 developmental patterns. Both progress through a gradual developmental transition towards full C4 photosynthesis, with an associated increase in levels of photosynthetic enzymes. Analysis of longitudinal sections showed undeveloped domains at the leaf base, with Rubisco rbcL mRNA and protein contained within all chloroplasts. The two domains were first distinguishable in chlorenchyma cells at the leaf mid-regions, but still contained structurally similar chloroplasts with equivalent amounts of rbcL mRNA and protein; while mitochondria had become confined to just one domain (proto-C4-D). The C4 state was fully formed towards the leaf tips, Rubisco transcripts and protein were compartmentalized specifically to structurally distinct chloroplasts in the C4-D domains indicating selective regulation of Rubisco expression may occur by control of transcription or stability of rbcL mRNA. Determination of CO2 compensation points showed young leaves were not functionally C4, consistent with cytological observations of the developmental progression from C3 default to intermediate to C4 photosynthesis.

Kranz and single-cell forms of C4 plants in the subfamily Suaedoideae show kinetic C4 convergence for PEPC and Rubisco with divergent amino acid substitutions

The two carboxylation reactions performed by phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) are vital in the fixation of inorganic carbon for C4 plants. The abundance of PEPC is substantially elevated in C4 leaves, while the location of Rubisco is restricted to one of two chloroplast types. These differences compared with C3 leaves have been shown to result in convergent enzyme optimization in some C4 species. Investigation into the kinetic properties of PEPC and Rubisco from Kranz C4, single cell C4, and C3 species in Chenopodiaceae s. s. subfamily Suaedoideae showed that these major carboxylases in C4 Suaedoideae species lack the same mutations found in other C4 systems which have been examined; but still have similar convergent kinetic properties. Positive selection analysis on the N-terminus of PEPC identified residues 364 and 368 to be under positive selection with a posterior probability >0.99 using Bayes empirical Bayes. Compared with previous analyses on other C4 species, PEPC from C4 Suaedoideae species have different convergent amino acids that result in a higher K m for PEP and malate tolerance compared with C3 species. Kinetic analysis of Rubisco showed that C4 species have a higher catalytic efficiency of Rubisco (k catc in mol CO2 mol(-1) Rubisco active sites s(-1)), despite lacking convergent substitutions in the rbcL gene. The importance of kinetic changes to the two-carboxylation reactions in C4 leaves related to amino acid selection is discussed.

Temperature Responses of C4 Photosynthesis: Biochemical Analysis of Rubisco, Phosphoenolpyruvate Carboxylase, and Carbonic Anhydrase in Setaria viridis

The photosynthetic assimilation of CO2 in C4 plants is potentially limited by the enzymatic rates of Rubisco, phosphoenolpyruvate carboxylase (PEPc), and carbonic anhydrase (CA). Therefore, the activity and kinetic properties of these enzymes are needed to accurately parameterize C4 biochemical models of leaf CO2 exchange in response to changes in CO2 availability and temperature. There are currently no published temperature responses of both Rubisco carboxylation and oxygenation kinetics from a C4 plant, nor are there known measurements of the temperature dependency of the PEPc Michaelis-Menten constant for its substrate HCO3 (-), and there is little information on the temperature response of plant CA activity. Here, we used membrane inlet mass spectrometry to measure the temperature responses of Rubisco carboxylation and oxygenation kinetics, PEPc carboxylation kinetics, and the activity and first-order rate constant for the CA hydration reaction from 10°C to 40°C using crude leaf extracts from the C4 plant Setaria viridis. The temperature dependencies of Rubisco, PEPc, and CA kinetic parameters are provided. These findings describe a new method for the investigation of PEPc kinetics, suggest an HCO3 (-) limitation imposed by CA, and show similarities between the Rubisco temperature responses of previously measured C3 species and the C4 plant S. viridis.