Biology subjects

von Caemmerer, S.

Publications and source records attributed to von Caemmerer, S..

17 recordsLinked to original sources

bioRxiv2026

PsbS confers limited adaptive benefit to C4 photosynthesis under fluctuating light

Adaptation of plant photosynthesis to dynamic light conditions experienced in natural environments is achieved through specific protective mechanisms. Energy-dependent non-photochemical quenching (qE), regulated by Photosystem II Subunit S (PsbS), is a key process facilitating acclimation to fluctuating light in C3 plants, which operate conventional photosynthesis. C4 plants, which include some of the worlds most productive and agriculturally important crops, have evolved a distinct high-efficiency photosynthetic pathway. Little is known about the role of specific processes, like qE, in acclimation of C4 plants to dynamic light environments. We generated gene-edited lines of the model C4 grass Setaria viridis lacking PsbS, which were found to be deficient in qE. This deficiency resulted in a modest increase in PSII photoinhibition and a CO2 assimilation penalty under light stress in short-term experiments, but photosynthesis and growth under fluctuating light were unaffected. Instead, keeping Photosystem I oxidised through photosynthetic control, negative feedback regulation of the Cytochrome b6f complex, was critical. Therefore, unlike in C3 plants, qE does not provide a significant adaptive advantage to C4 plants under dynamic light conditions. These findings provide important insights into the biology of C4 plants and help prioritise future strategies for improving the productivity and resilience of C4 crops.

plant biology↗

bioRxiv2026

C4 photosynthetic pathway fluxes in transgenic rice plants

Most land plants photosynthesize using the C3 pathway, in which ribulose bisphosphate carboxylase/oxygenase (Rubisco) fixes CO2 into 3-carbon acids. The C4 pathway, a biochemical CO2-concentrating mechanism that operates in the context of specialized leaf anatomy to concentrate CO2 around Rubisco, is more efficient. Introduction of the C4 pathway into the C3 crop rice could increase yield by 50%. Expression of five C4 enzymes in transgenic rice previously led to flux through the first step. However, there was no evidence for flux later in the cycle. Here we developed new transgenic rice lines and novel protocols to detect C4 cycle activity: CO2 fixation into C4 acids by carboxylation of a C3 compound, decarboxylation, refixation of CO2 by Rubisco, and regeneration of the C3 donor. We demonstrate that these four core C4 reactions are operating in rice, establishing the in vivo flux framework needed to progress towards a functional carbon-concentrating mechanism

plant biology↗

bioRxiv2026

The Role of Phosphoenolpyruvate Carboxylase-Protein Kinase in C4 Photosynthesis: Insights from Zea mays Mutant Analysis

Understanding the regulation of enzyme activity involved in photosynthesis is essential for engineering enhanced carbon fixation in crops. In C4 plants, the enzyme phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) is one of the most abundant leaf enzymes and plays an essential role in photosynthetic carbon dioxide (CO2) fixation. The enzyme also plays a key role in central metabolism (e.g., providing intermediates to the citric acid cycle) and therefore must be highly regulated to coordinate its activity. The regulation of PEPC activity can occur allosterically by glucose 6-phosphate activation and malate inhibition, which is in part influenced by reversible phosphorylation. A specific light-dependent phosphorylation of PEPC at an N-terminal serine residue by the PEPC-protein kinase (PEPC-PK) can regulate its sensitivity to this allosteric regulation. However, the impact of this PEPC phosphorylation has not been tested in a C4 crop. Therefore, we created PEPC-PK mutant lines in Zea mays to assess the impact of PEPC phosphorylation on its allosteric regulation, photosynthesis, and growth. While the maximum PEPC activity was unchanged, PEPC in the PEPC-PK mutant plants was not phosphorylated under light and was more sensitive to malate inhibition. However, gas exchange, electron transport, and field biomass analyses showed no differences in the PEPC-PK mutant plants. These results demonstrate that in Z. mays PEPC phosphorylation affects enzyme sensitivity to malate in vitro but does not substantially alert photosynthetic performance or growth under field conditions suggesting additional regulation of PEPC activity in planta.

plant biology↗

bioRxiv2026

Trade-offs between photosynthetic capacity, mesophyll conductance stability and leaf anatomy shape heat and water deficit resilience in Gossypium.

O_LIMesophyll conductance (gm) governs CO2 diffusion to Rubisco and is a key determinant of photosynthetic performance, yet the mechanisms underlying its sensitivity to heat and water stress remain unresolved. C_LIO_LIWe quantified gm temperature responses across diverse Gossypium species and examined anatomical drivers of gm plasticity in cultivated cotton (G. hirsutum) and the wild Australian species G. bickii under elevated temperature and soil water deficit. C_LIO_LISpecies exhibited contrasting gm strategies: G. hirsutum exhibited high gm and carbon assimilation near thermal optima but showed greater sensitivity under combined heat and water deficit, whereas G. bickii maintained comparatively stable gm and photosynthesis across stress conditions. C_LIO_LIUnder water deficit, structural adjustments in G. hirsutum (increased leaf porosity, cell wall thickness and mesophyll surface exposure to intercellular airspaces) were insufficient to sustain gm, suggesting that liquid-phase resistances impose dominant constraints on CO2 diffusion under extreme climatic stress. C_LIO_LIThese results identify gm as a dynamic, multi-component trait and a key physiological vulnerability in cotton, shaped by coordinated anatomical characteristics and potentially cell wall properties and membrane-associated processes, with major implications for mechanistic photosynthesis modelling and improving climate resilience in cotton and other C3 species. C_LI

plant biology↗

bioRxiv2025

Fitting photosynthetic carbon dioxide and irradiance response curves for C4 leaves

Gas exchange measurements provide crucial insights into the complex mechanisms of photosynthesis. Responses of CO2 assimilation rate (A) to intercellular CO2 partial pressure (Ci) and irradiance (I) link gas exchange measurements to the underlying photosynthetic biochemistry of a leaf. The unique biochemistry and leaf anatomy which distinguish C4 photosynthesis make it necessary to apply models and fitting routines which appropriately parameterise and incorporate these characteristics. Here we provide updates to the C4 photosynthesis model by improving the parameterisation of cyclic electron flow in C4 photosynthesis using experimentally derived values from S. viridis. We additionally describe two fitting routines for assessing C4 photosynthesis based on the updated model. Fitting of a CO2 response curve (A/Ci) provides estimates of maximum PEP carboxylase activity (Vpmax) and maximum Rubisco activity (Vcmax), and calculates the electron transport rate (J) needed to sustain the measured CO2 assimilation rate (A). Fitting of an irradiance response curve (A/I) provides estimates for the maximum electron transport rate (Jmax), day respiration rate (Rd), the quantum yield ({phi}CO2) and light compensation point ({Gamma}light). Values of the above output parameters are provided at both the measurement temperature and at 25 {degrees}C for ease of comparative reporting. The fitting tool has been designed in Microsoft Excel to minimise barriers to entry and enable simplicity of fitting while simultaneously catering to individuals with diverse expertise and experience in C4 gas exchange modelling.

plant biology↗

bioRxiv2024

PGR5 promotes energy-dependent non-photochemical quenching to enable efficient C4 photosynthesis under fluctuating light

PROTON GRADIENT REGULATION 5 (PGR5) is essential for generating proton motive force across thylakoid membranes in C3 plants and supporting photoprotection under fluctuating light conditions. It is proposed that this function is achieved by regulating cyclic electron flow around Photosystem I. During the evolutionary transition from C3 to C4 photosynthesis, the leaf abundance of PGR5 has increased, coinciding with a rise in the cyclic electron flow rate. To investigate the contribution of PGR5 to photoprotection in C4 photosynthesis, we generated model C4 monocot Setaria viridis with null pgr5 alleles. We show that plants lacking PGR5 struggle to establish proton motive force and energy-dependent non-photochemical quenching (qE) at higher irradiances during instantaneous measurements. This leads to a progressive decline in maximum Photosystem I activity when leaves are exposed to repeated cycles of high irradiance. Additionally, plants without PGR5 exhibit severely reduced growth and photosynthesis compared to wild type plants when grown under fluctuating daylight but not under constant daylight. In the absence of PGR5, a slower-relaxing, zeaxanthin-dependent form of non-photochemical quenching supports growth under fluctuating light, albeit at the cost of reduced photochemical efficiency and assimilation rate. Our findings underscore the role of PGR5 in enabling efficient C4 photosynthesis under fluctuating light by establishing proton motive force for the rapid upregulation of qE and preventing photodamage to the electron transport machinery. This study highlights the importance of various non-photochemical quenching mechanisms for C4 photosynthesis and emphasises the role of PGR5 in the evolution of C4 plants.

plant biology↗

bioRxiv2024

Automated and high throughput measurement of leaf stomatal traits in canola

BackgroundAutomating stomatal trait measurement has gained popularity because of their inherent importance for field phenotyping application as stomata are critical for both carbon capture and water use efficiency in plants. Such tool has been reported for rice, wheat, tomato, barley and oil palm. However, none exist yet for canola, which is an important economic and agronomic crop globally. ResultsWe developed a new toolkit called Stomatal Comprehensive Automated Neural Network or SCAN by combining the use of high-resolution portable digital microscopy with machine learning based on You Only Look Once algorithm (YOLOv8). Digital micrographs of leaf surfaces enter the SCAN pipeline, which includes stomata detection, stomata segmentation and stomatal pore segmentation models, to measure stomatal density, stomatal size and stomatal pore area, respectively. In addition to SCANs ability to measure leaf stomatal traits in canola at 89 to 94% accuracy, we also showed that SCAN can be used to predict stomatal density even in species not included in the training set such as Arabidopsis, tobacco, rice, wheat, maize and proso millet. SCAN was designed for the biological science community with the premise that users are not required to possess advanced programming capabilities to manage dependency prerequisites, execute the models, and integrate the analysis. This was achieved by packaging the models into a desktop application system that can be accessed offline. ConclusionOverall, SCAN provides a non-destructive, real-time, portable, and high-throughput measurement of leaf stomatal traits in canola. The minimised hardware requirement and user-friendly desktop application system make SCAN suitable for field phenotyping application.

plant biology↗

bioRxiv2023

Systems analysis of long-term heat stress responses in the C4 grass Setaria viridis

A substantial number of C4 plants are utilized as food and fodder crops and often display improved resource use efficiency compared to C3 counterparts. However, their response to future extreme climates such as heatwaves is less understood. Setaria viridis, an emerging C4 model grass closely related to important C4 crops, was grown under high temperature for two weeks (42{degrees}C as compared to 28{degrees}C). High temperature resulted in stunted growth, but surprisingly had little impact on leaf area based photosynthetic rates. Rates of dark respiration significantly increased and there were major alterations in carbon and nitrogen metabolism in the heat-stressed plants, including reduced starch levels, accumulation of soluble sugars and an increase in leaf nitrogen content. Measurements of major phytohormones revealed a dramatic increase in abscisic acid in the heat-stressed plants. Leaf transcriptomics, proteomics and metabolomics analyses were carried out and mapped onto metabolic pathways of photosynthesis, respiration, carbon/nitrogen metabolism and hormone synthesis and signaling. Overall, upregulation of a number of stress-signaling pathways was observed, consistent with multiple potent signals leading to reduced plant growth. A systems model of plant response is presented based on oxidative stress, hormone and sugar signaling pathways.

plant biology↗

bioRxiv2023

NDH complex-mediated cyclic electron flow in bundle sheath cells enables C4 photosynthesis

The superior productivity of C4 plants is achieved via a metabolic C4 cycle which acts as a CO2 pump across mesophyll and bundle sheath (BS) cells and requires an additional input of energy in the form of ATP. Chloroplast NADH dehydrogenase-like complex (NDH) increases ATP production in C3 plants by operating cyclic electron flow (CEF) around Photosystem I (PSI), and its importance for C4 photosynthesis has been proposed from evolutionary and reverse genetics studies. We used the gene-edited C4 species Setaria viridis with null ndhO alleles lacking NDH to study a contribution of the complex to the cell-level electron transport. Our results indicate that NDH is the primary PSI electron acceptor mediating the majority of CEF in BS cells whilst the contribution of the complex to CEF in mesophyll cells is minimal. Moreover, the reduced leaf CO2 assimilation rate and growth of plants lacking the complex cannot be rescued by supplying additional CO2, indicating that NDH is essential for generating ATP required for CO2 fixation by the C3 cycle. Hereby we resolve a cell-level mechanism for the contribution of NDH to supporting high CO2 assimilation rates in C4 photosynthesis.

plant biology↗

bioRxiv2022

Faster responses of photosynthesis to light transitions increase biomass and grain yield in transgenic Sorghum bicolor overexpressing Rieske FeS

Sorghum is one of the most important crops providing food and feed in many of the worlds harsher environments. Sorghum utilises the C4 pathway of photosynthesis in which a biochemical carbon concentrating mechanism results in high CO2 assimilation rates. Overexpressing the Rieske subunit of the Cytochrome b6f complex was previously shown to increase the rate of photosynthetic electron transport and stimulate CO2 assimilation in the model C4 plant Setaria viridis. To test whether productivity of C4 crops could be improved by Rieske overexpression, we created transgenic Sorghum bicolor plants with increased Rieske content. The transgenic plants showed no marked changes in abundance of other photosynthetic proteins or chlorophyll content. Increases in yield of Photosystem II and CO2 assimilation rate as well as faster responses of non-photochemical quenching during transient photosynthetic responses were observed as a result of an elevated in vivo Cytochrome b6f activity in plants overexpressing Rieske. The steady-state rates of electron transport and CO2 assimilation did not differ between transgenic and control plants, suggesting that Cytochrome b6f is not the only factor limiting electron transport in sorghum at high light and high CO2. Nevertheless, more agile responses of photosynthesis to light transitions led to increases in biomass and grain yield in plants overexpressing Rieske. Our results indicate that increasing Rieske content could boost productivity of C4 crops by improving the efficiency of light utilisation and conversion to biomass.

plant biology↗

bioRxiv2022

Rieske FeS overexpression in tobacco provides increased abundance and activity of Cytochrome b6f

Photosynthesis is fundamental for plant growth and yield. The Cytochrome b6f complex catalyses a rate-limiting step in thylakoid electron transport and therefore represents an important point of regulation of photosynthesis. Here we show that overexpression of a single core subunit of Cytochrome b6f, the Rieske FeS protein, led to up to a 40% increase in the abundance of the complex in Nicotiana tabacum (tobacco) and was accompanied by an enhanced in vitro Cytochrome f activity, indicating a full functionality of the complex. Analysis of transgenic plants overexpressing Rieske FeS by the light-induced fluorescence transients technique revealed a more oxidised primary quinone acceptor of Photosystem II (QA) and plastoquinone pool and a faster electron transport from the plastoquinone pool to Photosystem I upon changes in irradiance, compared to control plants. A faster establishing of qE, the energy-dependent component of non-photochemical quenching, in transgenic plants suggested a more rapid build-up of the transmembrane proton gradient, also supporting the increased in vivo Cytochrome b6f activity. However, there was no consistent increase in steady-state rates of electron transport or CO2 assimilation in plants overexpressing Rieske FeS grown in either laboratory conditions or in field trials, suggesting that the in vivo activity of the complex was only transiently increased upon changes in irradiance. Our results show that overexpression of Rieske FeS in tobacco enhances abundance of functional Cytochrome b6f and electron transport capacity and may have a potential to increase plant productivity if combined with other traits. One-sentence summaryIncreased abundance of Cytochrome b6f complex leads to transient increases in photosynthetic electron transport rate in tobacco.

plant biology↗

bioRxiv2022

Increased sedoheptulose-1,7-bisphosphatase content in the C4 species Setaria viridis does not affect photosynthesis

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and, in C3 plants, increasing the abundance of SBPase is known to provide higher photosynthetic rates and stimulate biomass and yield. C4 plants usually have higher photosynthetic rates because they operate a biochemical CO2 concentrating mechanism between mesophyll and bundle sheath cells. In the C4 system, SBPase and other enzymes of Calvin cycle are localised to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C4 photosynthesis. Using Setaria viridis, a model C4 plant of NADP-ME subtype, we created transgenic plants with 1.5 to 3.2-times higher SBPase content, compared to wild type plants. Transcripts of the transgene were found predominantly in the bundle sheaths suggesting the correct cellular localisation of the protein. Abundance of RBCL, the large subunit of Rubisco, was not affected in transgenic plants overexpressing SBPase, and neither was relative chlorophyll content or photosynthetic electron transport parameters. We found no correlation between SBPase content in S. viridis and saturating rates of CO2 assimilation. Moreover, detailed analysis of CO2 assimilation rates at different CO2 partial pressure, irradiance and leaf temperature, showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss potential implications of these results for understanding the regulation of C4 photosynthesis.

plant biology↗

bioRxiv2021

Expression of a CO2-permeable aquaporin enhances mesophyll conductance in the C4 species Setaria viridis

A fundamental limitation of photosynthetic carbon fixation is the availability of CO4. In C4 plants, primary carboxylation occurs in mesophyll cytosol, and little is known about the role of CO2 diffusion in facilitating C4 photosynthesis. We have examined the expression, localization, and functional role of selected plasma membrane intrinsic aquaporins (PIPs) from Setaria italica (foxtail millet) and discovered that SiPIP2;7 is CO2-permeable. When ectopically expressed in mesophyll cells of S. viridis (green foxtail), SiPIP2;7 was localized to the plasma membrane and caused no marked changes in leaf biochemistry. Gas-exchange and C18O16O discrimination measurements revealed that targeted expression of SiPIP2;7 enhanced the conductance to CO2 diffusion from the intercellular airspace to the mesophyll cytosol. Our results demonstrate that mesophyll conductance limits C4 photosynthesis at low pCO2 and that SiPIP2;7 is a functional CO2 permeable aquaporin that can improve CO2 diffusion at the airspace/mesophyll interface and enhance C4 photosynthesis.

plant biology↗

bioRxiv2021

Updating the steady state model of C4 photosynthesis

C4 plants play a key role in world agriculture. For example, C4 crops such as maize and sorghum are major contributors to both first and third world food production and the C4 grasses sugarcane; miscanthus and switchgrass are major plant sources of bioenergy. In the challenge to manipulate and enhance C4 photosynthesis, steady state models of leaf photosynthesis provide and important tool for gas exchange analysis and thought experiments that can explore photosynthetic pathway changes. Here the C4 photosynthetic model by von Caemmerer and Furbank (1999) has been updated with new kinetic parameterisation and temperature dependencies added. The parameterisation was derived from experiments on the C4 monocot, Setaria viridis, which for the first time provides a cohesive parametrisation. Mesophyll conductance and its temperature dependence have also been included, as this is an important step in the quantitative correlation between the initial slope of the CO2 response curve of CO2 assimilation and in vitro PEP carboxylase activity. Furthermore, the equations for chloroplast electron transport have been updated to include cyclic electron transport flow and equations have been added to calculate electron transport rate from measured CO2 assimilation rates. HighlightThe C4 photosynthesis model by von Caemmerer and Furbank (1999) has been updated. It now includes temperature dependencies and equations to calculate electron transport rate from measured CO2 assimilation rates.

plant biology↗

bioRxiv2021

Up-regulation of bundle sheath electron transport capacity under limiting light in C4 Setaria viridis

C4 photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO2 concentration at the site of CO2 fixation. C4 plants benefit from high irradiance but their efficiency decreases under shade causing a loss of productivity in crop canopies. We investigated shade acclimation responses of a model NADP-ME monocot Setaria viridis focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO2 assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio all increased in LL BS cells indicating a higher capacity for linear electron flow. PSI, ATP synthase, Cytochrome b6f and the chloroplastic NAD(P) dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were all upregulated in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts was associated with the more oxidised redox state of the plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results provide evidence of a redox regulation of the supramolecular composition of Photosystem II in BS cells in response to shading. This newly identified link contributes to understanding the regulation of PSII activity in C4 plants and will support strategies for crop improvement including the engineering of C4 photosynthesis into C3 plants. Significance statementThe efficiency of C4 photosynthesis decreases under low irradiance causing a loss of productivity in crop canopies. We investigate shade acclimation of a model NADP-ME monocot, analysing cell-specific protein expression and electron transport capacity. We propose a regulatory pathway controlling abundance and activity of Photosystem II in bundle sheath cells in response to irradiance.

plant biology↗

bioRxiv2020

The maize Aliphatic Suberin Feruloyl Transferase genes affect leaf water movement but are dispensable for bundle sheath CO2 concentration

C4 grasses often outperform C3 species under hot, arid conditions due to superior water and nitrogen use efficiencies and lower rates of photorespiration. A method of concentrating CO2 around the site of carbon fixation in the bundle sheath (BS) is required to realize these gains. In NADP-malic enzyme (NADP-ME)-type C4 grasses such as maize, suberin deposition in the BS cell wall is hypothesized to act as a diffusion barrier to CO2 escape and O2 entry from surrounding mesophyll cells. Suberin is a heteropolyester comprised of acyl-lipid-derived aliphatic and phenylpropanoid-derived aromatic components. To disrupt BS suberization, we mutated two paralogously duplicated, unlinked maize orthologues of Arabidopsis thaliana ALIPHATIC SUBERIN FERULOYL TRANSFERASE, ZmAsft1 and ZmAsft2, using closely linked Dissociation transposons. Loss-of-function double mutants revealed a 97% reduction in suberin-specific omega-hydroxy fatty acids without a stoichiometric decrease in ferulic acid. However, BS suberin lamellae were deficient in electron opaque material, and cohesion between the suberin lamellae and polysaccharide cell walls was attenuated in double mutants. There were no other morphological phenotypes under ambient conditions. Furthermore, there was no significant effect on net CO2 assimilation at any intercellular CO2 concentration, and no effect on 13C isotope discrimination relative to wild type. Thus, ZmAsft expression is not required to establish a functional CO2 concentrating mechanism in in maize. Double mutant leaves exhibit elevated cell wall elasticity, transpirational, and stomatal conductance relative to WT. Thus, the ZmAsft genes are dispensable for gas exchange barrier function but may be involved in regulation of leaf water movement. One-sentence SummaryDouble mutants of two paralogously duplicated maize Aliphatic Suberin Feruloyl Transferase (ZmAsft) genes exhibit reduced aliphatic suberin content, cell wall cohesion defects, and elevated leaf transpiration, but no changes in CO2 assimilation relative to wild type.

plant biology↗

bioRxiv2019

Overexpression of the Rieske FeS protein of the Cytochrome b6f complex increases C4 photosynthesis

C4 plants contribute 20% to the global primary productivity despite representing only 4% of higher plant species. Their CO2 concentrating mechanism operating between mesophyll and bundle sheath cells increases CO2 partial pressure at the site of Rubisco and hence photosynthetic efficiency. Electron transport chains in both cell types supply ATP and NADPH for C4 photosynthesis. Since Cytochrome b6f is a key point of control of electron transport in C3 plants, we constitutively overexpressed the Rieske FeS subunit in Setaria viridis to study the effects on C4 photosynthesis. Rieske FeS overexpression resulted in a higher content of Cytochrome b6f in both mesophyll and bundle sheath cells without marked changes in abundances of other photosynthetic complexes and Rubisco. Plants with higher Cytochrome b6f abundance showed better light conversion efficiency in both Photosystems and could generate higher proton-motive force across the thylakoid membrane. Rieske FeS abundance correlated with CO2 assimilation rate and plants with a 10% increase in Rieske FeS content showed a 10% increase in CO2 assimilation rate at ambient and saturating CO2 and high light. Our results demonstrate that Cytochrome b6f controls the rate of electron transport in C4 plants and that removing electron transport limitations can increase the rate of C4 photosynthesis.

plant biology↗