Stopping the leaks: new insights into C4 photosynthesis at low light

C4 monocots and C4 dicots exhibit rapid photosynthetic induction response in contrast to C3 plants

Considering the prevalence of ever-changing conditions in the natural world, investigation of photosynthetic responses in C4 plants under fluctuating light is needed. Here, we studied the effect of dynamic illumination on photosynthesis in totally 10 C3, C3-C4 intermediate, C4-like and C4 dicots and monocots at CO2 concentrations of 400 and 800 μmol mol-1. C4 and C4-like plants had faster photosynthetic induction and light-induced stomatal dynamics than C3 plants at 400 μmol mol-1, but not at 800 μmol mol-1 CO2, at which the CO2 supply rarely limits photosynthesis. C4 and C4-like plants had a higher water use efficiency than C3 plants at both CO2 concentrations. There were positive correlations between photosynthetic induction and light-induced stomatal response, together with CO2 compensation point, which was a parameter of the CO2-concentrating mechanism of C4 photosynthesis. These results clearly show that C4 photosynthesis in both monocots and dicots adapts to fluctuating light conditions more efficiently than C3 photosynthesis. The rapid photosynthetic induction response in C4 plants can be attributed to the rapid stomatal dynamics, the CO2-concentrating mechanism or both.

Differential eco-physiological performance to declining groundwater depth in Central Asian C3 and C4 shrubs in the Gurbantunggut Desert

Resources in water-limited ecosystems are highly variable and unpredictable, and the maintenance of functional diversity among coexisting species is a crucial ecological strategy through which plants mitigate environmental stress. The comparison of differential eco-physiological responses among co-occurring plants in harsh environments could help provide deep insights into the coexistence mechanisms of competing species. Two coexisting desert shrubs with different photosynthetic pathways (Haloxylon ammodendron and Tamarix ramosissima) were selected in the Gurbantunggut Desert located in northwest China. This study detected variations in the water sources, photosynthetic parameters, stem water status, and non-structural carbohydrates of the two shrubs at three sites with different groundwater table depths during the growing seasons of 2015 and 2016 to identify distinct eco-physiological performances in coexisting plants with different functional types under fluctuating water conditions. The water sources of H. ammodendron shifted from soil water to groundwater, while T. ramosissima extracted water mainly from deep soil layers at both sites. Significant reductions in carbon assimilation and stomatal conductance in H. ammodendron with deeper groundwater table depth were detected during most drought periods, but no significant decreases in transpiration rate were detected with declining groundwater table depth. For T. ramosissima, all of these gas exchange parameters decreased with the progression of summer drought, and their relative reduction rates were larger compared with those of H. ammodendron. The stem water status of H. ammodendron deteriorated, and the relative reduction rates of water potential increased with deeper groundwater, whereas those of T. ramosissima did not differ with greater groundwater depth. These findings indicated that prolonged drought would intensify the impact of declining groundwater depth on the eco-physiology of both shrubs, but the extent to which the shrubs would respond differed. The two shrubs were segregated along the water-carbon balance continuum: the C3 shrub T. ramosissima maximized its carbon fixation at an enormous cost of water, while greater carbon fixation was achieved with far greater water economy for H. ammodendron. These results demonstrated that the two shrubs prioritized carbon gain and water loss differently when faced with limited water sources. These mechanisms might mitigate competitive stress and enable their coexistence.

Forty years of research into crassulacean acid metabolism in the genus Clusia: anatomy, ecophysiology and evolution

Clusia is the only genus containing dicotyledonous trees with a capacity to perform crassulacean acid metabolism (CAM). Since the discovery of CAM in Clusia 40 years ago, several studies have highlighted the extraordinary plasticity and diversity of life forms, morphology and photosynthetic physiology of this genus. In this review, we revisit aspects of CAM photosynthesis in Clusia and hypothesize about the timing, the environmental conditions and potential anatomical characteristics that led to the evolution of CAM in the group. We discuss the role of physiological plasticity in influencing species distribution and ecological amplitude in the group. We also explore patterns of allometry of leaf anatomical traits and their correlations with CAM activity. Finally, we identify opportunities for further research on CAM in Clusia, such as the role of elevated nocturnal accumulation of citric acid, and gene expression in C3-CAM intermediate phenotypes.

13CO2 labeling kinetics in maize reveal impaired efficiency of C4 photosynthesis under low irradiance

C4 photosynthesis allows faster photosynthetic rates and higher water and nitrogen use efficiency than C3 photosynthesis, but at the cost of lower quantum yield due to the energy requirement of its biochemical carbon concentration mechanism. It has also been suspected that its operation may be impaired in low irradiance. To investigate fluxes under moderate and low irradiance, maize (Zea mays) was grown at 550 µmol photons m-2 s-l and 13CO2 pulse-labeling was performed at growth irradiance or several hours after transfer to 160 µmol photons m-2 s-1. Analysis by liquid chromatography/tandem mass spectrometry or gas chromatography/mass spectrometry provided information about pool size and labeling kinetics for 32 metabolites and allowed estimation of flux at many steps in C4 photosynthesis. The results highlighted several sources of inefficiency in low light. These included excess flux at phosphoenolpyruvate carboxylase, restriction of decarboxylation by NADP-malic enzyme, and a shift to increased CO2 incorporation into aspartate, less effective use of metabolite pools to drive intercellular shuttles, and higher relative and absolute rates of photorespiration. The latter provides evidence for a lower bundle sheath CO2 concentration in low irradiance, implying that operation of the CO2 concentration mechanism is impaired in this condition. The analyses also revealed rapid exchange of carbon between the Calvin-Benson cycle and the CO2-concentration shuttle, which allows rapid adjustment of the balance between CO2 concentration and assimilation, and accumulation of large amounts of photorespiratory intermediates in low light that provides a major carbon reservoir to build up C4 metabolite pools when irradiance increases.

Plant families exhibit unique geographic trends in C4 richness and cover in Australia

Numerous studies have analysed the relationship between C4 plant cover and climate. However, few have examined how different C4 taxa vary in their response to climate, or how environmental factors alter C4:C3 abundance. Here we investigate (a) how proportional C4 plant cover and richness (relative to C3) responds to changes in climate and local environmental factors, and (b) if this response is consistent among families. Proportional cover and richness of C4 species were determined at 541 one-hectare plots across Australia for 14 families. C4 cover and richness of the most common and abundant families were regressed against climate and local parameters. C4 richness and cover in the monocot families Poaceae and Cyperaceae increased with latitude and were strongly positively correlated with January temperatures, however C4 Cyperaceae occupied a more restricted temperature range. Seasonal rainfall, soil pH, soil texture, and tree cover modified proportional C4 cover in both families. Eudicot families displayed considerable variation in C4 distribution patterns. Proportional C4 Euphorbiaceae richness and cover were negatively correlated with increased moisture availability (i.e. high rainfall and low aridity), indicating they were more common in dry environments. Proportional C4 Chenopodiaceae richness and cover were weakly correlated with climate and local environmental factors, including soil texture. However, the explanatory power of C4 Chenopodiaceae models were poor, suggesting none of the factors considered in this study strongly influenced Chenopodiaceae distribution. Proportional C4 richness and cover in Aizoaceae, Amaranthaceae, and Portulacaceae increased with latitude, suggesting C4 cover and richness in these families increased with temperature and summer rainfall, but sample size was insufficient for regression analysis. Results demonstrate the unique relationships between different C4 taxa and climate, and the significant modifying effects of environmental factors on C4 distribution. Our work also revealed C4 families will not exhibit similar responses to local perturbations or climate.

C4 trees have a broader niche than their close C3 relatives

Previous studies have demonstrated the ecological sorting of herbaceous C3 and C4 species along gradients of precipitation and temperature: C4 herbaceous species typically occupy drier and warmer environments than their C3 relatives. However, it is unclear if this pattern holds true for C4 tree species, which are unique to Euphorbiaceae and found only on the Hawaiian Islands. Here, we combine occurrence data with local environmental and soil datasets to, for the first time, distinguish the ecological factors associated with photosynthetic diversification in the tree life form. These data are presented within a phylogenetic framework. We show that C3 and C4 trees inhabit similar environments, but that C4 photosynthesis expands the ecological niche in trees relative to that of C3 tree species. In particular, when compared with C3 trees, C4 trees moved into higher elevation habitats with characteristically sparse vegetation (and thus greater sunlight) and cooler temperatures, a pattern which contrasts with that of herbaceous species. Understanding the relationship between C4 photosynthesis and ecological niche in tree species has implications for establishing how C4 photosynthesis has, in this rare instance, evolved in trees, and whether this unique combination of traits could be exploited from an engineering perspective.

C4 species utilize fluctuating light less efficiently than C3 species

Reactivation of photosynthetic carbon assimilation during high light after a low light interval is slower in C4 than in C3 leaves.

Upregulation 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 Setaria viridis, a model monocot of NADP-dependent malic enzyme subtype, 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 were enhanced in LL BS cells, indicating a higher capacity for linear electron flow. Abundances of PSI, ATP synthase, Cytochrome b₆ f and the chloroplast NAD(P)H dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were also increased in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts were associated with a more oxidised plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results suggest that the supramolecular composition of PSII in BS cells is adjusted according to the redox state of the plastoquinone pool. This discovery contributes to the understanding of the acclimation of PSII activity in C₄ plants and will support the development of strategies for crop improvement, including the engineering of C₄ photosynthesis into C₃ plants.

Why is C4 photosynthesis so rare in trees?

Since C4 photosynthesis was first discovered >50 years ago, researchers have sought to understand how this complex trait evolved from the ancestral C3 photosynthetic machinery on >60 occasions. Despite its repeated emergence across the plant kingdom, C4 photosynthesis is notably rare in trees, with true C4 trees only existing in Euphorbia. Here we consider aspects of the C4 trait that could limit but not preclude the evolution of a C4 tree, including reduced quantum yield, increased energetic demand, reduced adaptive plasticity, evolutionary constraints, and a new theory that the passive symplastic phloem loading mechanism observed in trees, combined with difficulties in maintaining sugar and water transport over a long pathlength, could make C4 photosynthesis largely incompatible with the tree lifeform. We conclude that the transition to a tree habit within C4 lineages as well as the emergence of C4 photosynthesis within pre-existing trees would both face a series of challenges that together explain the global rarity of C4 photosynthesis in trees. The C4 trees in Euphorbia are therefore exceptional in how they have circumvented every potential barrier to the rare C4 tree lifeform.

A genetic link between leaf carbon isotope composition and whole-plant water use efficiency in the C4 grass Setaria

Genetic selection for whole-plant water use efficiency (yield per transpiration; WUE_plant ) in any crop-breeding programme requires high-throughput phenotyping of component traits of WUE_plant such as intrinsic water use efficiency (WUE_i ; CO₂ assimilation rate per stomatal conductance). Measuring WUE_i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE_i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ¹³ C_leaf ) has been suggested as a potential time-integrated proxy for WUE_i that may provide a tool to screen for WUE_plant . However, a genetic link between δ¹³ C_leaf and WUE_plant in a C₄ species has not been well established. Therefore, to determine if there is a genetic relationship in a C₄ plant between δ¹³ C_leaf and WUE_plant under well watered and water-limited growth conditions, a high-throughput phenotyping facility was used to measure WUE_plant in a recombinant inbred line (RIL) population created between the C₄ grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for δ¹³ C_leaf were found and co-localized with transpiration, biomass accumulation, and WUE_plant . Additionally, WUE_plant for each of the δ¹³ C_leaf QTL allele classes was negatively correlated with δ¹³ C_leaf , as would be predicted when WUE_i influences WUE_plant . These results demonstrate that δ¹³ C_leaf is genetically linked to WUE_plant , likely to be through their relationship with WUE_i , and can be used as a high-throughput proxy to screen for WUE_plant in these C₄ species.

TWI1 regulates cell-to-cell movement of OSH15 to control leaf cell fate

Cell pattern formation in plant leaves has attracted much attention from both plant biologists and breeders. However, in rice, the molecular mechanism remains unclear. Here, we describe the isolation and functional characterization of TWISTED-LEAF1 (TWI1), a critical gene involved in the development of the mestome sheath, vascular bundle sheath, interveinal mesophyll and sclerenchyma in rice leaves. Mutant twi1 plants have twisted leaves which might be caused by the compromised development and disordered patterning of bundle sheath, sclerenchyma and interveinal mesophyll cells. Expression of TWI1 can functionally rescue these mutant phenotypes. TWI1 encodes a transcription factor binding protein that interacts with OSH15, a class I KNOTTED1-like homeobox (KNOX) transcription factor. The cell-to-cell trafficking of OSH15 is restricted through its interaction with TWI1. Knockout or knockdown of OSH15 in twi1 rescues the twisted leaf phenotype. These studies reveal a key factor controlling cell pattern formation in rice leaves.

Steal the light: shade vs fire adapted vegetation in forest-savanna mosaics

Shade cast by trees, which suppresses grass growth, and fire fuelled by grass biomass, which prevents tree sapling establishment, are mutually exclusive and self-reinforcing drivers of biome distribution in savanna-forest mosaics. We investigated how shade depth, represented by canopy leaf area index (LAI), is generated by adult trees across savanna-forest boundaries and how a shade gradient filters tree functioning, and grass composition and biomass. Forest trees exerted greater shading through increased stem density and greater light interception per unit biomass. A critical transition at LAI c. 1.5 was linked to tree shifts from savanna to forest species, functional shifts from fire-tolerant to light-competitive species, and grass composition shifts from C₄ to C₃ pathways. A second transition to grass fuel loads too low to support fires, occurred at a lower canopy density (LAI > 0.5), accompanied by shifts in C₄ subtype dominance. This pattern suggests that shade suppression of grass biomass is an essential first step for the maintenance of alternative stable states.

Shade compromises the photosynthetic efficiency of NADP-ME less than that of PEP-CK and NAD-ME C4 grasses

The high energy cost and apparently low plasticity of C4 photosynthesis compared with C3 photosynthesis may limit the productivity and distribution of C4 plants in low light (LL) environments. C4 photosynthesis evolved numerous times, but it remains unclear how different biochemical subtypes perform under LL. We grew eight C4 grasses belonging to three biochemical subtypes [NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PEP-CK)] under shade (16% sunlight) or control (full sunlight) conditions and measured their photosynthetic characteristics at both low and high light. We show for the first time that LL (during measurement or growth) compromised the CO2-concentrating mechanism (CCM) to a greater extent in NAD-ME than in PEP-CK or NADP-ME C4 grasses by virtue of a greater increase in carbon isotope discrimination (∆P) and bundle sheath CO2 leakiness (ϕ), and a greater reduction in photosynthetic quantum yield (Φmax). These responses were partly explained by changes in the ratios of phosphoenolpyruvate carboxylase (PEPC)/initial Rubisco activity and dark respiration/photosynthesis (Rd/A). Shade induced a greater photosynthetic acclimation in NAD-ME than in NADP-ME and PEP-CK species due to a greater Rubisco deactivation. Shade also reduced plant dry mass to a greater extent in NAD-ME and PEP-CK relative to NADP-ME grasses. In conclusion, LL compromised the co-ordination of the C4 and C3 cycles and, hence, the efficiency of the CCM to a greater extent in NAD-ME than in PEP-CK species, while CCM efficiency was less impacted by LL in NADP-ME species. Consequently, NADP-ME species are more efficient at LL, which could explain their agronomic and ecological dominance relative to other C4 grasses.

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.

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.

NDH-Mediated Cyclic Electron Flow Around Photosystem I is Crucial for C4 Photosynthesis

C₄ photosynthesis exhibits efficient CO₂ assimilation in ambient air by concentrating CO₂ around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) through a metabolic pathway called the C₄ cycle. It has been suggested that cyclic electron flow (CEF) around PSI mediated by chloroplast NADH dehydrogenase-like complex (NDH), an alternative pathway of photosynthetic electron transport (PET), plays a crucial role in C₄ photosynthesis, although the contribution of NDH-mediated CEF is small in C₃ photosynthesis. Here, we generated NDH-suppressed transformants of a C₄ plant, Flaveria bidentis, and showed that the NDH-suppressed plants grow poorly, especially under low-light conditions. CO₂ assimilation rates were consistently decreased in the NDH-suppressed plants under low and medium light intensities. Measurements of non-photochemical quenching (NPQ) of Chl fluorescence, the oxidation state of the reaction center of PSI (P700) and the electrochromic shift (ECS) of pigment absorbance indicated that proton translocation across the thylakoid membrane is impaired in the NDH-suppressed plants. Since proton translocation across the thylakoid membrane induces ATP production, these results suggest that NDH-mediated CEF plays a role in the supply of ATP which is required for C₄ photosynthesis. Such a role is more crucial when the light that is available for photosynthesis is limited and the energy production by PET becomes rate-determining for C₄ photosynthesis. Our results demonstrate that the physiological contribution of NDH-mediated CEF is greater in C₄ photosynthesis than in C₃ photosynthesis, suggesting that the mechanism of PET in C₄ photosynthesis has changed from that in C₃ photosynthesis accompanying the changes in the mechanism of CO₂ assimilation.

Anatomical constraints to C4 evolution: light harvesting capacity in the bundle sheath

In C4 photosynthesis CO2 assimilation and reduction are typically coordinated across mesophyll (M) and bundle sheath (BS) cells, respectively. This system consequently requires sufficient light to reach BS to generate enough ATP to allow ribulose-1,5-bisphosphate (RuBP) regeneration in BS. Leaf anatomy influences BS light penetration and therefore constrains C4 cycle functionality. Using an absorption scattering model (coded in Excel, and freely downloadable) we simulate light penetration profiles and rates of ATP production in BS across the C3 , C3 -C4 and C4 anatomical continua. We present a trade-off for light absorption between BS pigment concentration and space allocation. C3 BS anatomy limits light absorption and benefits little from high pigment concentrations. Unpigmented BS extensions increase BS light penetration. C4 and C3 -C4 anatomies have the potential to generate sufficient ATP in the BS, whereas typical C3 anatomy does not, except some C3 taxa closely related to C4 groups. Insufficient volume of BS, relative to M, will hamper a C4 cycle via insufficient BS light absorption. Thus, BS ATP production and RuBP regeneration, coupled with increased BS investments, allow greater operational plasticity. We propose that larger BS in C3 lineages may be co-opted for C3 -C4 and C4 biochemistry requirements.

Why are there no C4 forests?

C₄ photosynthesis is absent from the arborescent life form, with the exception of seven Hawaiian Euphorbia species and a few desert shrubs that become arborescent with age. As a consequence, wherever C₃ trees can establish, their height advantage enables them to outcompete low stature C₄ vegetation. Had C₄ photosynthesis been able to evolve in an arborescent life form, forest cover (by C₄ trees) could have been much more extensive than today, with significant consequences for the biosphere. Here, we address why there are so few C₄ trees. Physiological explanations associated with low light performance of C₄ photosynthesis are not supported, because C₄ shade-tolerant species exhibit similar performance as shade-tolerant C₃ species in terms of quantum yield, steady-state photosynthetic and use of sunflecks. Hence, hypothetical C₄ trees could occur in the regeneration niche of forests. Constraints associated with the evolutionary history of the C₄ lineages are more plausible. Most C₄ species are grasses and sedges, which lack meristems needed for arborescence, while most C₄ eudicots are highly specialized for harsh (arid, saline, hot) or disturbed habitats where arborescence may be maladapted. Most C₄ eudicot clades are also young, and have not had sufficient time to radiate beyond the extreme environments where C₄ evolution is favored. In the case of the Hawaiian Euphorbia species, they belong to one of the oldest and most diverse C₄ lineages, which primed this group to evolve arborescence in a low-competition environment that appeared on the remote Hawaiian Islands.

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.

An Excel tool for deriving key photosynthetic parameters from combined gas exchange and chlorophyll fluorescence: theory and practice

Combined photosynthetic gas exchange and modulated fluorometres are widely used to evaluate physiological characteristics associated with phenotypic and genotypic variation, whether in response to genetic manipulation or resource limitation in natural vegetation or crops. After describing relatively simple experimental procedures, we present the theoretical background to the derivation of photosynthetic parameters, and provide a freely available Excel-based fitting tool (EFT) that will be of use to specialists and non-specialists alike. We use data acquired in concurrent variable fluorescence-gas exchange experiments, where A/Ci and light-response curves have been measured under ambient and low oxygen. From these data, the EFT derives light respiration, initial PSII (photosystem II) photochemical yield, initial quantum yield for CO2 fixation, fraction of incident light harvested by PSII, initial quantum yield for electron transport, electron transport rate, rate of photorespiration, stomatal limitation, Rubisco (ribulose 1·5-bisphosphate carboxylase/oxygenase) rate of carboxylation and oxygenation, Rubisco specificity factor, mesophyll conductance to CO2 diffusion, light and CO2 compensation point, Rubisco apparent Michaelis-Menten constant, and Rubisco CO2 -saturated carboxylation rate. As an example, a complete analysis of gas exchange data on tobacco plants is provided. We also discuss potential measurement problems and pitfalls, and suggest how such empirical data could subsequently be used to parameterize predictive photosynthetic models.

Integrating Hormone- and Micromolecule-Mediated Signaling with Plasmodesmal Communication

Intercellular and supracellular communications through plasmodesmata are involved in vital processes for plant development and physiological responses. Micro- and macromolecules, including hormones, RNA, and proteins, serve as biological information vectors that traffic through the plasmodesmata between cells. Previous studies demonstrated that the plasmodesmata are elaborately regulated, whereby a long queue of multiple signaling molecules forms. However, the mechanism by which these signals are coupled or coordinated in terms of simultaneous transport in a single channel remains a puzzle. In the last few years, several phytohormones that could function as both non-cell-autonomous signals and plasmodesmal regulators have been disclosed. Plasmodesmal regulators such as auxin, salicylic acid, reactive oxygen species, gibberellic acids, chitin, and jasmonic acid could regulate intercellular trafficking by adjusting plasmodesmal permeability. Here, callose, along with β-glucan synthase and β-glucanase, plays a critical role in regulating plasmodesmal permeability. Interestingly, most of the previously identified regulators are capable of diffusing through the plasmodesmata. Given the small sizes of these molecules, the plasmodesmata are prominent intercellular channels that allow diffusion-based movement of those signaling molecules. Obviously, intercellular communication is under the control of a major mechanism, named a feedback loop, at the plasmodesmata, which mediates complicated biological behaviors. Prospective research on the mechanism of coupling micromolecules at the plasmodesmata for developmental signaling and nutrient provision will help us to understand how plants coordinate their development and photosynthetic assimilation, which is important for agriculture.

Photosynthetic diversity meets biodiversity: the C4 plant example

Physiological diversification reflects adaptation for specific environmental challenges. As the major physiological process that provides plants with carbon and energy, photosynthesis is under strong evolutionary selection that gives rise to variability in nearly all parts of the photosynthetic apparatus. Here, we discuss how plants, notably those using C4 photosynthesis, diversified in response to environmental challenges imposed by declining atmospheric CO2 content in recent geological time. This reduction in atmospheric CO2 increases the rate of photorespiration and reduces photosynthetic efficiency. While plants have evolved numerous mechanisms to compensate for low CO2, the most effective are the carbon concentration mechanisms of C4, C2, and CAM photosynthesis; and the pumping of dissolved inorganic carbon, mainly by algae. C4 photosynthesis enables plants to dominate warm, dry and often salinized habitats, and to colonize areas that are too stressful for most plant groups. Because C4 lineages generally lack arborescence, they cannot form forests. Hence, where they predominate, C4 plants create a different landscape than would occur if C3 plants were to predominate. These landscapes (mostly grasslands and savannahs) present unique selection environments that promoted the diversification of animal guilds able to graze upon the C4 vegetation. Thus, the rise of C4 photosynthesis has made a significant contribution to the origin of numerous biomes in the modern biosphere.

Assessing the metabolic impact of nitrogen availability using a compartmentalized maize leaf genome-scale model

Maize (Zea mays) is an important C4 plant due to its widespread use as a cereal and energy crop. A second-generation genome-scale metabolic model for the maize leaf was created to capture C4 carbon fixation and investigate nitrogen (N) assimilation by modeling the interactions between the bundle sheath and mesophyll cells. The model contains gene-protein-reaction relationships, elemental and charge-balanced reactions, and incorporates experimental evidence pertaining to the biomass composition, compartmentalization, and flux constraints. Condition-specific biomass descriptions were introduced that account for amino acids, fatty acids, soluble sugars, proteins, chlorophyll, lignocellulose, and nucleic acids as experimentally measured biomass constituents. Compartmentalization of the model is based on proteomic/transcriptomic data and literature evidence. With the incorporation of information from the MetaCrop and MaizeCyc databases, this updated model spans 5,824 genes, 8,525 reactions, and 9,153 metabolites, an increase of approximately 4 times the size of the earlier iRS1563 model. Transcriptomic and proteomic data have also been used to introduce regulatory constraints in the model to simulate an N-limited condition and mutants deficient in glutamine synthetase, gln1-3 and gln1-4. Model-predicted results achieved 90% accuracy when comparing the wild type grown under an N-complete condition with the wild type grown under an N-deficient condition.