C4 Crop Species Research Articles

Reducing stomatal density by expression of a synthetic epidermal patterning factor increases leaf intrinsic water use efficiency and reduces plant water use in a C4 crop.

Enhancing crop water use efficiency (WUE) is a key target trait for climatic resilience and expanding cultivation on marginal lands. Engineering lower stomatal density to reduce stomatal conductance (gs) has improved WUE in multiple C3 crop species. However, reducing gs in C3 species often reduces photosynthetic carbon gain. A different response is expected in C4 plants because they possess specialized anatomy and biochemistry which concentrates CO2 at the site of fixation. This modifies the relationship of photosynthesis (AN) with intracellular CO2 concentration (ci), such that photosynthesis is CO2 saturated and reductions in gs are unlikely to limit AN. To test this hypothesis, genetic strategies were investigated to reduce stomatal density in the C4 crop sorghum. Constitutive expression of a synthetic epidermal patterning factor (EPF) transgenic allele in sorghum led to reduced stomatal densities, reduced gs, reduced plant water use, and avoidance of stress during a period of water deprivation. In addition, moderate reduction in stomatal density did not increase stomatal limitation to AN. However, these positive outcomes were associated with negative pleiotropic effects on reproductive development and photosynthetic capacity. Avoiding pleiotropy by targeting expression of the transgene to specific tissues could provide a pathway to improved agronomic outcomes.

Diurnal decline in photosynthesis and stomatal conductance in several tropical species.

Photosynthesis (A) and stomatal conductance (gs) change diurnally due to internal signals, but the effects of diurnal rhythms on dynamic photosynthetic behavior are understudied. We examined diurnal changes in A and gs in ten tropical species: across species, there was a tendency for A and gs to decline diurnally when these were repeatedly measured under either steady-state or fluctuating irradiance conditions. We then examined in more detail the irradiance-induced kinetics of gas exchange in a C3 and C4 crop species each, namely fig (Ficus carica) and sugarcane (Saccharum officinarum). During the day, fig showed significantly slower photosynthetic induction and lower gs, as well as a slower gs increase, in the afternoon than in the morning and noon. Sugarcane showed a reduction in steady-state A reached under high irradiance and slower gs increase as well as lower gs reached under high irradiance, but no changes in the rate of photosynthetic induction, in the afternoon, compared to morning and noon. These reductions in the afternoon were not reverted by a dark treatment in the middle of the day, suggesting that the decrease was not proportional to diurnal time-integrated carbon fixation. Repeated exposure to light- and shadeflecks (1000 and 50 μmol m-2 s-1, lasting 20 min each) revealed fundamental differences in stomatal regulation between species: in fig, stomata opened and closed slowly, and their opening became progressively slower under a series of lightflecks, whereas sugarcane showed much faster stomatal opening than closure that was unchanged during the course of the day. Our results highlight that steady-state rates and irradiance-induced kinetics of photosynthesis and stomatal movement change diurnally in most species studied, and that they do so differently in fig and sugarcane.

Evaluation of Ecosystem Water Use Efficiency Based on Coupled and Uncoupled Remote Sensing Products for Maize and Soybean

Accurate quantification of ecosystem water use efficiency (eWUE) over agroecosystems is crucial for managing water resources and assuring food security. Currently, the uncoupled Moderate Resolution Imaging Spectroradiometer (MODIS) product is the most widely applied dataset for simulating local, regional, and global eWUE across different plant functional types. However, it has been rarely investigated as to whether the coupled product can outperform the uncoupled product in eWUE estimations for specific C4 and C3 crop species. Here, the eWUE as well as gross primary production (GPP) and evapotranspiration (ET) from the uncoupled MODIS product and the coupled Penman–Monteith–Leuning version 2 (PMLv2) product were evaluated against the in-situ observations on eight-day and annual scales (containing 1902 eight-day and 61 annual samples) for C4 maize and C3 soybean at the five cropland sites from the FLUXNET2015 and AmeriFlux datasets. Our results show the following: (1) For GPP estimates, the PMLv2 product showed paramount improvements for C4 maize and slight improvements for C3 soybean, relative to the MODIS product. (2) For ET estimates, both products performed similarly for both crop species. (3) For eWUE estimates, the coupled PMLv2 product achieved higher-accuracy eWUE estimates than the uncoupled MODIS product at both eight-day and annual scales. Taking the result at an eight-day scale for example, compared to the MODIS product, the PMLv2 product could reduce the root mean square error (RMSE) from 2.14 g C Kg−1 H2O to 1.36 g C Kg−1 H2O and increase the coefficient of determination (R2) from 0.06 to 0.52 for C4 maize, as well as reduce the RMSE from 1.33 g C Kg−1 H2O to 0.89 g C Kg−1 H2O and increase the R2 from 0.05 to 0.49 for C3 soybean. (4) Despite the outperformance of the PMLv2 product in eWUE estimations, both two products failed to differentiate C4 and C3 crop species in their model calibration and validation processes, leading to a certain degree of uncertainties in eWUE estimates. Our study not only provides an important reference for applying remote sensing products to derive reliable eWUE estimates over cropland but also indicates the future modification of the current remote sensing models for C4 and C3 crop species.

Isolation of gametes and zygotes from Setaria viridis.

Setaria viridis, the wild ancestor of foxtail millet (Setaria italica), is an effective model plant for larger C4 crops because S. viridis has several desirable traits, such as short generation time, prolific seed production and a small genome size. These advantages are well suited for investigating molecular mechanisms in angiosperms, especially C4 crop species. Here, we report a procedure for isolating gametes and zygotes from S. viridis flowers. To isolate egg cells, ovaries were harvested from unpollinated mature flowers and cut transversely, which allowed direct access to the embryo sac. Thereafter, an egg cell was released from the cut end of the basal portion of the dissected ovary. To isolate sperm cells, pollen grains released from anthers were immersed in a mannitol solution, resulting in pollen-grain bursting, which released sperm cells. Additionally, S. viridis zygotes were successfully isolated from freshly pollinated flowers. Isolated zygotes cultured in a liquid medium developed into globular-like embryos and cell masses. Thus, isolated S. viridis gametes, zygotes and embryos are attainable for detailed observations and investigations of fertilization and developmental events in angiosperms.

Response of Multiple Tissues to Drought Revealed by a Weighted Gene Co-Expression Network Analysis in Foxtail Millet [Setaria italica (L.) P. Beauv.

Characterization of drought-tolerance mechanisms during the jointing stage in foxtail millet under water-limited conditions is essential for improving the grain yield of this C4 crop species. In this trial, two drought-tolerant and two drought-sensitive cultivars were examined using transcriptomic dissections of three tissues (root, stem, and leaf) under naturally occurring water-limited conditions. We detected a total of 32,170 expressed genes and characterized 13,552 differentially expressed genes (DEGs) correlated with drought treatment. The majority of DEGs were identified in the root tissue, followed by leaf and stem tissues, and the number of DEGs identified in the stems of drought-sensitive cultivars was about two times higher than the drought-tolerant ones. A total of 127 differentially expressed transcription factors (DETFs) with different drought-responsive patterns were identified between drought-tolerant and drought-sensitive genotypes (including MYB, b-ZIP, ERF, and WRKY). Furthermore, a total of 34 modules were constructed for all expressed genes using a weighted gene co-expression network analysis (WGCNA), and seven modules were closely related to the drought treatment. A total of 1,343 hub genes (including RAB18, LEA14, and RD22) were detected in the drought-related module, and cell cycle and DNA replication-related transcriptional pathways were identified as vital regulators of drought tolerance in foxtail millet. The results of this study provide a comprehensive overview of how Setaria italica copes with drought-inflicted environments during the jointing stage through transcriptional regulating strategies in different organs and lays a foundation for the improvement of drought-tolerant cereal cultivars through genomic editing approaches in the future.

An actinomycete strain of Nocardiopsis lucentensis reduces arsenic toxicity in barley and maize

Accumulation of arsenic in plant tissues poses a substantial threat to global crop yields. The use of plant growth-promoting bacterial strains to mitigate heavy metal toxicity has been illustrated before. However, its potential to reduce plant arsenic uptake and toxicity has not been investigated to date. Here, we describe the identification and characterization of a Nocardiopsis lucentensis strain isolated from heavy metal contaminated soil. Inoculation with this bioactive actinomycete strain decreased arsenic root and shoot bioaccumulation in both C3 and C4 crop species namely barley and maize. Upon arsenate treatment, N. lucentensis S5 stimulated root citric acid production and the plant’s innate detoxification capacity in a species-specific manner. In addition, this specific strain promoted biomass gain, despite substantial tissue arsenic levels. Detoxification (metallothionein, phytochelatin, glutathione-S-transferase levels) was upregulated in arsenate-exposed shoot and roots, and this response was further enhanced upon S5 supplementation, particularly in barley and maize roots. Compared to barley, maize plants were more tolerant to arsenate-induced oxidative stress (less H2O2 and lipid peroxidation levels). However, barley plants invested more in antioxidative capacity induction (ascorbate-glutathione turnover) to mitigate arsenic oxidative stress, which was strongly enhanced by S5. We quantify and mechanistically discuss the physiological and biochemical basis of N. lucentensis-mediated plant biomass recovery on arsenate polluted soils. Our findings substantiate the potential applicability of a bactoremediation strategy to mitigate arsenic-induced yield loss in crops.

Genetic Diversity of C4 Photosynthesis Pathway Genes in Sorghum bicolor (L.).

C4 photosynthesis has evolved in over 60 different plant taxa and is an excellent example of convergent evolution. Plants using the C4 photosynthetic pathway have an efficiency advantage, particularly in hot and dry environments. They account for 23% of global primary production and include some of our most productive cereals. While previous genetic studies comparing phylogenetically related C3 and C4 species have elucidated the genetic diversity underpinning the C4 photosynthetic pathway, no previous studies have described the genetic diversity of the genes involved in this pathway within a C4 crop species. Enhanced understanding of the allelic diversity and selection signatures of genes in this pathway may present opportunities to improve photosynthetic efficiency, and ultimately yield, by exploiting natural variation. Here, we present the first genetic diversity survey of 8 known C4 gene families in an important C4 crop, Sorghum bicolor (L.) Moench, using sequence data of 48 genotypes covering wild and domesticated sorghum accessions. Average nucleotide diversity of C4 gene families varied more than 20-fold from the NADP-malate dehydrogenase (MDH) gene family (θπ = 0.2 × 10−3) to the pyruvate orthophosphate dikinase (PPDK) gene family (θπ = 5.21 × 10−3). Genetic diversity of C4 genes was reduced by 22.43% in cultivated sorghum compared to wild and weedy sorghum, indicating that the group of wild and weedy sorghum may constitute an untapped reservoir for alleles related to the C4 photosynthetic pathway. A SNP-level analysis identified purifying selection signals on C4 PPDK and carbonic anhydrase (CA) genes, and balancing selection signals on C4 PPDK-regulatory protein (RP) and phosphoenolpyruvate carboxylase (PEPC) genes. Allelic distribution of these C4 genes was consistent with selection signals detected. A better understanding of the genetic diversity of C4 pathway in sorghum paves the way for mining the natural allelic variation for the improvement of photosynthesis.

C4 and crassulacean acid metabolism within a single leaf: deciphering key components behind a rare photosynthetic adaptation.

Although biochemically related, C4 and crassulacean acid metabolism (CAM) systems are expected to be incompatible. However, Portulaca species, including P.oleracea, operate C4 and CAM within a single leaf, and the mechanisms behind this unique photosynthetic arrangement remain largely unknown. Here, we employed RNA-seq to identify candidate genes involved exclusively or shared by C4 or CAM, and provided an in-depth characterization of their transcript abundance patterns during the drought-induced photosynthetic transitions in P.oleracea. Data revealed fewer candidate CAM-specific genes than those recruited to function in C4 . The putative CAM-specific genes were predominantly involved in night-time primary carboxylation reactions and malate movement across the tonoplast. Analysis of gene transcript-abundance regulation and photosynthetic physiology indicated that C4 and CAM coexist within a single P.oleracea leaf under mild drought conditions. Developmental and environmental cues were shown to regulate CAM expression in stems, whereas the shift from C4 to C4 -CAM hybrid photosynthesis in leaves was strictly under environmental control. Moreover, efficient starch turnover was identified as part of the metabolic adjustments required for CAM operation in both organs. These findings provide insights into C4 /CAM connectivity and compatibility, contributing to a deeper understanding of alternative ways to engineer CAM into C4 crop species.

Foxtail millet (Setaria italica (L.) P. Beauv) CIPKs are responsive to ABA and abiotic stresses.

CBL-interacting protein kinases (CIPKs) have been shown to regulate a variety of environmental stress-related signalling pathways in plants. Foxtail millet (Setaria italica (L.) P. Beauv) is known worldwide as a relatively stress-tolerant C4 crop species. Although the foxtail millet genome sequence has been released, little is known about the functions of CIPKs in foxtail millet. Therefore, a systematic genome-wide analysis of CIPK genes in foxtail millet was performed. In total, 35 CIPK members were identified in foxtail millet and divided into four subgroups (I to IV) on the basis of their phylogenetic relationships. Phylogenetic and gene structure analyses clearly divided all SiCIPKs into intron-poor and intron-rich clades. Cis-element analysis subsequently indicated that these SiCIPKs may be involved in responses to abiotic stimuli, hormones, and light signalling during plant growth and development, and stress-induced expression profile analysis revealed that all the SiCIPKs are involved in various stress signalling pathways. These results suggest that the CIPK genes in foxtail millet exhibit the basic characteristics of CIPK family members and play important roles in response to abiotic stresses. The results of this study will contribute to future functional characterization of abiotic stress responses mediated by CIPKs in foxtail millet.

Reference gene identification for reliable normalisation of quantitative RT-PCR data in Setaria viridis

BackgroundQuantitative real-time polymerase chain reaction (RT-qPCR) is the key platform for the quantitative analysis of gene expression in a wide range of experimental systems and conditions. However, the accuracy and reproducibility of gene expression quantification via RT-qPCR is entirely dependent on the identification of reliable reference genes for data normalisation. Green foxtail (Setaria viridis) has recently been proposed as a potential experimental model for the study of C4 photosynthesis and is closely related to many economically important crop species of the Panicoideae subfamily of grasses, including Zea mays (maize), Sorghum bicolor (sorghum) and Sacchurum officinarum (sugarcane). Setaria viridis (Accession 10) possesses a number of key traits as an experimental model, namely; (i) a small sized, sequenced and well annotated genome; (ii) short stature and generation time; (iii) prolific seed production, and; (iv) is amendable to Agrobacterium tumefaciens-mediated transformation. There is currently however, a lack of reference gene expression information for Setaria viridis (S. viridis). We therefore aimed to identify a cohort of suitable S. viridis reference genes for accurate and reliable normalisation of S. viridis RT-qPCR expression data.ResultsEleven putative candidate reference genes were identified and examined across thirteen different S. viridis tissues. Of these, the geNorm and NormFinder analysis software identified SERINE/THERONINE-PROTEIN PHOSPHATASE 2A (PP2A), 5′-ADENYLYLSULFATE REDUCTASE 6 (ASPR6) and DUAL SPECIFICITY PHOSPHATASE (DUSP) as the most suitable combination of reference genes for the accurate and reliable normalisation of S. viridis RT-qPCR expression data. To demonstrate the suitability of the three selected reference genes, PP2A, ASPR6 and DUSP, were used to normalise the expression of CINNAMYL ALCOHOL DEHYDROGENASE (CAD) genes across the same tissues.ConclusionsThis approach readily demonstrated the suitably of the three selected reference genes for the accurate and reliable normalisation of S. viridis RT-qPCR expression data. Further, the work reported here forms a highly useful platform for future gene expression quantification in S. viridis and can also be potentially directly translatable to other closely related and agronomically important C4 crop species.

Biogenic volatile organic compound emissions from senescent maize leaves and a comparison with other leaf developmental stages

Plants are the major source of Biogenic Volatile Organic Compounds (BVOCs) which have a large influence on atmospheric chemistry and the climate system. Therefore, understanding of BVOC emissions from all abundant plant species at all developmental stages is very important. Nevertheless, investigations on BVOC emissions from even the most widespread agricultural crop species are rare and mainly confined to the healthy green leaves. Senescent leaves of grain crop species could be an important source of BVOCs as almost all the leaves senesce on the field before being harvested. For these reasons, BVOC emission measurements have been performed on maize (Zea mays L.), one of the most cultivated crop species in the world, at all the leaf developmental stages. The measurements were performed in controlled environmental conditions using dynamic enclosures and proton transfer reaction mass spectrometry (PTR-MS). The main compounds emitted by senescent maize leaves were methanol (31% of the total cumulative BVOC emission on a mass of compound basis) and acetic acid (30%), followed by acetaldehyde (11%), hexenals (9%) and m/z 59 compounds (acetone/propanal) (7%). Important differences were observed in the temporal emission profiles of the compounds, and both yellow leaves during chlorosis and dry brown leaves after chlorosis were identified as important senescence-related BVOC sources. Total cumulative BVOC emissions from senescent maize leaves were found to be among the highest for senescent Poaceae plant species. BVOC emission rates varied strongly among the different leaf developmental stages, and senescent leaves showed a larger diversity of emitted compounds than leaves at earlier stages. Methanol was the compound with the highest emissions for all the leaf developmental stages and the contribution from the young-growing, mature, and senescent stages to the total methanol emission by a typical maize leaf was 61, 13, and 26%, respectively. This study shows that BVOC emissions from senescent maize leaves cannot be neglected and further investigations in field conditions are recommended to further constrain the BVOC emissions from this important C4 crop species.

Yield Characteristics Of Melissa Officinalis L. In Response To The Foliar Application Of Methanol And Ethanol Solutions

Lemon balm, one of the important medicinal plant species mainly grown in natural flora, especially in the Mediterranean region of Turkey, is native to southern Europe and northern Africa, and east as far as the Caucasus and northern Iran [1]. Short chain alcohols such as ethanol and methanol are reported to enhance biomass accumulation and seed germination for several C3 crop species [2]. Foliar sprays of aqueous 10 – 50% methanol increased growth and development of C3 crop plants [3]. The aims of this study were evaluation of the yield characteristics of lemon balm in response to the methanol and ethanol solutions as a new and safe technology in production of medicinal plants, based on a randomized complete block design with 12 treatments and 3 replications in the experimental field of Institute of Medicinal Plants (IMP) in Karaj at 2010. The treatments of this study include: control (distilled and without distilled water), ethanol and methanol aqueous solutions each one of them with 10, 20, 30, 40 and 50% (v/v). The results were indicated that the effects of hydroalcoholic solutions had significant differences (P < 0.01) on 15 agronomical and morphophysiological traits. The results indicated that the most amount of plant height in methanol 30%, stem diameter, stem number, stem fresh weight, stem dry weight, leaf number, leaf area, leaf fresh weight, leaf weight, shoot fresh weight, shoot dry weight, seed weight per plant and essential oil yield in methanol 40%, chlorophyll content and thousand seed weight in methanol 50% were obtained.

Uncertainty in simulating gross primary production of cropland ecosystem from satellite-based models

Accurate estimates of gross primary production (GPP) for croplands are needed to assess carbon cycle and crop yield. Satellite-based models have been developed to monitor spatial and temporal GPP patterns. However, there are still large uncertainties in estimating cropland GPP. This study compares three light use efficiency (LUE) models (MODIS-GPP, EC-LUE, and VPM) with eddy-covariance measurements at three adjacent AmeriFlux crop sites located near Mead, Nebraska, USA. These sites have different crop-rotation systems (continuous maize vs. maize and soybean rotated annually) and water management practices (irrigation vs. rainfed). The results reveal several major uncertainties in estimating GPP which need to be sufficiently considered in future model improvements. Firstly, the C4 crop species (maize) shows a larger photosynthetic capacity compared to the C3 species (soybean). LUE models need to use different model parameters (i.e., maximal light use efficiency) for C3 and C4 crop species, and thus, it is necessary to have accurate species-distribution products in order to determine regional and global estimates of GPP. Secondly, the 1km sized MODIS fPAR and EVI products, which are used to remotely identify the fraction of photosynthetically active radiation absorbed by the vegetation canopy, may not accurately reflect differences in phenology between maize and soybean. Such errors will propagate in the GPP model, reducing estimation accuracy. Thirdly, the water-stress variables in the remote sensing models do not fully characterize the impacts of water availability on vegetation production. This analysis highlights the need to improve LUE models with regard to model parameters, vegetation indices, and water-stress inputs.

Productivity, absorbed photosynthetically active radiation, and light use efficiency in crops: Implications for remote sensing of crop primary production

Vegetation productivity metrics such as gross primary production (GPP) at the canopy scale are greatly affected by the efficiency of using absorbed radiation for photosynthesis, or light use efficiency (LUE). Thus, close investigation of the relationships between canopy GPP and photosynthetically active radiation absorbed by vegetation is the basis for quantification of LUE. We used multiyear observations over irrigated and rainfed contrasting C3 (soybean) and C4 (maize) crops having different physiology, leaf structure, and canopy architecture to establish the relationships between canopy GPP and radiation absorbed by vegetation and quantify LUE. Although multiple LUE definitions are reported in the literature, we used a definition of efficiency of light use by photosynthetically active “green” vegetation (LUEgreen) based on radiation absorbed by “green” photosynthetically active vegetation on a daily basis. We quantified, irreversible slowly changing seasonal (constitutive) and rapidly day-to-day changing (facultative) LUEgreen, as well as sensitivity of LUEgreen to the magnitude of incident radiation and drought events. Large (2–3-fold) variation of daily LUEgreen over the course of a growing season that is governed by crop physiological and phenological status was observed. The day-to-day variations of LUEgreen oscillated with magnitude 10–15% around the seasonal LUEgreen trend and appeared to be closely related to day-to-day variations of magnitude and composition of incident radiation. Our results show the high variability of LUEgreen between C3 and C4 crop species (1.43gC/MJ vs. 2.24gC/MJ, respectively), as well as within single crop species (i.e., maize or soybean). This implies that assuming LUEgreen as a constant value in GPP models is not warranted for the crops studied, and brings unpredictable uncertainties of remote GPP estimation, which should be accounted for in LUE models. The uncertainty of GPP estimation due to facultative and constitutive changes in LUEgreen can be considered as a critical component of the total error budget in the context of remotely sensed based estimations of GPP. The quantitative framework of LUEgreen estimation presented here offers a way of characterizing LUEgreen in plants that can be used to assess their phenological and physiological status and vulnerability to drought under current and future climatic conditions and is essential for calibration and validation of globally applied LUE algorithms.

Characterisation of cyanobacterial bicarbonate transporters in E. coli shows that SbtA homologs are functional in this heterologous expression system.

Cyanobacterial HCO3 - transporters BCT1, SbtA and BicA are important components of cyanobacterial CO2-concentration mechanisms. They also show potential in applications aimed at improving photosynthetic rates and yield when expressed in the chloroplasts of C3 crop species. The present study investigated the feasibility of using Escherichia coli to assess function of a range of SbtA and BicA transporters in a heterologous expression system, ultimately for selection of transporters suitable for chloroplast expression. Here, we demonstrate that six β-forms of SbtA are active in E. coli, although other tested bicarbonate transporters were inactive. The sbtA clones were derived from Synechococcus sp. WH5701, Cyanobium sp. PCC7001, Cyanobium sp. PCC6307, Synechococcus elongatus PCC7942, Synechocystis sp. PCC6803, and Synechococcus sp. PCC7002. The six SbtA homologs varied in bicarbonate uptake kinetics and sodium requirements in E. coli. In particular, SbtA from PCC7001 showed the lowest uptake affinity and highest flux rate and was capable of increasing the internal inorganic carbon pool by more than 8 mM relative to controls lacking transporters. Importantly, we were able to show that the SbtB protein (encoded by a companion gene near sbtA) binds to SbtA and suppresses bicarbonate uptake function of SbtA in E. coli, suggesting a role in post-translational regulation of SbtA, possibly as an inhibitor in the dark. This study established E. coli as a heterologous expression and analysis system for HCO3 - transporters from cyanobacteria, and identified several SbtA transporters as useful for expression in the chloroplast inner envelope membranes of higher plants.

Can the cyanobacterial carbon-concentrating mechanism increase photosynthesis in crop species? A theoretical analysis.

Experimental elevation of [CO₂] around C₃ crops in the field has been shown to increase yields by suppressing the Rubisco oxygenase reaction and, in turn, photorespiration. Bioengineering a cyanobacterial carbon-concentrating mechanism (CCM) into C₃ crop species provides a potential means of elevating [CO₂] at Rubisco, thereby decreasing photorespiration and increasing photosynthetic efficiency and yield. The cyanobacterial CCM is an attractive alternative relative to other CCMs, because its features do not require anatomical changes to leaf tissue. However, the potential benefits of engineering the entire CCM into a C₃ leaf are unexamined. Here, a CO₂ and HCO₃⁻ diffusion-reaction model is developed to examine how components of the cyanobacterial CCM affect leaf light-saturated CO₂ uptake (A(sat)) and to determine whether a different Rubisco isoform would perform better in a leaf with a cyanobacterial CCM. The results show that the addition of carboxysomes without other CCM components substantially decreases A(sat) and that the best first step is the addition of HCO₃⁻ transporters, as a single HCO₃⁻ transporter increased modeled A(sat) by 9%. Addition of all major CCM components increased A(sat) from 24 to 38 µmol m⁻² s⁻¹. Several Rubisco isoforms were compared in the model, and increasing ribulose bisphosphate regeneration rate will allow for further improvements by using a Rubisco isoform adapted to high [CO₂]. Results from field studies that artificially raise [CO₂] suggest that this 60% increase in A(sat) could result in a 36% to 60% increase in yield.

Soil Carbon Sequestration in Sorghum Cropping Systems

Management practices can influence both the quantity of soil organic carbon (SOC) and its distribution into different fractions or pools. We investigated SOC sequestration potentials of cropping systems in near-surface (0–5 cm) samples through soil size and density fractionation coupled with acid hydrolysis and natural abundance of stable isotopes (δ13C) in a 20-year field study in the southern Great Plains in 2002. Treatments included two tillage regimens, conventional and no tillage (NT), in combination with two cropping systems: continuous grain sorghum [Sorghum bicolor (L.) Moench.] (CS) and a sorghum-wheat (Triticum aestivum L.)-soybean [Glycine max (L.) Merr.] (SWSoy) rotation, and with or without N fertilization. Tillage and cropping sequence significantly affected SOC distribution and the natural abundance of δ13C in different fractions. Samples from CS exhibited δ13C values ranging from −15‰ to −20‰, suggesting most SOC was derived from this C4 crop species. The δ13C values for soils from SWSoy varied from −20‰ to −22‰, reflecting a mixed input from C3-derived and C4-derived residue input. For whole soil and all aggregate-size fractions, SOC concentrations were significantly higher for NT than conventional tillage. However, the effects of cropping system and N fertilization on SOC interacted with tillage. Greater SOC for enhanced cropping (SWSoy) or N fertilization was observed only under NT. The fraction of <53 μm represented a greater proportion of soil than other aggregate-size fractions. Our long-term study indicated that SOC and its various fractions, including more resistant, can be increased by NT with enhanced cropping and N fertilization.

A vacuolar H+-pyrophosphatase differential activation and energy coupling integrate the responses of weeds and crops to drought stress

BackgroundCyperus rotundus L. is a C4 weed of large vegetative and reproductive vigor endowed with competitive advantages over most crop species mainly under adverse environmental conditions. Vacuole functions are critical for the mechanisms of drought resistance, and here the modulation of the primary system of vacuolar ion transport is investigated during a transient water stress imposed to this weed and to C4 crop species (Zea mays L.). MethodsThe vacuolar H+ pumps, the H+-ATPase and H+-PPiase, expression, activities and the energy coupling were spectrophotometrically investigated as key elements in the differential drought-resistance mechanisms developed by weeds and crops. ResultsIn C. rotundus tonoplasts, ATP hydrolysis was more sensitive to drought than its coupled H+ transport, which was in turn at least 3-folds faster than that mediated by the H+-PPiase. Its PPi hydrolysis was only slightly affected by severe water deficit, contrasting with the disruption induced in the PPi-dependent H+-gradient. This effect was antagonized by plant rehydration as the H+-PPiase activity was highly stimulated, reassuming a coupled PPi-driven H+ pumping. Maize tonoplasts exhibited 2–4 times lower hydrolytic activities than that of C. rotundus, but were able to overactivate specifically PPi-dependent H+ pumping in response to stress relief, resulting in an enhanced H+-pumps coupling efficiency. ConclusionThese results together with immunoanalysis revealed profiles consistent with pre- and post-translational changes occurring on the tonoplast H+-pumps, which differ between weeds and crops upon water deficit. General significanceThe evidences highlight an unusual modulation of the H+-PPiase energy coupling as a key biochemical change related to environmental stresses adaptive capacity of plants.

Assessing the impact of increasing carbon dioxide and temperature on crop-weed interactions for tomato and a C3 and C4 weed species

Based on the carboxylation kinetics of the C3 and C4 photosynthetic pathway, it is anticipated that C3 crops may be favored over C4 weeds as atmospheric CO2 increases. In the current study, tomato (Lycopersicon esculentum), a C3 crop species, was grown at ambient (~400μmolmol−1) and enhanced carbon dioxide (~800μmolmol−1) with and without two common weeds, lambsquarters (Chenopodium album), a C3 weed, and redroot pigweed (Amaranthus retroflexus), a C4 weed, from seedling emergence until mutual shading of crop-weed leaves. Because growth temperature is also likely to change in concert with rising CO2, the experiment was repeated at day/night temperatures of 21/12 and 26/18°C. For both day/night temperatures, elevated CO2 exacerbated weed competition from both the C3 and C4 weed species. A model based on relative leaf area following emergence was used to calculate potential crop losses from weeds. This analysis indicated that potential crop losses increased from 33 to 55% and from 32 to 61% at the 21/12 and 26/18°C day/night temperatures, for ambient and elevated CO2, respectively. For the current study, reductions in biomass and projected yield of tomato appeared independent of the photosynthetic pathway of the competing weed species. This may be due to inherent variation and overlap in the growth response of C3 and C4 species, whether weeds or crops, to increasing CO2 concentration. Overall, these results suggest that as atmospheric CO2 and/or temperature increases, other biological interactions, in addition to photosynthetic pathway, deserve additional consideration in predicting competitive outcomes between weeds and crops.

Will C3 crops enhanced with the C4 CO2-concentrating mechanism live up to their full potential (yield)?

Sustainably feeding the world's growing population in future is a great challenge and can be achieved only by increasing yield per unit land surface. Efficiency of light interception and biomass partitioning into harvestable parts (harvest index) has been improved substantially via plant breeding in modern crops. The conversion efficiency of intercepted light into biomass still holds promise for yield increase. This conversion efficiency is to a great extent constrained by the metabolic capacity of photosynthesis, defined by the characteristics of its components. Genetic manipulations are increasingly applied to lift these constraints, by improving CO2 or substrate availability for the photosynthetic carbon reduction cycle. Although these manipulations can lead to improved potential growth rates, this increase might be offset by a decrease in performance under stress conditions. In this review, we assess possible positive or negative effects of the introduction of a CO2-concentrating mechanism in C3 crop species on crop potential productivity and yield robustness.