Kranz Anatomy Research Articles

Diversity in structure and forms of carbon assimilation in photosynthetic organs in Cleome (Cleomaceae).

Photosynthesis in different organs of Cleome was analysed in four species known to have differences in leaf photosynthesis: Cleome africana Botsch. (C3), Cleome paradoxa R.Br. (C3-C4 intermediate), Cleome angustifolia Forssk. and Cleome gynandra L. (C4). The chlorophyll content, carbon isotope composition, stomatal densities, anatomy, levels and compartmentation of some key photosynthetic enzymes, and the form and function of photosynthesis were determined in different organs of these species. In the three xerophytes, C. africana, C. paradoxa, and C. angustifolia, multiple organs contribute to photosynthesis (cotyledons, leaves, petioles, stems and pods) which is considered important for their survival under arid conditions. In C. africana, all photosynthetic organs have C3 photosynthesis. In C. paradoxa, cotyledons, leaves, stems and petioles have C3-C4 type features. In C. angustifolia, the pods have C3 photosynthesis, whereas all other organs have C4 photosynthesis with Kranz anatomy formed by a continuous, dual layer of chlorenchyma cells. In the subtropical C4 species C. gynandra, cotyledons, leaves, and pods develop C4 photosynthesis, with Kranz anatomy around individual veins; but not in stems and petioles which have limited function of photosynthesis. The diversity in forms and the capacity of photosynthesis in organs of these species to contribute to their carbon economy is discussed.

Elevated CO2 leads to carbon sequestration by modulating C4 photosynthesis pathway enzyme (PPDK) in Suaeda monoica and S. fruticosa

The C4 halophytic species Suaeda monoica and S. fruticosa, possess the C4 photosynthesis pathway without Kranz anatomy were grown at ambient (470ppm CO2) and elevated (850ppm CO2) atmospheric CO2 under control containment facility to study the plant response under CO2 stress condition. The relative growth of both Suaeda species was enhanced with atmospheric CO2 enrichment compared to control (ambient) condition. The photosynthesis rate was found 2.5μmolCO2m−2s−1 in both species under stress condition compared to about 1.9μmolCO2m−2s−1 under control conditions. About 0.3molH2Om−2s−1 conductance was detected under an unstressed condition which decreased significantly to ~0.07molH2Om−2s−1 on the 6th day of stress treatment. Similarly, transpiration rate was also decreased significantly from 4.4–5.2mmolH2Om−2s−1 to 1.7–1.9 under stress condition. In contrast, VpdL increased significantly from 1.9kPa to 2.5kPa under stress condition. A higher total chlorophyll content observed in S. monoica (56.36mgg−1 tissue) compared to S. fruticosa (33.12mgg−1 tissue) under unstressed (control) condition. A significant increase was found in the total chlorophyll content of S. fruticosa (45.47mgg−1 tissue) with stress treatment compared to control (33.12mgg−1 tissue). In contrast, the total chlorophyll decreased in S. monoica (51.58mgg−1 tissue) under similar stress condition compared to control plants (56.36mgg−1 tissue). About 6–6.8mg total sugar per gram tissue found under control condition which enhanced further (7.5 to 11mgg−1 tissue) under stress condition. Similarly, total reducing sugar (~2mgg−1 tissue) and total starch content (6.5–11mgg−1 tissue) increased under stress condition. About 6.5- and 3- fold higher expression of PPDK gene was observed for S. monoica and S. fruticosa, respectively under CO2 stress condition. PPDK (1.2- and 1.5- fold) and antioxidant enzymes; APX (12.7- and two-fold), CAT (2.2- and 6.4- fold) and SOD (4.6- and 94- fold) enhanced significantly in S. fruticosa and S. monoica, respectively under high CO2 stress condition compared to control plants. Overall, it was observed that PPDK enzyme plays a key role in C4 photosynthesis pathway and S. monoica is a potential candidate to be explored further for the saline agricultural and CO2 capture.

Changes in redox regulation during transition from C3 to single cell C4 photosynthesis in Bienertia sinuspersici

Bienertia sinuspersici performs single cell C4 photosynthesis without Kranz anatomy. Peripheral and central cytoplasmic compartments in a single chlorenchyma cell act as mesophyll cells and bundle sheath cells. Development of this specialized mechanism is gradual during plant development. Young leaves perform C3 photosynthesis, while mature leaves have complete C4 cycle. The aim of this work was to investigate changes in redox regulation and antioxidant defence during transition from C3 to single cell C4 photosynthesis in B. sinuspersici leaves. First, we confirmed gradual development of C4 with protein blot and qRT-PCR analysis of C4 enzymes. After this activities and isoenzymes of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and H2O2 and TBARS and glutathione pool and redox status (GSH/GSSG) were determined in young, developing and mature leaves during transition from C3 to single cell C4 photosynthesis. Activities of SOD, APX and POX decrease, while GR and DHAR were increased. However, most striking results were the changes in isoenzyme patterns of SOD, CAT and GR which were gradual through transition to C4 photosynthesis.

Biology of Amaranths

Amaranthus, a cosmopolitan genus including endangered species, restricted endemics and widespread weeds, is often difficult to characterize taxonomically and thus has generally been considered by systematists as a “difficult” genus. Species in this genus have high genetic variability, with diversity in growth form, plant height, number of inflorescences, seed colour, protein content, seed yield, resistance to pests and diseases, and adaptation to soil type, pH, climate, rainfall and day-length. The combination of various anatomical characteristics of Amaranthus, such as Kranz anatomy, well developed root system, stomatal conductance, and maintenance of leaf area, results in increased efficiency of using CO2 under a wide range of temperatures, and higher light intensity and moisture stress environments which enables this plant to adapt under diverse geographic and environmental conditions. Buried seeds of Amaranthus constitute an important part of the soil seed bank and position, distribution and dormancy type of these seeds in the soil play an important role in their germination and subsequent emergence, which is further influenced by factors like temperature, soil moisture, and light availability. The current review highlights the positive as well as negative role of the various species of genus Amaranthus. Many species of the genus are medicinally important and bear antiallergic, anticancer, antihypertensive and antioxidant properties, thus being used in the treatment of several aliments. Amaranthus being a rich source of fatty acids, proteins, micronutrients, vitamins and squalene, are used as cereals, dye plants, forages, medicinal plants, ornamentals, and as vegetables. However some of the Amaranthus species are noxious weeds which are known to compete with many economic crops in different parts of the world and cause great yield losses. Thus, further research is warranted to strike a balance between the beneficial and harmful species of this Pseudocereal. Moreover, understanding the weedy behaviour of these plants would provide valuable information for improving our mechanistic models of crop-weed competition and weed population dynamics.

The Presence of Silica Bodies in the Foliar Epidermis of Zoysiagrass

The presence of silica bodies (SBs) in the foliar epidermis of zoysiagrass (Zoysia spp. Willd.) is a special characteristic, the function of which is not well understood. To quantify the importance of SB, we evaluated foliar SB densities and sizes of five Zoysia species [Z. japonica Steud., Z. matrella (L.) Merr., Z. pacifica (Goudsw.) M. Hotta & S. Kuroki, Z. minima (Colenso) Zotov, and Z. macrostachya Franch. & Sav.] including three ecotypes of each species using tabletop scanning electron microscopy. Our results demonstrated that abaxial SB density was significantly higher (mean: 734.3 mm−2) than that of adaxial SB (mean: 317.7mm−2) in all species. Abaxial SB size was also significantly larger (mean: 80.0 μm2) than that of the adaxial SB (mean: 61.4 μm2) in all species. Zoysia japonica had significantly higher SB density and size than the other tested Zoysia spp. Location‐specific sizes of foliar abaxial SB were identified: large on the midrib, medium on other ribs, and small in the costal zone. These results support the hypothesis that foliar SBs of zoysiagrass function to protect rib bundles and Kranz anatomy from physical damages, and to assist light absorbance for C4 photosynthesis through inhibiting leaves from twisting. Further work assessing SB impact on growth efficiency, survival strategy, and the utility of zoysiagrass is needed.

Making Roots, Shoots, and Seeds: IDD Gene Family Diversification in Plants.

The INDETERMINATE DOMAIN (IDD) family of transcriptional regulators controls a diversity of processes in a variety of plant tissues and organs and at different stages of plant development. Several recent reports describe the genetic characterization of IDD family members, including those that are likely to regulate C4 kranz anatomy, with implications for the engineering of C4 traits into C3 crops. In this review we summarize the reported functions of IDD members in the regulation of metabolic sensing and leaf, root, seed, and inflorescence development. We also provide an IDD phylogeny for the grasses and suggest future directions and strategies to define the function of IDDs in C4 photosynthesis and other developmental processes.

Study of anatomical parameters of leaf and brix content of some promising commercial Indian varieties of sugarcane

The structural and quantitative diversity of Kranz anatomy of nineteen Indian varieties of sugarcane was studied in relation to brix content or yield of sugar. The nature and distribution of Kranz tissue and types of vascular bundle in the leaf blade have also been described and discussed. In hand made transverse sections of leaf blade of sugarcane varieties, two different sizes (large and small) of vascular bundles were found. The large vascular bundles were characterized by the presence of metaxylem vessels on the either side of protoxylem. The small vascular bundles entirely consist of metaxylem but lack of protoxylem. Variation of Brix content of the basal, middle and top part of Culm of nineteen varieties was also observed. Statistically, average Brix content was positively correlated with the total area of the photosynthetic cells as well as bundle sheath cells and the linear regression curve followed the straight line.

Carbon Di Oxide Fertilization: Effects on Plant Productivity

Increasing Carbon dioxide (CO2) is an important component of global climate change that has drawn the attention of environmentalists worldwide in the last few decades. Besides acting as an important greenhouse gas, it also produces a stimulatory effect, its instantaneous impact being a significant increase in the plant productivity. Atmospheric CO2 levels have linearly increased from approximately 280 parts per million (ppm) during pre-industrial times to the current level of more than 390 ppm. In past few years, anthropogenic activities led to a rapid increase in global CO2 concentration. Current Intergovernmental Panel on Climate Change (IPCC) projection indicates that atmospheric CO2 concentration will increase over this century, reaching 730-1020 ppm by 2100. An increase in global temperature, ranging from 1.1 to 6.4oC depending on global emission scenarios, will accompany the rise in atmospheric CO2. As CO2 acts as a limiting factor in photosynthesis, the immediate effect of increasing atmospheric CO2 is improved plant productivity, a feature commonly termed as “CO2 fertilization”. Variability in crop responses to the elevated CO2 made the agricultural productivity and food security vulnerable to the climate change. Several studies have shown significant CO2 fertilization effect on crop growth and yield. An increase of 30 % in plant growth and yield has been reported when CO2 concentration has been doubled from 330 to 660 ppm. However, the fertilization effect of elevated CO2 is not very much effective in case of C4 plants which already contain a CO2 concentration mechanism, owing to their specific leaf 2 anatomy called kranz anatomy. As a result, yield increments observed in C4plants are comparatively lower than the C3 plants under similar elevated CO2 concentrations. This review discusses the trends and the causes of increasing CO2 concentration in the atmosphere, its effects on the crop productivity and the discrepancies in the response of C3 and C4 plants to increasing CO2 concentrations.

An rbcL mRNA-binding protein is associated with C3 to C4 evolution and light-induced production of Rubisco in Flaveria.

Nuclear-encoded RLSB protein binds chloroplastic rbcL mRNA encoding the Rubisco large subunit. RLSB is highly conserved across all groups of land plants and is associated with positive post-transcriptional regulation of rbcL expression. In C3 leaves, RLSB and Rubisco occur in all chlorenchyma cell chloroplasts, while in C4 leaves these accumulate only within bundle sheath (BS) chloroplasts. RLSB's role in rbcL expression makes modification of its localization a likely prerequisite for the evolutionary restriction of Rubisco to BS cells. Taking advantage of evolutionarily conserved RLSB orthologs in several C3, C3-C4, C4-like, and C4 photosynthetic types within the genus Flaveria, we show that low level RLSB sequence divergence and modification to BS specificity coincided with ontogeny of Rubisco specificity and Kranz anatomy during C3 to C4 evolution. In both C3 and C4 species, Rubisco production reflected RLSB production in all cell types, tissues, and conditions examined. Co-localization occurred only in photosynthetic tissues, and both proteins were co-ordinately induced by light at post-transcriptional levels. RLSB is currently the only mRNA-binding protein to be associated with rbcL gene regulation in any plant, with variations in sequence and acquisition of cell type specificity reflecting the progression of C4 evolution within the genus Flaveria.

High spatial resolution mass spectrometry imaging reveals the genetically programmed, developmental modification of the distribution of thylakoid membrane lipids among individual cells of maize leaf.

Metabolism in plants is compartmentalized among different tissues, cells and subcellular organelles. Mass spectrometry imaging (MSI) with matrix-assisted laser desorption ionization (MALDI) has recently advanced to allow for the visualization of metabolites at single-cell resolution. Here we applied 5- and 10μm high spatial resolution MALDI-MSI to the asymmetric Kranz anatomy of Zea mays (maize) leaves to study the differential localization of two major anionic lipids in thylakoid membranes, sulfoquinovosyldiacylglycerols (SQDG) and phosphatidylglycerols (PG). The quantification and localization of SQDG and PG molecular species, among mesophyll (M) and bundle sheath (BS) cells, are compared across the leaf developmental gradient from four maize genotypes (the inbreds B73 and Mo17, and the reciprocal hybrids B73×Mo17 and Mo17×B73). SQDG species are uniformly distributed in both photosynthetic cell types, regardless of leaf development or genotype; however, PG shows photosynthetic cell-specific differential localization depending on the genotype and the fatty acyl chain constituent. Overall, 16:1-containing PGs primarily contribute to the thylakoid membranes of M cells, whereas BS chloroplasts are mostly composed of 16:0-containing PGs. Furthermore, PG32:0 shows genotype-specific differences in cellular distribution, with preferential localization in BS cells for B73, but more uniform distribution between BS and M cells in Mo17. Maternal inheritance is exhibited within the hybrids, such that the localization of PG 32:0 in B73×Mo17 is similar to the distribution in the B73 parental inbred, whereas that of Mo17×B73 resembles the Mo17 parent. This study demonstrates the power of MALDI-MSI to reveal unprecedented insights on metabolic outcomes in multicellular organisms at single-cell resolution.

Walking the C4 pathway: past, present, and future.

The year 2016 marks 50 years since the publication of the seminal paper by Hatch and Slack describing the biochemical pathway we now know as C4 photosynthesis. This review provides insight into the initial discovery of this pathway, the clues which led Hatch and Slack and others to these definitive experiments, some of the intrigue which surrounds the international activities which led up to the discovery, and personal insights into the future of this research field. While the biochemical understanding of the basic pathways came quickly, the role of the bundle sheath intermediate CO2 pool was not understood for a number of years, and the nature of C4 as a biochemical CO2 pump then linked the unique Kranz anatomy of C4 plants to their biochemical specialization. Decades of "grind and find biochemistry" and leaf physiology fleshed out the regulation of the pathway and the differences in physiological response to the environment between C3 and C4 plants. The more recent advent of plant transformation then high-throughput RNA and DNA sequencing and synthetic biology has allowed us both to carry out biochemical experiments and test hypotheses in planta and to better understand the evolution-driven molecular and genetic changes which occurred in the genomes of plants in the transition from C3 to C4 Now we are using this knowledge in attempts to engineer C4 rice and improve the C4 engine itself for enhanced food security and to provide novel biofuel feedstocks. The next 50 years of photosynthesis will no doubt be challenging, stimulating, and a drawcard for the best young minds in plant biology.

Unique photosynthetic phenotypes inPortulaca(Portulacaceae): C3-C4intermediates and NAD-ME C4species with Pilosoid-type Kranz anatomy

Portulacaceae is a family that has considerable diversity in photosynthetic phenotypes. It is one of 19 families of terrestrial plants where species having C4 photosynthesis have been found. Most species in Portulaca are in the alternate-leaved (AL) lineage, which includes one clade (Cryptopetala) with taxa lacking C4 photosynthesis and three clades having C4 species (Oleracea, Umbraticola and Pilosa). All three species in the Cryptopetala clade lack Kranz anatomy, the leaves have C3-like carbon isotope composition and they have low levels of C4 cycle enzymes. Anatomical, biochemical and physiological analyses show they are all C3-C4 intermediates. They have intermediate CO2 compensation points, enrichment of organelles in the centripetal position in bundle sheath (BS) cells, with selective localization of glycine decarboxylase in BS mitochondria. In the three C4 clades there are differences in Kranz anatomy types and form of malic enzyme (ME) reported to function in C4 (NAD-ME versus NADP-ME): Oleracea (Atriplicoid, NAD-ME), Umbraticola (Atriplicoid, NADP-ME) and Pilosa (Pilosoid, NADP-ME). Structural and biochemical analyses were performed on Pilosa clade representatives having Pilosoid-type leaf anatomy with Kranz tissue enclosing individual peripheral vascular bundles and water storage in the center of the leaf. In this clade, all species except P. elatior are NADP-ME-type C4 species with grana-deficient BS chloroplasts and grana-enriched M chloroplasts. Surprisingly, P. elatior has BS chloroplasts enriched in grana and NAD-ME-type photosynthesis. The results suggest photosynthetic phenotypes were probably derived from an ancestor with NADP-ME-type C4, with two independent switches to NAD-ME type.

Regulatory gateways for cell-specific gene expression in C4 leaves with Kranz anatomy.

C4 photosynthesis is a carbon-concentrating mechanism that increases delivery of carbon dioxide to RuBisCO and as a consequence reduces photorespiration. The C4 pathway is therefore beneficial in environments that promote high photorespiration. This pathway has evolved many times, and involves restricting gene expression to either mesophyll or bundle sheath cells. Here we review the regulatory mechanisms that control cell-preferential expression of genes in the C4 cycle. From this analysis, it is clear that the C4 pathway has a complex regulatory framework, with control operating at epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels. Some genes of the C4 pathway are regulated at multiple levels, and we propose that this ensures robust expression in each cell type. Accumulating evidence suggests that multiple genes of the C4 pathway may share the same regulatory mechanism. The control systems for C4 photosynthesis gene expression appear to operate in C3 plants, and so it appears that pre-existing mechanisms form the basis of C4 photosynthesis gene expression.

The draft genome of the C3 panicoid grass species Dichanthelium oligosanthes.

BackgroundComparisons between C3 and C4 grasses often utilize C3 species from the subfamilies Ehrhartoideae or Pooideae and C4 species from the subfamily Panicoideae, two clades that diverged over 50 million years ago. The divergence of the C3 panicoid grass Dichanthelium oligosanthes from the independent C4 lineages represented by Setaria viridis and Sorghum bicolor occurred approximately 15 million years ago, which is significantly more recent than members of the Bambusoideae, Ehrhartoideae, and Pooideae subfamilies. D. oligosanthes is ideally placed within the panicoid clade for comparative studies of C3 and C4 grasses.ResultsWe report the assembly of the nuclear and chloroplast genomes of D. oligosanthes, from high-throughput short read sequencing data and a comparative transcriptomics analysis of the developing leaf of D. oligosanthes, S. viridis, and S. bicolor. Physiological and anatomical characterizations verified that D. oligosanthes utilizes the C3 pathway for carbon fixation and lacks Kranz anatomy. Expression profiles of transcription factors along developing leaves of D. oligosanthes and S. viridis were compared with previously published data from S. bicolor, Zea mays, and Oryza sativa to identify a small suite of transcription factors that likely acquired functions specifically related to C4 photosynthesis.ConclusionsThe phylogenetic location of D. oligosanthes makes it an ideal C3 plant for comparative analysis of C4 evolution in the panicoid grasses. This genome will not only provide a better C3 species for comparisons with C4 panicoid grasses, but also highlights the power of using high-throughput sequencing to address questions in evolutionary biology.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-016-1080-3) contains supplementary material, which is available to authorized users.

Size matters for single-cell C4 photosynthesis in Bienertia.

Bienertia cycloptera belongs to a diverse set of plants, recently discovered to perform C4 photosynthesis within individual mesophyll cells. How these plants accomplish high photosynthetic efficiency without adopting Kranz anatomy remains unanswered. By modelling the processes of diffusion, capture, and release of carbon dioxide and oxygen inside a typical Bienertia mesophyll cell geometry, we show that a spatial separation as low as 10 μm between the primary and the secondary carboxylases, can, on its own, provide enough diffusive resistance to sustain a viable C4 pathway at 20 °C, with a CO2 leakage <35%. This critical separation corresponds to a cell diameter of 50 μm, consistent with the observed range where Bienertia's mesophyll cells start to develop their characteristic mature anatomy. Our results are robust to significant alterations in model assumptions and environmental conditions, their applicability extending even to aquatic plants.

Walking the C4 pathway: past, present, and future.

The year 2016 marks 50 years since the publication of the seminal paper by Hatch and Slack describing the biochemical pathway we now know as C4 photosynthesis. This review provides insight into the initial discovery of this pathway, the clues which led Hatch and Slack and others to these definitive experiments, some of the intrigue which surrounds the international activities which led up to the discovery, and personal insights into the future of this research field. While the biochemical understanding of the basic pathways came quickly, the role of the bundle sheath intermediate CO2 pool was not understood for a number of years, and the nature of C4 as a biochemical CO2 pump then linked the unique Kranz anatomy of C4 plants to their biochemical specialization. Decades of "grind and find biochemistry" and leaf physiology fleshed out the regulation of the pathway and the differences in physiological response to the environment between C3 and C4 plants. The more recent advent of plant transformation then high-throughput RNA and DNA sequencing and synthetic biology has allowed us both to carry out biochemical experiments and test hypotheses in planta and to better understand the evolution-driven molecular and genetic changes which occurred in the genomes of plants in the transition from C3 to C4 Now we are using this knowledge in attempts to engineer C4 rice and improve the C4 engine itself for enhanced food security and to provide novel biofuel feedstocks. The next 50 years of photosynthesis will no doubt be challenging, stimulating, and a drawcard for the best young minds in plant biology.

Taxonomic novelties from Irano-Turanian region and NE Iran: Oreosalsola, a new segregate from Salsola s.l., two new species in Anabasis and Salvia, and two new combinations in Caroxylon and Seseli

Oreosalsola, a non-C4 lineage is described as a new genus segregated from Salsola s.l. (Chenopodiaceae). The phylogenetic position and anatomical structure of the new genus in relation to photosynthetic types are discussed. Anabasis firouzii (Chenopodiaceae) and Salvia shahkuhmahalei (Lamiaceae) are described from Golestan National Park (GNP) and Ghorkhod Protected Area (GPA), NE Iran. The new Anabasis is a C4 plant according to its Salsoloid Kranz anatomy and carbon isotope composition. Anabasis firouzii grows on species-poor marl outcrops of Kopet Dagh Mountains in GNP and GPA. The phylogenetic relationship of the new species is discussed using ITS sequence data. The position of Anabasis setifera within the genus Anabasis (not Salsola) is confirmed by updated molecular data. Salvia shahkuhmahalei is closely related to S. rhytidea that is native of Afghanistan and S Iran. Salsola iljinii is transferred to Caroxylon following a new phylogenetic concept of Salsoleae. Seseli tortusosum subsp. kiabii (Apiaceae) is elevated to the species level, which is justified by the recent knowledge. The new species are illustrated; their habitat, plant communities, associated species, threat categories and phytogeographic importance are discussed.

Validation of reference genes for quantitative RT-PCR normalization in Suaeda aralocaspica, an annual halophyte with heteromorphism and C4 pathway without Kranz anatomy.

Reverse transcription quantitative real-time polymerase chain reaction (qRT-PCR) is a powerful analytical technique for the measurement of gene expression, which depends on the stability of the reference gene used for data normalization. Suaeda aralocaspica, an annual halophyte with heteromorphic seeds and possessing C4 photosynthesis pathway without Kranz anatomy, is an ideal plant species to identify stress tolerance-related genes and compare relative expression at transcriptional level. So far, no molecular information is available for this species. In the present study, six traditionally used reference genes were selected and their expression stability in two types of seeds of S. aralocaspica under different experimental conditions was evaluated. Three analytical programs, geNorm, NormFinder and BestKeeper, were used to assess and rank the stability of reference gene expression. Results revealed that although some reference genes may display different transcriptional profiles between the two types of seeds, β-TUB and GAPDH appeared to be the most suitable references under different developmental stages and tissues. GAPDH was the appropriate reference gene under different germination time points and salt stress conditions, and ACTIN was suitable for various abiotic stress treatments for the two types of seeds. For all the sample pools, β-TUB served as the most stable reference gene, whereas 18S rRNA and 28S rRNA performed poorly and presented as the least stable genes in our study. UBQ seemed to be unsuitable as internal control under different salt treatments. In addition, the expression of a photosynthesis-related gene (PPDK) of C4 pathway and a salt tolerance-related gene (SAT) of S. aralocaspica were used to validate the best performance reference genes. This is the first systematic comparison of reference gene selection for qRT-PCR work in S. aralocaspica and these data will facilitate further studies on gene expression in this species and other euhalophytes.

A two-dimensional microscale model of gas exchange during photosynthesis in maize (Zea mays L.) leaves

CO2 exchange in leaves of maize (Zea mays L.) was examined using a microscale model of combined gas diffusion and C4 photosynthesis kinetics at the leaf tissue level. Based on a generalized scheme of photosynthesis in NADP-malic enzyme type C4 plants, the model accounted for CO2 diffusion in a leaf tissue, CO2 hydration and assimilation in mesophyll cells, CO2 release from decarboxylation of C4 acids, CO2 fixation in bundle sheath cells and CO2 retro-diffusion from bundle sheath cells. The transport equations were solved over a realistic 2-D geometry of the Kranz anatomy obtained from light microscopy images. The predicted responses of photosynthesis rate to changes in ambient CO2 and irradiance compared well with those obtained from gas exchange measurements. A sensitivity analysis showed that the CO2 permeability of the mesophyll-bundle sheath and airspace–mesophyll interfaces strongly affected the rate of photosynthesis and bundle sheath conductance. Carbonic anhydrase influenced the rate of photosynthesis, especially at low intercellular CO2 levels. In addition, the suberin layer at the exposed surface of the bundle sheath cells was found beneficial in reducing the retro-diffusion. The model may serve as a tool to investigate CO2 diffusion further in relation to the Kranz anatomy in C4 plants.

Minor millets as model system to study C&lt;sub&gt;4&lt;/sub&gt; photosynthesis -A review

C&lt;sub&gt;4&lt;/sub&gt; photosynthesis is the primary mode of carbon capture and drives productivity in several major food crops and bioenergy grasses. Gains in productivity associated with C&lt;sub&gt;4&lt;/sub&gt; photosynthesis include improved water and nitrogen use efficiencies. Within grasses rice and brachypodium are used as model species. Since these two crops are using C&lt;sub&gt;3&lt;/sub&gt; photosynthesis for their growth and development, it cannot be used as model for to study C&lt;sub&gt;4&lt;/sub&gt; photosynthesis. In order to characterize the evolutionary innovations and to provide genomic insight into crop improvement for the many important crop species, a new genomic and genetic model species is required. Minor millets have small diploid genomes, shorter life cycles, self pollination and prolific seed production. Due to these characteristics it gains importance over major C&lt;sub&gt;4&lt;/sub&gt; species which lack all of these traits. Within Minor millets, &lt;italic&gt;Setaria italica&lt;/italic&gt; and &lt;italic&gt;Setaria viridis&lt;/italic&gt; are used as model systems since these crops fulfils all the traits responsible to be a model species. Importantly, &lt;italic&gt;Setaria&lt;/italic&gt; species uses NADP-Malic enzyme subtype C&lt;sub&gt;4&lt;/sub&gt; photosynthetic system to fix carbon and therefore is a potential powerful model system for dissecting C&lt;sub&gt;4&lt;/sub&gt; photosynthesis. C&lt;sub&gt;4&lt;/sub&gt; grasses have a shorter distance between longitudinal veins in the leaves than C&lt;sub&gt;3&lt;/sub&gt; grasses. The C&lt;sub&gt;4&lt;/sub&gt; grasses have denser transverse and small longitudinal veins than the C&lt;sub&gt;3&lt;/sub&gt; grasses. It indicates that C&lt;sub&gt;4&lt;/sub&gt; grasses have a structurally superior photosynthate translocation and water distribution system by developing denser networks of small longitudinal and transverse veins. &lt;italic&gt;Setaria&lt;/italic&gt; has high vein density and kranz anatomy that helps to concentrate CO&lt;sub&gt;2&lt;/sub&gt; in the bundle sheath cells. This minimizes photorespiration thereby prevents the loss of energy.