Photorespiration: metabolic pathways and their role in stress protection

Photorespiration results from the oxygenase reaction catalysed by ribulose-1,5-bisphosphate carboxylase/ oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus making photorespiration a wasteful process. However, precisely because of this ine¤ciency, photorespiration could serve as an energy sink preventing the overreduction of the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO2 assimilation. Furthermore, photorespiration provides metabolites for other metabolic processes, e.g. glycine for the synthesis of glutathione, which is also involved in stress protection. In this review, we describe the use of photorespiratory mutants to study the control and regulation of photorespiratory pathways. In addition, we discuss the possible role of photorespiration under stress conditions, such as drought, high salt concentrations and high light intensities encountered by alpine plants

Geometric Mechanics of Curved Crease Origami

Folding a sheet of paper along a curve can lead to structures seen in decorative art and utilitarian packing boxes. Here we present a theory for the simplest such structure: an annular circular strip that is folded along a central circular curve to form a three-dimensional buckled structure driven by geometrical frustration. We quantify this shape in terms of the radius of the circle, the dihedral angle of the fold and the mechanical properties of the sheet of paper and the fold itself. When the sheet is isometrically deformed everywhere except along the fold itself, stiff folds result in creases with constant curvature and oscillatory torsion. However, relatively softer folds inherit the broken symmetry of the buckled shape with oscillatory curvature and torsion. Our asymptotic analysis of the isometrically deformed state is corroborated by numerical simulations which allow us to generalize our analysis to study multiply folded structures

NADPH oxidase, NOX1, mediates vascular injury in ischemic retinopathy

<b>Aims:</b> Ischemic retinal diseases such as retinopathy of prematurity are major causes of blindness due to damage to the retinal microvasculature. Despite this clinical situation, retinopathy of prematurity is mechanistically poorly understood. Therefore, effective preventative therapies are not available. However, hypoxic-induced increases in reactive oxygen species (ROS) have been suggested to be involved with NADPH oxidases (NOX), the only known dedicated enzymatic source of ROS. Our major aim was to determine the contribution of NOX isoforms (1, 2, and 4) to a rodent model of retinopathy of prematurity. <b>Results:</b> Using a genetic approach, we determined that only mice with a deletion of NOX1, but not NOX2 or NOX4, were protected from retinal neovascularization and vaso-obliteration, adhesion of leukocytes, microglial accumulation, and the increased generation of proangiogenic and proinflammatory factors and ROS. We complemented these studies by showing that the specific NOX inhibitor, GKT137831, reduced vasculopathy and ROS levels in retina. The source of NOX isoforms was evaluated in retinal vascular cells and neuro-glial elements. Microglia, the immune cells of the retina, expressed NOX1, 2, and 4 and responded to hypoxia with increased ROS formation, which was reduced by GKT137831. <b>Innovation:</b> Our studies are the first to identify the NOX1 isoform as having an important role in the pathogenesis of retinopathy of prematurity. <b>Conclusions:</b> Our findings suggest that strategies targeting NOX1 have the potential to be effective treatments for a range of ischemic retinopathie

Allosteric “beta-blocker” isolated from a DNA-encoded small molecule library

The present study reports the discovery of a small-molecule negative allosteric modulator for the β2-adrenergic receptor (β2AR) via in vitro affinity-based iterative selection of highly diverse DNA-encoded small-molecule libraries. Characterization of the compound demonstrates its selectivity for the β2AR and that it negatively modulates a wide range of receptor functions. More importantly, our findings establish a generally applicable, proof-of-concept strategy for screening DNA-encoded small-molecule libraries against purified G-protein–coupled receptors (GPCRs), which holds great potential for discovering therapeutic molecules

A major grain protein content locus on barley (Hordeum vulgare L.) chromosome 6 influences flowering time and sequential leaf senescence

Timing of various developmental stages including anthesis and whole-plant (‘monocarpic’) senescence influences yield and quality of annual crops. While a correlation between flowering/seed filling and whole-plant senescence has been observed in many annuals, it is unclear how the gene networks controlling these processes interact. Using near-isogenic germplasm, it has previously been demonstrated that a grain protein content (GPC) locus on barley chromosome 6 strongly influences the timing of post-anthesis flag leaf senescence, with high-GPC germplasm senescing early. Here, it is shown that the presence of high-GPC allele(s) at this locus also accelerates pre-anthesis plant development. While floral transition at the shoot apical meristem (SAM; determined by the presence of double ridges) occurred simultaneously, subsequent development was faster in the high- than in the low-GPC line, and anthesis occurred on average 5 d earlier. Similarly, sequential (pre-anthesis) leaf senescence was slightly accelerated, but only after differences in SAM development became visible. Leaf expression levels of four candidate genes (from a list of genes differentially regulated in post-anthesis flag leaves) were much higher in the high-GPC line even before faster development of the SAM became visible. One of these genes may be a functional homologue of Arabidopsis glycine-rich RNA-binding protein 7, which has previously been implicated in the promotion of flowering. Together, the data establish that the GPC locus influences pre- and post-anthesis barley development and senescence, and set the stage for a more detailed analysis of the interactions between the molecular networks controlling these important life history traits

Global gene flow releases invasive plants from environmental constraints on genetic diversity

When plants establish outside their native range, their ability to adapt to the new environment is influenced by both demography and dispersal. However, the relative importance of these two factors is poorly understood. To quantify the influence of demography and dispersal on patterns of genetic diversity underlying adaptation, we used data from a globally distributed demographic research network comprising 35 native and 18 nonnative populations of Plantago lanceolata. Species-specific simulation experiments showed that dispersal would dilute demographic influences on genetic diversity at local scales. Populations in the native European range had strong spatial genetic structure associated with geographic distance and precipitation seasonality. In contrast, nonnative populations had weaker spatial genetic structure that was not associated with environmental gradients but with higher within-population genetic diversity. Our findings show that dispersal caused by repeated, long-distance, human-mediated introductions has allowed invasive plant populations to overcome environmental constraints on genetic diversity, even without strong demographic changes. The impact of invasive plants may, therefore, increase with repeated introductions, highlighting the need to constrain future introductions of species even if they already exist in an area

Role of plant glyoxylate reductases during stress: a hypothesis

Molecular modelling suggests that a group of proteins in plants known as the β-hydroxyacid dehydrogenases, or the hydroxyisobutyrate dehydrogenase superfamily, includes enzymes that reduce succinic semialdehyde and glyoxylate to γ-hydroxybutyrate and glycolate respectively. Recent biochemical and expression studies reveal that NADPH-dependent cytosolic (termed GLYR1) and plastidial (termed GLYR2) isoforms of succinic semialdehyde/glyoxylate reductase exist in Arabidopsis. Succinic semialdehyde and glyoxylate are typically generated in leaves via two distinct metabolic pathways, γ-aminobutyrate and glycolate respectively. In the present review, it is proposed that the GLYRs function in the detoxification of both aldehydes during stress and contribute to redox balance. Outstanding questions are highlighted in a scheme for the subcellular organization of the detoxification mechanism in Arabidopsis

Identification and characterization of a plastid-localized Arabidopsis glyoxylate reductase isoform: comparison with a cytosolic isoform and implications for cellular redox homeostasis and aldehyde detoxification

Enzymes that reduce the aldehyde chemical grouping (i.e. H-C=O) to its corresponding alcohol could be crucial in maintaining plant health. Recently, recombinant expression of a cytosolic enzyme from Arabidopsis thaliana (L.) Heynh (designated as glyoxylate reductase 1 or AtGR1) revealed that it effectively catalyses the in vitro reduction of both glyoxylate and succinic semialdehyde (SSA). In this paper, web-based bioinformatics tools revealed a second putative GR cDNA (GenBank Accession No. AAP42747; designated herein as AtGR2) that is 57% identical on an amino acid basis to GR1. Sequence encoding a putative targeting signal (N-terminal 43 amino acids) was deleted from the full-length GR2 cDNA and the resulting truncated gene was co-expressed with the molecular chaperones GroES/EL in Escherichia coli, enabling production and purification of soluble recombinant protein. Kinetic analysis revealed that recombinant GR2 catalysed the conversion of glyoxylate to glycolate (Km glyoxylate=34 μM), and SSA to γ-hydroxybutyrate (Km SSA=8.96 mM) via an essentially irreversible, NADPH-based mechanism. GR2 had a 350-fold higher preference for glyoxylate than SSA, based on the performance constants (kcat/Km). Fluorescence microscopic analysis of tobacco (Nicotiana tabacum L.) suspension cells transiently transformed with GR1 linked to the green fluorescent protein (GFP) revealed that GR1 was localized to the cytosol, whereas GR2-GFP was localized to plastids via targeting information contained within its N-terminal 45 amino acids. The identification and characterization of distinct plastidial and cytosolic glyoxylate reductase isoforms is discussed with respect to aldehyde detoxification and the plant stress response