Excitons and stacking order in h-BN

The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN) makes this material one of the most promising candidate for light emitting devices in the far ultraviolet (UV). However, single excitons occur only in perfect monocrystals that are extremely hard to synthesize, while regular h-BN samples present a complex emission spectrum with several additional peaks. The microscopic origin of these additional emissions has not yet been understood. In this work we address this problem using an experimental and theoretical approach that combines nanometric resolved cathodoluminescence, high resolution transmission electron microscopy and state of the art theoretical spectroscopy methods. We demonstrate that emission spectra are strongly inhomogeneus within individual flakes and that additional excitons occur at structural deformations, such as faceted plane folds, that lead to local changes of the h-BN stacking order

Phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxylase kinase isoenzymes play an important role in the filling and quality of Arabidopsis thaliana seed

Three plant-type phosphoenolpyruvate carboxylase (PPC1 to PPC3) and two phosphoenolpyruvate carboxylase kinase (PPCKs: PPCK1 and 2) genes are present in the Arabidopsis thaliana genome. In seeds, all PPC genes were found to be expressed. Examination of individual ppc mutants showed little reduction of PEPC protein and global activity, with the notable exception of PPC2 which represent the most abundant PEPC in dry seeds. Ppc mutants exhibited moderately lower seed parameters (weight, area, yield, germination kinetics) than wild type. In contrast, ppck1-had much altered (decreased) yield. At the molecular level, ppc3-was found to be significantly deficient in global seed nitrogen (nitrate, amino-acids, and soluble protein pools). Also, N-deficiency was much more marked in ppck1-, which exhibited a tremendous loss of 95% and 90% in nitrate and proteins, respectively. The line ppck2-had accumulated amino-acids but lower levels of soluble proteins. Regarding carboxylic acid pools, Krebs cycle intermediates were found to be diminished in all mutants; this was accompanied by a consistent decrease in ATP. Lipids were stable in ppc mutants, however ppck1-seeds accumulated more lipids while ppck2-seeds showed high level of polyunsaturated fatty acid oleic and linolenic (omega 3). Altogether, the results indicate that the complete PEPC and PPCK family are needed for normal C/N metabolism ratio, growth, development, yield and quality of the seed.Ministerio de Economía y Competitividad AGL2012-35708, AGL2016-75413-PJunta de Andalucía P12-FQM-48

In vivo monoubiquitination of anaplerotic phosphoenolpyruvate carboxylase occurs at Lys624 in germinating sorghum seeds

Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) is an important cytosolic regulatory enzyme that plays a pivotal role in numerous physiological processes in plants, including seed development and germination. Previous studies demonstrated the occurrence of immunoreactive PEPC polypeptides of ~110kDa and 107kDa (p110 and p107, respectively) on immunoblots of clarified extracts of germinating sorghum (Sorghum bicolor) seeds. In order to establish the biochemical basis for this observation, a 460kDa PEPC heterotetramer composed of an equivalent ratio of p110 and p107 subunits was purified to near homogeneity from the germinated seeds. Mass spectrometry established that p110 and p107 are both encoded by the same plant-type PEPC gene (CP21), but that p107 was in vivo monoubiquitinated at Lys624 to form p110. This residue is absolutely conserved in vascular plant PEPCs and is proximal to a PEP-binding/catalytic domain. Anti-ubiquitin IgG immunodetected p110 but not p107, whereas incubation with a deubiquitinating enzyme (USP-2 core) efficiently converted p110 into p107, while relieving the enzyme’s feedback inhibition by l-malate. Partial PEPC monoubiquitination was also detected during sorghum seed development. It is apparent that monoubiquitination at Lys624 is opposed to phosphorylation at Ser7 in terms of regulating the catalytic activity of sorghum seed PEPC. PEPC monoubiquitination is hypothesized to fine-tune anaplerotic carbon flux according to the cell’s immediate physiological requirements for tricarboxylic acid cycle intermediates needed in support of biosynthesis and carbon–nitrogen interactions.España, Ministerio de Economía y Competitividad AGL2012-35708España Junta de Andalucía P06-CVI-02186 BIO29

Formación de Recursos Humanos Docentes

Agencia Española de Cooperación Internacional para el Desarrollo (AECID

Calcium and the control of C4 photosynthesis

http://www.sherpa.ac.uk/romeo/search.ph

La fosfoenolpiruvato carboxilasa (PEPC): enzima clave de los metabolismos fotosintéticos C4 y CAM

http://digital.csic.es/bitstream/10261/29768/13/echevarria.pdfLa fosfoenolpiruvato carboxilasa (PEPC; EC 4.1.1.31) cataliza la β-carboxilación del fosfoenolpiruvato (PEP) en presencia de HCO3 - y Mg2+, para producir oxaloacetato (OAA) y Pi (Chollet et al., 1996). La PEPC está ampliamente distribuida en plantas, algas verdes y microorganismos pero ausente en levaduras y animales (Chollet et al., 1996). En plantas vasculares su papel estelar está relacionado con la fotosíntesis C4 y CAM («Crassulacean acid metabolism»), sin embargo desempeña otras funciones como la anaplerótica, en relación a la síntesis de proteínas, homeostasis del pH citosólico, electroneutralidad y osmolaridad. Está formada por una pequeña familia multigénica algunos de cuyos representantes están regulados a nivel transcripcional por factores como luz, hormonas y metabolitos (Chollet et al., 1996; Vidal y Chollet, 1997). La naturaleza alostérica de la enzima permite una regulación fina en relación a diferentes ambientes metabólicos. La PEPC está regulada por fosforilación reversible, proceso ligado a una cascada de transducción de señales de alta complejidad. En la actualidad es uno de los mejores modelos de señalización descritos en plantas. Este capítulo se centra en los eventos relacionados con este proceso en plantas C4 y CAM, los dos sistemas mejor estudiados en la actualidad (Chollet et al., 1996; Echevarría y Vidal, 2003; Izui et al., 2004; Nimmo, 2000; Vidal y Chollet, 1997).Phosphoenolpyruvate carboxylase (EC 4.1.1.31, PEPC) catalyzes the b-carboxylation of phosphoenolpyruvate (PEP) by HCO3 - in the presence of Mg2+ to yiel oxaloacetate and Pi (Chollet et al., 1996). PEPC is a widely distributed enzyme in plants, green algae and micro-organisms but absent in yeast and animals (Chollet et al., 1996). In higher plants, it catalyses a pivotal reaction related to such important processes as C4 and Crassulacean acid metabolism (CAM) photosynthesis, the anaplerotic pathway linked to amino acid synthesis, homeostasis of cytosolic pH, electroneutrality and osmolarity. PEPC belongs to a small multigenic family (Chollet et al., 1996; Vidal y Chollet, 1997). At the transcriptional level, some PEPC genes respond to external and internal factors (light, hormones and metabolites), while at the protein level, the allosteric nature of the enzyme allows its activity to be fine-tuned in relation to a varying metabolic environment. PEPC undergoes a posttranslational control by a phosphorylation process linked to a highly complex signal transduction cascade. Today, it is one of the best-described models of plant signaling. This chapter will focus on what is known about these processes in leaves of C4 and CAM plants, the two systems that have been studied in detail so far (Chollet et al., 1996; Echevarría y Vidal, 2003; Izui et al., 2004; Nimmo, 2000; Vidal y Chollet, 1997)

The Arabidopsis Nitrate Transporter NRT2.4 Plays a Double Role in Roots and Shoots of Nitrogen-Starved Plants

Plants have evolved a variety of mechanisms to adapt to N starvation. NITRATE TRANSPORTER2.4 (NRT2.4) is one of seven NRT2 family genes in Arabidopsis thaliana, and NRT2.4 expression is induced under N starvation. Green fluorescent protein and β-glucuronidase reporter analyses revealed that NRT2.4 is a plasma membrane transporter expressed in the epidermis of lateral roots and in or close to the shoot phloem. The spatiotemporal expression pattern of NRT2.4 in roots is complementary with that of the major high-affinity nitrate transporter NTR2.1. Functional analysis in Xenopus laevis oocytes and in planta showed that NRT2.4 is a nitrate transporter functioning in the high-affinity range. In N-starved nrt2.4 mutants, nitrate uptake under low external supply and nitrate content in shoot phloem exudates was decreased. In the absence of NRT2.1 and NRT2.2, loss of function of NRT2.4 (triple mutants) has an impact on biomass production under low nitrate supply. Together, our results demonstrate that NRT2.4 is a nitrate transporter that has a role in both roots and shoots under N starvation