The glutamine synthetase (CS) gene family of Medicago truncatula Caertn. contains three genes related to cytosolic GS (MtCSa, MtCSb, and MtCSc), although one of these (MtGSc) appears not to be expressed. Sequence analysis suggests that the genes are more highly conserved interspecifically rather than intraspecifically: MtCSa and MtCSb are more similar to their homologs in Medicago sativa and Pisum sativum than to each other. Studies in which gene-specific probes are used show that both MtCSa and MtCSb are induced during symbiotic root nodule development, although not coordinately. MtCSa is the most highly expressed CS gene in nodules but is also expressed to lower extents in a variety of other organs. MtCSb shows higher levels of expression in roots and the photosynthetic cotyledons of seedlings than in nodules or other organs. In roots, both genes are expressed in the absence of an exogenous nitrogen source. However the addition of nitrate leads to a short-term, 2-to 3-fold increase in the abundance of both mRNAs, and the addition of ammonium leads to a 2-fold increase in MtCSb mRNA. The nitrogen supply, therefore, influentes the expression of the two genes in roots, but it is clearly not the major effector of their expression. In the discussion section, the expression of the CS gene family of the model legume M. truncatula is compared to those of other leguminous plants.GS (EC 6.3.1.2) is a key enzyme in the nitrogen metabolism of higher plants, catalyzing the assimilation of ammonium to form Gln. This ammonium is derived from the primary nitrogen sources of the plant (through the reduction of soil nitrate and, in the case of legumes, by the symbiotic fixation of atmospheric nitrogen), as well as from other metabolic pathways such as photorespiration, phenylpropanoid metabolism, and the catabolism of amino acids (Lea et al., 1990). These pathways occur to varying extents in different tissues and subcellular locations, which is reflected by the fact that in higher plants GS exists as a number of distinct isoenzymes located in both the cytosol and the chloroplast, which have different activities in different organs (McNally and Hirel, 1983). These multiple isoenzymes are encoded by a small family of genes that, in tum, have been shown to be differentially expressed in both a developmental-and organ-'This work was supported by a fellowship to A.C.S. from the
Double quantum dots (DQDs) hold great promise as building blocks for quantum technology as they allow for two electronic states to coherently couple. Defining QDs with materials rather than using electrostatic gating allows for QDs with a hard-wall confinement potential and more robust charge and spin states. An unresolved problem is how to individually address these quantum dots, which is necessary for controlling quantum states. We here report the fabrication of double quantum dot devices defined by the conduction band edge offset at the interface of the wurtzite and zinc blende crystal phases of InAs in nanowires. By using sacrifical epitaxial GaSb markers selectively forming on one crystal phase, we are able to precisely align gate electrodes allowing us to probe and control each QD independently. We hence observe textbook-like charge stability diagrams, a discrete energy spectrum and electron numbers consistent with theoretical estimates and investigate the tunability of the devices, finding that changing the electron number can be used to tune the tunnel barrier as expected by simple band diagram arguments.When electrons are spatially confined in semiconductor quantum dots (QDs), they form bound states with discrete energy levels. These systems have drawn much attention both experimentally and theoretically [1,2] as they form atom-like structures in solid-state. One system of particular importance is the double quantum dot (DQD) where two discrete electronic states couple coherently, making it the building block of charge and spin qubits [1][2][3][4][5][6]. QDs are also elemental in other semiconductor quantum systems that are promising for use in quantum computers and quantum systems in general [4,5,7], such as Majorana fermions [8][9][10][11]. QDs are commonly defined by gate depletion [2,5,12], but progress has been made in material-defined QDs as well [13][14][15]. The material-defined approach allows for more well-defined features and less coupling to external noise. In this letter, we utilize recently developed InAs polytype bandgap engineering [15][16][17] to define DQDs with a hard-wall potential. With epitaxial markers, we gain control of the individual dots and, demonstrate the honeycomb-shaped charge stability diagrams of material-defined DQDs and the robustness of the system with a wide range of electron populations.The most common approach to forming quantum dots for transport experiments is to start from a material system that is already structurally confined in one or two dimensions and use electrostatic gating to confine the remaining dimensions.Examples here include two-dimensional electron gases [2,5], one-dimensional carbon nanotubes and semiconductor nanowires [12]. These partially gate-defined QDs have a smoothly changing confinement potential and hence their size *
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