Glutamine Synthetase Isoforms in Pea Leaves: Intracellular Localization

Confirmation and inheritance of glufosinate resistance in an Amaranthus palmeri population from North Carolina

A putative glufosinate-resistant Amaranthus palmeri population was reported in 2015 in Anson County, North Carolina. The results from dose-response assays conducted in the field suggested plants were surviving lethal rates of glufosinate. Dose-response assays conducted in the glasshouse determined the Anson County accession exhibited reduced susceptibility to glufosinate compared to three glufosinate-susceptible populations. The LD50 values (210-316 g ai ha-1) for the Anson County population were always higher than the LD50 values (118-158 g ai ha-1) for the tested susceptible populations from the dose-response assays. Anson County plants that survived lethal glufosinate rates were reciprocally crossed with susceptible plants to create F1 genotypes and treated with a lethal rate of glufosinate (267 g ai ha-1; ascertained from glasshouse dose-response assay) to determine the distribution of injury and survival for each cross compared to a cross of susceptible parents. The distribution of injury was non-normal for the crosses containing an Anson County plant compared to the cross with a susceptible parent. Survival was 68%-84% for crosses containing an Anson County plant, whereas the survival was significantly reduced to 35% for the susceptible plant cross. Chi-square goodness of fit tests were used to test inheritance models to describe the responses of the genotypes. The resistant × susceptible crosses were best described with a heterozygous two loci with incomplete dominance model compared to the resistant × resistant cross that was best described with a heterozygous single locus with incomplete dominance model. The Anson County population has evolved resistance to glufosinate that is heritable and likely conferred by an oligogenic mechanism with incomplete dominance.

Using glutamine synthetase 1 to evaluate the symbionts' potential of ammonia assimilation and their responses to illumination in five organs of the giant clam, Tridacna squamosa

Nitrogen-deficient symbiotic dinoflagellates (zooxanthellae) living inside the fluted giant clam, Tridacna squamosa, need to obtain nitrogen from the host. Glutamine synthetase 1 (GS1) is a cytosolic enzyme that assimilates ammonia into glutamine. We determined the transcript levels of zooxanthellal GS1 (Zoox-GS1), which represented comprehensively GS1 transcripts of Symbiodinium, Cladocopium and Durusdinium, in five organs of T. squamosa. The outer mantle had significantly higher transcript level of Zoox-GS1 than the inner mantle, foot muscle, hepatopancreas and ctenidium, but the transcript ratios of Zoox-GS1 to zooxanthellal form II ribulose-1,5-bisphosphate carboxylase/oxygenase (Zoox-rbcII), which represented the potential of ammonia assimilation relative to the phototrophic potential, were comparable among these five organs. Based on transcript ratios of Zoox-GS1 to zooxanthellal Urease (Zoox-URE), the outer mantle had the highest potential of urea degradation relative to ammonia assimilation among the five organs, probably because urea degradation could furnish CO₂ and NH₃ for photosynthesis and amino acid synthesis, respectively, in the symbionts therein. The protein abundance of Zoox-GS1 was upregulated in the outer mantle and the inner mantle during illumination. Zoox-GS1 could catalyze light-enhanced glutamine formation using ammonia absorbed from the host or ammonia released through urea degradation in the cytoplasm. The glutamine produced could be used to synthesize other nitrogenous compounds, including amino acids in the cytoplasm or in the plastid of the dinoflagellates. Some of the amino acids synthesized by the symbionts in the inner mantle and foot muscle could be donated to the host to support shell organic matrix formation and muscle production, respectively.

Glutamine synthetases of green and etiolated leaves ofSinapis alba : Evidence of the identity of the respective enzyme proteins

Studies on the glutamine synthetases (GS, EC 6.3.1.2) of green (GS2) and etiolated leaves (GSet) ofSinapis alba L. (cv. Steinacher) revealed striking similarities between the respective enzyme proteins. The enzymes showed corresponding chromatographic properties, both on dimethylaminoethyl-Sephacel and on hydroxylapatite columns. The purified GS proteins were also identical with regard to the molecular weight of their subunits. Isoelectrofocusing of pure GSet yielded two distinct polypeptide bands in the pH 5.6 region of the gels. This pattern corresponded to the two strong bands of GS2. Two charge variants of GS polypeptides could be detected by Western-blot analysis of the soluble protein of green leaves using antibodies against mustard GS2. In immunoprecipitation experiments, the holoenzymes of GS2 and GSet were recognized with identical affinities by this antiserum. We conclude that strong similarities exist between the proteins of the GS enzymes in green and etiolated leaves of mustard. Most probably only one GS form, namely the plastidic enzyme, can be found in the epigeal organs ofSinapis. The polypeptides of the GS2 subunits showed no differences in the hydrophobicity of the polypeptide chains. Neither glucosyl nor mannosyl residues could be detected.

Chromatographic and immunological evidence that chloroplastic and cytosolic pea (Pisum sativum L.) NADP-isocitrate dehydrogenases are distinct isoenzymes

Two NADP-isocitrate dehydrogenase isoenzymes designated as NADP-IDH1 and NADP-IDH2 (EC 1.1.1.42) were identified in pea (Pisum sativum) leaf extracts by diethylaminoethylcellulose chromatography. The predominant form was found to be NADP-IDH1 while NADP-IDH2 represented only about 4% of the total leaf enzyme activity. These enzymes share few common epitopes as NADP-IDH2 was poorly recognized by the specific polyclonal antibodies raised against NADP-IDH1, and as a consequence NADP-IDH2 does not result from a post-translational modification of NADP-IDH1. Subcellular fractionation and isolation of chloroplasts through a Percoll gradient, followed by the identification of the associated enzymes, showed that NADP-IDH1 is restricted to the cytosol and NADP-IDH2 to the chloroplasts. Compared with the cytosolic isoenzyme, NADP-IDH2 was more thermolabile and exhibited a lower optimum pH. The data reported in this paper constitute the first report that the chloroplastic NADP-IDH and the cytosolic NADP-IDH are two distinct isoenzymes. The possible functions of the two isoenzymes are discussed.

Glutamine synthetase isoforms in the green hydra symbiosis

The occurrence of glutamine synthetase isoforms in the green hydra symbiosis has been investigated using ion-exchange chromatography. There were three isoforms which were designated algal cytosolic, chloroplastic and host isoenzymes. Comparison of the proportions of algal isoforms from the intact association with those from nitrogen-replete and nitrogen-starved cultures of a Chlorella strain originally isolated from green hydra, suggests that symbionts in the intact association may be nitrogen-deficient.

Regulation of the appearance of glutamine synthetase in mustard (Sinapis alba L.) cotyledons by light, nitrate and ammonium

During transformation of mustard seedlings cotyledons from storage organs to photosynthetically competent leaves, a process which occurs during the first 4 d after sowing, total glutamine-synthetase (GS, EC 6.3.1.2) activity increases from zero to the high level usually observed in green leaves. In the present study we have used ion-exchange chromatography to separate possible isoforms of GS during the development of the cotyledons. The approach failed since we could only detect a single form of GS, presumably plastidic GS, under all circumstances tested. The technique of selective photooxidative destruction of plastids in situ was applied to solve the problem of GS localization. It was inferred from the data that the GS as detected by ion-exchange chromatography is plastidic GS.The regulatory role, if any, of light, nitrate and ammonium in the process of the appearance of GS in the developing cotyledons was investigated. The results show that nitrate and ammonium play only minor roles. Light, operating via phytochrome, is the major regulatory factor.

Changes in the activities of chloroplast and cytosolic isoenzymes of glutamine synthetase during normal leaf growth and plastid development in wheat

Soluble protein extracts and chloroplasts from a serial sequence of transverse sections of a 7-d-old wheat leaf (Triticum aestivum cv. Maris Huntsman) were used to study changes in the activity of glutamine synthetase (GS; EC 6.3.1.2) during cell and chloroplast development. Glutamine synthetase activity increased more than 50-fold per cell from the base to the tip of the wheat leaf. Two isoenzymes of GS were separated using fast protein liquid chromatography (FPLC). Glutamine synthetase localized in the cytoplasm (GS1) eluted at about 0.21 M NaCl, and the isoenzyme localized in the chloroplast (GS2) eluted at about 0.33 M NaCl. The increase in GS activity during leaf development was found to be caused primarily by an increase in the activity of the chloroplast GS2. The activity of the cytoplasmic GS1 remained constant as the cells were displaced from the base to the tip of the leaf, whereas GS2 activity increased within the chloroplast throughout development. At the base of the leaf, 26% of total GS activity was cytoplasmic; the remaining 74% was in the chloroplast. At 10 cm from the base, only 4% of the activity was cytoplasmic, and 96% was in the chloroplast. The results indicate that the chloroplast GS2 is probably responsible for most of the ammonia assimilation in the mature wheat leaf, whereas cytoplasmic GS1 may serve a role in immature developing leaf cells.

Cytosolic glutamine synthetase in higher plants. A comparative immunological study

Cytosolic glutamine synthetase (GS1) was purified to homogeneity from etiolated barley leaves by DEAE-Sephacel and hydroxyapatite chromatography, gel filtration and polyacrylamide gel electrophoresis. Specific antibodies against the purified protein were raised by the immunization of rabbits. Immunoprecipitation experiments demonstrated that cytosolic glutamine synthetases isolated from the leaves of different plant species were very similar proteins. Good recognition of other cytosolic glutamine synthetases from roots, root nodular tissue and seeds by barley GS1 antibodies was obtained, suggesting that they too are all quite similar proteins. In contrast, chloroplast glutamine synthetase (GS2) was considered to be a different protein in view of its low level of recognition by barley GS1 antibodies.

Four isoforms of glutamine synthetase in light-grown soybeans

Glutamine synthetase (GS) activity recovered from linear sucrose gradients was associated with the cytosol of cells isolated from etiolated soybean hypocotyls whereas light-grown tissue contained increased GS activity localized in both the cytosol and chloroplasts. DEAE-cellulose chromatography indicated two GS isoforms in etiolated hypocotyls whereas light-grown hypocotyls and primary leaves contained four isoforms. Only one GS isoform was recovered from both etiolated and light-grown cotyledons.

Evidence for a cytosolic-dependent light induction of chloroplastic glutamine synthetase during greening of etiolated rice leaves

During the greening of etiolated rice leaves, total glutamine synthetase activity increases about twofold, and after 48 h the level of activity usually observed in green leaves is obtained. A density-labeling experiment with deuterium demonstrates that the increase in enzyme activity is due to a synthesis of the enzyme. The enhanced activity obtained upon greening is the result of two different phenomena: there is a fivefold increase of chloroplastic glutamine synthetase content accompanied by a concommitant decrease (twofold) of the cytosolic glutamine synthetase. The increase of chloroplastic glutamine synthetase (GS2) is only inhibited by cycloheximide and not by lincomycin. This result indicates a cytosolic synthesis of GS2. The synthesis of GS2 was confirmed by a quantification of the protein by an immunochemical method. It was demonstrated that GS2 protein content in green leaves is fivefold higher than in etiolated leaves.