Enhancement of Symbiotic Dinitrogen Fixation by a Toxin-Releasing Plant Pathogen

Knight, Thomas J.; Langston-Unkefer, Pat J.

doi:10.1126/science.241.4868.951

Cited by 67 publications

(21 citation statements)

References 24 publications

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“…Therefore, we have shown that the increase in α-amino-nitrogen in bacteroids correlates with a decrease in nitrogenase activity (Mendoza et al, 1995). Knight & Langston-Unkefer (1988) have found that the glutamate and glutamine content of nodules affects nitrogenase activity. The nitrogen assimilated into amino acids in the bacteroids may contribute to this effect.…”

Section: Discussionmentioning

confidence: 99%

Role of GOGAT in carbon and nitrogen partitioning in Rhizobium etli The GenBank accession number for the sequence reported in this paper is AF107264.

et al. 2000

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The isolation and characterization of a Rhizobium etli glutamate auxotroph, TAD12, harbouring a single Tn5 insertion, is reported. This mutant produced no detectable glutamate synthase (GOGAT) activity. The cloning and physical characterization of a 72 kb fragment of R. etli DNA harbouring the structural genes gltB and gltD encoding the two GOGAT subunits GltB and GltD is also reported. In comparison with the wild-type strain (CFN42), the GOGAT mutant strain utilized less succinate and glutamate and grew less with this and other amino acids as nitrogen source. R. etli assimilates ammonium by the glutamine synthetase (GS)-GOGAT pathway and a GOGAT mutant prevents the cycling of glutamine by this pathway, something that impairs nitrogen and carbon metabolism and explains the decrease in the amino-nitrogen during exponential growth, with glutamate as nitrogen source. GOGAT activity also has a role in ammonium turnover and in the synthesis of amino acids and proteins, processes that are necessary to sustain cell viability in non-growing conditions. The assimilation of ammonium is important during symbiosis and glutamate constitutes 20-40 % of the total amino-nitrogen. In symbiosis, the blockage of ammonium assimilation by a GOGAT mutation significantly decreases the amino-nitrogen pool of the bacteroids and may explain why more N 2 is fixed in ammonium, excreted to the plant cell, transported to the leaves and stored in the seeds.

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Section: Discussionmentioning

confidence: 99%

Role of GOGAT in carbon and nitrogen partitioning in Rhizobium etli The GenBank accession number for the sequence reported in this paper is AF107264.

et al. 2000

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“…In legumes, an accumulation of Asn was observed when GS activity was impaired, suggesting that other enzymes (e.g., Asn synthetase) may be important in bypassing the flux of reduced nitrogen to avoid a toxic accumulation of NH 4 þ , thus ensuring plant survival (Carvalho et al, 2003;Harrison et al, 2003;Wong et al, 2004). The question as to whether these alternative metabolic pathways are important in the control of plant productivity has been extensively studied in legumes and nonlegumes (Knight and Langston-Unkefer, 1988;Brears et al, 1993;Lam et al, 2003) but has still not been fully elucidated, particularly in cereals.…”

Section: Physiological Impact Of Decreased Gs Activitymentioning

confidence: 99%

Two Cytosolic Glutamine Synthetase Isoforms of Maize Are Specifically Involved in the Control of Grain Production

et al. 2006

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The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.

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“…mate in GS biosynth were found between GS1a and GS1d. ATP is known to be an allosteric effector of plant GS (33). Therefore, the K m value for ATP could not be determined.…”

Section: Fig 4 Sds-page Of Gs Purified From Maize Rootsmentioning

confidence: 99%

Molecular Identification and Characterization of Cytosolic Isoforms of Glutamine Synthetase in Maize Roots

Sakakibara

Shimizu

Hase

et al. 1996

Journal of Biological Chemistry

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In maize, a small multigene family encodes the cytosolic isoforms of glutamine synthetase (GS), and five cDNAs, designated pGS1a, pGS1b, pGS1c, pGS1d, and pGS1e, have been cloned (Sakakibara, H., Kawabata, S., Takahashi . This report describes the identification and enzymatic characterization of the cytosolic isoforms of GS in maize roots, namely GS1 and GSr. The purified isoforms, as well as recombinant enzymes that had been overexpressed in Escherichia coli, were analyzed by capillary liquid chromatography/electrospray ionization-mass spectrometry, and GS1 and GSr were identified as the products of the GS1a/GS1b and GS1c/ GS1d genes, respectively. Upon the addition of ammonia to the culture medium, significant amounts of GSr accumulated and a preferential increase in GS synthetase activity, as compared to GS transferase activity, was found in the root extract. Assays with the purified recombinant enzymes confirmed that the specific biosynthetic and synthetase activities of GSr were 1.6-fold higher than those of GS1. Marked differences in stability were also found between the two isoforms: GSr was more sensitive to heat than GS1 and octameric aggregates of the subunits of GSr were easily dissociated to monomers than those of GS1 at low concentrations of Mn 2؉ and Mg 2؉ ions. These characteristics of the ammonia-induced isoform of GS seem to be physiologically important for the primary assimilation of external ammonia by roots.

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Enhancement of Symbiotic Dinitrogen Fixation by a Toxin-Releasing Plant Pathogen

Cited by 67 publications

References 24 publications

Role of GOGAT in carbon and nitrogen partitioning in Rhizobium etli The GenBank accession number for the sequence reported in this paper is AF107264.

Role of GOGAT in carbon and nitrogen partitioning in Rhizobium etli The GenBank accession number for the sequence reported in this paper is AF107264.

Two Cytosolic Glutamine Synthetase Isoforms of Maize Are Specifically Involved in the Control of Grain Production

Molecular Identification and Characterization of Cytosolic Isoforms of Glutamine Synthetase in Maize Roots

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