Down-regulation of specific members of the glutamine synthetase gene family in alfalfa by antisense RNA technology

Comparison of alfalfa plants overexpressing glutamine synthetase with those overexpressing sucrose phosphate synthase demonstrates a signaling mechanism integrating carbon and nitrogen metabolism between the leaves and nodules

Alfalfa, like other legumes, establishes a symbiotic relationship with the soil bacteria, Sinorhizobium meliloti, which results in the formation of the root nodules. Nodules contain the bacteria enclosed in a membrane-bound vesicle, the symbiosome where it fixes atmospheric N2 and converts it into ammonia using the bacterial enzyme, nitrogenase. The ammonia released into the cytoplasm from the symbiosome is assimilated into glutamine (Gln) using carbon skeletons produced by the metabolism of sucrose (Suc), which is imported into the nodules from the leaves. The key enzyme involved in the synthesis of Suc in the leaves is sucrose phosphate synthase (SPS) and glutamine synthetase (GS) is the enzyme with a role in ammonia assimilation in the root nodules. Alfalfa plants, overexpressing SPS or GS, or both showed increased growth and an increase in nodule function. The endogenous genes for the key enzymes in C/N metabolism showed increased expression in the nodules of both sets of transformants. Furthermore, the endogenous SPS and GS genes were also induced in the leaves and nodules of the transformants, irrespective of the transgene, suggesting that the two classes of plants share a common signaling pathway regulating C/N metabolism in the nodules. This study reaffirms the utility of the nodulated legume plant to study C/N interaction and the cross talk between the source and sink for C and N.

Rootstock-scion interaction affecting citrus response to CTV infection: a proteomic view

Citrus tristeza virus (CTV) is the causal agent of various diseases with dramatic effects on citrus crops worldwide. Most Citrus species, grown on their own roots, are symptomless hosts for many CTV isolates. However, depending on different scion-rootstock combination, CTV infection should result in distinct syndromes, being 'tristeza' the more severe one, leading to a complete decline of the susceptible plants in a few weeks. Transcriptomic analyses revealed several genes involved either in defense response, or systemic acquired resistance, as well as transcription factors and components of the phosphorylation cascades, to be differentially regulated during CTV infection in Citrus aurantifolia species. To date little is known about the molecular mechanism of this host-pathogen interaction, and about the rootstock effect on citrus response to CTV infection. In this work, the response to CTV infection has been investigated in tolerant and susceptible scion-rootstock combinations by two-dimensional gel electrophoresis (2DE). A total of 125 protein spots have been found to be differently accumulated and/or phosphorylated between the two rootstock combinations. Downregulation in tolerant plants upon CTV infection was detected for proteins involved in reactive oxygen species (ROS) scavenging and defense response, suggesting a probable acclimation response able to minimize the systemic effects of virus infection. Some of these proteins resulted to be modulated also in absence of virus infection, revealing a rootstock effect on scion proteome modulation. Moreover, the phospho-modulation of proteins involved in ROS scavenging and defense response, further supports their involvement either in scion-rootstock crosstalk or in the establishment of tolerance/susceptibility to CTV infection.

Complementary DNA cloning of the pear 1-aminocyclopropane-1-carboxylic acid oxidase gene and agrobacterium-mediated anti-sense genetic transformation

The aim of the present study was to genetically modify plantlets of the Chinese yali pear to reduce their expression of ripening-associated 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and therefore increase the shelf-life of the fruit. Primers were designed with selectivity for the conserved regions of published ACO gene sequences, and yali complementary DNA (cDNA) cloning was performed by reverse transcription quantitative polymerase chain reaction (PCR). The obtained cDNA fragment contained 831 base pairs, encoding 276 amino acid residues, and shared no less than 94% nucleotide sequence identity with other published ACO genes. The cDNA fragment was inversely inserted into a pBI121 expression vector, between the cauliflower mosaic virus 35S promoter and the nopaline synthase terminator, in order to construct the anti‑sense expression vector of the ACO gene; it was transfected into cultured yali plants using Agrobacterium LBA4404. Four independent transgenic lines of pear plantlets were obtained and validated by PCR analysis. A Southern blot assay revealed that there were three transgenic lines containing a single copy of exogenous gene and one line with double copies. The present study provided germplasm resources for the cultivation of novel storage varieties of pears, therefore providing a reference for further applications of anti‑sense RNA technology in the genetic improvement of pears and other fruit.

Glutamine synthetase is a molecular target of nitric oxide in root nodules of Medicago truncatula and is regulated by tyrosine nitration

Nitric oxide (NO) is emerging as an important regulatory player in the Rhizobium-legume symbiosis, but its biological role in nodule functioning is still far from being understood. To unravel the signal transduction cascade and ultimately NO function, it is necessary to identify its molecular targets. This study provides evidence that glutamine synthetase (GS), a key enzyme for root nodule metabolism, is a molecular target of NO in root nodules of Medicago truncatula, being regulated by tyrosine (Tyr) nitration in relation to active nitrogen fixation. In vitro studies, using purified recombinant enzymes produced in Escherichia coli, demonstrated that the M. truncatula nodule GS isoenzyme (MtGS1a) is subjected to NO-mediated inactivation through Tyr nitration and identified Tyr-167 as the regulatory nitration site crucial for enzyme inactivation. Using a sandwich enzyme-linked immunosorbent assay, it is shown that GS is nitrated in planta and that its nitration status changes in relation to active nitrogen fixation. In ineffective nodules and in nodules fed with nitrate, two conditions in which nitrogen fixation is impaired and GS activity is reduced, a significant increase in nodule GS nitration levels was observed. Furthermore, treatment of root nodules with the NO donor sodium nitroprusside resulted in increased in vivo GS nitration accompanied by a reduction in GS activity. Our results support a role of NO in the regulation of nitrogen metabolism in root nodules and places GS as an important player in the process. We propose that the NO-mediated GS posttranslational inactivation is related to metabolite channeling to boost the nodule antioxidant defenses in response to NO.

Metabolome and photochemical analysis of rice plants overexpressing Arabidopsis NAD kinase gene

The chloroplastic NAD kinase (NADK2) is reported to stimulate carbon and nitrogen assimilation in Arabidopsis (Arabidopsis thaliana), which is vulnerable to high light. Since rice (Oryza sativa) is a monocotyledonous plant that can adapt to high light, we studied the effects of NADK2 expression in rice by developing transgenic rice plants that constitutively expressed the Arabidopsis chloroplastic NADK gene (NK2 lines). NK2 lines showed enhanced activity of NADK and accumulation of the NADP(H) pool, while intermediates of NAD derivatives were unchanged. Comprehensive analysis of the primary metabolites in leaves using capillary electrophoresis mass spectrometry revealed elevated levels of amino acids and several sugar phosphates including ribose-1,5-bisphosphate, but no significant change in the levels of the other metabolites. Studies of chlorophyll fluorescence and gas change analyses demonstrated greater electron transport and CO2 assimilation rates in NK2 lines, compared to those in the control. Analysis of oxidative stress response indicated enhanced tolerance to oxidative stress in these transformants. The results suggest that NADP content plays a critical role in determining the photosynthetic electron transport rate in rice and that its enhancement leads to stimulation of photosynthesis metabolism and tolerance of oxidative damages.

Effects of the overexpression of a soybean cytosolic glutamine synthetase gene (GS15) linked to organ-specific promoters on growth and nitrogen accumulation of pea plants supplied with ammonium

A soybean cytosolic glutamine synthetase gene (GS15) fused to a constitutive promoter (CaMV 35S), a putative nodule-specific promoter (LBC(3)), or a putative root-specific promoter (rolD) was transformed into Pisum sativum L. cv. Greenfeast. Four lines with single copies (Lines 1, 7, 8 and 9) and four lines with two copies each of GS15 (Lines 2, 4, 6 and 11) were compared to the wild-type (WT) parental line for levels of cytosolic glutamine synthetase (GS1), glutamine synthetase (GS) activity, N accumulation, N derived form the atmosphere (NDFA), and biomass of plants grown on 0.0, 0.1, 1.0 or 10.0 mM NH(4)(+). Enhanced levels of GS1 were detected in leaves of one of the two lines transformed with the 35S-GS15 construct, and all three lines containing the rolD-GS15 construct. All three lines containing the LBC(3)-GS15 construct had increased levels of GS1 in nodules. Despite the increased levels of GS1 in many transformants, only the roots of lines containing the rolD-GS15 construct consistently demonstrated enhanced levels of GS activity (up to 12-fold). Positive responses in plant N content, NDFA, and biomass were rare, but increases in plant biomass and N content of up to 17% and 54%, respectively, occurred in some of the rolD-GS15 lines at certain levels of ammonium. In general, GS15 copy number did not seem to differentially affect phenotype of the transformants, and transformants respond to ammonium concentrations in similar patterns to that previously observed with nitrate. Despite the fact that the rolD-GS15 transformants consistently resulted in increased GS activity in roots and resulted in some occurrences of increases in biomass and plant N content, the lack of consistent positive growth effect across all transformants indicates that the generalized overexpression of GS1 in tissues holds little potential for positive growth responses in pea.

The 3' untranslated region of a soybean cytosolic glutamine synthetase (GS1) affects transcript stability and protein accumulation in transgenic alfalfa

Higher plants assimilate nitrogen in the form of ammonia through the concerted activity of glutamine synthetase (GS) and glutamate synthase (GOGAT). The GS enzyme is either located in the cytoplasm (GS1) or in the chloroplast (GS2). Glutamine synthetase 1 is regulated in different plants at the transcriptional level and there are some reports of regulation at the level of protein stability. Here we present data that clearly establish that GS1 in plants is also regulated at the level of transcript turnover and at the translational level. Using a Glycine max (soybean) GS1 transgene, with and without its 3' untranslated region (UTR), driven by the constitutive CaMV 35S promoter in Medicago sativa (alfalfa) and Nicotiana tabacum (tobacco), we show that the 3' UTR plays a major role in both transcript turnover and translation repression in both the leaves and the nodules. Our data suggest that the 3' UTR mediated turnover of the transcript is regulated by a nitrogen metabolite or carbon/nitrogen ratios. We also show that the 3' UTR of the gene for the soybean GS1 confers post-transcriptional regulation on a reporter gene. Our dissection of post-transcriptional and translational levels of regulation of GS in plants shows that the situation in plants strongly resembles that in other organisms where GS is regulated at almost all levels. Multistep regulation of GS shows the high priority given by organisms to regulating and ensuring optimal control of nitrogen substrates and preventing overproduction of glutamine and drainage of the glutamate pool.

Phosphorylation and subsequent interaction with 14-3-3 proteins regulate plastid glutamine synthetase in Medicago truncatula

In this report we demonstrate that plastid glutamine synthetase of Medicago truncatula (MtGS2) is regulated by phosphorylation and 14-3-3 interaction. To investigate regulatory aspects of GS2 phosphorylation, we have produced non-phosphorylated GS2 proteins by expressing the plant cDNA in E. coli and performed in vitro phosphorylation assays. The recombinant isoenzyme was phosphorylated by calcium dependent kinase(s) present in leaves, roots and nodules. Using an (His)6-tagged 14-3-3 protein column affinity purification method, we demonstrate that phosphorylated GS2 interacts with 14-3-3 proteins and that this interaction leads to selective proteolysis of the plastid located isoform, resulting in inactivation of the isoenzyme. By site directed mutagenesis we were able to identify a GS2 phosphorylation site (Ser97) crucial for the interaction with 14-3-3s. Phosphorylation of this target residue can be functionally mimicked by replacing Ser97 by Asp, indicating that the introduction of a negative charge contributes to the interaction with 14-3-3 proteins and subsequent specific proteolysis. Furthermore, we document that plant extracts contain protease activity that cleaves the GS2 protein only when it is bound to 14-3-3 proteins following either phosphorylation or mimicking of phosphorylation by Ser97Asp.

Identification and characterization of a QTL on chromosome 2 for cytosolic glutamine synthetase content and panicle number in rice

A quantitative trait locus (QTL) associated with the protein content of cytosolic glutamine synthetase (GS1; EC 6.3.1.2) in senescing leaves, panicle number, and panicle weight was characterized in rice (Oryza sativa L.). A near-isogenic line (NIL), C-22, developed by marker-assisted selection was grown under different nitrogen levels in the greenhouse and in a paddy field. Chromosome 2 of C-22 had an approximately 50-cM segment substituted from the Kasalath (indica) chromosome in a Koshihikari (japonica) genetic background. C-22 showed a 12-37% lower content of GS1 protein in leaf blades than Koshihikari, which was in good agreement with a QTL region positively affected by the japonica chromosome. At an early vegetative stage, C-22 had more active tillers than Koshihikari in the greenhouse. At the reproductive stage, both panicle number and total panicle weight of C-22 were significantly higher than those of Koshihikari, particularly when the plants were grown under a low-nitrogen condition. These traits of C-22 were further confirmed in a paddy field. Thus, tiller development was positively affected by the Kasalath chromosome at an early vegetative stage, which resulted in an increased panicle number and panicle weight at the mature stage in C-22. These data indicate that the target QTL (Pnn1; panicle number 1) is important in the development of tillers and panicles in rice. Linkage analyses for panicle number and ratio of developing tiller formation in the second axil (RDT) revealed that Pnn1 was delimited at the 6.7-cM region.

Nodule-specific modulation of glutamine synthetase in transgenic Medicago truncatula leads to inverse alterations in asparagine synthetase expression

Transgenic Medicago truncatula plants were produced harboring chimeric gene constructs of the glutamine synthetase (GS) cDNA clones (MtGS1a or MtGS1b) fused in sense or antisense orientation to the nodule-specific leghemoglobin promoter Mtlb1. A series of transgenic plants were obtained showing a 2- to 4-fold alteration in nodule GS activity when compared with control plants. Western and northern analyses revealed that the increased or decreased levels of GS activity correlate with the amount of cytosolic GS polypeptides and transcripts present in the nodule extracts. An analysis of the isoenzyme composition showed that the increased or decreased levels of GS activity were attributable to major changes in the homo-octameric isoenzyme GS1a. Nodules of plants transformed with antisense GS constructs showed an increase in the levels of both asparagine synthetase (AS) polypeptides and transcripts when compared with untransformed control plants, whereas the sense GS transformants showed decreased AS transcript levels but polypeptide levels similar to control plants. The polypeptide abundance of other nitrogen metabolic enzymes NADH-glutamic acid synthase and aspartic acid amino-transferase as well as those of major carbon metabolic enzymes phosphoenolpyruvate carboxylase, carbonic anhydrase, and sucrose synthase were not affected by the GS-gene manipulations. Increased levels of AS polypeptides and transcripts were also transiently observed in nodules by inhibiting GS activity with phosphinothricin. Taken together, the results presented here suggest that GS activity negatively regulates the level of AS in root nodules of M. truncatula. The potential role of AS in assimilating ammonium when GS becomes limiting is discussed.

Does lowering glutamine synthetase activity in nodules modify nitrogen metabolism and growth of Lotus japonicus?

A cDNA encoding cytosolic glutamine synthetase (GS) from Lotus japonicus was fused in the antisense orientation relative to the nodule-specific LBC3 promoter of soybean (Glycine max) and introduced into L. japonicus via transformation with Agrobacterium tumefaciens. Among the 12 independent transformed lines into which the construct was introduced, some of them showed diminished levels of GS1 mRNA and lower levels of GS activity. Three of these lines were selected and their T(1) progeny was further analyzed both for plant biomass production and carbon and nitrogen (N) metabolites content under symbiotic N-fixing conditions. Analysis of these plants revealed an increase in fresh weight in nodules, roots and shoots. The reduction in GS activity was found to correlate with an increase in amino acid content of the nodules, which was primarily due to an increase in asparagine content. Thus, this study supports the hypothesis that when GS becomes limiting, other enzymes (e.g. asparagine synthetase) that have the capacity to assimilate ammonium may be important in controlling the flux of reduced N in temperate legumes such as L. japonicus. Whether these alternative metabolic pathways are important in the control of plant biomass production still remains to be fully elucidated.

Antisense inhibition of NADH glutamate synthase impairs carbon/nitrogen assimilation in nodules of alfalfa (Medicago sativa L.)

Legumes acquire significant amounts of nitrogen for growth from symbiotic nitrogen fixation. The glutamine synthetase (GS)/NADH-dependent glutamate synthase (NADH-GOGAT) cycle catalyzes initial nitrogen assimilation. This report describes the impact of specifically reducing nodule NADH-GOGAT activity on symbiotic performance of alfalfa (Medicago sativa L.). Four independent transgenic alfalfa lines, designated GA89, GA87, GA88, and GA82 (for GOGATantisense), containing an antisense NADH-GOGAT cDNA fragment under the control of the soybean leghemoglobin (lbc3) promoter were evaluated. The GA plants were fertile and showed normal growth in non-symbiotic conditions. The NADH-GOGAT antisense transgene was heritable and the T1 plants showed phenotypic alterations - similar to primary transformants. Clonally propagated plants were inoculated with Sinorhizobium meliloti after rooting and the symbiotic phenotype was analyzed 21 days post-inoculation. Nodules of each GA line had reduced NADH-GOGAT activity, ranging from 33 to 87% of control plants, that was accompanied by comparable decreases in RNA and protein. Plants from the GA89 line, with the lowest NADH-GOGAT activity (c. 30%), presented a strikingly altered symbiotic phenotype: concomitantly activities of key enzyme for carbon and nitrogen assimilation decreased; nodule amino acids and amides were reduced while sucrose accumulated. Antisense GOGAT plants were chlorotic, reduced in fresh weight, and had a lower N content than control plants. Photosynthesis was also impaired in antisense plants. Specifically, reducing NADH-GOGAT in nodules resulted in plants having impaired nitrogen assimilation and altered carbon/nitrogen metabolic flux.

Overexpression of alfalfa cytosolic glutamine synthetase in nodules and flowers of transgenic Lotus japonicus plants

Legumes can obtain nitrogen from symbiotic nitrogen fixation in root nodules. The glutamine synthetase/glutamate synthase cycle is responsible for the initial nitrogen assimilation. This work reports the analysis of transgenic Lotus japonicus plants with the chimeric gene containing the alfalfa cytosolic glutamine synthetase (GS1) (EC 6.3.1.2) gene controlled by the Sesbania rostrata leghemoglobin gene promoter (Srglb3p). Surprisingly, all of the transgenic primary transformants analysed were sterile. Two transformants designated GS39 and GS44 were further analysed. GS in nodules of GS39 and GS44 plants was upregulated, at the level of transcript and protein. The transgenic plants had 2-fold higher nodule GS activity and similar root GS activity compared to control plants. The GS39 and GS44 sterile plants showed morphological alterations in pollen grains and in ovules. An increase in GS transcript abundance and enzyme activity was measured during early and late stages of flower development of GS plants. Flowers of GS plants showed higher glutamine content, resulting in an increased glutamine/glutamate ratio. The GS transcript and protein were detected in ovules. These data indicate that overexpression of GS1 in reproductive organs critically affects their development and might be a reason for sterility of L. japonicus plants.

Genetic manipulation and quantitative-trait loci mapping for nitrogen recycling in rice

Immunocytological studies in this laboratory have suggested that NADH-dependent glutamate synthase (NADH-GOGAT; EC 1.4.1.14) in developing organs of rice (Oryza sativa L. cv. Sasanishiki) is involved in the utilization of glutamine remobilized from senescing organs through the phloem. Because most of the indica cultivars contained less NADH-GOGAT in their sink organs than japonica cultivars, over-expression of NADH-GOGAT gene from japonica rice was investigated using Kasalath, an indica cultivar. Several T0 transgenic Kasalath lines over-producing NADH-GOGAT under the control of a NADH-GOGAT promoter of Sasanishiki, a japonica rice, showed an increase in grain weight (80% as a maximum), indicating that NADH-GOGAT is indeed a key step for nitrogen utilization and grain filling in rice. A genetic approach using 98 backcross-inbred lines (BC(1)F(6)) developed between Nipponbare (a japonica rice) and Kasalath were employed to detect putative quantitative trait loci (QTLs) associated with the contents of cytosolic glutamine synthetase (GS1; EC 6.3.1.2), which is probably involved in the export of nitrogen from senescing organs and those of NADH-GOGAT. Immunoblotting analyses showed transgressive segregations toward lower or greater contents of these enzyme proteins in these BC(1)F(6). Seven chromosomal QTL regions were detected for GS1 protein content and six for NADH-GOGAT. Some of these QTLs were located in QTL regions for various biochemical and agronomic traits affected by nitrogen recycling. The relationships between the genetic variability of complex agronomic traits and traits for these two enzymes are discussed.

Cytosolic glutamine synthetase in soybean is encoded by a multigene family, and the members are regulated in an organ-specific and developmental manner

Gln synthetase (GS) is the key enzyme in N metabolism and it catalyzes the synthesis of Gln from glutamic acid, ATP, and NH4+. There are two major isoforms of GS in plants, a cytosolic form (GS1) and a chloroplastic form (GS2). In leaves, GS2 functions to assimilate ammonia produced by nitrate reduction and photorespiration, and GS1 is the major isoform assimilating NH3 produced by all other metabolic processes, including symbiotic N2 fixation in the nodules. GS1 is encoded by a small multigene family in soybean (Glycine max), and cDNA clones for the different members have been isolated. Based on sequence divergence in the 3'-untranslated region, three distinct classes of GS1 genes have been identified (alpha, beta, and gamma). Genomic Southern analysis and analysis of hybrid-select translation products suggest that each class has two distinct members. The alpha forms are the major isoforms in the cotyledons and young roots. The beta forms, although constitutive in their expression pattern, are ammonia inducible and show high expression in N2-fixing nodules. The gamma1 gene appears to be more nodule specific, whereas the gamma2 gene member, although nodule enhanced, is also expressed in the cotyledons and flowers. The two members of the alpha and beta class of GS1 genes show subtle differences in the expression pattern. Analysis of the promoter regions of the gamma1 and gamma2 genes show sequence conservation around the TATA box but complete divergence in the rest of the promoter region. We postulate that each member of the three GS1 gene classes may be derived from the two ancestral genomes from which the allotetraploid soybean was derived.

Overlapping expression of cytosolic glutamine synthetase and phenylalanine ammonia-lyase in immature leaf blades of rice

In order to estimate whether cytosolic glutamine synthetase (GS1; EC 6.3.1.2) is partly coupled to the reaction of phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) in developing organs of rice (Oryza sativa L.), we compared the expression pattern of transcripts and proteins for GS1 and PAL in the tissue sections from leaf blades at various stages of development. In immature vascular bundles of unexpanded leaf blades, GS1 mRNA was mainly detected in xylem parenchyma cells, mestome-sheath cells, and sclerenchyma cells. PAL transcripts were also accumulated in these cell types. Vascular bundles in midribs of immature leaf blades contained mRNAs and proteins for both GS1 and PAL abundantly in sclerenchyma cells, although distribution of these two proteins was not completely overlapped. In immature vascular bundles in midribs, lignin deposition was observed in cell walls of xylem parenchyma cells, mestome-sheath cells and sclerenchyma cells. These results implied that a part of GS1 in unexpanded leaf blades is possibly involved in reassimilation of ammonia released from PAL reaction during the lignin production.

Constitutive overexpression of cytosolic glutamine synthetase (GS1) gene in transgenic alfalfa demonstrates that GS1 may be regulated at the level of RNA stability and protein turnover

Glutamine synthetase (GS) catalyzes the ATP-dependent condensation of NH4+ with glutanate to yield glutamine. Gene constructs consisting of the cauliflower mosaic virus (CaMV) 35S promoter driving a cytosolic isoform of GS (GS1) gene have been introduced into alfalfa (Medicago sativa). Although transcripts for the transgene were shown to accumulate to high levels in the leaves, they were undetectable in the nodules. However, significant amounts of beta-glucuronidase activity could be detected in nodules of plants containing the CaMV 35S promoter-beta-glucuronidase gene construct, suggesting that the transcript for the GS1 transgene is not stable in the root nodules. Leaves of alfalfa plants with the CaMV 35S promoter-GS1 gene showed high levels of accumulation of the transcript for the transgene when grown under low-nitrogen conditions and showed a significant drop in the level of GS1 transcripts when fed with high levels of NO3-. However, no increase in GS activity or polypeptide level was detected in the leaves of transgenic plants. The results suggest that GS1 is regulated at the level of RNA stability and protein turnover.

Decreased NADH glutamate synthase activity in nodules and flowers of alfalfa (Medicago sativa L.) transformed with an antisense glutamate synthase transgene

Legumes obtain a substantial portion of their nitrogen (N) from symbiotic N2 fixation in root nodules. The glutamine synthetase (GS, EC 6.3.1.2)/glutamate synthase (GOGAT) cycle is responsible for the initial N assimilation. This report describes the analysis of a transgenic alfalfa (Medicago sativa L.) line containing an antisense NADH-GOGAT (EC 1.4.1.14) under the control of the nodule-enhanced aspartate amino-transferase (AAT-2) promoter. In one transgenic line, NADH-GOGAT enzyme activity was reduced to approximately 50%, with a corresponding reduction in protein and mRNA. The transcript abundance for cytosolic GS, ferredoxin-dependent GOGAT (EC 1.4.7.1), AAT-2 (EC 2.6.1.1), asparagine synthase (EC 6.3.5.4), and phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) were unaffected, as were enzyme activities for AAT, PEPC and GS. Antisense NADH-GOGAT plants grown under symbiotic conditions were moderately chlorotic and reduced in growth and N content, even though symbiotic N2 fixation was not significantly reduced. The addition of nitrate relieved the chlorosis and restored growth and N content. Surprisingly, the antisense NADH-GOGAT plants were male sterile resulting from inviable pollen. A reduction in NADH-GOGAT enzyme activity and transcript abundance in the antisense plants was measured during the early stages of flower development. Inheritance of the transgene was stable and resulted in progeny with a range of NADH-GOGAT activity. These data indicate that NADH-GOGAT plays a critical role in the assimilation of symbiotically fixed N and during pollen development.

NADH-Glutamate synthase in alfalfa root nodules. Immunocytochemical localization

In root nodules of alfalfa (Medicago sativa L.), N2 is reduced to NH4+ in the bacteroid by the nitrogenase enzyme and then released into the plant cytosol. The NH4+ is then assimilated by the combined action of glutamine synthetase (EC 6.3.1.2) and NADH-dependent Glu synthase (NADH-GOGAT; EC 1.4.1.14) into glutamine and Glu. The alfalfa nodule NADH-GOGAT protein has a 101-amino acid presequence, but the subcellular location of the protein is unknown. Using immunocytochemical localization, we determined first that the NADH-GOGAT protein is found throughout the infected cell region of both 19- and 33-d-old nodules. Second, in alfalfa root nodules NADH-GOGAT is localized predominantly to the amyloplast of infected cells. This finding, together with earlier localization and fractionation studies, indicates that in alfalfa the infected cells are the main location for the initial assimilation of fixed N2.