Can glutamine synthetase activity levels be modulated in transgenic plants by the use of recombinant DNA 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.

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.

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.