Expression of a ferredoxin-dependent glutamate synthase gene in mesophyll and vascular cells and functions of the enzyme in ammonium assimilation in Nicotiana tabacum (L.)

Identification of Wheat Glutamate Synthetase Gene Family and Expression Analysis under Nitrogen Stress

Nitrogen (N), as the main component of biological macromolecules, maintains the basic process of plant growth and development. GOGAT, as a key enzyme in the N assimilation process, catalyzes α-ketoglutaric acid and glutamine to form glutamate. In this study, six GOGAT genes in wheat (Triticum aestivum L.) were identified and classified into two subfamilies, Fd-GOGAT (TaGOGAT2s) and NADH-GOGAT (TaGOGAT3s), according to the type of electron donor. Subcellular localization prediction showed that TaGOGAT3-D was localized in mitochondria and that the other five TaGOGATs were localized in chloroplasts. Via the analysis of promoter elements, many binding sites related to growth and development, hormone regulation and plant stress resistance regulations were found on the TaGOGAT promoters. The tissue-specificity expression analysis showed that TaGOGAT2s were mainly expressed in wheat leaves and flag leaves, while TaGOGAT3s were highly expressed in roots and leaves. The expression level of TaGOGATs and the enzyme activity of TaGOGAT3s in the leaves and roots of wheat seedlings were influenced by the treatment of N deficiency. This study conducted a systematic analysis of wheat GOGAT genes, providing a theoretical basis not only for the functional analysis of TaGOGATs, but also for the study of wheat nitrogen use efficiency (NUE).

Distinct physiological and transcriptional responses of leaves of paper mulberry (Broussonetia kazinoki × B. papyrifera) under different nitrogen supply levels

Paper mulberry, a vigorous pioneer species used for ecological reclamation and a high-protein forage plant for economic development, has been widely planted in China. To further develop its potential value, it is necessary to explore the regulatory mechanism of nitrogen metabolism for rational nitrogen utilization. In this study, we investigated the morphology, physiology and transcriptome of a paper mulberry hybrid (Broussonetia kazinoki × B. papyrifera) in response to different nitrogen concentrations. Moderate nitrogen promoted plant growth and biomass accumulation. Photosynthetic characteristics, concentration of nitrogenous compounds and activities of enzymes were stimulated under nitrogen treatment. However, these enhancements were slightly or severely inhibited under excessive nitrogen supply. Nitrite reductase and glutamate synthase were more sensitive than nitrate reductase and glutamine synthetase and more likely to be inhibited under high nitrogen concentrations. Transcriptome analysis of the leaf transcriptome identified 161,961 unigenes. The differentially expressed genes associated with metabolism of nitrogen, alanine, aspartate, glutamate and glycerophospholipid showed high transcript abundances after nitrogen application, whereas those associated with glycerophospholipid, glycerolipid, amino sugar and nucleotide sugar metabolism were down-regulated. Combined with weighted gene coexpression network analysis, we uncovered 16 modules according to similarity in expression patterns. Asparagine synthetase and inorganic pyrophosphatase were considered two hub genes in two modules, which were associated with nitrogen metabolism and phosphorus metabolism, respectively. The expression characteristics of these genes may explain the regulation of morphological, physiological and other related metabolic strategies harmoniously. This multifaceted study provides valuable insights to further understand the mechanism of nitrogen metabolism and to guide utilization of paper mulberry.

Expression Analysis of Nitrogen Metabolism-Related Genes Reveals Differences in Adaptation to Low-Nitrogen Stress between Two Different Barley Cultivars at Seedling Stage

The excess use of nitrogen fertilizers causes many problems, including higher costs of crop production, lower nitrogen use efficiency, and environmental damage. Crop breeding for low-nitrogen tolerance, especially molecular breeding, has become the major route to solving these issues. Therefore, in crops such as barley (Hordeum vulgare L.), it is crucial to understand the mechanisms of low-nitrogen tolerance at the molecule level. In the present study, two barley cultivars, BI-04 (tolerant to low nitrogen) and BI-45 (sensitive to low nitrogen), were used for gene expression analysis under low-nitrogen stress, including 10 genes related to primary nitrogen metabolism. The results showed that the expressions of HvNIA2 (nitrite reductase), HvGS2 (chloroplastic glutamine synthetase), and HvGLU2 (ferredoxin-dependent glutamate synthase) were only induced in shoots of BI-04 under low-nitrogen stress, HvGLU2 was also only induced in roots of BI-04, and HvGS2 showed a rapid response to low-nitrogen stress in the roots of BI-04. The expression of HvASN1 (asparagine synthetase) was reduced in both cultivars, but it showed a lower reduction in the shoots of BI-04. In addition, gene expression and regulation differences in the shoots and roots were also compared between the barley cultivars. Taken together, the results indicated that the four above-mentioned genes might play important roles in low-nitrogen tolerance in barley.

ES7, encoding a ferredoxin-dependent glutamate synthase, functions in nitrogen metabolism and impacts leaf senescence in rice

Glutamate synthase (GOGAT) is a key enzyme for nitrogen metabolism and ammonium assimilation in plants. In this study, an early senescence 7 (es7) mutant was identified and characterized. The leaves of the es7 mutant begin to senesce at the tillering stage about 60day after sowing, and become increasingly senescent as the plants develop at the heading stage. When es7 plants are grown under photorespiration-suppressed conditions (high CO₂), the senescence phenotype and chlorophyll content are rescued. qRT-PCR analysis showed that senescence- associated genes were up-regulated significantly in es7. A map-based cloning strategy was used to identify ES7, which encodes a ferredoxin-dependent glutamate synthase (Fd-GOGAT). ES7 was expressed constitutively, and the ES7 protein was localized in chloroplast. qRT-PCR analysis indicated that several genes related to nitrogen metabolism were differentially expressed in es7. Further, we also demonstrated that chlorophyll synthesis-associated genes were significantly down-regulated in es7. In addition, when seedlings are grown under increasing nitrogen concentrations (NH₄NO₃) for 15days, the contents of chlorophyll a, chlorophyll b and total chlorophyll were significantly lower in es7. Our results demonstrated that ES7 is involved in nitrogen metabolism, effects chlorophyll synthesis, and may also associated with photorespiration, impacting leaf senescence in rice.

GNC and CGA1 modulate chlorophyll biosynthesis and glutamate synthase (GLU1/Fd-GOGAT) expression in Arabidopsis

Chloroplast development is an important determinant of plant productivity and is controlled by environmental factors including amounts of light and nitrogen as well as internal phytohormones including cytokinins and gibberellins (GA). The paralog GATA transcription factors GNC and CGA1/GNL up-regulated by light, nitrogen and cytokinin while also being repressed by GA signaling. Modifying the expression of these genes has previously been shown to influence chlorophyll content in Arabidopsis while also altering aspects of germination, elongation growth and flowering time. In this work, we also use transgenic lines to demonstrate that GNC and CGA1 exhibit a partially redundant control over chlorophyll biosynthesis. We provide novel evidence that GNC and CGA1 influence both chloroplast number and leaf starch in proportion to their transcript level. GNC and CGA1 were found to modify the expression of chloroplast localized GLUTAMATE SYNTHASE (GLU1/Fd-GOGAT), which is the primary factor controlling nitrogen assimilation in green tissue. Altering GNC and CGA1 expression was also found to modulate the expression of important chlorophyll biosynthesis genes (GUN4, HEMA1, PORB, and PORC). As previously demonstrated, the CGA1 transgenic plants demonstrated significantly altered timing to a number of developmental events including germination, leaf production, flowering time and senescence. In contrast, the GNC transgenic lines we analyzed maintain relatively normal growth phenotypes outside of differences in chloroplast development. Despite some evidence for partial divergence, results indicate that regulation of both GNC and CGA1 by light, nitrogen, cytokinin, and GA acts to modulate nitrogen assimilation, chloroplast development and starch production. Understanding the mechanisms controlling these processes is important for agricultural biotechnology.

Glutamate induces series of action potentials and a decrease in circumnutation rate in Helianthus annuus

Reports concerning the function of glutamate (Glu) in the electrical and movement phenomena in plants are scarce. Using the method of extracellular measurement, we recorded electrical potential changes in the stem of 3-week-old Helianthus annuus L. plants after injection of Glu solution. Simultaneously, circumnutation movements of the stem were measured with the use of time-lapse images. Injection of Glu solution at millimolar (200, 50, 5 mM) concentrations in the basal part of the stem evoked a series of action potentials (APs). The APs appeared in the site of injection and in different parts of the stem and were propagated acropetally and/or basipetally along the stem. Glu injection also resulted in a transient, approximately 5-h-long decrease in the stem circumnutation rate. The APs initiated and propagating in the sunflower stem after Glu injection testify the existence of a Glu perception system in vascular plants and suggest its involvement in electrical, long-distance signaling. Our experiments also demonstrated that Glu is a factor affecting circumnutation movements.

Assimilation of excess ammonium into amino acids and nitrogen translocation in Arabidopsis thaliana--roles of glutamate synthases and carbamoylphosphate synthetase in leaves

This study was aimed at investigating the physiological role of ferredoxin-glutamate synthases (EC 1.4.1.7), NADH-glutamate synthase (EC 1.4.1.14) and carbamoylphosphate synthetase (EC 6.3.5.5) in Arabidopsis. Phenotypic analysis revealed a high level of photorespiratory ammonium, glutamine/glutamate and asparagine/aspartate in the GLU1 mutant lacking the major ferredoxin-glutamate synthase, indicating that excess photorespiratory ammonium was detoxified into amino acids for transport out of the veins. Consistent with these results, promoter analysis and in situ hybridization demonstrated that GLU1 and GLU2 were expressed in the mesophyll and phloem companion cell-sieve element complex. However, these phenotypic changes were not detected in the GLU2 mutant defective in the second ferredoxin-glutamate synthase gene. The impairment in primary ammonium assimilation in the GLT mutant under nonphotorespiratory high-CO(2) conditions underlined the importance of NADH-glutamate synthase for amino acid trafficking, given that this gene only accounted for 3% of total glutamate synthase activity. The excess ammonium from either endogenous photorespiration or the exogenous medium was shifted to arginine. The promoter analysis and slight effects on overall arginine synthesis in the T-DNA insertion mutant in the single carbamoylphosphate synthetase large subunit gene indicated that carbamoylphosphate synthetase located in the chloroplasts was not limiting for ammonium assimilation into arginine. The data provided evidence that ferredoxin-glutamate synthases, NADH-glutamate synthase and carbamoylphosphate synthetase play specific physiological roles in ammonium assimilation in the mesophyll and phloem for the synthesis and transport of glutamine, glutamate, arginine, and derived amino acids.

Chloroplast nitrite uptake is enhanced in Arabidopsis PII mutants

In higher plants, the PII protein is a nuclear-encoded plastid protein that regulates the activity of a key enzyme of arginine biosynthesis. We have previously observed that Arabidopsis PII mutants are more sensitive to nitrite toxicity. Using intact chloroplasts isolated from Arabidopsis leaves and (15)N-labelled nitrite we show that a light-dependent nitrite uptake into chloroplasts is increased in PII knock-out mutants when compared to the wild-type. This leads to a higher incorporation of (15)N into ammonium and amino acids in the mutant chloroplasts. However, the uptake differences do not depend on GS/GOGAT activities. Our observations suggest that PII is involved in the regulation of nitrite uptake into higher plant chloroplasts.

Glutamine synthetase-glutamate synthase pathway and glutamate dehydrogenase play distinct roles in the sink-source nitrogen cycle in tobacco

Glutamate (Glu) metabolism and amino acid translocation were investigated in the young and old leaves of tobacco (Nicotiana tabacum L. cv Xanthi) using [15N]ammonium and [2-15N]Glu tracers. Regardless of leaf age, [15N]ammonium assimilation occurred via glutamine synthetase (GS; EC 6.1.1.3) and Glu synthase (ferredoxin [Fd]-GOGAT; EC 1.4.7.1; NADH-GOGAT; EC 1.4.1.14), both in the light and darkness, and it did not depend on Glu dehydrogenase (GDH; EC 1.4.1.2). The [15N]ammonium and ammonium accumulation patterns support the role of GDH in the deamination of [2-15N]Glu to provide 2-oxoglutarate and [15N]ammonium. In the dark, excess [15N]ammonium was incorporated into asparagine that served as an additional detoxification molecule. The constant Glu levels in the phloem sap suggested that Glu was continuously synthesized and supplied into the phloem regardless of leaf age. Further study using transgenic tobacco lines, harboring the promoter of the GLU1 gene (encoding Arabidopsis [Arabidopsis thaliana] Fd-GOGAT) fused to a GUS reporter gene, revealed that the expression of Fd-GOGAT remained higher in young leaves compared to old leaves, and higher in the veins compared to the mesophyll. Confocal laser-scanning microscopy localized the Fd-GOGAT protein to the phloem companion cells-sieve element complex in the leaf veins. The results are consistent with a role of Fd-GOGAT in supplying Glu for the synthesis and transport of amino acids. Taken together, the data provide evidence that the GS-GOGAT pathway and GDH play distinct roles in the source-sink nitrogen cycle of tobacco leaves.