Possible role of glutamine synthetase of the prokaryotic type (GSI-like) in nitrogen signaling in Medicago truncatula

Bioengineering Techniques to Improve Nitrogen Transformation and Utilization: Implications for Nitrogen Use Efficiency and Future Sustainable Crop Production

Nitrogen (N) is crucial for plant growth and development, especially in physiological and biochemical processes such as component of different proteins, enzymes, nucleic acids, and plant growth regulators. Six categories, such as transporters, nitrate absorption, signal molecules, amino acid biosynthesis, transcription factors, and miscellaneous genes, broadly encompass the genes regulating NUE in various cereal crops. Herein, we outline detailed research on bioengineering modifications of N metabolism to improve the different crop yields and biomass. We emphasize effective and precise molecular approaches and technologies, including N transporters, transgenics, omics, etc., which are opening up fascinating opportunities for a complete analysis of the molecular elements that contribute to NUE. Moreover, the detection of various types of N compounds and associated signaling pathways within plant organs have been discussed. Finally, we highlight the broader impacts of increasing NUE in crops, crucial for better agricultural yield and in the greater context of global climate change.

Theanine, a tea-plant-specific non-proteinogenic amino acid, is involved in the regulation of lateral root development in response to nitrogen status

Glutamine synthetase type I (GSI)-like proteins are proposed to mediate nitrogen signaling and developmental fate by synthesizing yet unidentified metabolites. Theanine, the most abundant non-proteinogenic amino acid in tea plants, is the first identified metabolite synthesized by a GSI-like protein (CsTSI) in a living system. However, the roles of theanine in nitrogen signaling and development are little understood. In this study we found that nitrogen deficiency significantly reduced theanine accumulation and increased lateral root development in tea plant seedlings. Exogenous theanine feeding significantly repressed lateral root development of seedlings of tea plants and the model plant Arabidopsis. The transcriptomic analysis revealed that the differentially expressed genes in the roots under theanine feeding were enriched in the apoplastic pathway and H₂O₂ metabolism. Consistently, theanine feeding reduced H₂O₂ levels in the roots. Importantly, when co-treated with H₂O₂, theanine abolished the promoting effect of H₂O₂ on lateral root development in both tea plant and Arabidopsis seedlings. The results of histochemical assays confirmed that theanine inhibited reactive oxygen species accumulation in the roots. Further transcriptomic analyses suggested the expression of genes encoding enzymes involved in H₂O₂ generation and scavenging was down- and upregulated by theanine, respectively. Moreover, the expression of genes involved in auxin metabolism and signaling, cell division, and cell expansion was also regulated by theanine. Collectively, these results suggested that CsTSI-synthesized theanine is likely involved in the regulation of lateral root development, via modulating H₂O₂ accumulation, in response to nitrogen levels in tea plants. This study also implied that the module consisting of GSI-like protein and theanine-like metabolite is probably conserved in regulating development in response to nitrogen status in plant species.

Tea plant-legume intercropping simultaneously improves soil fertility and tea quality by changing bacillus species composition

Tea plant is an economically important crop in China, but long-term monoculture and substantial chemical nitrogen fertilizer input cause soil acidification, which in turn affects the nutrient supply and tea quality. Intercropping has drawn more attention in tea gardens because this pattern is expected to improve soil fertility and tea quality and change the soil microbial community composition. However, the roles of some key microorganisms in rhizosphere soils have not been well characterized. Hereby, a "soybean in summer and smooth vetch in winter" mode was selected to investigate the effects of intercropped legumes in a tea garden on soil fertility, tea quality, and the potential changes in beneficial bacteria such as Bacillus. Our data showed that when soybeans were turned into soil, intercropping system exhibited higher soil organic matter (SOM), total nitrogen (TN), tea quality indices and the expression of Camellia sinensis glutamine synthetase gene (CsGS). Notably, intercropping significantly affected the bacterial communities and decreased the relative abundance of Bacillus but increased its absolute abundance. Bacillus amyloliquefaciens BM1 was isolated from intercropped soil and showed outstanding plant growth-promoting (PGP) properties when coinoculated with rhizobia. In winter, intercropping with smooth vetch had a beneficial effect on soil properties and tea quality. Comparably, coinoculation with strain BM1 and Rhizobium leguminosarum Vic5 on smooth vetch (Vicia villosa) showed huge improvements in SOM, TN and quality of tea leaves, accompanied by the highest level of amino acids and lowest levels of polyphenol and caffeine (p < 0.05). According to these results, our findings demonstrate that intercropping with some legumes in the tea garden is a strategy that increases SOM, TN and tea quality, and some PGP Bacillus species are optional to obtain an amplification effect.

Characterization of CsTSI in the Biosynthesis of Theanine in Tea Plants (Camellia sinensis)

Theanine is a unique major amino acid in tea plants responsible for umami taste and mental health benefits of tea. However, theanine biosynthesis and physiological role in tea plants are not fully understood. Here, we demonstrate that tea plant theanine synthetase is encoded by a glutamine synthetase gene CsTSI. The expression pattern of CsTSI is closely correlated with theanine and glutamine levels in various tissues. CsTSI transcripts were accumulated in root tip epidermal cells, pericycle and procambial cells, where CsTSI presents as a cytosolic protein. Ectopic expression of the gene in Arabidopsis led to greater glutamine and theanine production than controls when fed with ethylamine (EA). RNAi knockdown or overexpression of CsTSI in tea plant hairy roots reduced or enhanced theanine and glutamine contents, respectively, compared with controls. The CsTSI recombinant enzymes used glutamate as an acceptor and ammonium or EA as a donor to synthesize glutamine and theanine, respectively. CsTSI expression in tea roots responded to nitrogen supply and deprivation and was correlated with theanine contents. This study provides fresh insights into the molecular basis for the biosynthesis of theanine, which may facilitate the breeding of high-theanine tea plants for improving the nutritional benefit of tea.

Endophytic Bacteria as Contributors to Theanine Production in Camellia sinensis

Theanine is the most abundant non-protein amino acid in Camellia sinensis, but it is not known how a tea plant accumulates such high levels of theanine. The endophyte isolated from in vitro grown plantlets of C. sinensis cultivars was identified as Luteibacter spp., showing strong biocatalytic activity for converting both glutamine and ethylamine to theanine. Theanine was secreted outside of the bacteria. The endophyte isolated from in vitro plantlets of Camellia oleifera cultivar was identified as Bacillus safensis and did not convert glutamine and ethylamine to theanine. Enzymatic assays in vitro indicated that γ-glutamyltranspeptidases rCsEGGTs from the endophyte Luteibacter strains converted glutamine and ethylamine to theanine at higher rates than rCsGGTs from C. sinensis. This is the first report on theanine biosynthesis by an endophyte from C. sinensis, which provides a new pathway to explore the mechanism of theanine biosynthesis in C. sinensis and the interactions between an endophyte and tea plants.

Cisgenic overexpression of cytosolic glutamine synthetase improves nitrogen utilization efficiency in barley and prevents grain protein decline under elevated CO2

Cytosolic glutamine synthetase (GS1) plays a central role in nitrogen (N) metabolism. The importance of GS1 in N remobilization during reproductive growth has been reported in cereal species but attempts to improve N utilization efficiency (NUE) by overexpressing GS1 have yielded inconsistent results. Here, we demonstrate that transformation of barley (Hordeum vulgare L.) plants using a cisgenic strategy to express an extra copy of native HvGS1-1 lead to increased HvGS1.1 expression and GS1 enzyme activity. GS1 overexpressing lines exhibited higher grain yields and NUE than wild-type plants when grown under three different N supplies and two levels of atmospheric CO₂ . In contrast with the wild-type, the grain protein concentration in the GS1 overexpressing lines did not decline when plants were exposed to elevated (800-900 μL/L) atmospheric CO₂ . We conclude that an increase in GS1 activity obtained through cisgenic overexpression of HvGS1-1 can improve grain yield and NUE in barley. The extra capacity for N assimilation obtained by GS1 overexpression may also provide a means to prevent declining grain protein levels under elevated atmospheric CO₂ .

Proteomics Reveal the Profiles of Color Change in Brunfelsia acuminata Flowers

Brunfelsia acuminata is a popular ornamental plant with different colors resulted from the rapid change of color after blooming. The petals at day one (purple), day three (white and purple) and day five (white) were used to analyze the reason of flower color change by a comparative proteomics approach, gas chromatography coupled to a time-of-flight mass analyzer (GC-TOF-MS) and quantitative real-time PCR (qRT-PCR). The results showed that the 52 identified proteins were classified into eight functional groups, 6% of which were related to the anthocyanin metabolic pathway. The expression levels of all anthocyanin proteins from the first day to fifth day were remarkably down-regulated, which was consistent with the changing patterns of the key genes (CHS, CHI and F3′5′H) in petals. Simultaneously, the main floral volatile components including Linalool and 2-Hexenal (E) were identified, and the contents of 2-Hexenal at day five increased dramatically. Moreover, the content of flavonoids and total phenolic increased at day five. The majority of the proteins associated with stress defense and senescence proteins were up-regulated and the activities of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) in the petals at day five were significantly higher than others. It was concluded that the competition in the precursors of metabolic pathways occurs and causes the flow of metabolite to the pathways of floral scent and lignin derived from the shikimate pathway or degrade into others. Therefore, the anthocyanin content significantly decreased, and the petal color changed from deep purple to white.

The Early Asexual Development Regulator fluG Codes for a Putative Bifunctional Enzyme

FluG is a long recognized early regulator of asexual development in Aspergillus nidulans. fluG null mutants show profuse aerial growth and no conidial production. Initial studies reported sequence homology of FluG with a prokaryotic type I glutamine synthetase, but catalytic activity has not been demonstrated. In this study, we conducted an in-depth analysis of the FluG sequence, which revealed a single polypeptide containing a putative N-terminal amidohydrolase region linked to a putative C-terminal γ-glutamyl ligase region. Each region corresponded, separately and completely, to respective single function bacterial enzymes. Separate expression of these regions confirmed that the C-terminal region was essential for asexual development. The N-terminal region alone did not support conidial development, but contributed to increased conidial production under high nutrient availability. Point mutations directed at respective key catalytic residues in each region demonstrated that they were essential for biological function. Moreover, the substitution of the N- and C-terminal regions with homologs from Lactobacillus paracasei and Pseudomonas aeruginosa, respectively, maintained functionality, albeit with altered characteristics. Taken together, the results lead us to conclude that FluG is a bifunctional enzyme that participates in an as yet unidentified metabolic or signaling pathway involving a γ-glutamylated intermediate that contributes to developmental fate.

Draft genome sequence of Camellia sinensis var. sinensis provides insights into the evolution of the tea genome and tea quality

A high-quality genome assembly of Camellia sinensis var. sinensis facilitates genomic, transcriptomic, and metabolomic analyses of the quality traits that make tea one of the world’s most-consumed beverages. The specific gene family members critical for biosynthesis of key tea metabolites, monomeric galloylated catechins and theanine, are indicated and found to have evolved specifically for these functions in the tea plant lineage. Two whole-genome duplications, critical to gene family evolution for these two metabolites, are identified and dated, but are shown to account for less amplification than subsequent paralogous duplications. These studies lay the foundation for future research to understand and utilize the genes that determine tea quality and its diversity within tea germplasm.

Tea, one of the world’s most important beverage crops, provides numerous secondary metabolites that account for its rich taste and health benefits. Here we present a high-quality sequence of the genome of tea, Camellia sinensis var. sinensis (CSS), using both Illumina and PacBio sequencing technologies. At least 64% of the 3.1-Gb genome assembly consists of repetitive sequences, and the rest yields 33,932 high-confidence predictions of encoded proteins. Divergence between two major lineages, CSS and Camellia sinensis var. assamica (CSA), is calculated to ∼0.38 to 1.54 million years ago (Mya). Analysis of genic collinearity reveals that the tea genome is the product of two rounds of whole-genome duplications (WGDs) that occurred ∼30 to 40 and ∼90 to 100 Mya. We provide evidence that these WGD events, and subsequent paralogous duplications, had major impacts on the copy numbers of secondary metabolite genes, particularly genes critical to producing three key quality compounds: catechins, theanine, and caffeine. Analyses of transcriptome and phytochemistry data show that amplification and transcriptional divergence of genes encoding a large acyltransferase family and leucoanthocyanidin reductases are associated with the characteristic young leaf accumulation of monomeric galloylated catechins in tea, while functional divergence of a single member of the glutamine synthetase gene family yielded theanine synthetase. This genome sequence will facilitate understanding of tea genome evolution and tea metabolite pathways, and will promote germplasm utilization for breeding improved tea varieties.

How do plants sense their nitrogen status?

The primary processes that contribute to the efficient capture of soil nitrate are the development of a root system that effectively explores the soil and the expression of high-affinity nitrate uptake systems in those roots. Both these processes are highly regulated to take into account the availability and distribution of external nitrate pools and the endogenous N status of the plant. While significant progress has been made in elucidating the early steps in sensing and responding to external nitrate, there is much less clarity about how the plant monitors its N status. This review specifically addresses the questions of what N compounds are sensed and in which part of the plant, as well as the identity of the signalling pathways responsible for their detection. Candidates that are considered for the role of N sensory systems include the target of rapamycin (TOR) signalling pathway, the general control non-derepressible 2 (GCN2) pathway, the plastidic PII-dependent pathway, and the family of glutamate-like receptors (GLRs). However, despite significant recent progress in elucidating the function and mode of action of these signalling systems, there is still much uncertainty about the extent to which they contribute to the process by which plants monitor their N status. The possibility is discussed that the large GLR family of Ca2+ channels, which are gated by a wide range of different amino acids and expressed throughout the plant, could act as amino acid sensors upstream of a Ca2+-regulated signalling pathway, such as the TOR pathway, to regulate the plant's response to changes in N status.

Carbon dioxide assimilation and photosynthetic electron transport of tea leaves under nitrogen deficiency

BACKGROUND

Tea plant is famed in humid and sub-humid of tropical regions, sub-tropical regions, and is a leaf-harvested crop. Nitrogen is the most important nutrient for increasing quality of tea leaves. Therefore, large amounts of nitrogen fertilizer are increasingly applied by tea farmers. Appropriate application of nitrogen fertilizer aroused people's concern. This research of physiological response to N deficiency stress will be helpful for appropriate application of nitrogen fertilizer for tea farmers and elucidate a mechanistic basis for the reductions in carbon dioxide (CO2) assimilation.

RESULTS

To elucidate a mechanistic basis for the reductions in carbon dioxide (CO2) assimilation under nitrogen (N) deficiency tea leaves, changes in chlorophyll (Chl), carbohydrates, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and chlorophyll fluorescence transient were examined together with six N treatment (0, 50, 100, 300, 1200 or 6000 μM N). Root, stem and leaves dry weight (DW) increased as N supply increased from 0 to 300 μM, then remained unchanged. The reductions in CO2 assimilation of N-deficient leaves paralleled with high intercellular CO2 concentration. Rubisco activity, protein and Chl content increased linearly or curvilinearly over the range of leaf N content examined except unchanged as leaf N from 2.15 to 2.79 g m-2. Chlorophyll fluorescence transient from N-deficient leaves displayed a depression at the P-step, accompanied by a new step at about 150 μs (L-step). Fv/Fm, REo/ETo, ETo/ABS, Sm, ETo/CSo, PIabs, PItot, abs, were decreased in N-deficient leaves but increased DIo/CSo, DIo/RC and DIo/ABS. Regressive analysis showed that CO2 assimilation decreased linearly or curvilinearly with decreasing initial rubisco, PIabs and Leaf Chl, respectively. Therefore, we concluded the decreased photosynthetic electron transport capacity, leaf chl content and initial rubisco activity are probably the main factors contributing to decreased CO2 assimilation under N deficiency.

CONCLUSIONS

The decreased photosynthetic electron transport capacity, leaf Chl content and initial rubisco activity are probably the main factors contributing to decreased CO2 assimilation under N deficiency.