Glutamate signalling in roots

Involvement of GLR-mediated nitric oxide effects on ROS metabolism in Arabidopsis plants under salt stress

Plant glutamate receptor-like channels (GLRs) play important roles in plant development, immune response, defense signaling and Nitric oxide (NO) production. However, their involvement in abiotic stress responses, particularly in regulating Reactive Oxygen Species (ROS), is not well understood. This study aimed to investigate GLR-mediated NO production on ROS regulation in salt-stressed cells. To achieve this, Arabidopsis thaliana Columbia (Col-0) were treated with NaCl, glutamate antagonists [(DNQX (6,7-dinitroquinoxaline-2,3-dione and AP-5(D-2-amino-5-phosphono pentanoic acid)], and NO scavenger [cPTIO (2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt)]. Salt-stressed plants in combination with DNQX and AP-5 have exhibited higher increase in lipid peroxidation (TBARS), hydrogen peroxide (H2O2) and superoxide radical (O-2) contents as compared to solely NaCl-treated plants. Furthermore, NO and total glutathione contents, and S-nitrosoglutathione reductase (GSNOR) activity decreased with these treatments. AP-5 and DNQX increased the activities of NADPH oxidase (NOX), catalase (CAT), peroxidase (POX), cell wall peroxidase (CWPOX) in salt-stressed Arabidopsis leaves. However, their activities (except NOX) were significantly inhibited by cPTIO. Conversely, the combination of NaCl and GLR antagonists, NO scavenger decreased the activities of ascorbate peroxidase (APX), superoxide dismutase (SOD), glutathione reductase (GR), dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) resulting in elevated GSSG levels, a low GSH/GSSG ratio, impaired ROS scavenging, excessive ROS accumulation and cell membrane damage. The findings of this study provide evidence that GLR-mediated NO plays a crucial role in improvement of the tolerance of Arabidopsis plants to salt-induced oxidative stress. It helps to maintain cellular redox homeostasis by reducing ROS accumulation and increasing the activity of SOD, GSNOR, and the ASC-GSH cycle enzymes.

Genome-wide identification and comparative analysis of the Amino Acid Transporter (AAT) gene family and their roles during Phaseolus vulgaris symbioses

Amino acid transporters (AATs) are essential integral membrane proteins that serve multiple roles, such as facilitating the transport of amino acids across cell membranes. They play a crucial role in the growth and development of plants. Phaseolus vulgaris, a significant legume crop, serves as a valuable model for studying root symbiosis. In this study, we have conducted an exploration of the AAT gene family in P. vulgaris. In this research, we identified 84 AAT genes within the P. vulgaris genome sequence and categorized them into 12 subfamilies based on their similarity and phylogenetic relationships with AATs found in Arabidopsis and rice. Interestingly, these AAT genes were not evenly distributed across the chromosomes of P. vulgaris . Instead, there was an unusual concentration of these genes located toward the outer edges of chromosomal arms. Upon conducting motif analysis and gene structural analysis, we observed a consistent presence of similar motifs and an intron-exon distribution pattern among the subfamilies. When we analyzed the expression profiles of PvAAT genes, we noted tissue-specific expression patterns. Furthermore, our investigation into AAT gene expression under rhizobial and mycorrhizal symbiotic conditions revealed that certain genes exhibited high levels of expression. Specifically, ATLa5 and LHT2 was notably upregulated under both symbiotic conditions. These findings point towards a potential role of AATs in the context of rhizobial and mycorrhizal symbiosis in P. vulgaris, in addition to their well-established regulatory functions.

Comparative genomics reveals a unique nitrogen-carbon balance system in Asteraceae

The Asteraceae (daisy family) is one of the largest families of plants. The genetic basis for its high biodiversity and excellent adaptability has not been elucidated. Here, we compare the genomes of 29 terrestrial plant species, including two de novo chromosome-scale genome assemblies for stem lettuce, a member of Asteraceae, and Scaevola taccada, a member of Goodeniaceae that is one of the closest outgroups of Asteraceae. We show that Asteraceae originated ~80 million years ago and experienced repeated paleopolyploidization. PII, the universal regulator of nitrogen-carbon (N-C) assimilation present in almost all domains of life, has conspicuously lost across Asteraceae. Meanwhile, Asteraceae has stepwise upgraded the N-C balance system via paleopolyploidization and tandem duplications of key metabolic genes, resulting in enhanced nitrogen uptake and fatty acid biosynthesis. In addition to suggesting a molecular basis for their ecological success, the unique N-C balance system reported for Asteraceae offers a potential crop improvement strategy.

ʟ-glutamic acid modulates antioxidant defense systems and nutrient homeostasis in lentil (Lens culinaris Medik.) under copper toxicity

Copper (Cu), an essential micronutrient, can generate reactive oxygen species (ROS) at its supra-optimal level in living cells as a transition metal, thus producing oxidative stress in plants. Therefore, protecting plants from Cu-induced oxidative stress via the exogenous application of chemical substances, particularly L-glutamic acid (L-Glu), could be a viable strategy for mitigating the toxicity of Cu. The aim of our present study was to investigate how ʟ-Glu protects lentil seedlings from oxidative stress produced by toxic Cu and allows them to survive under Cu toxicity. The results exhibited that when lentil seedlings were exposed to excessive Cu, their growth was inhibited and their biomass decreased due to an increase in Cu accumulation and translocation to the root, shoot, and leaves. Exposure to toxic Cu also depleted photosynthetic pigments, imbalanced water content, and other essential nutrients, increased oxidative stress, and reduced enzymatic and non-enzymatic antioxidants. However, pre-treatment of ʟ-Glu improved the phenotypic appearance of lentil seedlings, which was distinctly evidenced by higher biomass production, maintenance of water balance, and an increase in photosynthetic pigments when exposed to toxic Cu. ʟ-Glu also protected the seedlings from Cu-induced oxidative stress by reducing the oxidative stress marker, specifically by the efficient action of enzymatic and non-enzymatic antioxidants, particularly ascorbate, catalase, monodehydroascorbate, and glutathione peroxidase and maintaining redox balance. Furthermore, ʟ-Glu assisted in maintaining the homeostasis of Cu and other nutrient in the roots, shoots, and leaves of lentil. Collectively, our results provide evidence of the mechanism of ʟ-Glu-mediated protective role in lentil against Cu toxicity, thus proposed as a potential chemical for managing Cu toxicity not only in lentil but also other plants.

Egg-Associated Germs Induce Salicylate Defenses but Not Render Plant Against a Global Invasive Fruit Fly Effectively

Germs associated with insect eggs can profoundly mediate interactions between host plants and herbivores, with the potential to coordinate plant physiological reactions with cascading effects on insect fitness. An experimental system was established including the oriental fruit fly (OFF, Bactrocera dorsalis) and tomato to examine the functions of egg-associated germs in mediating plant-herbivore interactions. OFF feeding resulted in significantly increased tannins, flavonoids, amino acids, and salicylic acid in the host tomato. These defensive responses of tomato were induced by the egg-associated germs, including Lactococcus sp., Brevundimonas sp., and Vagococcus sp. Tannins and flavonoids had no significant feedback effects on the pupal weight of OFF, while pupal biomass was significantly decreased by tannins and flavonoids in the germ-free treatment. Metabolome analysis showed that OFF mainly induced metabolic changes in carboxylic acid derivatives. Phenylalanine significantly induced downstream metabolic changes associated with phenylpropanoid accumulation. Finally, we conclude that the effects of egg-associated germs played an important role in facilitating OFF population adaptation and growth by mediating plant defenses, which provides a new paradigm for exploring the interaction of plant-pest and implementing effective pest biocontrol.

Pine has two glutamine synthetase paralogs, GS1b.1 and GS1b.2, exhibiting distinct biochemical properties

The enzyme glutamine synthetase (EC 6.3.1.2) is mainly responsible for the incorporation of inorganic nitrogen into organic molecules in plants. In the present work, a pine (Pinus pinaster) GS1 (PpGS1b.2) gene was identified, showing a high sequence identity with the GS1b.1 gene previously characterized in conifers. Phylogenetic analysis revealed that the presence of PpGS1b.2 is restricted to the genera Pinus and Picea and is not found in other conifers. Gene expression data suggest a putative role of PpGS1b.2 in plant development, similar to other GS1b genes from angiosperms, suggesting evolutionary convergence. The characterization of GS1b.1 and GS1b.2 at the structural, physicochemical, and kinetic levels has shown differences even though they have high sequence homology. GS1b.2 had a lower optimum pH (6 vs. 6.5) and was less thermally stable than GS1b.1. GS1b.2 exhibited positive cooperativity for glutamate and substrate inhibition for ammonium. However, GS1b.1 exhibited substrate inhibition behavior for glutamate and ATP. Alterations in the kinetic characteristics produced by site-directed mutagenesis carried out in this work strongly suggest an implication of amino acids at positions 264 and 267 in the active center of pine GS1b.1 and GS1b.2 being involved in affinity toward ammonium. Therefore, the amino acid differences between GS1b.1 and GS1b.2 would support the functioning of both enzymes to meet distinct plant needs.

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.

Mechanism of calcium signal response to cadmium stress in duckweed

Cadmium (Cd) causes serious damage to plants. Although calcium (Ca) signal has been found to respond to certain stress, the localization of Ca and molecular mechanisms underlying Ca signal in plants during Cd stress are largely unknown. In this study, Ca²⁺-sensing fluorescent reporter (GCaMP3) transgenic duckweed showed the Ca²⁺ signal response in Lemna turionifera 5511 (duckweed) during Cd stress. Subsequently, the subcellular localization of Ca²⁺ has been studied during Cd stress by transmission electron microscopy, showing the accumulation of Ca²⁺ in vacuoles. Also, Ca²⁺ flow during Cd stress has been measured. At the same time, the effects of exogenous glutamic acid (Glu) and γ-aminobutyric (GABA) on duckweed can better clarify the signal operation mechanism of plants to Cd stress. The molecular mechanism of Ca²⁺ signal responsed during Cd stress showed that Cd treatment promotes the positive response of Ca signaling channels in plant cells, and thus affects the intracellular Ca content. These novel signal studies provided an important Ca²⁺ signal molecular mechanism during Cd stress.

Roles of Glutamate Receptor-Like Channels (GLRs) in Plant Growth and Response to Environmental Stimuli

Plant glutamate receptor-like channels (GLRs) are the homologues of ionotropic glutamate receptors (iGluRs) that mediate neurotransmission in mammals, and they play important roles in various plant-specific physiological processes, such as pollen tube growth, sexual reproduction, root meristem proliferation, internode cell elongation, stomata aperture regulation, and innate immune and wound responses. Notably, these biological functions of GLRs have been mostly linked to the Ca²⁺-permeable channel activity as GLRs can directly channel the transmembrane flux of Ca²⁺, which acts as a key second messenger in plant cell responses to both endogenous and exogenous stimuli. Thus, it was hypothesized that GLRs are mainly involved in Ca²⁺ signaling processes in plant cells. Recently, great progress has been made in GLRs for their roles in long-distance signal transduction pathways mediated by electrical activity and Ca²⁺ signaling. Here, we review the recent progress on plant GLRs, and special attention is paid to recent insights into the roles of GLRs in response to environmental stimuli via Ca²⁺ signaling, electrical activity, ROS, as well as hormone signaling networks. Understanding the roles of GLRs in integrating internal and external signaling for plant developmental adaptations to a changing environment will definitely help to enhance abiotic stress tolerance.

Glutamic Acid and Poly-γ-glutamic Acid Enhanced the Heat Resistance of Chinese Cabbage (Brassica rapa L. ssp. pekinensis) by Improving Carotenoid Biosynthesis, Photosynthesis, and ROS Signaling

Heat stress is one of the most common agrometeorological risks in crop production in the middle and lower reaches of the Yangtze River in China. This study aimed to investigate whether glutamic acid (Glu) or poly-γ-glutamic acid (γ-PGA) biostimulants can improve the thermotolerance of a cool-season Chinese cabbage (Brassica rapa L. ssp. pekinensis) crop. Priming with Glu (2.0 mM) or γ-PGA (20 mg·L⁻¹) was conducted at the third leaf stage by applying as daily foliar sprays for 5 days before 5 days of heat stress (45 °C in 16-h light/35 °C in 8-h dark). Coupled with morpho-physiological and biochemical analyses, transcriptomes of Glu or γ-PGA-primed Chinese cabbage under heat stress were examined by RNA-seq analysis. The results showed that the thermotolerance conferred by Glu and γ-PGA priming was associated with the increased parameters of vegetative growth, gas exchange, and chlorophyll fluorescence. Compared with the control, the dry weights of plants treated with Glu and γ-PGA increased by 51.52% and 39.39%, respectively. Glu and γ-PGA application also significantly increased the contents of total chlorophyll by 42.21% and 23.12%, and carotenoid by 32.00% and 24.00%, respectively. In addition, Glu- and γ-PGA-primed plants markedly inhibited the levels of malondialdehyde, electrolyte leakage, and super-oxide anion radical, which was accompanied by enhanced activity levels of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD). Enrichment analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) categories within the differentially expressed genes (DEGs) functional clusters of RNA-seq data indicated that the expression levels of the genes for DNA replication, DNA repair system, linoleic acid metabolism, cysteine and methionine metabolism, glutathione metabolism, purine and pyrimidine metabolism, carotenoid biosynthesis, and plant–pathogen interaction were commonly up-regulated by both Glu and γ-PGA priming. Glu treatment enhanced the expression levels of the genes involved in aliphatic glucosinolate and 2-oxocarboxylic acid, while γ-PGA treatment activated carotenoid cleavage reaction to synthesize abscisic acid. Taken together, both Glu and γ-PGA have great potential for the preadaptation of Chinese cabbage seedlings to heat stress, with Glu being more effective than γ-PGA.

Genetic Circuit Design in Rhizobacteria

Genetically engineered plants hold enormous promise for tackling global food security and agricultural sustainability challenges. However, construction of plant-based genetic circuitry is constrained by a lack of well-characterized genetic parts and circuit design rules. In contrast, advances in bacterial synthetic biology have yielded a wealth of sensors, actuators, and other tools that can be used to build bacterial circuitry. As root-colonizing bacteria (rhizobacteria) exert substantial influence over plant health and growth, genetic circuit design in these microorganisms can be used to indirectly engineer plants and accelerate the design-build-test-learn cycle. Here, we outline genetic parts and best practices for designing rhizobacterial circuits, with an emphasis on sensors, actuators, and chassis species that can be used to monitor/control rhizosphere and plant processes.

Amino acid transporter gene TaATLa1 from Triticum aestivum L. improves growth under nitrogen sufficiency and is down regulated under nitrogen deficiency

TaATLa1 was identified to respond to nitrogen deprivation through transcriptome analysis of wheat seedlings. TaATLa1 specifically transports Gln, Glu, and Asp, and affects the biomass of Arabidopsis and wheat. Nitrogen is an essential macronutrient and plays a crucial role in wheat production. Amino acids, the major form of organic nitrogen, are remobilized by amino acid transporters (AATs) in plants. AATs are commonly described as central components of essential developmental processes and yield formation via taking up and transporting amino acids in plants. However, few studies have reported the detailed biochemical properties and biological functions of these AATs in wheat. In this study, key genes encoding AATs were screened from transcriptome analysis of wheat seedlings treated with normal nitrogen (NN) and nitrogen deprivation (ND). Among them, 21 AATs were down-regulated and eight AATs were up-regulated under ND treatment. Among the homoeologs, TaATLa1.1-3A, TaATLa1.1-3B, and TaATLa1.1-3D (TaATLa1.1-3A, -3B, and -3D), belonging to amino acid transporter-like a (ATLa) subfamily, were significantly down-regulated in response to ND in wheat, and accordingly were selected for functional analyses. The results demonstrated that TaATLa1.1-3A, -3B, and -3D effectively transported glutamine (Gln), glutamate (Glu), and aspartate (Asp) in yeast. Overexpression of TaAILa1.1-3A, -3B, and -3D in Arabidopsis thaliana L. significantly increased amino acid content in leaves, storage protein content in seeds and the plant biomass under NN. Knockdown of TaATLa1.1-3A, -3B, and -3D in wheat seedlings resulted in a significant block of amino acid remobilization and growth inhibition. Taken together, TaATLa1.1-3A, -3B, and -3D contribute substantially to Arabidopsis and wheat growth. We propose that TaATLa1.1-3A, -3B, and -3D may participate in the source-sink translocation of amino acid, and they may have profound implications for wheat yield improvement.

New insight into the effect of riluzole on cadmium tolerance and accumulation in duckweed (Lemna turionifera)

Cadmium (Cd) damages plant photosynthesis, affects roots and leaves growth, and triggers molecular responses. Riluzole (RIL), which protected neuronal damage via inhibiting excess Glu release in animals, has been found to improve Cd tolerance in duckweed in this study. Firstly, RIL treatment alleviated leaf chlorosis by protecting chlorophyll and decreased root abscission under Cd stress. Secondly, RIL declines Cd accumulation by alleviating excess Glu release during Cd shock. RIL mitigate Glu outburst in duckweed during Cd stress by a decline in Glu in roots. The Cd²⁺ influx was repressed by RIL addition with Cd shock. Finally, differentially expressed genes (DEGs) of duckweed under Cd stress with RIL have been investigated. 2141 genes were substantially up-regulated and 3282 genes were substantially down-regulated with RIL addition. RIL down-regulates the genes related to the Glu synthesis, and genes related to DNA repair have been up-regulated with RIL treatment under Cd stress. These results provide new insights into the possibility of RIL to reduce Cd accumulation and increase Cd tolerance in duckweed, and lay the foundation for decreasing Cd accumulation in crops.

Dysregulation of Vesicular Glutamate Transporter VGluT2 via BDNF/TrkB Pathway Contributes to Morphine Tolerance in Mice

Morphine is widely used in the treatment of moderate to severe pain. Long-term use of morphine leads to various adverse effects, such as tolerance and hyperalgesia. Vesicular glutamate transporter 2 (VGluT2) accumulates glutamate into synaptic vesicles and plays multiple roles in the central nervous system. However, the specific role of VGluT2 in morphine tolerance has not been fully elucidated. Here, we investigated the regulatory role of VGluT2 in morphine tolerance and assessed the potential role of the brain-derived neurotrophic factor (BDNF)/tyrosine kinase B (TrkB) pathway in VGluT2 mediated morphine antinociceptive tolerance in mice. In the present study, we found that VGluT2 is upregulated in the spinal cord after the development of morphine tolerance. Furthermore, inhibition of VGluT2 with its antagonist (Chicago sky blue 6 B, CSB6B) or knockdown of VGluT2 by lentivirus restored the analgesic effect of morphine, suppressed the activation of astrocytes and microglia, and decreased glial-derived pro-inflammatory cytokines. Overexpression of VGluT2 by lentivirus facilitated morphine tolerance and mechanical hyperalgesia. In addition, we found the expression of BDNF is correlated with VGluT2 expression in the spinal cord after chronic morphine administration. Intrathecal injection of the BDNF/TrkB pathway antagonist K252a attenuated the development of morphine tolerance and decreased the expression of VGluT2 in the spinal cord, which suggested the BDNF/TrkB pathway participates in the regulation of VGluT2 in morphine tolerance. This study elucidates the functional capability of VGluT2 in modulating morphine tolerance and identifies a novel mechanism and promising therapeutic target for morphine tolerance.

Glutamate: A multifunctional amino acid in plants

Glutamate (Glu) is a versatile metabolite and a signaling molecule in plants. Glu biosynthesis is associated with the primary nitrogen assimilation pathway. The conversion between Glu and 2-oxoglutarate connects Glu metabolism to the tricarboxylic acid cycle, carbon metabolism, and energy production. Glu is the predominant amino donor for transamination reactions in the cell. In addition to protein synthesis, Glu is a building block for tetrapyrroles, glutathione, and folate. Glu is the precursor of γ-aminobutyric acid that plays an important role in balancing carbon/nitrogen metabolism and various cellular processes. Glu can conjugate to the major auxin indole 3-acetic acid (IAA), and IAA-Glu is destined for oxidative degradation. Glu also conjugates with isochorismate for the production of salicylic acid. Accumulating evidence indicates that Glu functions as a signaling molecule to regulate plant growth, development, and defense responses. The ligand-gated Glu receptor-like proteins (GLRs) mediate some of these responses. However, many of the Glu signaling events are GLR-independent. The receptor perceiving extracellular Glu as a danger signal is still unknown. In addition to GLRs, Glu may act on receptor-like kinases or receptor-like proteins to trigger immune responses. Glu metabolism and Glu signaling may entwine to regulate growth, development, and defense responses in plants.

Rice organs concentrate cadmium by chelation of amino acids containing dicarboxyl groups and enhance risks to human and environmental health in Cd-contaminated areas

When rice plants grown in paddy fields with Cd content of 0.3-1.5 mg kg^-1, Cd quantities in roots and straws were 2-7 times higher than that in topsoil. Return of these vegetative organs to topsoil aggravated the ecological risk of Cd pollution. Cd content in rice grains was 0.1-1.3 mg kg^-1, and hazard quotients for local consumers by intake of these rice were 0.7-8.8. Planting low-Cd-accumulating (LCA) cultivar reduced hazard quotients for consumers by intake of rice, but had similar ecological risks as high-accumulating (HCA) cultivars. LCA cultivar had lower Cd content in grains as well as higher efficiency of altering Cd into insoluble forms in flag leaves and upmost nodes than HCA cultivars. Insoluble Cd content in nodes was linearly increased with soil Cd content, companied by significant decline of 4 amino acids with dicarboxyl groups. Glu or Asp can form a cyclic complex with Cd by two O atoms from α-COO- and side chain-COO-. These results indicate that roots and straws have high potential to concentrate Cd by forming complexes between amino acids and Cd ions, and Cd-enriched straw return to topsoil may aggravate the ecological risk of Cd contamination.

GLUTAMATE RECEPTOR-like gene OsGLR3.4 is required for plant growth and systemic wound signaling in rice (Oryza sativa)

Recent studies have revealed the physiological roles of glutamate receptor-like channels (GLRs) in Arabidopsis; however, the functions of GLRs in rice remain largely unknown. Here, we show that knockout of OsGLR3.4 in rice leads to brassinosteroid (BR)-regulated growth defects and reduced BR sensitivity. Electrophoretic mobility shift assays and transient transactivation assays indicated that OsGLR3.4 is the downstream target of OsBZR1. Further, agonist profile assays showed that multiple amino acids can trigger transient Ca²⁺ influx in an OsGLR3.4-dependent manner, indicating that OsGLR3.4 is a Ca²⁺ -permeable channel. Meanwhile, the study of internode cells demonstrated that OsGLR3.4-mediated Ca²⁺ flux is required for actin filament organization and vesicle trafficking. Following root injury, the triggering of both slow wave potentials (SWPs) in leaves and the jasmonic acid (JA) response are impaired in osglr3.4 mutants, indicating that OsGLR3.4 is required for root-to-shoot systemic wound signaling in rice. Brassinosteroid treatment enhanced SWPs and OsJAZ8 expression in root-wounded plants, suggesting that BR signaling synergistically regulates the OsGLR3.4-mediated systemic wound response. In summary, this article describes a mechanism of OsGLR3.4-mediated cell elongation and long-distance systemic wound signaling in plants and provides new insights into the contribution of GLRs to plant growth and responses to mechanical wounding.

Enhanced Yield of Pepper Plants Promoted by Soil Application of Volatiles From Cell-Free Fungal Culture Filtrates Is Associated With Activation of the Beneficial Soil Microbiota

Plants communicate with microorganisms by exchanging chemical signals throughout the phytosphere. Such interactions are important not only for plant productivity and fitness, but also for terrestrial ecosystem functioning. It is known that beneficial microorganisms emit diffusible substances including volatile organic compounds (VOCs) that promote growth. Consistently, soil application of cell-free culture filtrates (CF) of beneficial soil and plant-associated microorganisms enhances plant growth and yield. However, how this treatment acts in plants and whether it alters the resident soil microbiota, are largely unknown. In this work we characterized the responses of pepper (Capsicum annuum L.) plants cultured under both greenhouse and open field conditions and of soil microbiota to soil application of CFs of beneficial and phytopathogenic fungi. To evaluate the contribution of VOCs occurring in the CFs to these responses, we characterized the responses of plants and of soil microbiota to application of distillates (DE) of the fungal CFs. CFs and their respective DEs contained the same potentially biogenic VOCs, and application of these extracts enhanced root growth and fruit yield, and altered the nutritional characteristics of fruits. High-throughput amplicon sequencing of bacterial 16S and fungal ITS rRNA genes of the soil microbiota revealed that the CF and DE treatments altered the microbial community compositions, and led to strong enrichment of the populations of the same beneficial bacterial and fungal taxa. Our findings show that CFs of both beneficial and phytopathogenic fungi can be used as biostimulants, and provide evidence that VOCs occurring in the fungal CFs act as mediators of the plants' responses to soil application of fungal CFs through stimulation of the beneficial soil microbiota.

Proline-Mediated Drought Tolerance in the Barley (Hordeum vulgare L.) Isogenic Line Is Associated with Lateral Root Growth at the Early Seedling Stage

A vigorous root system in barley promotes water uptake from the soil under water-limited conditions. We investigated three spring barley genotypes with varying water stress responses using rhizoboxes at the seedling stage. The genotypes comprised two elite German cultivars, Barke and Scarlett, and a near-isogenic line, NIL 143. The isogenic line harbors the wild allele pyrroline-5-carboxylate synthase1-P5cs1. Root growth in rhizoboxes under reduced water availability conditions caused a significant reduction in total root length, rooting depth, root maximum width, and root length density. On average, root growth was reduced by more than 20% due to water stress. Differences in organ proline concentrations were observed for all genotypes, with shoots grown under water stress exhibiting at least a 30% higher concentration than the roots. Drought induced higher leaf and root proline concentrations in NIL 143 compared with any of the other genotypes. Under reduced water availability conditions, NIL 143 showed less severe symptoms of drought, higher lateral root length, rooting depth, maximum root width, root length density, and convex hull area compared with Barke and Scarlett. Within the same comparison, under water stress, NIL 143 had a higher proportion of lateral roots (+30%), which were also placed at deeper substrate horizons. NIL 143 had a less negative plant water potential and higher relative leaf water content and stomatal conductance compared with the other genotypes under water stress. Under these conditions, this genotype also maintained an enhanced net photosynthetic rate and exhibited considerable fine root growth (diameter class 0.05–0.35 mm). These results show that water stress induces increased shoot and root proline accumulation in the NIL 143 barley genotype at the seedling stage and that this effect is associated with increased lateral root growth.

Metabolite profiles of energy cane and sugarcane reveal different strategies during the axillary bud outgrowth

Commercial cultivation of sugarcane is usually carried out by planting culm segments (sett) carrying buds in their internodes. However, this is an inefficient practice due to high sprouting irregularity. In this work, we inspect the first stages of the physiological preparation of the culm for sprouting, trying to identify compounds that actively participate in this process. We compared, during the first 48 h, the metabolic profile of sugarcane against energy cane, a cultivar known to have higher sprouting speed and consistency. In fact, during this short period it was possible to observe that energy cane already had a higher physiological activity than sugarcane, with significant changes in the catabolism of amino acids, increased levels of reducing sugars, lipids and metabolic activity in the phenylpropanoid pathway. On the other hand, sugarcane samples had just begun their activity during this same period, with an increase in the level of glutamate as the most significant change, which may be linked to the strategy of these cultivars to develop their roots before leaves, opposite of what is seen for energy cane. These results contribute to the development of strategies for increasing the efficiency of sprouting in sugarcane.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

Integrative Transcriptomic and Proteomic Analysis Reveals an Alternative Molecular Network of Glutamine Synthetase 2 Corresponding to Nitrogen Deficiency in Rice (Oryza sativa L.)

Nitrogen (N) is an essential nutrient for plant growth and development. The root system architecture is a highly regulated morphological system, which is sensitive to the availability of nutrients, such as N. Phenotypic characterization of roots from LY9348 (a rice variety with high nitrogen use efficiency (NUE)) treated with 0.725 mM NH₄NO₃ (1/4N) was remarkable, especially primary root (PR) elongation, which was the highest. A comprehensive analysis was performed for transcriptome and proteome profiling of LY9348 roots between 1/4N and 2.9 mM NH₄NO₃ (1N) treatments. The results indicated 3908 differential expression genes (DEGs; 2569 upregulated and 1339 downregulated) and 411 differential abundance proteins (DAPs; 192 upregulated and 219 downregulated). Among all DAPs in the proteome, glutamine synthetase (GS2), a chloroplastic ammonium assimilation protein, was the most upregulated protein identified. The unexpected concentration of GS2 from the shoot to the root in the 1/4N treatment indicated that the presence of an alternative pathway of N assimilation regulated by GS2 in LY9348 corresponded to the low N signal, which was supported by GS enzyme activity and glutamine/glutamate (Gln/Glu) contents analysis. In addition, N transporters (NRT2.1, NRT2.2, NRT2.3, NRT2.4, NAR2.1, AMT1.3, AMT1.2, and putative AMT3.3) and N assimilators (NR2, GS1;1, GS1;2, GS1;3, NADH-GOGAT2, and AS2) were significantly induced during the long-term N-deficiency response at the transcription level (14 days). Moreover, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis demonstrated that phenylpropanoid biosynthesis and glutathione metabolism were significantly modulated by N deficiency. Notably, many transcription factors and plant hormones were found to participate in root morphological adaptation. In conclusion, our study provides valuable information to further understand the response of rice roots to N-deficiency stress.

Characterization of physiological and molecular responses of Zea mays seedlings to different urea-ammonium ratios

Despite the wide use of urea and ammonium as N-fertilizers, no information is available about the proper ratio useful to maximize the efficiency of their acquisition by crops. Ionomic analyses of maize seedlings fed with five different mixes of urea and ammonium indicated that after 7 days of treatment, the elemental composition of plant tissues was more influenced by ammonium in the nutrient solution than by urea. Within 24 h, similar high affinity influx rates of ammonium were measured in ammonium-treated seedlings, independently from the amount of the cation present in the nutrient solution (from 0.5 to 2.0 mM N), and it was confirmed by the similar accumulation of ¹⁵N derived from ammonium source. After 7 days, some changes in ammonium acquisition occurred among treatments, with the highest ammonium uptake efficiency when the urea-to-ammonium ratio was 3:1. Gene expression analyses of enzymes and transporters involved in N nutrition highlight a preferential induction of the cytosolic N-assimilatory pathway (via GS, ASNS) when both urea and ammonium were supplied in conjunction, this response might explain the higher N-acquisition efficiency when both sources are applied. In conclusion, this study provides new insights on plant responses to mixes of N sources that maximize the N-uptake efficiency by crops and thus could allow to adapt agronomic practices in order to limit the economic and environmental impact of N-fertilization.

Nitrogen Uptake in Plants: The Plasma Membrane Root Transport Systems from a Physiological and Proteomic Perspective

Nitrogen nutrition in plants is a key determinant in crop productivity. The availability of nitrogen nutrients in the soil, both inorganic (nitrate and ammonium) and organic (urea and free amino acids), highly differs and influences plant physiology, growth, metabolism, and root morphology. Deciphering this multifaceted scenario is mandatory to improve the agricultural sustainability. In root cells, specific proteins located at the plasma membrane play key roles in the transport and sensing of nitrogen forms. This review outlines the current knowledge regarding the biochemical and physiological aspects behind the uptake of the individual nitrogen forms, their reciprocal interactions, the influences on root system architecture, and the relations with other proteins sustaining fundamental plasma membrane functionalities, such as aquaporins and H⁺-ATPase. This topic is explored starting from the information achieved in the model plant Arabidopsis and moving to crops in agricultural soils. Moreover, the main contributions provided by proteomics are described in order to highlight the goals and pitfalls of this approach and to get new hints for future studies.

Association mapping reveals novel genomic regions controlling some root and stolon traits in tetraploid potato (Solanum tuberosum L.)

Tuber crops have measurable biological variation in root and stolon phenotyping and thus may be utilized to identify genomic regions associated with these variations. This is the first comprehensive association mapping study related to potato root and stolon traits. A diverse panel of 192 tetraploid potato (Solanum tuberosum L.) genotypes were grown in aeroponics to reveal a biologically significant variation and detection of genomic regions associated with the root and stolon traits. Phenotyping of root traits was performed by image analysis software "WinRHIZO" (a root scanning method), and stolon traits was measured manually, while SolCAP 25K potato array was used for genotyping. Significant variation was observed between the potato genotypes for root and stolon traits along with high heritabilities (0.80 in TNS to 0.95 in SL). For marker-trait associations, Q + K linear mixed model was implemented and 50 novel genomic regions were detected. Significantly associated SNPs with stolon traits were located on chr 4, chr 6, chr 7, chr 9, chr 11 and chr 12, while those linked to root traits on chr 1, chr 2, chr 3, chr 9, chr 11, and chr 12. Structure and PCA analysis grouped genotypes into four sub-populations disclosing population genetic diversity. LD decay was observed at 2.316 Mbps (r ² = 0.29) in the population. The identified SNPs were associated with genes performing vital functions such as root signaling and signal transduction in stress environments (GT-2 factors, protein kinases SAPK2-like and protein phosphatases "StPP1"), transcriptional and post-transcriptional gene regulation (RNA-binding proteins), sucrose synthesis and transporter families (UGPase, Sus3, SuSy, and StSUT1) and PVY resistance (Ry sto). The findings of our study can be employed in future breeding programs for improvement in potato production.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02727-6.

Absence of 4-Formylaminooxyvinylglycine Production by Pseudomonas fluorescens WH6 Results in Resource Reallocation from Secondary Metabolite Production to Rhizocompetence

Pseudomonas fluorescens WH6 produces the non-proteinogenic amino acid 4-formylaminooxyvinylglycine (FVG), a secondary metabolite with antibacterial and pre-emergent herbicidal activities. The gvg operon necessary for FVG production encodes eight required genes: one regulatory (gvgR), two of unknown functional potential (gvgA and C), three with putative biosynthetic function (gvgF, H, and I), and two small ORFs (gvgB and G). To gain insight into the role of GvgA and C in FVG production, we compared the transcriptome of knockout (KO) mutants of gvgR, A, and C to wild type (WT) to test two hypotheses: (1) GvgA and GvgC play a regulatory role in FVG production and (2) non-gvg cluster genes are regulated by GvgA and GvgC. Our analyses show that, collectively, 687 genes, including the gvg operon, are differentially expressed in all KO strains versus WT, representing >10% of the genome. Fifty-one percent of these genes were similarly regulated in all KO strains with GvgC having the greatest number of uniquely regulated genes. Additional transcriptome data suggest cluster regulation through feedback of a cluster product. We also discovered that FVG biosynthesis is regulated by L-glu, L-asp, L-gln, and L-asn and that resources are reallocated in KO strains to increase phenotypes involved in rhizocompetence including motility, biofilm formation, and denitrification. Altogether, differential transcriptome analyses of mutants suggest that regulation of the cluster is multifaceted and the absence of FVG production or its downregulation can dramatically shift the lifestyle of WH6.

Recent Advances in Carbon and Nitrogen Metabolism in C3 Plants

C and N are the most important essential elements constituting organic compounds in plants. The shoots and roots depend on each other by exchanging C and N through the xylem and phloem transport systems. Complex mechanisms regulate C and N metabolism to optimize plant growth, agricultural crop production, and maintenance of the agroecosystem. In this paper, we cover the recent advances in understanding C and N metabolism, regulation, and transport in plants, as well as their underlying molecular mechanisms. Special emphasis is given to the mechanisms of starch metabolism in plastids and the changes in responses to environmental stress that were previously overlooked, since these changes provide an essential store of C that fuels plant metabolism and growth. We present general insights into the system biology approaches that have expanded our understanding of core biological questions related to C and N metabolism. Finally, this review synthesizes recent advances in our understanding of the trade-off concept that links C and N status to the plant's response to microorganisms.

Fate of nitrogen in agriculture and environment: agronomic, eco-physiological and molecular approaches to improve nitrogen use efficiency

Nitrogen is the main limiting nutrient after carbon, hydrogen and oxygen for photosynthetic process, phyto-hormonal, proteomic changes and growth-development of plants to complete its lifecycle. Excessive and inefficient use of N fertilizer results in enhanced crop production costs and atmospheric pollution. Atmospheric nitrogen (71%) in the molecular form is not available for the plants. For world's sustainable food production and atmospheric benefits, there is an urgent need to up-grade nitrogen use efficiency in agricultural farming system. The nitrogen use efficiency is the product of nitrogen uptake efficiency and nitrogen utilization efficiency, it varies from 30.2 to 53.2%. Nitrogen losses are too high, due to excess amount, low plant population, poor application methods etc., which can go up to 70% of total available nitrogen. These losses can be minimized up to 15-30% by adopting improved agronomic approaches such as optimal dosage of nitrogen, application of N by using canopy sensors, maintaining plant population, drip fertigation and legume based intercropping. A few transgenic studies have shown improvement in nitrogen uptake and even increase in biomass. Nitrate reductase, nitrite reductase, glutamine synthetase, glutamine oxoglutarate aminotransferase and asparagine synthetase enzyme have a great role in nitrogen metabolism. However, further studies on carbon-nitrogen metabolism and molecular changes at omic levels are required by using "whole genome sequencing technology" to improve nitrogen use efficiency. This review focus on nitrogen use efficiency that is the major concern of modern days to save economic resources without sacrificing farm yield as well as safety of global environment, i.e. greenhouse gas emissions, ammonium volatilization and nitrate leaching.

Ralstonia solanacearum promotes pathogenicity by utilizing l-glutamic acid from host plants

Ralstonia solanacearum is an important bacterial pathogen that can infect a broad range of plants worldwide. A previous study showed that R. solanacearum could respond to exogenous organic acids or amino acids to modulate cell motility. However, it was unclear whether R. solanacearum uses these compounds to control infection. In this study, we found that R. solanacearum GMI1000 uses host plant metabolites to enhance the biosynthesis of virulence factors. We demonstrated that l-glutamic acid from host plants is the key active component associated with increased extracellular polysaccharide production, cellulase activity, swimming motility, and biofilm formation in R. solanacearum GMI1000. In addition, l-glutamic acid also promoted colonization of R. solanacearum cells in the roots and stems of tomato plants and accelerated disease incidence. Furthermore, genetic screening and biochemical analysis suggested that RS01577, a hybrid sensor histidine kinase/response regulator, is involved in l-glutamic acid signalling in R. solanacearum. Mutations in RS01577 and exogenous addition of l-glutamic acid to the GMI1000 wild-type strain had overlapping effects on both the transcriptome and biological functions of R. solanacearum, including on motility, biofilm formation, and virulence. Thus, our results have established a new interaction mechanism between R. solanacearum and host plants that highlights the complexity of the virulence regulation mechanism and may provide new insight into disease control.

Metabolomic Analyses Provide New Insights into Signaling Mechanisms for Nutrient Uptake by Lateral Roots of Pruned Tea Plant (Camellia sinensis)

Pruning is an important plant management practice in tea cultivation. However, the mechanism underlying the dynamics of nutrient uptake by roots of pruned tea is unknown. This study investigated the metabolic alterations in lateral roots of pruned tea to unveil the mechanism of nutrient uptake. Elemental analysis revealed that pruning significantly increases the uptake of nutrients by lateral roots. Metabolic profiling showed significant metabolic variations in lateral roots of pruned tea. Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis revealed that flavonoid biosynthesis, phenylpropanoid biosynthesis, and amino acid metabolism were differentially regulated in lateral roots. Caffeine metabolism was significantly hindered, while ethylene signaling was significantly induced in lateral roots of pruned plants. In addition, intermediates in the tricarboxylic acid (TCA) cycle were upregulated, indicating high rates of the TCA cycle. Therefore, pathways related to phenylpropanoid biosynthesis, TCA cycle, ethylene biosynthesis, and metabolism of amino acids contribute to higher nutrient uptake by lateral roots of the tea plant.

Phytoremediation of Cadmium: Physiological, Biochemical, and Molecular Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment, and has noxious effects on plant growth and production. Cd-accumulating plants showed reduced growth and productivity. Therefore, remediation of this non-essential and toxic pollutant is a prerequisite. Plant-based phytoremediation methodology is considered as one a secure, environmentally friendly, and cost-effective approach for toxic metal remediation. Phytoremediating plants transport and accumulate Cd inside their roots, shoots, leaves, and vacuoles. Phytoremediation of Cd-contaminated sites through hyperaccumulator plants proves a ground-breaking and profitable choice to combat the contaminants. Moreover, the efficiency of Cd phytoremediation and Cd bioavailability can be improved by using plant growth-promoting bacteria (PGPB). Emerging modern molecular technologies have augmented our insight into the metabolic processes involved in Cd tolerance in regular cultivated crops and hyperaccumulator plants. Plants’ development via genetic engineering tools, like enhanced metal uptake, metal transport, Cd accumulation, and the overall Cd tolerance, unlocks new directions for phytoremediation. In this review, we outline the physiological, biochemical, and molecular mechanisms involved in Cd phytoremediation. Further, a focus on the potential of omics and genetic engineering strategies has been documented for the efficient remediation of a Cd-contaminated environment.

Nitrate Signaling, Functions, and Regulation of Root System Architecture: Insights from Arabidopsis thaliana

Root system architecture (RSA) is required for the acquisition of water and mineral nutrients from the soil. One of the essential nutrients, nitrate (NO₃⁻), is sensed and transported by nitrate transporters NRT1.1 and NRT2.1 in the plants. Nitrate transporter 1.1 (NRT1.1) is a dual-affinity nitrate transporter phosphorylated at the T101 residue by calcineurin B-like interacting protein kinase (CIPKs); it also regulates the expression of other key nitrate assimilatory genes. The differential phosphorylation (phosphorylation and dephosphorylation) strategies and underlying Ca²⁺ signaling mechanism of NRT1.1 stimulate lateral root growth by activating the auxin transport activity and Ca²⁺-ANR1 signaling at the plasma membrane and the endosomes, respectively. NO₃⁻ additionally functions as a signal molecule that forms a signaling system, which consists of a vast array of transcription factors that control root system architecture that either stimulate or inhibit lateral and primary root development in response to localized and high nitrate (NO₃⁻), respectively. This review elucidates the so-far identified nitrate transporters, nitrate sensing, signal transduction, and the key roles of nitrate transporters and its downstream transcriptional regulatory network in the primary and lateral root development in Arabidopsis thaliana under stress conditions.

Transcriptional regulation of amino acid metabolism in response to nitrogen deficiency and nitrogen forms in tea plant root (Camellia sinensis L.)

Free amino acids, including theanine, glutamine and glutamate, contribute greatly to the pleasant taste and multiple health benefits of tea. Amino acids in tea plants are mainly synthesized in roots and transported to new shoots, which are significantly affected by nitrogen (N) level and forms. However, the regulatory amino acid metabolism genes have not been systemically identified in tea plants. Here, we investigated the dynamic changes of free amino acid contents in response to N deficiency and forms in tea plant roots, and systemically identified the genes associated amino acid contents in individual metabolism pathways. Our results showed that glutamate-derived amino acids are the most dynamic in response to various forms of N and N deficiency. We then performed transcriptomic analyses of roots treated with N deficiency and various forms of N, and differentially expressed amino acid metabolic genes in each pathway were identified. The analyses on expression patterns and transcriptional responses of metabolic genes to N treatments provided novel insights for the molecular basis of high accumulation of theanine in tea plant root. These analyses also identified potential regulatory genes in dynamic amino acid metabolism in tea plant root. Furthermore, our findings indicated that the dynamic expression levels of CsGDH, CsAlaDC, CsAspAT, CsSDH, CsPAL, CsSHMT were highly correlated with changes of amino acid contents in their corresponding pathways. Herein, this study provides comprehensive insights into transcriptional regulation of amino acid metabolism in response to nitrogen deficiency and nitrogen forms in tea plant root.

Interplay between 1-aminocyclopropane-1-carboxylic acid, γ-aminobutyrate and D-glucose in the regulation of high nitrate-induced root growth inhibition in maize

Nitrogen is one of the main factors that affect plant growth and development. However, high nitrogen concentrations can inhibit both shoot and root growth, even though the processes involved in this inhibition are still unknown. The aim of this work was to identify the metabolic alterations that induce the inhibition of root growth caused by high nitrate supply, when the whole plant growth is also reduced. High nitrate altered nitrogen and carbon metabolism, reducing the content of sugars and inducing the accumulation of Ca²⁺ and amino acids, such as glutamate, alanine and γ-aminobutyrate (GABA), that could act to replenish the succinate pool in the tricarboxylic acid cycle and maintain its activity. Other metabolic alterations found were the accumulation of the polyamines spermidine and spermine, and the reduction of jasmonic acid (JA) and the ethylene precursor aminocyclopropane-1-carboxylic acid (ACC). These results indicate that the growth root inhibition by high NO₃^- is a complex metabolic response that involves GABA as a key link between C and N metabolism which, together with plant growth regulators such as auxins, cytokinins, abscisic acid, JA, and the ethylene precursor ACC, is able to regulate the metabolic response of root grown under high nitrate concentrations.

Glutamate alleviates cadmium toxicity in rice via suppressing cadmium uptake and translocation

Cadmium (Cd), a naturally occurring heavy metal, is toxic to animals and plants. Minimization of Cd in rice grain is important to human health since rice is the main source of Cd intake for human populations feeding on it as staple food. Glutamate (Glu) is reportedly involved in plant abiotic stress responses, whereas the underlying molecular mechanism remains poorly understood. In this study, we showed that supplement of Glu, but not glutamine, significantly alleviated Cd toxicity in hydroponically grown rice plants. Cd accumulation was reduced by 44.1% and 65.6% in root and shoot of rice plants respectively, after Glu supplementation (3 mM). Glu supplement restored chlorophyll biosynthesis and significantly ameliorated Cd-induced oxidative stress with reduced levels of H₂O₂, ¹O₂, MDA, and increased activities of major anti-oxidant enzymes, catalase, peroxidase and glutathione S-transferase. Levels of stress-associated free amino acids proline, arginine and γ-aminobutyric acid were also reduced after Glu supplement. We further demonstrated that Glu supplement suppressed the Cd-induced expression of metal transporter genes OsNramp1, OsNramp5, OsIRT1, OsIRT2, OsHMA2 and OsHMA3 in roots of Cd-treated plants. Taken together, our results suggest that Glu supplement could alleviate Cd toxicity in rice by suppressing Cd uptake and translocation.

Loss of function of Arabidopsis NADP-malic enzyme 1 results in enhanced tolerance to aluminum stress

In acidic soils, aluminum (Al) toxicity is a significant limitation to crop production worldwide. Given its Al-binding capacity, malate allows internal as well as external detoxification strategies to cope with Al stress, but little is known about the metabolic processes involved in this response. Here, we analyzed the relevance of NADP-dependent malic enzyme (NADP-ME), which catalyzes the oxidative decarboxylation of malate, in Al tolerance. Plants lacking NADP-ME1 (nadp-me1) display reduced inhibition of root elongation along Al treatment compared with the wild type (wt). Moreover, wt roots exposed to Al show a drastic decrease in NADP-ME1 transcript levels. Although malate levels in seedlings and root exudates are similar in nadp-me1 and wt, a significant increase in intracellular malate is observed in roots of nadp-me1 after long exposure to Al. The nadp-me1 plants also show a lower H₂ O₂ content in root apices treated with Al and no inhibition of root elongation when exposed to glutamate, an amino acid implicated in Al signaling. Proteomic studies showed several differentially expressed proteins involved in signal transduction, primary metabolism and protection against biotic and other abiotic stimuli and redox processes in nadp-me1, which may participate directly or indirectly in Al tolerance. The results indicate that NADP-ME1 is involved in adjusting the malate levels in the root apex, and its loss results in an increased content of this organic acid. Furthermore, the results suggest that NADP-ME1 affects signaling processes, such as the generation of reactive oxygen species and those that involve glutamate, which could lead to inhibition of root growth.

Signaling Role of Glutamate in Plants

It is well known that glutamate (Glu), a neurotransmitter in human body, is a protein amino acid. It plays a very important role in plant growth and development. Nowadays, Glu has been found to emerge as signaling role. Under normal conditions, Glu takes part in seed germination, root architecture, pollen germination, and pollen tube growth. Under stress conditions, Glu participates in wound response, pathogen resistance, response and adaptation to abiotic stress (such as salt, cold, heat, and drought), and local stimulation (abiotic or biotic stress)-triggered long distance signaling transduction. In this review, in the light of the current opinion on Glu signaling in plants, the following knowledge was updated and discussed. 1) Glu metabolism; 2) signaling role of Glu in plant growth, development, and response and adaptation to environmental stress; as well as 3) the underlying research directions in the future. The purpose of this review was to look forward to inspiring the rapid development of Glu signaling research in plant biology, particularly in the field of stress biology of plants.

Nitrate and Ammonium Affect the Overall Maize Response to Nitrogen Availability by Triggering Specific and Common Transcriptional Signatures in Roots

Nitrogen (N) is an essential macronutrient for crops. Plants have developed several responses to N fluctuations, thus optimizing the root architecture in response to N availability. Nitrate and ammonium are the main inorganic N forms taken up by plants, and act as both nutrients and signals, affecting gene expression and plant development. In this study, RNA-sequencing was applied to gain comprehensive information on the pathways underlying the response of maize root, pre-treated in an N-deprived solution, to the provision of nitrate or ammonium. The analysis of the transcriptome shows that nitrate and ammonium regulate overlapping and distinct pathways, thus leading to different responses. Ammonium activates the response to stress, while nitrate acts as a negative regulator of transmembrane transport. Both the N-source repress genes related to the cytoskeleton and reactive oxygen species detoxification. Moreover, the presence of ammonium induces the accumulation of anthocyanins, while also reducing biomass and chlorophyll and flavonoids accumulation. Furthermore, the later physiological effects of these nutrients were evaluated through the assessment of shoot and root growth, leaf pigment content and the amino acid concentrations in root and shoot, confirming the existence of common and distinct features in response to the two nitrogen forms.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

Glutamate signaling enhances the heat tolerance of maize seedlings by plant glutamate receptor-like channels-mediated calcium signaling

Glutamate (Glu), a neurotransmitter in animal, is a novel signaling molecule in plants, which takes part in cellular metabolism, seed germination, plant growth, development, and long-distance information transfer. However, whether Glu can enhance the heat tolerance in maize seedlings and its relation to calcium signaling is still elusive. In this study, maize seedlings were pretreated with Glu and then exposed to heat stress. The results showed that Glu pretreatment enhanced the survival percentage of maize seedlings under heat tolerance, indicating that Glu could increase the heat tolerance of maize seedlings. The Glu-induced heat tolerance was weakened by exogenous calcium chloride, plasma membrane Ca²⁺ channel blocker (LaCl₃), Ca²⁺ chelator (ethylene glycol-bis(b-aminoethylether)-N,N, N΄,N΄-tetraacetic acid), calmodulin antagonists (trifluoperazine and chlopromazine), and plant glutamate receptor-like antagonists (MgCl₂ and 6,7-dinitroquinoxaline- 2,3-(1H,4H)- dione). These findings for the first time reported that Glu could increase the heat tolerance of maize seedlings by plant glutamate receptor-like channels-mediated calcium signaling.

Involvement of Medicago truncatula glutamate receptor-like channels in nitric oxide production under short-term water deficit stress

Early stages of plant development are highly susceptible to environmental cues, and seedlings have to develop sophisticated mechanisms to sense and respond to abiotic stresses. We have previously identified that abscisic acid (ABA), nitric oxide (NO) and modulation of nitrogen metabolism are involved in adaptive responses in Medicago truncatula seedlings under water deficit stress. Here, we investigated whether glutamate receptor-like channels (GLRs) played a role in the developmental physiological processes of Medicago seedlings during post-germination after a short-term water deficit stress. Twenty-nine independent MtGLR genes have been identified and then divided into four clades following a phylogenetic analysis; seventeen of them exhibited specific domains which are characteristic of animal ionotropic glutamate receptors. Under drought stress, ABA-induced NO accumulation was significantly reduced in presence of a GLR competitive antagonist, suggesting that this water deficit-induced endogenous NO production was mediated through a MtGLR-dependent pathway. Water deficit-induced inhibition of embryo axis elongation was strongly reduced whereas loss of water content was alleviated when MtGLRs were inhibited. These results suggest that glutamate receptors-like channels are required, through their involvement in NO production, in adaptive responses under short-term water-deficit stress during Medicago seedling establishment.

Exome sequencing highlights the role of wild-relative introgression in shaping the adaptive landscape of the wheat genome

Introgression is a potential source of beneficial genetic diversity. The contribution of introgression to adaptive evolution and improvement of wheat as it was disseminated worldwide remains unknown. We used targeted re-sequencing of 890 diverse accessions of hexaploid and tetraploid wheat to identify wild-relative introgression. Introgression, and selection for improvement and environmental adaptation, each reduced deleterious allele burden. Introgression increased diversity genome wide and in regions harboring major agronomic genes, and contributed alleles explaining a substantial proportion of phenotypic variation. These results suggest that historic gene flow from wild relatives made a substantial contribution to the adaptive diversity of modern bread wheat.

Exogenous glycine inhibits root elongation and reduces nitrate-N uptake in pak choi (Brassica campestris ssp. Chinensis L.)

Nitrogen (N) supply, including NO3--N and organic N in the form of amino acids can influence the morphological attributes of plants. For example, amino acids contribute to plant nutrition; however, the effects of exogenous amino acids on NO3--N uptake and root morphology have received little attention. In this study, we evaluated the effects of exogenous glycine (Gly) on root growth and NO3--N uptake in pak choi (Brassica campestris ssp. Chinensis L.). Addition of Gly to NO3--N agar medium or hydroponic solution significantly decreased pak choi seedling root length; these effects of Gly on root morphology were not attributed to the proportion of N supply derived from Gly. When pak choi seedlings were exposed to mixtures of Gly and NO3--N in hydroponic culture, Gly significantly reduced 15NO3--N uptake but significantly increased the number of root tips per unit root length, root activity and 15NO3--N uptake rate per unit root length. In addition, 15N-Gly was taken up into the plants. In contrast to absorbed NO3--N, which was mostly transported to the shoots, a larger proportion of absorbed Gly was retained in the roots. Exogenous Gly enhanced root 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and oxidase (ACO) activities and ethylene production. The ethylene antagonists aminoethoxyvinylglycine (0.5 μM AVG) and silver nitrate (10 μM AgNO3) partly reversed Gly-induced inhibition of primary root elongation on agar plates and increased the NO3--N uptake rate under hydroponic conditions, indicating exogenous Gly exerts these effects at least partly by enhancing ethylene production in roots. These findings suggest Gly substantially affects root morphology and N uptake and provide new information on the specific responses elicited by organic N sources.

Seed and Foliar Application of Amino Acids Improve Variables of Nitrogen Metabolism and Productivity in Soybean Crop

The application of amino acids in crops has been a common practice in recent years, although most of the time they are associated with products based on algae extracts or on fermented animal or vegetable wastes. However, little is known about the isolated effect of amino acids on the development of crops. Therefore, the objective of this research was to evaluate the effect of the application of isolated amino acids on the in some steps of the soybean nitrogen metabolism and on productivity. Experiments were carried out in a greenhouse and in the field with the application of the amino acids glutamate (Glu), phenylalanine (Phe), cysteine (Cys) and glycine (Gly) and as a set (Glu+Phe+Cys+Gly), as seed treatment (ST), as foliar application (FA) and both (ST+FA), at the V4 growth stage. Evaluations consisted of nitrate reductase and urease activities, nitrate, ureide, total amino acids and total nitrogen content in leaves, and productivity. The application of Glu to leaves, Cys as ST and a mixture of Glu+Cys+Phe+Gly as ST+FA in the greenhouse experiment increased the total amino acids content. In the field experiment all treatments increased the amino acid content in leaves. At the V6 stage in the field experiment, all modes of Gly application, Glu as ST and FA, Cys and Phe as ST+FA and Glu+Cys+Phe+Gly as FA increased the nitrate content in leaves. In the greenhouse, application of Cys and Phe as ST increased the production of soybean plants by at least 21%. The isolated application of Cys, Phe, Gly, Glu and the set of these amino acids as ST increased the productivity of soybean plants in the field experiment by at least 22%.

Mitogen activated protein kinase 6 and MAP kinase phosphatase 1 are involved in the response of Arabidopsis roots to L-glutamate

The function and components of L-glutamate signaling pathways in plants have just begun to be elucidated. Here, using a combination of genetic and biochemical strategies, we demonstrated that a MAPK module is involved in the control of root developmental responses to this amino acid. Root system architecture plays an essential role in plant adaptation to biotic and abiotic factors via adjusting signal transduction and gene expression. L-Glutamate (L-Glu), an amino acid with neurotransmitter functions in animals, inhibits root growth, but the underlying genetic mechanisms are poorly understood. Through a combination of genetic analysis, in-gel kinase assays, detailed cell elongation and division measurements and confocal analysis of expression of auxin, quiescent center and stem cell niche related genes, the critical roles of L-Glu in primary root growth acting through the mitogen-activated protein kinase 6 (MPK6) and the dual specificity serine-threonine-tyrosine phosphatase MKP1 could be revealed. In-gel phosphorylation assays revealed a rapid and dose-dependent induction of MPK6 and MPK3 activities in wild-type Arabidopsis seedlings in response to L-Glu. Mutations in MPK6 or MKP1 reduced or increased root cell division and elongation in response to L-Glu, possibly modulating auxin transport and/or response, but in a PLETHORA1 and 2 independent manner. Our data highlight MPK6 and MKP1 as components of an L-Glu pathway linking the auxin response, and cell division for primary root growth.

Mechanisms of cytosolic calcium elevation in plants: the role of ion channels, calcium extrusion systems and NADPH oxidase-mediated 'ROS-Ca2+ Hub'

Elevation in the cytosolic free calcium is crucial for plant growth, development and adaptation. Calcium influx into plant cells is mediated by Ca2+ depolarisation-activated, hyperpolarisation-activated and voltage-independent Ca2+-permeable channels (DACCs, HACCs and VICCs respectively). These channels are encoded by the following gene families: (1) cyclic nucleotide-gated channels (CNGCs), (2) ionotropic glutamate receptors (GLRs), (3) annexins, (4) 'mechanosensitive channels of small (MscS) conductance'-like channels (MSLs), (5) 'mid1-complementing activity' channels (MCAs), Piezo channels, and hyperosmolality-induced [Ca2+]cyt. channel 1 (OSCA1). Also, a 'tandem-pore channel1' (TPC1) catalyses Ca2+ efflux from the vacuole in response to the plasma membrane-mediated Ca2+ elevation. Recent experimental data demonstrated that Arabidopsis thaliana (L.) Heynh. CNGCs 2, 5-10, 14, 16 and 18, GLRs 1.2, 3.3, 3.4, 3.6 and 3.7, TPC1, ANNEXIN1, MSL9 and MSL10,MCA1 and MCA2, OSCA1, and some their homologues counterparts in other species, are responsible for Ca2+ currents and/or cytosolic Ca2+ elevation. Extrusion of Ca2+ from the cytosol is mediated by Ca2+-ATPases and Ca2+/H+ exchangers which were recently examined at the level of high resolution crystal structure. Calcium-activated NADPH oxidases and reactive oxygen species (ROS)-activated Ca2+ conductances form a self-amplifying 'ROS-Ca2+hub', enhancing and transducing Ca2+ and redox signals. The ROS-Ca2+ hub contributes to physiological reactions controlled by ROS and Ca2+, demonstrating synergism and unity of Ca2+ and ROS signalling mechanisms.

Exogenous trehalose improves growth under limiting nitrogen through upregulation of nitrogen metabolism

BACKGROUND

The trehalose (Tre) pathway has strong effects on growth and development in plants through regulation of carbon metabolism. Altering either Tre or trehalose 6-phosphate (T6P) can improve growth and productivity of plants as observed under different water availability. As yet, there are no reports of the effects of modification of Tre orT6P on plant performance under limiting nutrition.

RESULTS

Here we report that nitrogen (N) metabolism is positively affected by exogenous application of Tre in nitrogen-deficient growing conditions. Spraying foliage of tobacco (Nicotiana tabacum) with trehalose partially alleviated symptoms of nitrogen deficiency through upregulation of nitrate and ammonia assimilation and increasing activities of nitrate reductase (NR), glycolate oxidase (GO), glutamine synthetase (GS) and glutamine oxoglutarate aminotransferase (GOGAT) with concomitant changes in ammonium (NH4+) and nitrate (NO3-) concentrations, glutamine and amino acids. Chlorophyll and total nitrogen content of leaves and rates of photosynthesis were increased compared to nitrogen-deficient plants without applied Tre. Total plant biomass accumulation was also higher in Tre -fed nitrogen-deficient plants, with a smaller proportion of dry weight partitioned to roots, compared to nitrogen-deficient plants without applied Tre. Consistent with higher nitrogen assimilation and growth, Tre application reduced foliar starch. Minimal effects of Tre feeding were observed on nitrogen-sufficient plants.

CONCLUSIONS

The data show, for the first time, significant stimulatory effects of exogenous Tre on nitrogen metabolism and growth in plants growing under deficient nitrogen. Under such adverse conditions metabolism is regulated for survival rather than productivity. Application of Tre can alter this regulation towards maintenance of productive functions under low nitrogen. This has implications for considering approaches to modifying the Tre pathway for to improve crop nitrogen-use efficiency and production.

The imbalance between C and N metabolism during high nitrate supply inhibits photosynthesis and overall growth in maize (Zea mays L.)

Nitrogen (N) is an important regulator of photosynthetic carbon (C) flow in plants, and an adequate balance between N and C metabolism is needed for correct plant development. However, an excessive N supply can alter this balance and cause changes in specific organic compounds associated with primary and secondary metabolism, including plant growth regulators. In previous work, we observed that high nitrate supply (15 mM) to maize plants led to a decrease in leaf expansion and overall biomass production, when compared with low nitrate supply (5 mM). Thus, the aim of this work is to study how overdoses of nitrate can affect photosynthesis and plant development. The results show that high nitrate doses greatly increased amino acid production, which led to a decrease in the concentration of 2-oxoglutarate, the main source of C skeletons for N assimilation. The concentration of 1-aminocyclopropane-1-carboxylic acid (and possibly its product, ethylene) also rose in high nitrate plants, leading to a decrease in leaf expansion, reducing the demand for photoassimilates by the growing tissues and causing the accumulation of sugars in source leaves. This accumulation of sugars, together with the decrease in 2-oxoglutarate levels and the reduction in chlorophyll concentration, decreased plant photosynthetic rates. This work provides new insights into how high nitrate concentration alters the balance between C and N metabolism, reducing photosynthetic rates and disrupting whole plant development. These findings are particularly relevant since negative effects of nitrate in contexts other than root growth have rarely been studied.