Abiotic Stresses Downregulate Key Genes Involved in Nitrogen Uptake and Assimilation in Brassica juncea L

Impact of Water Stress on Metabolic Intermediates and Regulators in Broccoli Sprouts, and Cellular Defense Potential of Their Extracts

Drought and flood (water stress) alter plant metabolism, impacting the phytochemical content and biological effects. Using spectrophotometric, HPLC, and electrophoretic methods, we analyze the effects of water stress on broccoli (Brassica oleracea L. convar. botrytis (L.) Alef. var. cymosa Duch.) sprouts. Drought and flood differently influenced chlorophylls, carotenoids, and porphyrins, with drought having a stronger inhibitory effect on chlorophyll a, total chlorophyll, and porphyrins. Carotenoids and glucosinolates increased under drought but decreased with flooding, suggesting that these compounds play a crucial role in drought tolerance. Nitrate increased with drought from 13.11 ± 1.05 mg/g dw to 22.41 ± 1.20 mg/g dw but decreased under flooding to 5.17 ± 1.03 mg/g dw, and oxalic acid was reduced by drought only (from 48.94 ± 1.30 mg/g dw to 46.43 ± 0.64 mg/g dw). Flood reduced proteins by 29%, phenolics by 15%, flavonoids by 10%, flavonols by 11%, tannins by 36%, and proanthocyanidins by 19%, while drought decreased flavonoids by 23%. Total phenolics and proanthocyanidins were increased by drought by 29% and 7%, respectively, while flooding decreased hydroxycinnamic acids by 13%. Both stress types influenced individual polyphenols differently: drought diminished ferulic acid by 17% and increased sinapic acid by 30%, while flooding reversed these effects and enhanced kaempferol by 22%. These compounds, along with proline (which increased by 139% under drought), emerged as biomarkers of water stress. Flood impacted antioxidant capacity more significantly, while drought-stressed broccoli extracts better protected plasmid DNA against oxidative damage. These findings underline the metabolic plasticity of broccoli sprouts and their potential in targeted crop management for water stress resilience.

Cenococcum geophilum impedes cadmium toxicity in Pinus massoniana by modulating nitrogen metabolism

Nitrogen (N) is of great significance to the absorption, distribution and detoxification of cadmium (Cd). Ectomycorrhizal fungi (EMF) are able to affect the key processes of plant N uptake to resist Cd stress, while the mechanism is still unclear. Therefore, we explored potential strategies of Cenococcum geophilum (C. geophilum) symbiosis to alleviate Cd stress in Pinus massoniana (P. massoniana) from the perspective of plant N metabolism and soil N transformation. The results showed that inoculation of C. geophilum significantly increased the activities of NR, NiR and GS in the shoots and roots of P. massoniana, thereby promoting the assimilation of NO3- and NH4+ into amino acids. Moreover, C. geophilum promoted soil urease and protease activities, but decreased soil NH4+ content, indicating that C. geophilum might increase plant uptake of soil inorganic N. qRT-PCR results showed that C3 symbiosis significantly up-regulated the expression of genes encoding functions involved in NH4+ uptake (AMT3;1), NO3- uptake (NRT2.1, NRT2.4, NRT2.9), as well as Cd resistance (ABCC1 and ABCC2), meanwhile down-regulated the expression of NRT7.3, Cd transporter genes (HMA2 and NRAMP3) in the roots of P. massoniana seedlings. These results demonstrated that C. geophilum was able to alleviate Cd stress by increasing the absorption and assimilation of inorganic N in plants and inhibiting the transport of Cd from roots to shoots, which provided new insights into how EMF improved host resistance to abiotic stress.

Physiological and transcriptomic analyses reveal the molecular mechanism of PsAMT1.2 in salt tolerance

Soil salinization has become a global problem and high salt concentration in soil negatively affects plant growth. In our previous study, we found that overexpression of PsAMT1.2 from Populus simonii could improve the salt tolerance of poplar, but the physiological and molecular mechanism was not well understood. To explore the regulation pathway of PsAMT1.2 in salt tolerance, we investigated the morphological, physiological and transcriptome differences between the PsAMT1.2 overexpression transgenic poplar and the wild type under salt stress. The PsAMT1.2 overexpression transgenic poplar showed better growth with increased net photosynthetic rate and higher chlorophyll content compared with wild type under salt stress. The overexpression of PsAMT1.2 increased the catalase, superoxide dismutase, peroxidase and ascorbate peroxidase activities, and therefore probably enhanced the reactive oxygen species clearance ability, which also reduced the degree of membrane lipid peroxidation under salt stress. Meanwhile, the PsAMT1.2 overexpression transgenic poplar maintained a relatively high K+/Na+ ratio under salt stress. RNA-seq analysis indicated that PsAMT1.2 might improve plant salt tolerance by regulating pathways related to the photosynthetic system, chloroplast structure, antioxidant activity and anion transport. Among the 1056 differentially expressed genes, genes related to photosystem I and photosystem II were up-regulated and genes related to chloride channel protein-related were down-regulated. The result of the present study would provide new insight into regulation mechanism of PsAMT1.2 in improving salt tolerance of poplar.

Unraveling the effects of drying techniques on chaya leaves: Metabolomics analysis of nonvolatile and volatile metabolites, umami taste, and antioxidant capacity

Chaya (Cnidoscolus chayamansa) leaves are known for their strong umami taste and widespread use as a dried seasoning. This study aimed to assess the impact of different drying methods [freeze drying (FD), vacuum drying, oven drying at 50 °C and 120 °C (OD120) and pan roasting (PR)] on the metabolome using mass spectrometry, umami intensity, and antioxidant properties of chaya leaves. The predominant volatile compound among all samples, 3-methylbutanal, exhibited the highest relative odor activity value (rOAV), imparting a malt-like odor, while hexanal (green grass-like odor) and 2-methylbutanal (coffee-like odor) are the second highest rOAV in the FD and PR samples, respectively. OD120 and PR samples possessed the highest levels of umami-tasting amino acids and 5'-ribonucleotides as well as the most intense umami taste, whereas FD samples exhibited the highest antioxidant capacity. These findings enhance our understanding of the aroma characteristics, umami taste, and antioxidant potential of processed chaya leaves.

Salinity-driven nitrogen removal and bacteria community compositions in microbial fuel cell-integrated constructed wetlands

The effects of salinity gradients (500-4000 mg·L-1 NaCl) on electricity generation, nitrogen removal, and microbial community were investigated in a constructed wetland-microbial fuel cell (CW-MFC) system. The result showed that power density significantly increased from 7.77 mW m-2 to a peak of 34.27 mW m-2 as salinity rose, indicating enhanced electron transfer capabilities under saline conditions. At a moderate salinity level of 2000 mg·L-1 NaCl, the removal efficiencies of NH4+-N and TN reached their maximum at 77.34 ± 7.61% and 48.45 ± 8.14%, respectively. This could be attributed to increased microbial activity and the presence of critical nitrogen-removal organisms, such as Nitrospira and unclassified Betaproteobacteria at the anode, as well as Bacillus, unclassified Rhizobiales, Sphingobium, and Simplicispira at the cathode. Additionally, this salinity corresponded with the highest abundance of Exiguobacterium (3.92%), a potential electrogenic bacterium, particularly at the cathode. Other microorganisms, including Geobacter, unclassified Planctomycetaceae, and Thauera, adapted well to elevated salinity, thereby enhancing both electricity generation and nitrogen removal.

Systematic characterization of Gossypium GLN family genes reveals a potential function of GhGLN1.1a regulates nitrogen use efficiency in cotton

The enzyme glutamine synthetase (GLN) is mainly responsible for the assimilation and reassimilation of nitrogen (N) in higher plants. Although the GLN gene has been identified in various plants, there is little information about the GLN family in cotton (Gossypium spp.). To elucidate the roles of GLN genes in cotton, we systematically investigated and characterized the GLN gene family across four cotton species (G. raimondii, G. arboreum, G. hirsutum, and G. barbadense). Our analysis encompassed analysis of members, gene structure, cis-element, intragenomic duplication, and exploration of collinear relationships. Gene duplication analysis indicated that segmental duplication was the primary driving force for the expansion of the GhGLN gene family. Transcriptomic and quantitative real-time reverse-transcription PCR (qRT-PCR) analyses indicated that the GhGLN1.1a gene is responsive to N induction treatment and several abiotic stresses. The results of virus-induced gene silencing revealed that the accumulation and N use efficiency (NUE) of cotton were affected by the inactivation of GhGLN1.1a. This study comprehensively analyzed the GhGLN genes in Gossypium spp., and provides a new perspective on the functional roles of GhGLN1.1a in regulating NUE in cotton.

Partial root-zone drying combined with nitrogen treatments mitigates drought responses in rice

Drought is a major stress affecting rice yields. Combining partial root-zone drying (PRD) and different nitrogen fertilizers reduces the damage caused by water stress in rice. However, the underlying molecular mechanisms remain unclear. In this study, we combined treatments with PRD and ammonia:nitrate nitrogen at 0:100 (PRD0:100) and 50:50 (PRD50:50) ratios or PEG and nitrate nitrogen at 0:100 (PEG0:100) ratios in rice. Physiological, transcriptomic, and metabolomic analyses were performed on rice leaves to identify key genes involved in water stress tolerance under different nitrogen forms and PRD pretreatments. Our results indicated that, in contrast to PRD0:100, PRD50:50 elevated the superoxide dismutase activity in leaves to accelerate the scavenging of ROS accumulated by osmotic stress, attenuated the degree of membrane lipid peroxidation, stabilized photosynthesis, and elevated the relative water content of leaves to alleviate the drought-induced osmotic stress. Moreover, the alleviation ability was better under PRD50:50 treatment than under PRD0:100. Integrated transcriptome and metabolome analyses of PRD0:100 vs PRD50:50 revealed that the differences in PRD involvement in water stress tolerance under different nitrogen pretreatments were mainly in photosynthesis, oxidative stress, nitrogen metabolism process, phytohormone signaling, and biosynthesis of other secondary metabolites. Some key genes may play an important role in these pathways, including OsGRX4, OsNDPK2, OsGS1;1, OsNR1.2, OsSUS7, and YGL8. Thus, the osmotic stress tolerance mediated by PRD and nitrogen cotreatment is influenced by different nitrogen forms. Our results provide new insights into osmotic stress tolerance mediated by PRD and nitrogen cotreatment, demonstrate the essential role of nitrogen morphology in PRD-induced molecular regulation, and identify genes that contribute to further improving stress tolerance in rice.

Identification of candidate genes and residues for improving nitrogen use efficiency in the N-sensitive medicinal plant Panax notoginseng

BACKGROUND

Nitrogen (N) metabolism-related key genes and conserved amino acid sites in key enzymes play a crucial role in improving N use efficiency (NUE) under N stress. However, it is not clearly known about the molecular mechanism of N deficiency-induced improvement of NUE in the N-sensitive rhizomatous medicinal plant Panax notoginseng (Burk.) F. H. Chen. To explore the potential regulatory mechanism, the transcriptome and proteome were analyzed and the three-dimensional (3D) information and molecular docking models of key genes were compared in the roots of P. notoginseng grown under N regimes.

RESULTS

Total N uptake and the proportion of N distribution to roots were significantly reduced, but the NUE, N use efficiency in biomass production (NUEb), the recovery of N fertilizer (RNF) and the proportion of N distribution to shoot were increased in the N0-treated (without N addition) plants. The expression of N uptake- and transport-related genes NPF1.2, NRT2.4, NPF8.1, NPF4.6, AVP, proteins AMT and NRT2 were obviously up-regulated in the N0-grown plants. Meanwhile, the expression of CIPK23, PLC2, NLP6, TCP20, and BT1 related to the nitrate signal-sensing and transduction were up-regulated under the N0 condition. Glutamine synthetase (GS) activity was decreased in the N-deficient plants, while the activity of glutamate dehydrogenase (GDH) increased. The expression of genes GS1-1 and GDH1, and proteins GDH1 and GDH2 were up-regulated in the N0-grown plants, there was a significantly positive correlation between the expression of protein GDH1 and of gene GDH1. Glu192, Glu199 and Glu400 in PnGS1 and PnGDH1were the key amino acid residues that affect the NUE and lead to the differences in GDH enzyme activity. The 3D structure, docking model, and residues of Solanum tuberosum and P. notoginseng was similar.

CONCLUSIONS

N deficiency might promote the expression of key genes for N uptake (genes NPF8.1, NPF4.6, AMT, AVP and NRT2), transport (NPF1.2 and NRT2.4), assimilation (proteins GS1 and GDH1), signaling and transduction (genes CIPK23, PLC2, NLP6, TCP20, and BT1) to enhance NUE in the rhizomatous species. N deficiency might induce Glu192, Glu199 and Glu400 to improve the biological activity of GS1 and GDH, this has been hypothesized to be the main reason for the enhanced ability of N assimilation in N-deficient rhizomatous species. The key genes and residues involved in improving NUE provide excellent candidates for the breeding of medicinal plants.

Polyamines mediate the inhibitory effect of drought stress on nitrogen reallocation and utilization to regulate grain number in wheat

Drought stress poses a serious threat to grain formation in wheat. Nitrogen (N) plays crucial roles in plant organ development; however, the physiological mechanisms by which drought stress affects plant N availability and mediates the formation of grains in spikes of winter wheat are still unclear. In this study, we determined that pre-reproductive drought stress significantly reduced the number of fertile florets and the number of grains formed. Transcriptome analysis demonstrated that this was related to N metabolism, and in particular, the metabolism pathways of arginine (the main precursor for synthesis of polyamine) and proline. Continuous drought stress restricted plant N accumulation and reallocation rates, and plants preferentially allocated more N to spike development. As the activities of amino acid biosynthesis enzymes and catabolic enzymes were inhibited, more free amino acids accumulated in young spikes. The expression of polyamine synthase genes was down-regulated under drought stress, whilst expression of genes encoding catabolic enzymes was enhanced, resulting in reductions in endogenous spermidine and putrescine. Treatment with exogenous spermidine optimized N allocation in young spikes and leaves, which greatly alleviated the drought-induced reduction in the number of grains per spike. Overall, our results show that pre-reproductive drought stress affects wheat grain numbers by regulating N redistribution and polyamine metabolism.

Antibiotics induced changes in nitrogen metabolism and antioxidative enzymes in mung bean (Vigna radiata)

Excessive use and release of antibiotics into the soil environment in the developing world have resulted in altered soil processes affecting terrestrial organisms and posing a serious threat to crop growth and productivity. The present study investigated the influence of exogenously applied oxytetracycline (OXY) and levofloxacin (LEV) on plant physiological responses, key enzymes involved in nitrogen metabolism (e.g., nitrate reductase, glutamine synthetase), nitrogen contents and oxidative stress response of mung bean (Vigna radiata). Plants were irrigated weekly with antibiotics containing water for exposing the plants to different concentrations i.e., 1, 10, 20, 50, and 100 mg L^-1. Results showed a significant decrease in nitrate reductase activity in both antibiotic treatments and their mixtures and increased antioxidant enzymatic activities in plants. At lower concentrations of antibiotics (≤20 mg L^-1), 53.9 % to 78.4 % increase in nitrogen content was observed in levofloxacin and mixtures compared to the control, resulting in an increase in the overall plant biomass. Higher antibiotic (≥50 mg L^-1) concentration showed 58 % decrease in plant biomass content and an overall decrease in plant nitrogen content upon exposure to the mixtures. This was further complemented by 22 % to 42 % increase in glutamine synthetase activity observed in the plants treated with levofloxacin and mixtures. The application of low doses of antibiotics throughout the experiments resulted in lower toxicity symptoms in the plants. However, significantly higher malondialdehyde (MDA) concentrations at higher doses (20 mg L^-1 and above) than the control showed that plants' tolerance against oxidative stress was conceded with increasing antibiotic concentrations. The toxicity trend was: levofloxacin > mixture > oxytetracycline.

Enhanced growth performance of abi5 plants under high salt and nitrate is associated with reduced nitric oxide levels

Numerous environmental stresses have a significant impact on plant growth and development. By 2050, it is anticipated that high salinity will destroy more than fifty percent of the world's agricultural land. Understanding how plants react to the excessive use of nitrogen fertilizers and salt stress is crucial for enhancing crop yield. However, the effect of excessive nitrate treatment on plant development is disputed and poorly understood; so, we evaluated the effect of excessive nitrate supply and high salinity on abi5 plant growth performance. We demonstrated that abi5 plants are tolerant to the harmful environmental conditions of excessive nitrate and salt. abi5 plants have lower amounts of endogenous nitric oxide than Arabidopsis thaliana Columbia-0 plants due to their decreased nitrate reductase activity, caused by a decrease in the transcript level of NIA2, a gene encoding nitrate reductase. Nitric oxide appeared to have a critical role in reducing the salt stress tolerance of plants, which was diminished by an excess of nitrate. Discovering regulators such as ABI5 that can modulate nitrate reductase activity and comprehending the molecular activities of these regulators are crucial for the application of gene-editing techniques. This would result in the appropriate buildup of nitric oxide to increase the production of crops subjected to a variety of environmental stresses.

Magnesium oxide nanoparticles improved vegetative growth and enhanced productivity, biochemical potency and storage stability of harvested mustard seeds

A field study was conducted to investigate the influence of MgO-NPs priming on growth and development of mustard. Priming of mustard seeds before sowing with MgO-NPs at concentration 10, 50, 100, and 150 μg/ml enhanced the vegetative parameters of plants, with considerable increase in leaf area. MgO-NPs exposure increased the photosynthetic pigment accumulation in mustard that led to increase in biomass, carbohydrate content, and the yield in terms of total grain yield. Increased chlorophyll has simultaneously increased the oxidative stress in plants, and hence stimulated their antioxidant potential. A consistent increase was observed in the content of mustard polyphenols and activity of SOD, CAT, and APX on MgO-NPs exposure. MgO-NPs induced oxidative stress further reduced the protein content and bioavailability in mustard. We further, evaluated the influence of MgO-NPs on the quality of mustard harvested seeds. The seeds harvested from nanoprimed mustard possessed increased antioxidant potential and reduced oxidative stress. The carbohydrate and protein accumulation was significantly enhanced in response to nanopriming. Reduced chlorophyll content in seeds obtained from nanoprimed mustard indicated their potential for disease resistance and stability on long term storage. Therefore, the seeds harvested from MgO-NPs primed mustard were biochemically rich and more stable. Therefore, MgO-NPs priming can be potentially used as a novel strategy for growth promotion in plants where leaves are economically important and a strategy to enhance the seed quality under long term storage conditions.

Melatonin involves hydrogen sulfide in the regulation of H+-ATPase activity, nitrogen metabolism, and ascorbate-glutathione system under chromium toxicity

Contamination of soils with chromium (Cr) jeopardized agriculture production globally. The current study was planned with the aim to better comprehend how melatonin (Mel) and hydrogen sulfide (H₂S) regulate antioxidant defense system, potassium (K) homeostasis, and nitrogen (N) metabolism in tomato seedlings under Cr toxicity. The data reveal that application of 30 μM Mel to the seedlings treated with 25 μM Cr has a positive effect on H₂S metabolism that resulted in a considerable increase in H₂S. Exogenous Mel improved phytochelatins content and H⁺-ATPase activity with an associated increase in K content as well. Use of tetraethylammonium chloride (K⁺-channel blocker) and sodium orthovanadate (H⁺-ATPase inhibitor) showed that Mel maintained K homeostasis through regulating H⁺-ATPase activity under Cr toxicity. Supplementation of the stressed seedlings with Mel substantially scavenged excess reactive oxygen species (ROS) that maintained ROS homeostasis. Reduced electrolyte leakage and lipid peroxidation were additional signs of Mel's ROS scavenging effects. In addition, Mel also maintained normal functioning of nitrogen (N) metabolism and ascorbate-glutathione (AsA-GSH) system. Improved level of N fulfilled its requirement for various enzymes that have induced resilience during Cr stress. Additionally, the AsA-GSH cycle's proper operation maintained redox equilibrium, which is necessary for the biological system to function normally. Conversely, 1 mM hypotaurine (H₂S scavenger) abolished the Mel-effect and again Cr-induced impairment on the above-mentioned parameters was observed even in presence of Mel. Therefore, based on the observed findings, we concluded that Mel needs endogenous H₂S to alleviate Cr-induced impairments in tomato seedlings.

Sustaining nitrogen dynamics: A critical aspect for improving salt tolerance in plants

In the current changing environment, salt stress has become a major concern for plant growth and food production worldwide. Understanding the mechanisms of how plants function in saline environments is critical for initiating efforts to mitigate the detrimental effects of salt stress. Agricultural productivity is linked to nutrient availability, and it is expected that the judicious metabolism of mineral nutrients has a positive impact on alleviating salt-induced losses in crop plants. Nitrogen (N) is a macronutrient that contributes significantly to sustainable agriculture by maintaining productivity and plant growth in both optimal and stressful environments. Significant progress has been made in comprehending the fundamental physiological and molecular mechanisms associated with N-mediated plant responses to salt stress. This review provided an (a) overview of N-sensing, transportation, and assimilation in plants; (b) assess the salt stress-mediated regulation of N dynamics and nitrogen use- efficiency; (c) critically appraise the role of N in plants exposed to salt stress. Furthermore, the existing but less explored crosstalk between N and phytohormones has been discussed that may be utilized to gain a better understanding of plant adaptive responses to salt stress. In addition, the shade of a small beam of light on the manipulation of N dynamics through genetic engineering with an aim of developing salt-tolerant plants is also highlighted.

Life cycle and reproduction dynamics of Bangiales in response to environmental stresses

Red algae of the order Bangiales are notable for exhibiting flexible promotion of sexual and asexual reproductive processes by environmental stresses. This flexibility indicates that a trade-off between vegetative growth and reproduction occurs in response to environmental stresses that influence the timing of phase transition within the life cycle. Despite their high phylogenetic divergence, both filamentous and foliose red alga in the order Bangiales exhibit a haploid-diploid life cycle, with a haploid leafy or filamentous gametophyte (thallus) and a diploid filamentous sporophyte (conchocelis). Unlike haploid-diploid life cycles in other orders, the gametophyte in Bangiales is generated independently of meiosis; the regulation of this generation transition is not fully understood. Based on transcriptome and gene expression analyses, the originally proposed biphasic model for alternation of generations in Bangiales was recently updated to include a third stage. Along with the haploid gametophyte and diploid sporophyte, the triphasic framework recognizes a diploid conchosporophyte-a conchosporangium generated on the conchocelis-phase and previously considered to be part of the sporophyte. In addition to this sexual life cycle, some Bangiales species have an asexual life cycle in which vegetative cells of the thallus develop into haploid asexual spores, which are then released from the thallus to produce clonal thalli. Here, we summarize the current knowledge of the triphasic life cycle and life cycle trade-off in Neopyropia yezoensis and 'Bangia' sp. as model organisms for the Bangiales.

Genome-wide investigation and expression profiles of the NPF gene family provide insight into the abiotic stress resistance of Gossypium hirsutum

Membrane transporters encoded by NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER (NPF) genes, which play crucial roles in plant growth, development and resistance to various stresses, are involved in the transport of nitrate (NO₃ ^-) and peptides. In several plant species, NPF genes are involved in the resistance to abiotic stresses; however, whether the whole NPF gene family in cotton contributes to this resistance has not been systematically investigated. Here, 201 genes encoding NPF proteins with a peptide transporter (PTR) domain were confirmed in three different Gossypium species, namely, Gossypium hirsutum, Gossypium arboreum and Gossypium raimondii. The NPF proteins in these three Gossypium species and Arabidopsis thaliana were classified into three different subfamilies via phylogenetic analysis. Among the genes that encode these proteins, most GhNPF genes in the same subfamily contained similar gene structures and conserved domains. Predictions of the promoters of these genes revealed that the cis-acting elements included phytohormone- and light-responsive elements, indicating that some of these genes might be expressed in response to abiotic stress. Furthermore, 52 common potential candidate genes in 98 GhNPFs were predicted to exhibit specific spatiotemporal expression patterns in different tissues based on two RNA sequencing (RNA-seq) datasets. Finally, the gene expression profiles of abiotic stress indicated that 31 GhNPF genes were upregulated in at least one treatment period. Under abiotic stress for 12 and 24 h, the expression of GhNPF8 was upregulated upon cold treatment but downregulated with heat treatment, salt treatment and drought treatment. Furthermore, the expression of genes GhNPF8, GhNPF54 and GhNPF43 peaked at 6 h after heat and salt treatment. These results indicated that these genes exhibit underlying characteristics related to responses to abiotic stress. The verification of NPFs and analysis of their expression profiles in different tissues and in response to different abiotic stresses of cotton provide a basis for further studying the relationship between abiotic stress resistance and nitrogen (N) transport in cotton, as well as identifying candidate genes to facilitate their functional identification.

Regulatory Mechanisms of Plant Growth-Promoting Rhizobacteria and Plant Nutrition against Abiotic Stresses in Brassicaceae Family

Extreme environmental conditions, such as abiotic stresses (drought, salinity, heat, chilling and intense light), offer great opportunities to study how different microorganisms and plant nutrition can influence plant growth and development. The intervention of biological agents such as plant growth-promoting rhizobacteria (PGPRs) coupled with proper plant nutrition can improve the agricultural importance of different plant species. Brassicaceae (Cruciferae) belongs to the monophyletic taxon and consists of around 338 genera and 3709 species worldwide. Brassicaceae is composed of several important species of economical, ornamental and food crops (vegetables, cooking oils, forage, condiments and industrial species). Sustainable production of Brassicas plants has been compromised over the years due to several abiotic stresses and the unbalanced utilization of chemical fertilizers and uncertified chemicals that ultimately affect the environment and human health. This chapter summarized the influence of PGPRs and nutrient management in the Brassicaceae family against abiotic stresses. The use of PGPRs contributed to combating climate-induced change/abiotic factors such as drought, soil and water salinization and heavy metal contamination that limits the general performance of plants. Brassica is widely utilized as an oil and vegetable crop and is harshly affected by abiotic stresses. Therefore, the use of PGPRs along with proper mineral nutrients management is a possible strategy to cope with abiotic stresses by improving biochemical, physiological and growth attributes and the production of brassica in an eco-friendly environment.

Genome-Wide Identification of AMT2-Type Ammonium Transporters Reveal That CsAMT2.2 and CsAMT2.3 Potentially Regulate NH4+ Absorption among Three Different Cultivars of Camellia sinensis

Ammonium (NH₄⁺), as a major inorganic source of nitrogen (N) for tea plant growth, is transported and distributed across membranes by the proteins of ammonium transporters (AMTs). However, the AMT2-type AMTs from tea plants remain poorly understood. In this study, five CsAMT2 subfamily genes were identified in tea plant genomes, and their full-length coding sequences (CDS) were isolated from roots. Then, a NH₄⁺ uptake kinetic comparison of Fudingdabaicha (FD), Huangdan (HD), and Maoxie (MX) showed that FD was a high N efficiency (HNE) cultivar that had a wide range of adaptability to NH₄⁺, HD was a high N efficiency under high N conditions (HNEH) cultivar, in which it was easy to obtain higher yield in a high N environment, and MX was a high N efficiency under low N conditions (HNEL) cultivar, which had a higher affinity for NH₄⁺ than the other two. Tissue-specific expression analysis suggested that CsAMT2.2 and CsAMT2.3 were highly expressed in the roots, indicating that these two members may be unique in the CsAMT2 subfamily. This is further supported by our findings from the temporal expression profiles in the roots among these three different N adaptation cultivars. Expression levels of CsAMT2.2 and CsAMT2.3 in FD and HD were upregulated by a short time (2 h) under high NH₄⁺ treatment, while under low NH₄⁺ treatment, CsAMT2.2 and CsAMT2.3 were highly expressed at 0 h and 2 h in the HNEL-type cultivar-MX. Furthermore, the functional analysis illustrated that CsAMT2.2 and CsAMT2.3 could make a functional complementation of NH₄⁺-defective mutant yeast cells at low NH₄⁺ levels, and the transport efficiency of CsAMT2.3 was higher than that of CsAMT2.2. Thus, we concluded that CsAMT2.2 and CsAMT2.3 might play roles in controlling the NH₄⁺ uptake from the soil to the roots. These results will further the understanding of the NH₄⁺ signal networks of AMT2-type proteins in tea plants.

Salt stress decreases seed yield and postpones growth process of canola (Brassica napus L.) by changing nitrogen and carbon characters

Salt stress is a major challenge for plant growth and yield achievement in canola (Brassica napus L.). Nitrogen (N) is considered as an essential nutrient involved in many physiological processes, and carbon (C) is the most component of plant biomass. N and C assimilations of canola plants are always inhibited by salt stress. However, the knowledge of how salt stress affects biomass and seed yield through changing N and C characters is limited. A field experiment was conducted to investigate the growth process, N and C characters, photosynthetic performance, biomass accumulation and seed yield under the low and high soil salt-ion concentration conditions (LSSC and HSSC). The results indicated that HSSC postponed the time of early flowering stage and maturity stage by 4 ~ 5 days and 6 ~ 8 days, respectively, as compared with LSSC. Besides, HSSC decreased the N and C accumulation and C/N at both growing stages, suggesting that salt stress break the balance between C assimilation and N assimilation, with stronger effect on C assimilation. Although the plant N content under HSSC was increased, the photosynthesis rate at early flowering stage was decreased. The leaf area index at early flowering stage was also reduced. In addition, HSSC decreased N translocation efficiency especially in stem, and N utilization efficiency. These adverse effects of HSSC together resulted in reduced biomass accumulation and seed yield. In conclusion, the high soil salt-ion concentration reduced biomass accumulation and seed yield in canola through changing N and C characters.

Modulation of plant nitrogen remobilization and postflowering nitrogen uptake under environmental stresses

Plants are sessile organisms that take up nitrogen (N) from the soil for growth and development. At the postflowering stage, N that plants require for seed growth and filling derives from either root uptake or shoot remobilization. The balance between N uptake and N remobilization determines the final carbon (C) and N composition of the seed. The N uptake and N remobilization mechanisms are regulated by endogenous signals, including hormones, developmental stage, and carbon/nitrogen ratio, and by environmental factors. The cellular responses to the environment are relatively well known. However, the effects of environmental stresses on the balance between N uptake and N remobilization are still poorly understood. Thus, this study aims to analyze the impact of environmental stresses (drought, heat, darkness, triggered defense, and low nitrate) on N fluxes within plants during seed filling. Using publicly available Arabidopsis transcriptome data, expression of several marker genes involved in N assimilation, transport, and recycling was analyzed in relation to stress. Results showed that the responses of genes encoding inorganic N transporters, N assimilation, and N recycling are mainly regulated by N limitation, the genes encoding housekeeping proteases are principally sensitive to C limitation, and the response of genes involved in the transport of organic N is controlled by both C and N limitations. In addition, ¹⁵N data were used to examine the effects of severe environmental stresses on N remobilization and N uptake, and a schematic representation of the major factors that regulate the balance between N remobilization and N uptake under the stress and control conditions was provided.

GmTDN1 improves wheat yields by inducing dual tolerance to both drought and low-N stress

Genetically enhancing drought tolerance and nutrient use efficacy enables sustainable and stable wheat production in drought-prone areas exposed to water shortages and low soil fertility, due to global warming and declining natural resources. In this study, wheat plants, exhibiting improved drought tolerance and N-use efficacy, were developed by introducing GmTDN1, a gene encoding a DREB-like transcription factor, into two modern winter wheat varieties, cv Shi4185 and Jimai22. Overexpressing GmTDN1 in wheat resulted in significantly improved drought and low-N tolerance under drought and N-deficient conditions in the greenhouse. Field trials conducted at three different locations over a period of 2-3 consecutive years showed that both Shi4185 and Jimai22 GmTDN1 transgenic lines were agronomically superior to wild-type plants, and produced significantly higher yields under both drought and N-deficient conditions. No yield penalties were observed in these transgenic lines under normal well irrigation conditions. Overexpressing GmTDN1 enhanced photosynthetic and osmotic adjustment capacity, antioxidant metabolism, and root mass of wheat plants, compared to those of wild-type plants, by orchestrating the expression of a set of drought stress-related genes as well as the nitrate transporter, NRT2.5. Furthermore, transgenic wheat with overexpressed NRT2.5 can improve drought tolerance and nitrogen (N) absorption, suggesting that improving N absorption in GmTDN1 transgenic wheat may contribute to drought tolerance. These findings may lead to the development of new methodologies with the capacity to simultaneously improve drought tolerance and N-use efficacy in cereal crops to ensure sustainable agriculture and global food security.

Impact of microplastics from polyethylene and biodegradable mulch films on rice (Oryza sativa L.)

Extensive use of plastic film mulch has resulted in accumulation of a large amount of residual plastic, which will eventually fragment into microplastics in agricultural soils. However, it is unclear how microplastics from plastic mulch film affect crops. To address this issue, rice plants exposed to microplastics derived from poly(butyleneadipate-co-terephthalate) (PBAT)-based biodegradable mulch film (BM) and polyethylene (PE) mulch film (PM) were investigated for plant growth, physio-biochemical processes, and gene expressions. Both types of microplastics significantly reduced the height and dry weight of rice plant. Oxidative stress was induced by microplastics in rice shoot and root, with levels of ROS relatively higher under treatment PM than that under treatment BM. Transcriptomic data showed that more genes were down-regulated by treatment PM than that by treatment BM. Genes encoding ammonium and nitrate transporters were down-regulated by both types of microplastics in rice roots at vegetative stage, whereas up-regulated at reproductive stage, as compared to their respective treatment with no microplastics (CK). Similar results regarding phenylpropanoid biosynthesis pathway and lignin content were also observed in rice roots. Net photosynthetic rate and SPAD value were significantly inhibited by treatments BM and PM in rice shoot, and the expression of genes involved in light reaction was reduced at vegetative stage, whereas there were no differences of them at reproductive stage, as compared to their respective treatment CK. Our study suggests that microplastics from BM and PM both affect the growth of rice plants via nitrogen metabolism and photosynthesis. The negative effects imposed by both types of microplastics on rice plant can be mitigated with the growth of plants, and the negative effects of microplastics from PE mulch film on rice plant are relatively stronger than that from the PBAT-based biodegradable film.

Moderately Reducing Nitrogen Application Ameliorates Salt-Induced Growth and Physiological Damage on Forage Bermudagrass

Nitrogen (N) application is one of the most effective methods to alleviate salt-induced damage on plants. Forage bermudagrass has higher utilization potential on saline soil, but whether its N requirement changed under high salt stress has not been studied. Through examining plant growth-related traits, salt-stress-responsive physiological traits, photosynthesis, N metabolism, and forage quality supplied with different N concentrations under high salt stress (200 mM NaCl), we noticed that the optimum N requirement of forage bermudagrass reduced. When supplied with 10 mM N under higher salt stress, plants had a similar biomass, turf color, and chlorophyll content with plants supplied with 15 mM N, accompanied by a lower firing rate and Na+ content of leaves. The N content, crude protein, crude fat content, the expression of AMTs (ammonium transporters), NR (nitrate reductase), GS (glutamine synthetase), and GOGAT (glutamate synthetase), the chlorophyll fluorescence curve, and parameters of leaves (e.g., PIABS; PICS; ABS/RC; TRo/RC; ETo/RC) all peaked under 10 mM N under high salt stress instead of 15 mM N. Through exploring the proper N application under higher salt stress and its alleviation mechanisms, our results indicated that moderate reduction in N application under high salt level had a maximum promotion effect on the salt tolerance of forage bermudagrass without growth or forage quality inhibition. These response mechanisms obtained can provide a useful reference for N application in moderation rather than in excess on forage bermudagrass, especially in higher salinity areas.

Metabolic Pathways Involved in the Drought Stress Response of Nitraria tangutorum as Revealed by Transcriptome Analysis

Drought resistance in plants is controlled by multiple genes. To identify the genes that mediate drought stress responses and to assess the associated metabolic pathways in the desert shrub Nitraria tangutorum, we conducted a transcriptome analysis of plants under control (maximum field capacity) and drought (20% of the maximum field capacity) conditions. We analyzed differentially expressed genes (DEGs) of N. tangutorum and their enrichment in the KEGG metabolic pathways database, and explored the molecular biological mechanisms underlying the answer to its drought tolerance. Between the control and drought groups, 119 classified metabolic pathways annotated 3047 DEGs in the KEGG database. For drought tolerance, nitrate reductase (NR) gene expression was downregulated, indicating that NR activity was decreased to improve drought tolerance. In ammonium assimilation, drought stress inhibited glutamine formation. Protochlorophyllide reductase (1.3.1.33) expression was upregulated to promote chlorophyll a synthesis, whereas divinyl reductase (1.3.1.75) expression was downregulated to inhibit chlorophyll-ester a synthesis. The expression of the chlorophyll synthase (2.5.1.62) gene was downregulated, which affected the synthesis of chlorophyll a and b. Overall, drought stress appeared to improve the ability to convert chlorophyll b into chlorophyll a. Our data serve as a theoretical foundation for further elucidating the growth regulatory mechanism of desert xerophytes, thereby facilitating the development and cultivation of new, drought-resistant genotypes for the purpose of improving desert ecosystems.

Revealing the differential protein profiles behind the nitrogen use efficiency in popcorn (Zea mays var. everta)

We investigated the proteomic profiles of two popcorn inbred lines, P2 (N-efficient and N-responsive) and L80 (N-inefficient and nonresponsive to N), under low (10% of N supply) and high (100% of N supply) nitrogen environments, associated with agronomic- and physiological-related traits to NUE. The comparative proteomic analysis allowed the identification of 79 differentially accumulated proteins (DAPs) in the comparison of high/low N for P2 and 96 DAPs in the comparison of high/low N for L80. The NUE and N uptake efficiency (NUpE) presented high means in P2 in comparison to L80 at both N levels, but the NUE, NUpE, and N utilization efficiency (NUtE) rates decreased in P2 under a high N supply. DAPs involved in energy and carbohydrate metabolism suggested that N regulates enzymes of alternative pathways to adapt to energy shortages and that fructose-bisphosphate aldolase may act as one of the key primary nitrate responsive proteins in P2. Proteins related to ascorbate biosynthesis and nitrogen metabolism increased their regulation in P2, and the interaction of L-ascorbate peroxidase and Fd-NiR may play an important role in the NUE trait. Taken together, our results provide new insights into the proteomic changes taking place in contrasting inbred lines, providing useful information on the genetic improvement of NUE in popcorn.

The Loss of Function of the NODULE INCEPTION-Like PROTEIN 7 Enhances Salt Stress Tolerance in Arabidopsis Seedlings

Plants acquire nitrogen, an essential macronutrient, from the soil as nitrate. Since nitrogen availability is a major determinant of crop productivity, the soil is amended with nitrogenous fertilizers. Extensive use of irrigation can lead to the accumulation of salt in the soil, which compromises crop productivity. Our characterization of NODULE INCEPTION (NIN)-like PROTEIN 7 (NLP7), a transcription factor regulating the primary response to nitrate, revealed an intersection of salt stress and nitrate metabolism. The growth of loss-of-function mutant nlp7 was tolerant to high salinity that normally reduces the fresh weight and chlorophyll and protein content of wild type (Col-0). On a medium with high salinity, the nlp7 experienced less stress, accumulating less proline, producing less nitric oxide (NO) and reactive oxygen species (ROS), and expressing lower transcript levels of marker genes, such as RD29A and COR47, than Col-0. Nevertheless, more sodium ions were translocated to and accumulated in the shoots of nlp7 than that of Col-0. Since nlp7 also expressed less nitrate reductase (NR) activity, nitrate accumulated to abnormally high levels with or without salinity. We attributed the enhanced salt tolerance of nlp7 to the balanced accumulation of nitrate anions and sodium cations. Our results suggest that nitrate metabolism and signaling might be targeted to improve salt tolerance.

Nitrogen in plants: from nutrition to the modulation of abiotic stress adaptation

Nitrogen is one of the most important nutrient for plant growth and development; it is strongly associated with a variety of abiotic stress responses. As sessile organisms, plants have evolved to develop efficient strategies to manage N to support growth when exposed to a diverse range of stressors. This review summarizes the recent progress in the field of plant nitrate (NO3-) and ammonium (NH4+) uptake, which are the two major forms of N that are absorbed by plants. We explore the intricate relationship between NO3-/NH4+ and abiotic stress responses in plants, focusing on stresses from nutrient deficiencies, unfavorable pH, ions, and drought. Although many molecular details remain unclear, research has revealed a number of core signaling regulators that are associated with N-mediated abiotic stress responses. An in-depth understanding and exploration of the molecular processes that underpin the interactions between N and abiotic stresses is useful in the design of effective strategies to improve crop growth, development, and productivity.

Decreasing nitrogen assimilation under drought stress by suppressing DST-mediated activation of Nitrate Reductase 1.2 in rice

Nitrogen is an essential nutrient for plant growth and development, and plays vital roles in crop yield. Assimilation of nitrogen is thus fine-tuned in response to heterogeneous environments. However, the regulatory mechanism underlying this essential process remains largely unknown. Here, we report that a zinc-finger transcription factor, drought and salt tolerance (DST), controls nitrate assimilation in rice by regulating the expression of OsNR1.2. We found that loss of function of DST results in a significant decrease of nitrogen use efficiency (NUE) in the presence of nitrate. Further study revealed that DST is required for full nitrate reductase activity in rice and directly regulates the expression of OsNR1.2, a gene showing sequence similarity to nitrate reductase. Reverse genetics and biochemistry studies revealed that OsNR1.2 encodes an NADH-dependent nitrate reductase that is required for high NUE of rice. Interestingly, the DST-OsNR1.2 regulatory module is involved in the suppression of nitrate assimilation under drought stress, which contributes to drought tolerance. Considering the negative role of DST in stomata closure, as revealed previously, the positive role of DST in nitrogen assimilation suggests a mechanism coupling nitrogen metabolism and stomata movement. The discovery of this coupling mechanism will aid the engineering of drought-tolerant crops with high NUE in the future.

Molybdenum-induced endogenous nitric oxide (NO) signaling coordinately enhances resilience through chlorophyll metabolism, osmolyte accumulation and antioxidant system in arsenate stressed-wheat (Triticum aestivum L.) seedlings

There is little information available to decipher the interaction between molybdenum (Mo) and nitric oxide (NO) in mitigating arsenic (As^V) stress in plants. The present work highlights the associative role of exogenous Mo and endogenous NO signaling in regulating As^V tolerance in wheat seedlings. Application of Mo (1 μM) on 25-day-old wheat seedlings grown in the presence (5 μM) or absence of As^V stress caused improvement of photosynthetic pigment metabolism, reduction of electrolytic leakage and reactive oxygen species (ROS), and higher accumulation of osmolytes (proline and total soluble sugars). The molybdenum treatment upregulated antioxidative enzymes, such as superoxide dismutase, ascorbate peroxidase and glutathione reductase. In addition, the accumulation of nonenzymatic antioxidants (ascorbate and glutathione) was correlated with an increase in ascorbate peroxidase and glutathione reductase activity. The application of cPTIO (endogenous NO scavenger; 100 μM) reversed the Mo-mediated effects, thus indicating that endogenous NO may accompany Mo-induced mitigation of As^V stress. Mo treatment stimulated the accumulation of endogenous NO in the presence of As^V stress. Thus, it is evident that Mo and NO-mediated As^V stress tolerance in wheat seedlings are primarily operative through chlorophyll restoration, osmolytes accumulation, reduced electrolytic leakage, and ROS homeostasis.

Metabolic adjustments in forage oat (Avena sativa L.) genotypes under different sowing windows

The current investigation was aimed to study the influence of planting dates (9 Oct, 24 Oct, 13 Nov, 26 Nov, 11 Dec, and 26 Dec) and maturity stages (40, 50, and 60 DAS) on nitrogen metabolic enzymes and biochemical constituents. The experiment was laid out in randomized block design during the winter season of 2019 by using two oat genotypes (OL-10 and OL-11) with variable growing ability. The activity pattern of studied enzymes revealed upregulated nitrate assimilation during optimum (Oct 24) and sub-optimum (Oct 9 and Nov 13) planting dates but a reverse trend was observed during late planting dates; LPD (Nov 26, Dec 11, and Dec 26). The environmental constraints during LPD resulted in the accrual of nitrate above toxic levels (> 2000 ppm). The regression analysis depicted a significant relationship of nitrate assimilating enzymes and nitrate-N with temperature and sunshine hour. Examination of ammonia assimilation and transaminases suggested that the enzyme activities got uplifted during late planting dates but were stable or slightly low at optimum and sub-optimum ones. Additionally, OL-10 proved to be an N-efficient genotype in comparison with the OL-11 genotype because of its high N assimilation potential. Therefore, gaining a better understanding of planting time is crucial for sustainable livestock production.

Coordinated Role of Nitric Oxide, Ethylene, Nitrogen, and Sulfur in Plant Salt Stress Tolerance

Salt stress significantly contributes to major losses in agricultural productivity worldwide. The sustainable approach for salinity-accrued toxicity has been explored. The use of plant growth regulators/phytohormones, mineral nutrients and other signaling molecules is one of the major approaches for reversing salt-induced toxicity in plants. Application of the signaling molecules such as nitric oxide (NO) and ethylene (ETH) and major mineral nutrient such as nitrogen (N) and sulfur (S) play significant roles in combatting the major consequences of salt stress impacts in plants. However, the literature available on gaseous signaling molecules (NO/ETH) or/and mineral nutrients (N/S) stands alone, and major insights into the role of NO or/and ETH along with N and S in plant-tolerance to salt remained unclear. Thus, this review aimed to (a) briefly overview salt stress and highlight salt-induced toxicity, (b) appraise the literature reporting potential mechanisms underlying the role of gaseous signaling molecules and mineral nutrient in salt stress tolerance, and (c) discuss NO and ETH along with N and S in relation to salt stress tolerance. In addition, significant issues that have still to be investigated in this context have been mentioned.

Root NRT, NiR, AMT, GS, GOGAT and GDH expression levels reveal NO and ABA mediated drought tolerance in Brassica juncea L

Little is known about the interactive effects of exogenous nitric oxide (NO) and abscisic acid (ABA) on nitrogen (N) metabolism and related changes at molecular and biochemical levels under drought stress. The present study highlights the independent and combined effect of NO and ABA (grouped as "nitrate agonists") on expression profiles of representative key genes known to be involved in N-uptake and assimilation, together with proline metabolism, N-NO metabolism enzyme's activity and nutrient content in polyethylene glycol (PEG) treated roots of Indian mustard (B. juncea cv. Varuna). Here we report that PEG mediated drought stress negatively inhibited growth performance, as manifested by reduced biomass (fresh and dry weight) production. Total N content and other nitrogenous compounds (NO3-, NO2-) were decreased; however, NH4+, NH4+/ NO3- ratio and total free amino acids content were increased. These results were positively correlated with the PEG induced changes in expression of genes and enzymes involved in N-uptake and assimilation. Also, PEG supply lowered the content of macro- and micro-nutrients but proline level and the activity of ∆1-pyrroline-5-carboxylate synthetase increased indicating increased oxidative stress. However, all these responses were reversed upon the exogenous application of nitrate agonists (PEG + NO, PEG + NO + ABA, and PEG + ABA) where NO containing nitrate agonist treatment i.e. PEG + NO was significantly more effective than PEG + ABA in alleviating drought stress. Further, increases in activities of L-arginine dependent NOS-like enzyme and S-nitrosoglutathione reductase were observed under nitrate agonist treatments. This indicates that the balanced endogenous change in NO and ABA levels together during synthesis and degradation of NO mitigated the oxidative stress in Indian mustard seedlings. Overall, our results reveal that NO independently or together with ABA may contribute to improved crop growth and productivity under drought stress.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Improved resource allocation and stabilization of yield under abiotic stress

Sugars are the main building blocks for carbohydrate storage, but also serve as signaling molecules and protective compounds during abiotic stress responses. Accordingly, sugar transport proteins fulfill multiple roles as they mediate long distance sugar allocation, but also shape the subcellular and tissue-specific carbohydrate profiles by balancing the levels of these molecules in various compartments. Accordingly, transporter activity represents a target by classical or directed breeding approaches, to either, directly increase phloem loading or to increase sink strength in crop species. The relative subcellular distribution of sugars is critical for molecular signaling affecting yield-relevant processes like photosynthesis, onset of flowering and stress responses, while controlled long-distance sugar transport directly impacts development and productivity of plants. However, long-distance transport is prone to become unbalanced upon adverse environmental conditions. Therefore, we highlight the influence of stress stimuli on sucrose transport in the phloem and include the role of stress induced cellular carbohydrate sinks, like raffinose or fructans, which possess important roles to build up tolerance against challenging environmental conditions. In addition, we report on recent breeding approaches that resulted in altered source and sink capacities, leading to increased phloem sucrose shuttling in crops. Finally, we present strategies integrating the need of cellular stress-protection into the general picture of long-distance transport under abiotic stress, and point to possible approaches improving plant performance and resource allocation under adverse environmental conditions, leading to stabilized or even increased crop yield.

The Photosynthetic Bacterium Rhodopseudomonas palustris Strain PS3 Exerts Plant Growth-Promoting Effects by Stimulating Nitrogen Uptake and Elevating Auxin Levels in Expanding Leaves

Rhodopseudomonas palustris strain PS3, a phototrophic bacterium, was originally isolated from a paddy field located in Taipei city, Taiwan, and showed positive effects on the growth of leafy vegetables. The aim of this study was to clarify the mechanism of the beneficial effects exerted by PS3 on plants. An ineffective R. palustris strain, YSC3, isolated from a paddy field located in Yilan County, was used as the negative control for comparative analyses. We cultivated non-heading Chinese cabbage (Brassica rapa var. chinensis) in 1/2 strength Hoagland hydroponic solution, in which nitrate is the main nitrogen source. We evaluated various plant physiological responses to inoculation with different bacterial inoculants. The N use efficiency (NUE) of PS3-inoculated plants was dramatically higher than that of YSC3-inoculated plants. The nitrate uptake efficiency (NUpE) was significantly elevated in plants treated with PS3; however, no excess nitrate accumulation was observed in leaves. We also noticed that the endogenous indole-3-acetic acid (IAA) levels as well as the cell division rate in the leaves of PS3-inoculated plants were significantly higher than those in the leaves of YSC3-inoculated plants. We examined the bacterial transcription of some genes during root colonization, and found that the expression level of IAA synthesis related gene MAO was almost the same between these two strains. It suggests that the elevated endogenous IAA in the PS3-inoculated plants was not directly derived from the exogenous IAA produced by this bacterium. Taken together, we deduced that PS3 inoculation could promote plant growth by enhancing nitrate uptake and stimulating the accumulation of endogenous auxin in young expanding leaves to increase the proliferation of leaf cells during leaf development.

Moderately low nitrogen application mitigate the negative effects of salt stress on annual ryegrass seedlings

Appropriate application of nitrogen (N) can alleviate the salt stress-induced damage on plants. This study explores the changes of nitrogen requirement in feeding annual ryegrass seedlings under mild salt concentrations (50 mM, 100 mM) plus its underlying mitigation mechanism. Results showed that low salt concentration decreased N requirement as observed from the increment in plant height and biomass at a relative low N level (2.0 mM not 5.0 mM). Under salt treatment, especially at 50 mM NaCl, the OJIP (Chl a fluorescence induction transient) curve and a series of performance indexes (PIABS, RC/CS0, ET0/CS0, ϕE0, ϕ0) peaked whereas DI0/RC, Vj and M0 were the lowest under moderately low N level (2.0 mM). In addition, under salt stress, moderately low N application could maintain the expression of NR (nitrate reductase) and GS (glutamine synthetase) encoding genes at a relatively stable level but had no effect on the expression of detected NRT (nitrate transporter) gene. The seedlings cultured at 2.0 mM N also exhibited the highest activity of CAT and POD antioxidant enzymes and the lowest MDA content and EL under relative low level of salt treatment. These results indicated that mild salt treatment of annual ryegrass seedlings might reduce N requirement while moderately low N application could promote their growth via regulating photosynthesis, alleviating ROS-induced (reactive oxygen species) damage and maintenance of N metabolism. These results also can provide useful reference for nitrogen application in moderation rather than in excess on annual ryegrass in mild or medium salinity areas through understanding the underlying response mechanisms.

New gene functions are involved in the thermotolerance of the wild wheat relative Aegilops umbellulata

Wheat is one of the most important food crops in the world for human consumption, like all plants it is exposed to environmental stresses including high temperatures. The deleterious effect of high temperatures negatively affects plant growth and development, leading to reduced viability and yield. These effects can be reduced by improvement of thermotolerance through innovative breeding strategies, based on the expansion of the genetic pool available, by exploring important genetic functions from wheat wild progenitors. Improving the genetic thermotolerance characteristics of wheat requires greater understanding of genetic bases of thermotolerance, through identification of high temperature stress related genes. A good source of new useful alleles is given by Aegilops species characterized by thermotolerant habits. In this study we have classified as thermotolerant or thermosensitive, on the basis of physiologic tests, some accessions of wheat wild relative species belonging to Aegilops and Triticum genera. A thermotolerant accession of Aegilops umbellulata (AUM5) was selected, subjected to different thermal treatments and analyzed at transcriptional level. By differential display reverse transcriptase polymerase chain reaction (DDRT-PCR), we investigated modulation of gene expression elicited by heat treatments. This approach allowed the identification of various transcript-derived fragments (TDFs) produced by AUM5 in response to different thermal treatments. The functions of the inducible unique genes in the molecular determination of thermotolerance process are discussed.

Genetic regulation of water and nutrient transport in water stress tolerance in roots

Drought stress is one of the major abiotic factors affecting the growth and development of crops. The primary effect of drought is the alteration of water and nutrient uptake and transport by roots, related essentially with aquaporins and ion transporters of the plasma membrane. Therefore, the efficiency of water and nutrient transport across cell layers is a main factor in tolerance mechanisms. The regulation of this transport under water stress - in relation to the differing degrees of tolerance of crops and the effect of arbuscular mycorrhizae, together with signaling mechanisms - is reviewed here. Three different phases in the response to stress (immediate, short-term and long-term), involving different signals and levels of gene regulation, are highlighted.

Transcriptional and Metabolic Profiling of Potato Plants Expressing a Plastid-Targeted Electron Shuttle Reveal Modulation of Genes Associated to Drought Tolerance by Chloroplast Redox Poise

Water limitation represents the main environmental constraint affecting crop yield worldwide. Photosynthesis is a primary drought target, resulting in over-reduction of the photosynthetic electron transport chain and increased production of reactive oxygen species in plastids. Manipulation of chloroplast electron distribution by introducing alternative electron transport sinks has been shown to increase plant tolerance to multiple environmental challenges including hydric stress, suggesting that a similar strategy could be used to improve drought tolerance in crops. We show herein that the expression of the cyanobacterial electron shuttle flavodoxin in potato chloroplasts protected photosynthetic activities even at a pre-symptomatic stage of drought. Transcriptional and metabolic profiling revealed an attenuated response to the adverse condition in flavodoxin-expressing plants, correlating with their increased stress tolerance. Interestingly, 5-6% of leaf-expressed genes were affected by flavodoxin in the absence of drought, representing pathways modulated by chloroplast redox status during normal growth. About 300 of these genes potentially contribute to stress acclimation as their modulation by flavodoxin proceeds in the same direction as their drought response in wild-type plants. Tuber yield losses under chronic water limitation were mitigated in flavodoxin-expressing plants, indicating that the flavoprotein has the potential to improve major agronomic traits in potato.

Post-flowering biotic and abiotic stresses impact nitrogen use efficiency and seed filling in Arabidopsis thaliana

Nitrogen (N) is an essential nutrient that plants require for the synthesis of amino acids, proteins, and many other important metabolites. Plant metabolism and growth are consequently dependent on the amount of N that is assimilated and distributed from source leaves to developing sinks, such as fruits and seeds. The environmental stresses enhanced by climate change deeply influence seed yield and seed composition, and may disturb N use efficiency (NUE) in pants. We aimed to investigate plant responses to extreme climates with regard to NUE, N remobilization efficiency, and seed composition. By studying a collection of Arabidopsis genotypes showing a range of C:N ratios in seeds, we investigated the impact of different post-flowering growth conditions (control, heat, drought, low nitrate availability, induced senescence, and induced plant defense) on seed yield, N allocation in organs, NUE, and N remobilization efficiency. We analysed how post-flowering stresses could change seed filling and showed that post-flowering stresses change both the range of N and C concentrations and the C:N stoichiometry in seeds. Using a new trait, called delta seed composition, we measured the deviation in C:N stoichiometry of each genotype and revealed the genetic determinism of the C:N stoichiometry. Altogether, the results indicate that extreme climate impacts NUE dramatically in plants and generates different bottlenecks in N fluxes during seed filling.

A balancing act: how plants integrate nitrogen and water signals

Nitrogen (N) and water (W) are crucial inputs for plant survival as well as costly resources for agriculture. Given their importance, the molecular mechanisms that plants rely on to signal changes in either N or W status have been under intense scrutiny. However, how plants sense and respond to the combination of N and W signals at the molecular level has received scant attention. The purpose of this review is to shed light on what is currently known about how plant responses to N are impacted by W status. We review classic studies which detail how N and W combinations have both synergistic and antagonistic effects on key plant traits, such as root architecture and stomatal aperture. Recent molecular studies of N and W interactions show that mutations in genes involved in N metabolism affect drought responses, and vice versa. Specifically, perturbing key N signaling genes may lead to changes in drought-responsive gene expression programs, which is supported by a meta-analysis we conduct on available transcriptomic data. Additionally, we cite studies that show how combinatorial transcriptional responses to N and W status might drive crop phenotypes. Through these insights, we suggest research strategies that could help to develop crops adapted to marginal soils depleted in both N and W, an important task in the face of climate change.

A Review of Environment Effects on Nitrate Accumulation in Leafy Vegetables Grown in Controlled Environments

Excessive accumulation of nitrates in vegetables is a common issue that poses a potential threat to human health. The absorption, translocation, and assimilation of nitrates in vegetables are tightly regulated by the interaction of internal cues (expression of related genes and enzyme activities) and external environmental factors. In addition to global food security, food nutritional quality is recognized as being of strategic importance by most governments and other agencies. Therefore, the identification and development of sustainable, innovative, and inexpensive approaches for increasing vegetable production and concomitantly reducing nitrate concentration are extremely important. Under controlled environmental conditions, optimal fertilizer/nutrient element management and environmental regulation play vital roles in producing vegetables with low nitrate content. In this review, we present some of the recent findings concerning the effects of environmental factors (e.g., light, temperature, and CO2) and fertilizer/nutrient solution management strategies on nitrate reduction in vegetables grown under controlled environments and discuss the possible molecular mechanisms. We also highlight several perspectives for future research to optimize the yield and nutrition quality of leafy vegetables grown in controlled environments.

Low-temperature tolerance of the Antarctic species Deschampsia antarctica: A complex metabolic response associated with nutrient remobilization

The species Deschampsia antarctica (DA) is one of the only two native vascular species that live in Antarctica. We performed ecophysiological, biochemical, and metabolomic studies to investigate the responses of DA to low temperature. In parallel, we assessed the responses in a non-Antarctic reference species (Triticum aestivum [TA]) from the same family (Poaceae). At low temperature (4°C), both species showed lower photosynthetic rates (reductions were 70% and 80% for DA and TA, respectively) and symptoms of oxidative stress but opposite responses of antioxidant enzymes (peroxidases and catalase). We employed fused least absolute shrinkage and selection operator statistical modelling to associate the species-dependent physiological and antioxidant responses to primary metabolism. Model results for DA indicated associations with osmoprotection, cell wall remodelling, membrane stabilization, and antioxidant secondary metabolism (synthesis of flavonols and phenylpropanoids), coordinated with nutrient mobilization from source to sink tissues (confirmed by elemental analysis), which were not observed in TA. The metabolic behaviour of DA, with significant changes in particular metabolites, was compared with a newly compiled multispecies dataset showing a general accumulation of metabolites in response to low temperatures. Altogether, the responses displayed by DA suggest a compromise between catabolism and maintenance of leaf functionality.

Ammonium transporter 1 (AMT1) gene family in tomato (Solanum lycopersicum L.): Bioinformatics, physiological and expression analyses under drought and salt stresses

Nitrogen (N) is an essential macronutrient for plants, and mainly taken from the soil as ammonium (NH⁺⁴). It is particularly transported into the plants by AMmonium Transporters (AMTs), which are plasma membrane proteins. In the present study, genome-wide identification, physiological and expression analyses of tomato (Solanum lycopersicum L.) ammonium transporters 1 (SlAMT1) genes under drought and salt stresses were performed. Sequence analyses revealed the presence of variations in SlAMT1s at nucleotide and protein levels. While all the SlAMT1s comprise an ammonium transporter domain (PF00909), the numbers of their transmembrane helices were found to be diverse. Digital expression analyses proved that SlAMT1-3 gene had different expression patterns compared to the others, suggesting its functional diversities. The expression analyses revealed that SlAMT1 genes were 0.16 and 5.94 -fold down-regulated under drought and salt stresses, respectively. The results suggested that expression of SlAMT1 genes were adversely affected by abiotic stress conditions.

Nitrogen fertilization compensation the weak photosynthesis of Oilseed rape (Brassca napus L.) under haze weather

Haze and cloudy weather reduce photo-synthetically active radiation (PAR), which affects the formation of crop yield and nitrogen (N) fertilizer utilization.. We conducted field trails in normal year and severe winter haze year, aiming to compare the difference of photosynthesis and N uptake in winter rapeseed under different N levels. Daily sunshine hours and averaged radiation intensity in winter haze year decreased by 54.1% and 33.3% respectively as compared with the past 30 years. Diurnal variation of net photosynthetic rate in winter haze day was 16.2% lower than that of sunny day. Leaf area and photosynthetic capacity decreased significantly during winter haze year. The shoot biomass and N uptake at the rosette stage accounted for only 9.6% and 26.6% of the total growth period in winter haze year, while 24.4% and 70.5% in normal year, respectively. However, in winter haze year, as the top dressing of N application increasing after the rosette stage, shoot biomass increased gradually. In order to achieve the target yield of 2.5 t ha-1, after suffering winter haze, it is necessary to apply additional 73.1 kg N ha-1. In conclusion, the haze climate reduced the radiation intensity and stability, leading to a decline in photosynthetic productivity in winter oilseed rape. Applying higher N fertilizer after winter haze can compensate the negative influence and ensure rapeseed yield.

Using Tomato Recombinant Lines to Improve Plant Tolerance to Stress Combination Through a More Efficient Nitrogen Metabolism

The development of plant varieties with a better nitrogen use efficiency (NUE) is a means for modern agriculture to decrease environmental pollution due to an excess of nitrate and to maintain a sufficient net income. However, the optimum environmental conditions for agriculture will tend to be more adverse in the coming years, with increases in temperatures, water scarcity, and salinity being the most important productivity constrains for plants. NUE is inherently a complex trait, as each step, including N uptake, translocation, assimilation, and remobilization, is governed by multiple interacting genetic and environmental factors. In this study, two recombinant inbred lines (RIL-66 and RIL-76) from a cross between Solanum lycopersicum and Solanum pimpinellifoilum with different degree of tolerance to the combination of salinity and heat were subjected to a physiological, ionomic, amino acid profile, and gene expression study to better understand how nitrogen metabolism is affected in tolerant plants as compared to sensitive ones. The ionomics results showed a different profile between the two RILs, with K+ and Mg2+ being significantly lower in RIL-66 (low tolerant) as compared to RIL-76 (high tolerant) under salinity and heat combination. No differences were shown between the two RILs in N total content; however, N-NO3 - was significantly higher in RIL-66, whereas N-Norg was lower as compared to the other genotype, which could be correlated with its tolerance to the combination of salinity and heat. Total proteins and total amino acid concentration were significantly higher in RIL-76 as compared to the sensitive recombinant line under these conditions. Glutamate, but more importantly glutamine, was also highly synthesized and accumulated in RIL-76 under the combination of salinity and heat, which was in agreement with the upregulation of the nitrogen metabolism related transcripts studied (SlNR, SlNiR, SlGDH, SlGLT1, SlNRT1.2, SlAMT1, and SlAMT2). This study emphasized the importance of studying abiotic stress in combination and how recombinant material with different degrees of tolerance can be highly important for the improvement of nitrogen use efficiency in horticultural plants through the targeting of N-related markers.

Different forms of nitrogen application affect metabolite patterns in grapevine leaves and the sensory of wine

The quality of grapevine berries, must and wine is influenced by environmental and viticultural inputs and their complex interactions. Aroma and flavour are decisive for quality and are mainly determined by primary and secondary metabolites. In particular, phenolic compounds contribute to berry and wine quality. The influence of various nitrogen forms on i) the composition of phenolic compounds in leaves and wine and; ii) the resulting wine quality were studied in a vineyard system. Must and wine quality was evaluated by chemical analysis and sensory testing. Metabolomic profiling was also performed. Aroma and sensory profile were significantly changed by the application of nitrogen in contrast to no nitrogen fertilisation. The levels of 33 metabolites in leaves and 55 metabolites in wine were significantly changed altered by fertilisation with the various nitrogen forms. In leaves, more metabolites were increased by the use of calcium nitrate or ammonium but were decreased by the use of urea. In terms of wine, the used nitrogen forms decreased more metabolites compared with no fertilisation.

Nitric oxide alters nitrogen metabolism and PIN gene expressions by playing protective role in arsenic challenged Brassica juncea L

Plants have ability to adapt themselves through altering their growth process. In the present study, we examined exogenous application of nitric oxide (NO) on nitrogen metabolism and auxin (PIN) gene expression, and its possible role in alleviation of arsenic (As) toxicity in Brassica juncea seedlings. Seven days old hydroponically grown B. juncea seedlings were exposed to As^III (150 μM), Sodium nitroprusside (NO donor, 100 μM), As^III + SNP and control (without metal)for 48 h. Experimental results revealed that As^III stress: enhanced the level of nitrite, NiR activity, NO₃^- and NH₄⁺content as well as NADH-GOGAT activity; but GDH level decreased; enhanced content of amino acids; upregulated gene expression level of N metabolism and downregulated polar auxin transporter genes (PIN); inhibited plant growth and morphological parameters; increased MDA, H₂O_2, cysteine, proline content, enzymatic antioxidants (SOD, CAT, APX; GSH, TT, NPT); and decreased nutrient content. As^III + SNP combination reduced the accumulation of As; improved growth; chlorophyll, protein and mineral nutrient content by scavenging ROS generation; maintained amino acids content; downregulated expression of N metabolism genes and upregulated expression of auxin transporter (PIN) genes . Additional biochemical data depicts reduction in the level of nitrogen related enzymatic activities, and other stress related parameters. Overall, this study provides an integrated view that exogenous SNP (NO donor) supplementation alleviated the inhibitory role of As^III in B. juncea seedlings by altering nutrients, amino acids and auxin redistribution via expression of nitrogen and PIN gene profiling.

Transcriptome profiling of Puccinellia tenuiflora during seed germination under a long-term saline-alkali stress

BACKGROUND

Puccinellia tenuiflora is the most saline-alkali tolerant plant in the Songnen Plain, one of the three largest soda saline-alkali lands worldwide. Here, we investigated the physicochemical properties of saline-alkali soils from the Songnen Plain and sequenced the transcriptomes of germinated P. tenuiflora seedlings under long-term treatment (from seed soaking) with saline-alkali soil extracts.

RESULTS

We found that the soils from Songnen Plain were reasonably rich in salts and alkali; moreover, the soils were severely deficient in nitrogen [N], phosphorus [P], potassium [K] and several other mineral elements. This finding demonstrated that P. tenuiflora can survive from not only high saline-alkali stress but also a lack of essential mineral elements. To explore the saline-alkali tolerance mechanism, transcriptional analyses of P. tenuiflora plants treated with water extracts from the saline-alkali soils was performed. Interestingly, unigenes involved in the uptake of N, P, K and the micronutrients were found to be significantly upregulated, which indicated the existence of an efficient nutrition-uptake system in P. tenuiflora. Compared with P. tenuiflora, the rice Oryza sativa was hypersensitive to saline-alkali stress. The results obtained using a noninvasive microtest techniques confirmed that the uptake of NO3- and NH4+ and the regulatory flux of Na+ and H+ were significantly higher in the roots of P. tenuiflora than in those of O. sativa. In the corresponding physiological experiments, the application of additional nutrition elements significantly eliminated the sensitive symptoms of rice to saline-alkali soil extracts.

CONCLUSIONS

Our results imply that the survival of P. tenuiflora in saline-alkali soils is due to a combination of at least two regulatory mechanisms and the high nutrient uptake capacity of P. tenuiflora plays a pivotal role in its adaptation to those stress. Taken together, our results highlight the role of nutrition uptake in saline-alkali stress tolerance in plants.

Genetic analyses of nitrogen assimilation enzymes in Brassica juncea (L.) Czern & Coss

Nitrogen (N) is a critical input for plant growth and development. A better understanding of N uptake and utilization is important to develop plant breeding strategies for improving nitrogen use efficiency (NUE). With that objective in mind, we assayed a SNP-genotyped association panel comprising 92 inbred lines for the activities of nitrate reductase (NR), nitrite reductase (NIR), glutamine synthetase (GS) and glutamate synthase (GOGAT). All these enzymes are associated with N assimilation. The experiments were carried out at two levels of N application: no added N (N₀) and agrnomically recommened dose (100 kg/ha) of N application (N₁₀₀). Genome wide association studies (GWAS) helped to identify several marker-trait associations (MTAs), involving chromosomes A01, A06, A08, B02, B04, B05 and B08. These explained high phenotypic variation (up to 32%). Annotation of the genomic region(s) in or around significant SNPs allowed prediction of genes encoding high affinity nitrate transporters, glutamine synthetase 1.3, myb-like transcription factor family protein, bidirectional amino acid transporter 1, auxin signaling F-box 3 and oxidoreductases. This is the first attempt to use GWAS for identification of enzyme QTLs to explain variation for nitrogen assimilation enzymes in Brassica juncea.