Legume-type glutamate dehydrogenase: Structure, activity, and inhibition studies.

Glutamate dehydrogenases (GDHs) are key enzymes at the crossroads of N and C metabolism in plants. Legumes, whose N metabolism is particularly intricate, possess a unique type of GDH. This study presents an analysis of a legume-type GDH (isoform 2) from Medicago truncatula (MtGDH2). We measured MtGDH2 activity in both the Glu â†’ 2-oxoglutarate (2OG) and 2OG â†’ Glu reaction directions and obtained kinetic parameters for Glu, 2OG, NAD+, and NADH. Inhibition assays revealed that compounds possessing di- or tricarboxylates act as inhibitors of plant GDHs. Interestingly, 2,6-pyridinedicarboxylate (PYR) weakly inhibits MtGDH2 compared to Arabidopsis thaliana homologs. Furthermore, we explored tetrazole derivatives to discover 3-(1H-tetrazol-5-yl)benzoic acid (TBA) as an MtGDH2 inhibitor. The kinetic experiments are supported by six crystal structures, solved as: (i) unliganded enzyme, (ii) trapping the reaction intermediate 2-amino-2-hydroxyglutarate and NAD+, and also complexed with NAD+ and inhibitors such as (iii) citrate, (iv) PYR, (v) isophthalate, and (vi) TBA. The complex with TBA revealed a new mode of action that, in contrast to other inhibitors, prevents domain closure. This discovery points to TBA as a starting point for the development of novel GDH inhibitors to study the functions of GDH in plants and potentially boost biomass production.

Heterotrophic nitrification processes driven by glucose and sodium acetate: New insights into microbial communities, functional genes and nitrogen metabolism from metagenomics and metabolomics.

Heterotrophic nitrification (HN) bacteria use organic carbon sources to remove ammonia nitrogen (NH4+-N); however, the mechanisms of carbon and nitrogen metabolism are unknown. To understand this mechanism, HN functional microbial communities named MG and MA were enriched with glucose and sodium acetate, respectively. The NH4+-N removal efficiencies were 98.87 % and 98.91 %, with 88.06 % and 69.77 % nitrogen assimilation for MG and MA at 22 h and 10 h, respectively. Fungi (52.86 %) were more competitive in MG, and bacteria (99.99 %) were dominant in MA. Metagenomic and metabolomic analyses indicated that HN might be a signaling molecule (NO) in the production and detoxification processes when MG metabolizes glucose (amo, hao, and nosZ were not detected). MA metabolizes sodium acetate to produce less energy and promotes nitrogen oxidation reduction; however, genes (hao, hox, and NOS2) were not detected. These results suggest that NO and energy requirements induce microbial HN.

Long-chain fatty acids facilitate acidogenic fermentation of food waste: Attention to the microbial response and the change of core metabolic pathway under saturated and unsaturated fatty acids loading.

Long-chain fatty acids (LCFAs) are recognized as a significant inhibitory factor in anaerobic digestion of food waste (FW), yet they are inevitably present in FW due to lipid hydrolysis. Given their distinct synthesis mechanism from traditional anaerobic digestion, little is known about the effect of LCFAs on FW acidogenic fermentation. This study reveals that total volatile fatty acids (VFAs) production increased by 9.98 % and 4.03 % under stearic acid and oleic acid loading, respectively. Acetic acid production increased by 20.66 % under stearic acid loading compared to the control group (CK). However, the LCFA stress restricted the degradation of solid organic matter, particularly under oleic acid stress. Analysis of microbial community structure and quorum sensing (QS) indicates that LCFA stress enhanced the relative abundance of Lactobacillus and Klebsiella. In QS system, the relative abundance of luxS declined from 0.157 % to 0.116 % and 0.125 % under oleic acid and stearic acid stress, respectively. LCFA stress limited the Autoinducer-2 (AI-2) biosynthesis, suggesting that microorganisms cannot use QS to resist the LCFA stress. Metagenomic sequencing showed that LCFA stress promoted acetic acid production via the conversion of pyruvate and acetyl-CoA to acetate. Direct conversion of pyruvate to acetic acid increased by 47.23 % compared to the CK group, accounting for the enhanced acetic acid production under stearic acid loading. The abundance of ß-oxidation pathway under stearic acid loading was lower than under oleic acid loading. Overall, the stimulating direct conversion of pyruvate plays a pivotal role in enhancing acetic acid biosynthesis under stearic acid loading, providing insights into the effect of LCFA on mechanism of FW acidogenic fermentation.

Identification and genomic analysis of a thermophilic bacterial strain that reduces ammonia loss from composting.

Ammonia loss is the most severe during the high-temperature stage (>50°C) of aerobic composting. Regulating ammonia volatilization during this period via thermophilic microbes can significantly improve the nitrogen content of compost and reduce air pollution due to ammonia loss. In this study, an ammonia-assimilating bacterial strain named LL-8 was screened out as having the strongest ammonia nitrogen conversion rate (32.7%) at high temperatures (50°C); it is able to significantly reduce 42.9% ammonia volatile loss in chicken manure composting when applied at a high-temperature stage. Phylogenetic analysis revealed that LL-8 was highly similar (>98%) with Priestia aryabhattai B8W22T and identified as Priestia aryabhatta. Genomic analyses indicated that the complete genome of LL-8 comprised 5,060,316 base pairs with a GC content of 32.7% and encoded 5,346 genes. Genes, such as gudB, rocG, glnA, gltA, and gltB, that enable bacteria to assimilate ammonium nitrogen were annotated in the LL-8 genome based on the comparison to the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. The results implied that the application of thermophilic ammonia-assimilating strain P. aryabhatta LL-8 would be a promising solution to reduce ammonia loss and mitigate air pollution of aerobic composting.IMPORTANCEAerobic composting is one of the essential ways to recycle organic waste, but its ammonia volatilization is severe and results in significant nitrogen loss, especially during the high-temperature period, which is also harmful to the environment. The application of thermophilic bacteria that can use ammonia as a nitrogen source at high temperatures is helpful to reduce the ammonia volatilization loss of composting. In this study, we screened and identified a bacteria strain called LL-8 with high temperature (50°C) resistance and strong ammonia-assimilating ability. It also revealed significant effects on decreasing ammonia volatile loss in composting. The whole-genome analysis revealed that LL-8 could utilize ammonium nitrogen by assimilation to decrease ammonia volatilization. Our work provides a theoretical basis for the application of this functional bacteria in aerobic composting to control nitrogen loss from ammonia volatilization.

Core microbes in Cordyceps militaris sclerotia and their nitrogen metabolism-related ecological functions.

Cordyceps militaris infects insects and forms sclerotia within the insect remains, establishing insect-microbe complexes. Here, C. militaris sclerotia samples from a single location in China over a 5-year period were subjected to high-throughput DNA sequencing, and the core microbes (which were stably enriched in the sclerotia over the 5 years) were identified. Next, seven bacterial strains were isolated from the C. militaris sclerotia, their biochemical characteristics were assessed, and they were co-cultured with C. militaris to study their effects on C. militaris metabolite production and biomass. Furthermore, the effects of NH4, NO3, and peptone media on C. militaris were compared. The results showed that Rhodococcus, Phyllobacterium, Pseudomonas, Achromobacter, Ensifer, Stenotrophomonas, Sphingobacterium, Variovorax, and Acinetobacter were the core microbes. Although co-culture of C. militaris with the seven bacterial strains isolated from the sclerotia did not directly increase the cordycepin level, they all had NO3 reduction ability, and four had urea decomposition ability. Meanwhile, C. militaris in NH4 medium had an increased cordycepin level compared to C. militaris in the other two media. From this, we inferred that bacteria in the sclerotia can convert NO3 to NH4, and then cordycepin is produced using NH4, which was confirmed by RNA-seq and real-time fluorescence quantitative PCR. Thus, bacteria in the sclerotia may indirectly affect the C. militaris metabolite production by regulating nitrogen metabolism. In summary, there are stable core microbes in the C. militaris sclerotia, and they may directly and indirectly affect the growth and metabolite production of C. militaris. IMPORTANCE: The model Cordyceps species Cordyceps militaris is rich in therapeutic compounds. It has recently been demonstrated that symbiotic microbes in sclerotia affect Cordyceps' growth, development, and secondary metabolite production. In this study, core microbes were identified based on C. militaris sclerotia samples obtained from the same site over 5 years. Additionally, bacterial strains isolated from C. militaris sclerotia were found to affect metabolite production and nitrogen utilization, based on functional tests. Moreover, based on the bacterial nitrogen metabolism capacity in the sclerotia and its influence on C. militaris metabolite production, we deduced that bacteria in the sclerotia can indirectly affect C. militaris metabolite production by regulating nitrogen metabolism. This is the first report on how bacteria in the sclerotia affect C. militaris metabolite production from the perspective of the nitrogen cycle. The results increase our understanding of microbial functions in C. militaris sclerotia.

Circadian Rhythm and Nitrogen Metabolism Participate in the Response of Boron Deficiency in the Root of Brassica napus.

Boron (B) deficiency has been shown to inhibit root cell growth and division. However, the precise mechanism underlying B deficiency-mediated root tip growth inhibition remains unclear. In this study, we investigated the role of BnaA3.NIP5;1, a gene encoding a boric acid channel, in Brassica napus (B. napus). BnaA3.NIP5;1 is expressed in the lateral root cap and contributes to B acquisition in the root tip. Downregulation of BnaA3.NIP5;1 enhances B sensitivity in B. napus, resulting in reduced shoot biomass and impaired root tip development. Transcriptome analysis was conducted on root tips from wild-type B. napus (QY10) and BnaA3.NIP5;1 RNAi lines to assess the significance of B dynamics in meristematic cells during seedling growth. Differentially expressed genes (DEGs) were significantly enriched in plant circadian rhythm and nitrogen (N) metabolism pathways. Notably, the circadian-rhythm-related gene HY5 exhibited a similar B regulation pattern in Arabidopsis to that observed in B. napus. Furthermore, Arabidopsis mutants with disrupted circadian rhythm (hy5/cor27/toc1) displayed heightened sensitivity to low B compared to the wild type (Col-0). Consistent with expectations, B deficiency significantly disrupted N metabolism in B. napus roots, affecting nitrogen concentration, nitrate reductase enzyme activity, and glutamine synthesis. Interestingly, this disruption was exacerbated in BnaA3NIP5;1 RNAi lines. Overall, our findings highlight the critical role of B dynamics in root tip cells, impacting circadian rhythm and N metabolism, ultimately leading to retarded growth. This study provides novel insights into B regulation in root tip development and overall root growth in B. napus.

RNA-seq reveals the gene expression in patterns in Populus × euramericana 'Neva' plantation under different precision water and fertilizer-intensive management.

BACKGROUND: Populus spp. is a crucial fast-growing and productive tree species extensively cultivated in the mid-latitude plains of the world. However, the impact of intensive cultivation management on gene expression in plantation remains largely unexplored. RESULTS: Precision water and fertilizer-intensive management substantially increased key enzyme activities of nitrogen transport, assimilation, and photosynthesis (1.12-2.63 times than CK) in Populus × euramericana 'Neva' plantation. Meanwhile, this management approach had a significant regulatory effect on the gene expression of poplar plantations. 1554 differential expression genes (DEGs)were identified in drip irrigation (ND) compared with conventional irrigation. Relative to ND, 2761-4116 DEGs, predominantly up-regulated, were identified under three drip fertilization combinations, among which 202 DEGs were mainly regulated by fertilization. Moreover, drip irrigation reduced the expression of cell wall synthesis-related genes to reduce unnecessary water transport. Precision drip and fertilizer-intensive management promotes the synergistic regulation of carbon and nitrogen metabolism and up-regulates the expression of major genes in nitrogen transport and assimilation processes (5 DEGs), photosynthesis (15 DEGs), and plant hormone signal transduction (11 DEGs). The incorporation of trace elements further enhanced the up-regulation of secondary metabolic process genes. In addition, the co-expression network identified nine hub genes regulated by precision water and fertilizer-intensive management, suggesting a pivotal role in regulating the growth of poplar. CONCLUSION: Precision water and fertilizer-intensive management demonstrated the ability to regulate the expression of key genes and transcription factor genes involved in carbon and nitrogen metabolism pathways, plant hormone signal transduction, and enhance the activity of key enzymes involved in related processes. This regulation facilitated nitrogen absorption and utilization, and photosynthetic abilities such as light capture, light transport, and electron transport, which faintly synergistically regulate the growth of poplar plantations. These results provide a reference for proposing highly efficient precision intensive management to optimize the expression of target genes.

Unraveling the genetic potential of nitrous oxide reduction in wastewater treatment: insights from metagenome-assembled genomes.

This study explores the genetic landscape of nitrous oxide (N2O) reduction in wastewater treatment plants (WWTPs) by profiling 1,083 high-quality metagenome-assembled genomes (HQ MAGs) from 23 Danish full-scale WWTPs. The focus is on the distribution and diversity of nitrous oxide reductase (nosZ) genes and their association with other nitrogen metabolism pathways. A custom pipeline for clade-specific nosZ gene identification with higher sensitivity revealed 503 nosZ sequences in 489 of these HQ MAGs, outperforming existing Kyoto Encyclopedia of Genes and Genomes (KEGG) module-based methods. Notably, 48.7% of the total 1,083 HQ MAGs harbored nosZ genes, with clade II being predominant, accounting for 93.7% of these genes. Taxonomic profiling highlighted the prevalence of nosZ-containing taxa within Bacteroidota and Pseudomonadota. Chloroflexota exhibited unexpected affiliations with both the sec and tat secretory pathways, and all were found to contain the accessory nosB gene, underscoring the importance of investigating the secretory pathway. The majority of non-denitrifying N2O reducers were found within Bacteroidota and Chloroflexota. Additionally, HQ MAGs with genes for dissimilatory nitrate reduction to ammonium and assimilatory nitrate reduction frequently co-occurred with the nosZ gene. Traditional primers targeting nosZ often focus on short-length amplicons. Therefore, we introduced custom-designed primer sets targeting near-full-length nosZ sequences. These new primers demonstrate efficacy in capturing diverse and well-characterized sequences, providing a valuable tool with higher resolution for future research. In conclusion, this comprehensive analysis enhances our understanding of N2O-reducing organisms in WWTPs, highlighting their potential as N2O sinks with the potential for optimizing wastewater treatment processes and mitigating greenhouse gas emissions. IMPORTANCE: This study provides critical insights into the genetic diversity of nitrous oxide reductase (nosZ) genes and the microorganisms harboring them in wastewater treatment plants (WWTPs) by exploring 1,083 high-quality metagenome-assembled genomes (MAGs) from 23 Danish full-scale WWTPs. Despite the pivotal role of nosZ-containing organisms, their diversity remains largely unexplored in WWTPs. Our custom pipeline for detecting nosZ provides near-full-length genes with detailed information on secretory pathways and accessory nos genes. Using these genes as templates, we developed taxonomically diverse clade-specific primers that generate nosZ amplicons for phylogenetic annotation and gene-to-MAG linkage. This approach improves detection and expands the discovery of novel sequences, highlighting the prevalence of non-denitrifying N2O reducers and their potential as N2O sinks. These findings have the potential to optimize nitrogen removal processes and mitigate greenhouse gas emissions from WWTPs by fully harnessing the capabilities of the microbial communities.

Flux Calculation for Primary Metabolism Reveals Changes in Allocation of Nitrogen to Different Amino Acid Families When Photorespiratory Activity Changes.

Photorespiration, caused by oxygenation of the enzyme Rubisco, is considered a wasteful process, because it reduces photosynthetic carbon gain, but it also supplies amino acids and is involved in amelioration of stress. Here, we show that a sudden increase in photorespiratory activity not only reduced carbon acquisition and production of sugars and starch, but also affected diurnal dynamics of amino acids not obviously involved in the process. Flux calculations based on diurnal metabolite profiles suggest that export of proline from leaves increases, while aspartate family members accumulate. An immense increase is observed for turnover in the cyclic reaction of glutamine synthetase/glutamine-oxoglutarate aminotransferase (GS/GOGAT), probably because of increased production of ammonium in photorespiration. The hpr1-1 mutant, defective in peroxisomal hydroxypyruvate reductase, shows substantial alterations in flux, leading to a shift from the oxoglutarate to the aspartate family of amino acids. This is coupled to a massive export of asparagine, which may serve in exchange for serine between shoot and root.

Potential effects of Cu2+ stress on nitrogen removal performance, microbial characteristics, and metabolism pathways of biofilm reactor.

Sequencing batch biofilm reactors (SBBR) were utilized to investigate the impact of Cu2+ on nitrogen (N) removal and microbial characteristics. The result indicated that the low concentration of Cu2+ (0.5 mg L-1) facilitated the removal of ammonia nitrogen (NH4+-N), total nitrogen (TN), nitrate nitrogen (NO3--N), and chemical oxygen demand (COD). In comparison to the average effluent concentration of the control group, the average effluent concentrations of NH4+-N, NO3--N, COD, and TN were found to decrease by 40.53%, 17.02%, 10.73%, and 15.86%, respectively. Conversely, the high concentration of Cu2+ (5 mg L-1) resulted in an increase of 94.27%, 55.47%, 22.22%, and 14.23% in the aforementioned parameters, compared to the control group. Low concentrations of Cu2+ increased the abundance of nitrifying bacteria (Rhodanobacter, unclassified-o-Sacharimonadales), denitrifying bacteria (Thermomonas, Comamonas), denitrification-associated genes (hao, nosZ, norC, nffA, nirB, nick, and nifD), and heavy-metal-resistant genes related to Cu2+ (pcoB, cutM, cutC, pcoA, copZ) to promote nitrification and denitrification. Conversely, high concentration Cu2+ hindered the interspecies relationship among denitrifying bacteria genera, nitrifying bacteria genera, and other genera, reducing denitrification and nitrification efficiency. Cu2+ involved in the N and tricarboxylic acid (TCA) cycles, as evidenced by changes in the abundance of key enzymes, such as (EC:1.7.99.1), (EC:1.7.2.4), and (EC:1.1.1.42), which initially increased and then decreased with varying concentrations of Cu2+. Conversely, the abundance of EC1.7.2.1, associated with the accumulation of nitrite nitrogen (NO2--N), gradually declined. These findings provided insights into the impact of Cu2+ on biological N removal.

Microbial community succession and responses to internal environmental drivers throughout the operation of constructed wetlands.

Constructed wetlands (CWs) have been widely used to ensure effective domestic wastewater treatment. Microorganisms-derived CWs have received extensive attention as they play a crucial role. However, research on the succession patterns of microbial communities and the influencing mechanisms of internal environmental factors throughout entire CW operations remains limited. In this context, three parallel-operated CWs were established in this study to assess the microbial communities and their influencing environmental factors at different substrate depths throughout the operation process using 16S rRNA gene high-throughput sequencing and metagenomic sequencing. The results showed gradual reproduction and accumulation of the microbial communities throughout the CW operation. Although gradual increases in the richness and diversity of the microbial communities were found, there were decreases in the functional expression of the dominant microbial species. The excessive accumulation of microorganisms will decrease the oxidation-reduction potential (ORP) within CWs and attenuate their influence on effluent. Dissolved oxygen (DO) was the major factor influencing the microbial community succession over the CW operation. The main identified functional bacterial genera responsible for the ammonium oxidation, nitrification, and denitrification processes in the CWs were Nitrosospira, Nitrobacter, Nitrospira, Rhodanobacter, and Nakamurella. The narG gene was identified as a key functional gene linking various components of nitrogen cycling, while pH, electrical conductivity (EC), and ORP were the major environmental factors affecting the metabolism characteristics of nitrogen functional microorganisms. This study provides a theoretical basis for the effective regulation of related microbial communities to achieve long-term, efficient, and stable CW operations.

Aromatic compound 2-acetyl-1-pyrroline coordinates nitrogen assimilation and methane mitigation in fragrant rice.

Rice paddy has been the main source of anthropogenic methane (CH4) emissions, with significant variations among rice varieties. 2-Acetyl-1-pyrroline (2-AP) is the key component of the pleasant aroma in fragrant rice. Here, we show that fragrant rice is metabolically active in nitrogen assimilation and exhibits high levels of 2-AP and that CH4 fluxes at the booting stage and cumulative emissions are 25.5% and 14.8% lower, respectively, in fragrant rice paddies compared with nonfragrant rice paddies. Three precursors involved in 2-AP synthesis-proline, glutamic acid, and ornithine-are identified as crucial nitrogen compounds that significantly promote CH4 oxidation in the rhizosphere. Augmenting 2-AP synthesis, either through foliar spraying or by utilizing CRISPR-Cas9 technology to generate knockout lines of BETAINE ALDEHYDE DEHYDROGENASE 2 gene, effectively enhances CH4 oxidation and reduces CH4 fluxes. Our findings reveal that the 2-AP metabolic pathway coordinates the carbon/nitrogen cycle to improve nitrogen assimilation along with high 2-AP levels and mitigate CH4 emissions in paddy ecosystems.

Synergistic mechanisms of denitrification in FeS2-based constructed wetlands: Effects of organic carbon availability under day-night alterations.

In constructed wetlands (CWs), carbon source availability profoundly affected microbial metabolic activities engaged in both iron cycle and nitrogen metabolism. However, research gaps existed in understanding the biotransformation of nitrogen and iron in response to fluctuations in organic carbon content under day-night alterations. Results demonstrated increased removal efficiency of NO3--N (95.7 %) and NH4+-N (75.70 %) under light conditions, attributed to increased total organic carbon (TOC). This enhancement promoted the relative abundance of bacteria involved in nitrogen and iron processes, establishing a more stable microbial network. Elevated TOC content also upregulated genes for iron metabolism and glycolysis, facilitating denitrification. Spearman correlation analysis supported the synergistic mechanisms between FeS2-based autotrophic denitrification and TOC-mediated heterotrophic denitrification under light conditions. The significant impact of carbon sources on microbial activities underscores the critical role of organic carbon availability in enhancing nitrogen removal efficiency, providing valuable insights for optimizing FeS2-based CWs design and operation strategies.

Dietary crude protein and protein solubility manipulation enhances intestinal nitrogen absorption and mitigates reactive nitrogen emissions through gut microbiota and metabolome reprogramming in sheep.

Dietary nutrient manipulation (e.g. protein fractions) could lower the environmental footprints of ruminants, especially reactive nitrogen (N). This study investigated the impacts of dietary soluble protein (SP) levels with decreased crude protein (CP) on intestinal N absorption, hindgut N metabolism, fecal microbiota and metabolites, and their linkage with N metabolism phenotype. Thirty-two male Hu sheep, with an age of six months and an initial BW of 40.37 ± 1.18 kg, were randomly assigned to four dietary groups. The control diet (CON), aligning with NRC standards, maintained a CP content of 16.7% on a dry matter basis. Conversely, the experimental diets (LPA, LPB, and LPC) featured a 10% reduction in CP compared with CON, accompanied by SP adjustments to 21.2%, 25.9%, and 29.4% of CP, respectively. Our results showed that low-protein diets led to significant reductions in the concentrations of plasma creatinine, ammonia, urea N, and fecal total short-chain fatty acids (SCFA) (P < 0.05). Notably, LPB and LPC exhibited increased total SCFA and propionate concentrations compared with LPA (P < 0.05). The enrichment of the Prevotella genus in fecal microbiota associated with energy metabolism and amino acid (AA) biosynthesis pathways was evident with SP levels in low-protein diets of approximately 25% to 30%. Moreover, LPB and LPC diets demonstrated a decrease in fecal NH 4 + -N and NO 2 - -N contents as well as urease activity, compared with CON (P < 0.05). Concomitantly, reductions in fecal glutamic acid dehydrogenase gene (gdh), nitrite reductase gene (nirS), and nitric oxide reductase gene (norB) abundances were observed (P < 0.05), pointing towards a potential reduction in reactive N production at the source. Of significance, the up-regulation of mRNA abundance of AA and peptide transporters in the small intestine (duodenum, jejunum, and ileum) and the elevated concentration of plasma AA (e.g. arginine, methionine, aspartate, glutamate, etc.) underscored the enhancement of N absorption and N efficiency. In summary, a 10% reduction in CP, coupled with an SP level of approximately 25% to 30%, demonstrated the potential to curtail reactive N emissions through fecal Prevotella enrichment and improve intestinal energy and N utilization efficiency.

The effect of biofertilizers on nickel accumulation, nitrogen metabolism and amino acid profile of corn (Zea mays L.) exposed to nickel stress.

The issue of heavy metal pollution such as nickel poses a significant environmental concern, exerting detrimental effects on the growth and viability of plant life. Plants have various mechanisms to effectively manage heavy metal stress, including the ability to modify their amino acid type and content. This adaptive response allows plants to mitigate the detrimental effects caused by excessive heavy metal accumulation. The aim of this study was to investigate the effect of biofertilizers on nickel accumulation, nitrogen metabolism and amino acid profile of corn (Zea mays L.) cv. 'PL438' exposed to Ni stress. After disinfecting and soaking in water for 24 h, corn seeds were primed with bacterial biofertilizers (T2: NPK + FZ), fungal biofertilizers (T3: Arbuscular mycorrhizal fungi (AMF) + Trichoderma (T)), or a combination of them (T4: NPK + FZ + AMF + T) and were cultured by the hydroponic method in completely controlled conditions. Then, they were simultaneously exposed to nickel chloride at various rates (0, 75, or 150 µM) at the three-leaf stage. They were harvested two weeks later and were subjected to the measurement of Ni content, nitrate and nitrite content, nitrate reductase activity, and amino acid profile by high-performance liquid chromatography. The results showed that the application of Ni at higher rates increased Ni, nitrate, and nitrite contents and nitrate reductase activity. The study of Ni accumulation and TF revealed that Ni accumulated in the roots to a greater extent than in the shoots and TF was < 1 in all treatments. The shoot amino acid profile showed that the treatment of Ni+2 increased som amino acids such as aspartic acid, asparagine, serine, histidine, and glycine versus the control, whereas T4 Ni+2 increased aspartic acid, glutamic acid, threonine and arginine. The change in amino acids in Ni-treated plants may play a key role in their adaptation to Ni stress. The findings indicate that biofertilizers played a crucial role in mitigating the negative impacts of Ni on corn plants through alterations in amino acid composition and decreased absorption and translocation of Ni.

Interaction of melatonin and H2S mitigates NaCl toxicity summer savory (Satureja hortensis L.) through Modulation of biosynthesis of secondary metabolites and physio-biochemical attributes.

As versatile signaling molecules, melatonin (ML) and hydrogen sulfide (H2S) are well-known for their roles in response to abiotic stresses. However, their cross-talk to the regulation of biochemical defence responses and secondary metabolite synthesis during salinity has received less attention. Here, the role of ML-H2S interplay in inducing defensive responses and the biosynthesis of essential oil compounds in summer savoury plants under NaCl treatment was investigated. NaCl treatment, by increasing Na accumulation, disrupting nitrogen metabolism, and inducing oxidative stress, lowered photosynthetic pigments and savoury growth. NaCl treatment also resulted in a decrease in γ-terpinene (10.3%), α-terpinene (21.9%), and p-cymene (15.3%), while an increase in carvacrol (9.1%) was observed over the control. ML and ML + H2S increased the activity of antioxidant enzymes and the level of total phenols and flavonoids, resulting in decreased levels of hydrogen peroxide and superoxide anion and alleviation of oxidative damage under salinity. ML and ML + H2S increased K uptake and restored K/Na homeostasis, thus protecting the photosynthetic apparatus against NaCl-induced toxicity. ML and ML + H2S treatments also improved nitrate/ammonium homeostasis and stimulated nitrogen metabolism, leading to improved summer savoury adaptation to NaCl stress. ML and ML + H2S changed the composition of essential oils, leading to an increase in the monoterpene hydrocarbons and oxygenated monoterpenes in plants stressed with NaCl. However, the addition of an H2S scavenger, hypotaurine, inhibited the protective effects of the ML and ML + H2S treatments under NaCl stress, which could confirm the function of H2S as a signaling molecule in the downstream defence pathway induced by ML.

Regulation of AreA on lipid biosynthesis under different nitrogen sources and C/N ratios in the model oleaginous fungus Mucor circinelloides.

Mucor circinelloides has been exploited as model filamentous fungi for studies of genetic manipulation of lipogenesis. It is widely recognized that lipid accumulation is increased when there is a lack of nitrogen source in oleaginous microorganism. Nitrogen metabolism in filamentous fungi is a complex process that can be regulated by the global nitrogen regulator AreA. In this study, we cultivated the areA-knockout and -overexpression strains obtained in our previous study, using 20 different nitrogen sources. It emerged that the disruption of AreA in M. circinelloides reduced its sensitivity to nitrogen availability, resulting in increased lipid synthesis. Specially, the areA-knockout strain was unable to fully utilize many nitrogen sources but the ammonium and glutamate. We continued to investigate lipid production at different molar C/N ratios using glucose as sole carbon source and ammonium sulfate as sole nitrogen source, of which the high C/N ratios activate high lipid accumulation. By comparing the experimental results with transcriptional analysis, we were able to identify the optimal process conditions suitable for lipid accumulation and potential targets for future metabolic engineering.

Effects of superoxide radical on photosynthesis and K+ and redox homeostasis in quinoa and spinach.

Methyl viologen (MV), also known as paraquat, is a widely used herbicide but has also been reported as highly toxic to different life forms. The mode of its operation is related to superoxide radical (O2.-) production and consequent oxidative damage. However, besides the damage to key macromolecules, reactive oxygen species (ROS; to which O2.- belongs) are also known as regulators of numerous ion transport systems located at cellular membranes. In this study, we used MV as a tool to probe the role of O2.- in regulating membrane-transport activity and systemic acquired tolerance in halophytic Chenopodium quinoa and glycophytic spinach plants. Both plant species showed growth reduction in terms of reduced shoot length, lower shoot fresh and dry weight, photosynthesis rate, and chlorophyll contents; however, quinoa showed less reduction in growth compared with spinach. This whole plant response was further examined by measuring the ion concentration, gene expression of ion transporters, activation of antioxidants, and osmolyte accumulation. We observed that at the mechanistic level, the differences in growth in response to MV were conferred by at least four complementary physiological mechanisms: (1) higher K+ loss from spinach leaves resulted from higher expression of MV-induced plasma membrane-based depolarization-activated K+ efflux GORK channel, (2) higher activation of high-affinity K+ uptake transporter HAK5 in quinoa, (3) higher antioxidant production and osmolyte accumulation in quinoa as compared with spinach, and (4) maintaining a higher rate of photosynthesis due to higher chlorophyll contents, and efficiency of photosystem II and reduced ROS and MDA contents. Obtained results also showed that MV induced O2.- significantly reduced N contents in both species but with more pronounced effects in glycophytic spinach. Taken together this study has shown the role of O2.- in regulating membrane ion transport and N metabolism in the leaves of halophyte vs. glycophyte in the context of oxidative stress tolerance.

Magnesium alleviates growth inhibition under low potassium by enhancing photosynthesis and carbon-nitrogen metabolism in apple plants.

Potassium (K) and magnesium (Mg) play analogous roles in regulating plant photosynthesis and carbon and nitrogen (C-N) metabolism. Based on this consensus, we hypothesize that appropriate Mg supplementation may alleviate growth inhibition under low K stress. We monitored morphological, physiological, and molecular changes in G935 apple plants under different K (0.1 and 6 mmol L-1) and Mg supply (3 and 6 mmol L-1) conditions. Low K stress caused changes in root and leaf structure, inhibited photosynthesis, and limited the root growth of the apple rootstock. Further study on Mg supplementation showed that it could promote the uptake of K+ and NO3- by upregulating the expression of K+ transporter proteins such as Arabidopsis K+ transporter 1 (MdAKT1), high-affinity K+ transporter 1 (MdHKT1), and potassium transporter 5 (MdPT5) and nitrate transporters such as nitrate transporter 1.1/1.2/2.1/2.4 (MdNRT 1.1/1.2/2.1/2.4). Mg promoted the translocation of 15N from roots to leaves and enhanced photosynthetic N utilization efficiency (PNUE) by increasing the proportion of photosynthetic N and alleviating photosynthetic restrictions. Furthermore, Mg supplementation improved the synthesis of photosynthates by enhancing the activities of sugar-metabolizing enzymes (Rubisco, SS, SPS, S6PDH). Mg also facilitated the transport of sucrose and sorbitol from leaves to roots by upregulating the expression of sucrose transporter 1.1/1.2/4.1/4.2 (MdSUT 1.1/1.2/4.1/4.2) and sorbitol transporter 1.1/1.2 (MdSOT 1.1/1.2). Overall, Mg effectively alleviated growth inhibition in apple rootstock plants under low K stress by facilitating the uptake of N and K uptake, optimizing nitrogen partitioning, enhancing nitrogen use efficiency (NUE) and PNUE, and promoting the photosynthate synthesis and translocation.

Selenite improves growth by modulating phytohormone pathways and reprogramming primary and secondary metabolism in tomato plants.

Selenium (Se) is an essential micronutrient in organisms that has a significant impact on physiological activity and gene expression in plants, thereby affecting growth and development. Humans and animals acquire Se from plants. Tomato (Solanum lycopersicum L.) is an important vegetable crop worldwide. Improving the Se nutrient level not only is beneficial for growth, development and stress resistance in tomato plants but also contributes to improving human health. However, the molecular basis of Se-mediated tomato plant growth has not been fully elucidated. In this study, using physiological and transcriptomic analyses, we investigated the effects of a low dosage of selenite [Se(Ⅳ)] on tomato seedling growth. Se(IV) enhanced the photosynthetic efficiency and increased the accumulation of soluble sugars, dry matter and organic matter, thereby promoting tomato plant growth. Transcriptome analysis revealed that Se(IV) reprogrammed primary and secondary metabolic pathways, thus modulating plant growth. Se(IV) also increased the concentrations of auxin, jasmonic acid and salicylic acid in leaves and the concentration of cytokinin in roots, thus altering phytohormone signaling pathways and affecting plant growth and stress resistance in tomato plants. Furthermore, exogenous Se(IV) alters the expression of genes involved in flavonoid biosynthesis, thereby modulating plant growth and development in tomato plants. Taken together, these findings provide important insights into the regulatory mechanisms of low-dose Se(IV) on tomato growth and contribute to the breeding of Se-accumulating tomato cultivars.