High ammonium supply impairs photosynthetic efficiency in rice exposed to excess light

Enhanced Stomatal Conductance Supports Photosynthesis in Wheat to Improved NH4+ Tolerance

The impact of ammonium (NH4+) stress on plant growth varies across species and cultivars, necessitating an in-depth exploration of the underlying response mechanisms. This study delves into elucidating the photosynthetic responses and differences in tolerance to NH4+ stress by investigating the effects on two wheat (Triticum aestivum L.) cultivars, Xumai25 (NH4+-less sensitive) and Yangmai20 (NH4+-sensitive). The cultivars were grown under hydroponic conditions with either sole ammonium nitrogen (NH4+, AN) or nitrate nitrogen (NO3-, NN) as the nitrogen source. NH4+ stress exerted a profound inhibitory effect on seedling growth and photosynthesis in wheat. However, these effects were less pronounced in Xumai25 than in Yangmai20. Dynamic photosynthetic analysis revealed that the suppression in photosynthesis was primarily attributed to stomatal limitation associated with a decrease in leaf water status and osmotic potential. Compared to Yangmai20, Xumai25 exhibited a significantly higher leaf K+ concentration and TaAKT1 upregulation, leading to a stronger stomatal opening and, consequently, a better photosynthetic performance under NH4+ stress. In conclusion, our study suggested stomatal limitation as the primary factor restricting photosynthesis under NH4+ stress. Furthermore, we demonstrated that improved regulation of osmotic substances contributed to higher stomatal conductance and enhanced photosynthetic performance in Xumai25.

Nitrogen sources differentially affect respiration, growth, and carbon allocation in Andean and Lowland ecotypes of Chenopodium quinoa Willd

Chenopodium quinoa Willd. is a native species that originated in the High Andes plateau (Altiplano) and its cultivation spread out to the south of Chile. Because of the different edaphoclimatic characteristics of both regions, soils from Altiplano accumulated higher levels of nitrate (NO3-) than in the south of Chile, where soils favor ammonium (NH₄ ⁺) accumulation. To elucidate whether C. quinoa ecotypes differ in several physiological and biochemical parameters related to their capacity to assimilate NO3- and NH₄ ⁺, juvenile plants of Socaire (from Altiplano) and Faro (from Lowland/South of Chile) were grown under different sources of N (NO3- or NH₄ ⁺). Measurements of photosynthesis and foliar oxygen-isotope fractionation were carried out, together with biochemical analyses, as proxies for the analysis of plant performance or sensitivity to NH₄ ⁺. Overall, while NH₄ ⁺ reduced the growth of Socaire, it induced higher biomass productivity and increased protein synthesis, oxygen consumption, and cytochrome oxidase activity in Faro. We discussed that ATP yield from respiration in Faro could promote protein production from assimilated NH₄ ⁺ to benefit its growth. The characterization of this differential sensitivity of both quinoa ecotypes for NH₄ ⁺ contributes to a better understanding of nutritional aspects driving plant primary productivity.

Soil ammonium (NH4+) toxicity thresholds for restoration grass species

Ionic rare earth mining has resulted in large amounts of bare soils, and revegetation success plays an important role in mine site rehabilitation and environmental management. However, the mining soils still maintain high NH₄⁺ concentrations that inhibit plant growth and NH₄⁺ toxicity thresholds for restoration plants have not been established. Here we investigated the NH₄⁺ toxicological effects and provided toxicity thresholds for grasses (Lolium perenne L. and Medicago sativa L.) commonly used in restoration. The results show that high NH₄⁺ concentration not only reduces the plant biomass and soluble sugars in leaves but also increases the H₂O₂ and MDA content, and SOD, POD, and GPX activities in roots. The SOD activities and root biomass can be adopted as the most NH₄⁺ sensitive biomarkers. Six ecotoxicological endpoints (root biomass, soluble sugars, proline, H₂O₂, MDA, and GSH) of ryegrass, eight ecotoxicological endpoints (root biomass, soluble sugars, proline, MDA, SOD, POD, GPX, and GSH) of alfalfa were selected to determine the threshold concentrations. The toxicity thresholds of NH₄⁺ concentrations were proposed as 171.9 (EC₅), 207.8 (EC₁₀), 286.6 (EC₂₅), 382.3 (EC₅₀) mg kg^-1 for ryegrass and 171.9 (EC₅), 193.2 (EC₁₀), 234.7 (EC₂₅), 289.6 (EC₅₀) mg kg^-1 for alfalfa. The toxicity thresholds and the relation between plant physiological indicators and NH₄⁺ concentrations can be used to assess the suitability of the investigated plants for ecological restoration strategies.

CBL-Interacting Protein Kinase OsCIPK18 Regulates the Response of Ammonium Toxicity in Rice Roots

Ammonium ( NH 4 + ) is one of the major nitrogen sources for plants. However, excessive ammonium can cause serious harm to the growth and development of plants, i.e., ammonium toxicity. The primary regulatory mechanisms behind ammonium toxicity are still poorly characterized. In this study, we showed that OsCIPK18, a CBL-interacting protein kinase, plays an important role in response to ammonium toxicity by comparative analysis of the physiological and whole transcriptome of the T-DNA insertion mutant (cipk18) and the wild-type (WT). Root biomass and length of cipk18 are less inhibited by excess NH 4 + compared with WT, indicating increased resistance to ammonium toxicity. Transcriptome analysis reveals that OsCIPK18 affects the NH 4 + uptake by regulating the expression of OsAMT1;2 and other NH 4 + transporters, but does not affect ammonium assimilation. Differentially expressed genes induced by excess NH 4 + in WT and cipk18 were associated with functions, such as ion transport, metabolism, cell wall formation, and phytohormones signaling, suggesting a fundamental role for OsCIPK18 in ammonium toxicity. We further identified a transcriptional regulatory network downstream of OsCIPK18 under NH 4 + stress that is centered on several core transcription factors. Moreover, OsCIPK18 might function as a transmitter in the auxin and abscisic acid (ABA) signaling pathways affected by excess ammonium. These data allowed us to define an OsCIPK18-regulated/dependent transcriptomic network for the response of ammonium toxicity and provide new insights into the mechanisms underlying ammonium toxicity.

Improved Utilization of Nitrate Nitrogen Through Within-Leaf Nitrogen Allocation Trade-Offs in Leymus chinensis

The Sharply increasing atmospheric nitrogen (N) deposition may substantially impact the N availability and photosynthetic capacity of terrestrial plants. Determining the trade-off relationship between within-leaf N sources and allocation is therefore critical for understanding the photosynthetic response to nitrogen deposition in grassland ecosystems. We conducted field experiments to examine the effects of inorganic nitrogen addition (sole NH₄ ⁺, sole NO₃ ^- and mixed NH₄ ⁺/NO₃ ^-: 50%/50%) on N assimilation and allocation by Leymus chinensis. The leaf N allocated to the photosynthetic apparatus (N_PSN) and chlorophyll content per unit area (Chl_area) were significantly positively correlated with the photosynthetic N-use efficiency (PNUE). The sole NO₃ ^- treatment significantly increased the plant leaf PNUE and biomass by increasing the photosynthetic N allocation and Chl_area. Under the NO₃ treatment, L. chinensis plants devoted more N to their bioenergetics and light-harvesting systems to increase electron transfer. Plants reduced the cell wall N allocation or increased their soluble protein concentrations to balance growth and defense under the NO₃ treatment. In the sole NH₄ ⁺ treatment, however, plants decreased their N allocation to photosynthetic components, but increased their N allocation to the cell wall and elsewhere. Our findings demonstrated that within-leaf N allocation optimization is a key adaptive mechanism by which plants maximize their PNUE and biomass under predicted future global changes.

Transcriptomic analysis dissects the regulatory strategy of toxic cyanobacterium Microcystis aeruginosa under differential nitrogen forms

The critical role of nitrogen in the global proliferation of cyanobacterial blooms is arousing increasing attention. However, the mechanism underlying the algal responses to differential nitrogen forms remains unclarified. The physiological and transcriptomic changes of Microcystis aeruginosa supplied with different nitrogen forms (nitrate and ammonium) were highlighted in this study. The results indicated that ammonium behaves better in stimulating the initial growth in N-limited cells than nitrate. However, a concomitant side effect is that cellular growth and photosynthesis decreased due to photosystem II damage induced by excess absorbed light energy under 10 mg L^-1 ammonium. By contrast, adequate nitrate supply favored more efficient photosynthesis, higher biomass yield and microcystin quotas than ammonium. Depending on the supplied nitrogen form, different transcriptomic patterns were observed in M. aeruginosa. Under nitrate, the upregulation of genes involved in Arg biosynthesis, ornithine-urea cycle and photosynthesis increased nitrogen storage and cellular growth, while genes involved in cyclic electron flow around photosystem I and CO₂-concentrating mechanism were heightened to dissipate excess energy under high ammonium. These insights provided important clues for understanding the physiological and molecular effects of available nitrogen forms on the frequent outbreaks of cyanobacteria.

Transcriptome and Proteomics Analysis of Wheat Seedling Roots Reveals That Increasing NH4 +/NO3 - Ratio Induced Root Lignification and Reduced Nitrogen Utilization

Wheat growth and nitrogen (N) uptake gradually decrease in response to high NH₄ ⁺/NO₃ ^- ratio. However, the mechanisms underlying the response of wheat seedling roots to changes in NH₄ ⁺/NO₃ ^- ratio remain unclear. In this study, we investigated wheat growth, transcriptome, and proteome profiles of roots in response to increasing NH₄ ⁺/NO₃ ^- ratios (N _a : 100/0; N _r1: 75/25, N _r2: 50/50, N _r3: 25/75, and N _n : 0/100). High NH₄ ⁺/NO₃ ^- ratio significantly reduced leaf relative chlorophyll content, Fv/Fm, and ΦII values. Both total root length and specific root length decreased with increasing NH₄ ⁺/NO₃ ^- ratios. Moreover, the rise in NH₄ ⁺/NO₃ ^- ratio significantly promoted O₂ ^- production. Furthermore, transcriptome sequencing and tandem mass tag-based quantitative proteome analyses identified 14,376 differentially expressed genes (DEGs) and 1,819 differentially expressed proteins (DEPs). The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicated that glutathione metabolism and phenylpropanoid biosynthesis were the main two shared enriched pathways across ratio comparisons. Upregulated DEGs and DEPs involving glutathione S-transferases may contribute to the prevention of oxidative stress. An increment in the NH₄ ⁺/NO₃ ^- ratio induced the expression of genes and proteins involved in lignin biosynthesis, which increased root lignin content. Additionally, phylogenetic tree analysis showed that both A0A3B6NPP6 and A0A3B6LM09 belong to the cinnamyl-alcohol dehydrogenase subfamily. Fifteen downregulated DEGs were identified as high-affinity nitrate transporters or nitrate transporters. Upregulated TraesCS3D02G344800 and TraesCS3A02G350800 were involved in ammonium transport. Downregulated A0A3B6Q9B3 is involved in nitrate transport, whereas A0A3B6PQS3 is a ferredoxin-nitrite reductase. This may explain why an increase in the NH₄ ⁺/NO₃ ^- ratio significantly reduced root NO₃ ^--N content but increased NH₄ ⁺-N content. Overall, these results demonstrated that increasing the NH₄ ⁺/NO₃ ^- ratio at the seedling stage induced the accumulation of reactive oxygen species, which in turn enhanced root glutathione metabolism and lignification, thereby resulting in increased root oxidative tolerance at the cost of reducing nitrate transport and utilization, which reduced leaf photosynthetic capacity and, ultimately, plant biomass accumulation.

Comparative effects of sodium hydrosulfide and proline on functional repair in rice chloroplast through the D1 protein and thioredoxin system under simulated thiocyanate pollution

Various environmental contaminants can find their way to enter plant cells and disturb and/or damage the essential components of PSII repair cycle in chloroplast, thereby resulting in dysfunction of chloroplast. In the current research, a microcosm hydroponic experiment was set up to evaluate the comparative effects of sodium hydrosulfide (NaHS)- and proline (Pro)-mediated functional repairing of chloroplast in rice plants under SCN^- stress. Our results displayed that when exposed to environmental realistic SCN^- concentrations (24-300 mg L^-1), foist significant (p < 0.05) gene-dose repercussion on the pathways of photosynthetic reactions and energy metabolism in rice shoots, and a downturn in the level of total soluble starch, sugar, and chlorophyll. Sodium hydrosulfide application effectively mitigated (p < 0.05) the toxic effects of SCN^- in rice seedlings by stimulating the processes of phosphorylation, dephosphorylation and new-synthesis of D1 protein in the PSII repair cycle, and increased the turnover of D1 protein to recover CO₂ assimilation. Evidently, Pro treatment mainly enhanced (p < 0.05) the expression of magnesium chelatase (MgCh) and ribulose-1,5-bisphosphate carboxylase (Rubisco) related genes under simulated SCN^- stress, suggesting that the targeted repairing site in chloroplast by Pro was different from NaHS. The outcome of the present research contributes to a better understanding at molecular level for repairing of chloroplast functional disorder by NaHS and Pro at different key nodes under SCN^- stress.

Efficient Photosynthetic Functioning of Arabidopsis thaliana Through Electron Dissipation in Chloroplasts and Electron Export to Mitochondria Under Ammonium Nutrition

An improvement in photosynthetic rate promotes the growth of crop plants. The sink-regulation of photosynthesis is crucial in optimizing nitrogen fixation and integrating it with carbon balance. Studies on these processes are essential in understanding growth inhibition in plants with ammonium ( NH 4 + ) syndrome. Hence, we sought to investigate the effects of using nitrogen sources with different states of reduction (during assimilation of NO 3 - versus NH 4 + ) on the photosynthetic performance of Arabidopsis thaliana. Our results demonstrated that photosynthetic functioning during long-term NH 4 + nutrition was not disturbed and that no indication of photoinhibition of PSII was detected, revealing the robustness of the photosynthetic apparatus during stressful conditions. Based on our findings, we propose multiple strategies to sustain photosynthetic activity during limited reductant utilization for NH 4 + assimilation. One mechanism to prevent chloroplast electron transport chain overreduction during NH 4 + nutrition is for cyclic electron flow together with plastid terminal oxidase activity. Moreover, redox state in chloroplasts was optimized by a dedicated type II NAD(P)H dehydrogenase. In order to reduce the amount of energy that reaches the photosynthetic reaction centers and to facilitate photosynthetic protection during NH 4 + nutrition, non-photochemical quenching (NPQ) and ample xanthophyll cycle pigments efficiently dissipate excess excitation. Additionally, high redox load may be dissipated in other metabolic reactions outside of chloroplasts due to the direct export of nucleotides through the malate/oxaloacetate valve. Mitochondrial alternative pathways can downstream support the overreduction of chloroplasts. This mechanism correlated with the improved growth of A. thaliana with the overexpression of the alternative oxidase 1a (AOX1a) during NH 4 + nutrition. Most remarkably, our findings demonstrated the capacity of chloroplasts to tolerate NH 4 + syndrome instead of providing redox poise to the cells.

Utilization of GC-MS untargeted metabolomics to assess the delayed response of glufosinate treatment of transgenic herbicide resistant (HR) buffalo grasses (Stenotaphrum secundatum L.)

INTRODUCTION

Herbicide resistant (HR) buffalo grasses were genetically engineered to resist the non-selective herbicide, glufosinate in order to facilitate a modern, 'weeding program' which is highly effective in terms of minimizing costs and labor. The resistant trait was conferred by an insertion of the pat gene to allow for the production of the enzyme phosphinothricin acetyltransferase (PAT) to detoxify the glufosinate inhibitive effect. To date, there are only a few reports using metabolomics as well as molecular characterizations published for glufosinate-resistant crops with no reports on HR turfgrass. Therefore, for the first time, this study examines the metabolome of glufosinate-resistant buffalo grasses which not only will be useful to future growers but also the scientific community.

OBJECTIVE

A major aim of this present work is to characterize and evaluate the metabolic alterations which may arise from a genetic transformation of HR buffalo grasses by comprehensively using gas chromatography-mass spectrometry (GC-MS) based untargeted metabolomics.

METHODS

Eight-week old plants of 4 HR buffalo grasses, (93-1A, 93-2B, 93-3C and 93-5A) and 3 wild type varieties (WT 8-4A, WT 9-1B and WT 9-1B) were selected for physiological, molecular and metabolomics experiments. Plants were either sprayed with 1, 5, 10 and 15% v/v of glufosinate to evaluate the visual injuries or submerged in 5% v/v of glufosinate 3 days prior to a GC-MS based untargeted metabolomics analysis. In contrast, the control group was treated with distilled water. Leaves were extracted in 1:1 methanol:water and then analysed, using an in-house GC-MS untargeted workflow.

RESULTS

Results identified 199 metabolites with only 6 of them (cis-aconitic acid, allantoin, cellobiose, glyceric acid, maltose and octadecanoic acid) found to be statistically significant (p < 0.05) between the HR and wild type buffalo grass varieties compared to the control experiment. Among these metabolites, unusual accumulation of allantoin was prominent and was an unanticipated effect of the pat gene insertion. As expected, glufosinate treatment caused significant metabolic alterations in the sensitive wild type, with the up-regulation of several amino acids (e.g. phenylalanine and isoleucine) which was likely due to glufosinate-induced senescence. The aminoacyl-tRNA biosynthetic pathway was identified as the most significant enriched pathway as a result of glufosinate effects because a number of its intermediates were amino acids.

CONCLUSION

HR buffalo grasses were very similar to its wild type comparator based on a comprehensive GC-MS based untargeted metabolomics and therefore, should guarantee the safe use of these HR buffalo grasses. The current metabolomics analyses not only confirmed the effects of glufosinate to up-regulate free amino acid pools in the sensitive wild type but also several alterations in sugar, sugar phosphate and organic acid metabolism have been reported.