Abstract
Plants use nitrate, ammonium, and organic nitrogen in the soil as nitrogen sources. Since the elevated CO
2
environment predicted for the near future will reduce nitrate utilization by C
3
...species, ammonium is attracting great interest. However, abundant ammonium nutrition impairs growth, i.e., ammonium toxicity, the primary cause of which remains to be determined. Here, we show that ammonium assimilation by GLUTAMINE SYNTHETASE 2 (GLN2) localized in the plastid rather than ammonium accumulation is a primary cause for toxicity, which challenges the textbook knowledge. With exposure to toxic levels of ammonium, the shoot GLN2 reaction produced an abundance of protons within cells, thereby elevating shoot acidity and stimulating expression of acidic stress-responsive genes. Application of an alkaline ammonia solution to the ammonium medium efficiently alleviated the ammonium toxicity with a concomitant reduction in shoot acidity. Consequently, we conclude that a primary cause of ammonium toxicity is acidic stress.
•High CO2 and nutrient supply enhanced biomass and amino acid levels in lettuce.•Fourteen amino acids in leaves were increased by irradiation of red LED for a week.•Irradiation of blue LED reduced ...biomass and organic acids.•Combination of red light, high CO2 and nutrients improved amino acids.
Carbon dioxide (CO2), nutrient supply, and light quality are amongst the major controlling factors to improve the biomass production and nutritional outputs in plant factory. The present study employed CE-MS to investigate the effects of high CO2, nutrient formulation, and LED on the accumulation of primary metabolites in head lettuce. Results suggested that high CO2 (1000ppm) and nutrient supply enhanced both the biomass and some amino acids. Hierarchical clustering analysis was used to evaluate effects of red LED in combination with high CO2 and Hoagland’s formulation; distinctive cluster formation contained 14 amino acids (mostly branched-chain and aromatic amino acids, histidine and arginine). Thus, simultaneous treatments of monochromatic LED, high CO2 and nutrient formulation improved the amino acids accumulation, and likely reduced the inorganic nitrogen sources in planta.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Plants require a high concentration of ascorbate as a redox buffer for survival under stress conditions, such as high light. Dehydroascorbate reductases (DHARs) are enzymes that catalyze the ...reduction of DHA to ascorbate using reduced glutathione (GSH) as an electron donor, allowing rapid ascorbate recycling. However, a recent study using an Arabidopsis (
) triple mutant lacking all three
genes (herein called ∆
) did not find evidence for their role in ascorbate recycling under oxidative stress. To further study the function of DHARs, we generated ∆
Arabidopsis plants as well as a quadruple mutant line combining ∆
with an additional
mutation that causes ascorbate deficiency. Measurements of ascorbate in these mutants under low- or high-light conditions indicated that DHARs have a nonnegligible impact on full ascorbate accumulation under high light, but that they are dispensable when ascorbate concentrations are low to moderate. Because GSH itself can reduce DHA nonenzymatically, we used the
mutant that contains ∼30% of the wild-type GSH level. The
mutant accumulated ascorbate at a wild-type level under high light; however, when the
mutation was combined with ∆
, there was near-complete inhibition of high-light-dependent ascorbate accumulation. The lack of ascorbate accumulation was consistent with a marked increase in the ascorbate degradation product threonate. These findings indicate that ascorbate recycling capacity is limited in ∆
plants, and that both DHAR activity and GSH content set a threshold for high-light-induced ascorbate accumulation.
Many enzymes involved in photosynthesis possess highly conserved cysteine residues that serve as redox switches in chloroplasts. These redox switches function to activate or deactivate enzymes during ...light-dark transitions and have the function of fine-tuning their activities according to the intensity of light. Accordingly, many studies on chloroplast redox regulation have been conducted under the hypothesis that "fine regulation of the activities of these enzymes is crucial for efficient photosynthesis." However, the impact of the regulatory system on plant metabolism is still unclear. To test this hypothesis, we here studied the impact of the ablation of a redox switch in chloroplast NADP-malate dehydrogenase (MDH). By genome editing, we generated a mutant plant whose MDH lacks one of its redox switches and is active even in dark conditions. Although NADPH consumption by MDH in the dark is expected to be harmful to plant growth, the mutant line did not show any phenotypic differences under standard long-day conditions. In contrast, the mutant line showed severe growth retardation under short-day or fluctuating light conditions. These results indicate that thiol-switch redox regulation of MDH activity is crucial for maintaining NADPH homeostasis in chloroplasts under these conditions.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Recent studies suggest that changes in leaf traits due to interactions between plants affect resource utilisation by herbivores, as well as herbivore distribution. However, this has not yet been ...confirmed experimentally. Here, we investigated the effects of phenotypic plasticity in leaf traits of Rumex obtusifolius (host plant) in response to intraspecific and interspecific interactions on the distribution of two leaf beetles, Gastrophysa atrocyanea (specialist herbivore) and Galerucella grisescens (generalist herbivore).
We investigated the local population density of R. obtusifolius plants and the presence of leaf beetles on the plants at five study sites. Leaf chemicals (condensed tannins and total phenolics) were compared between aggregated and solitary R. obtusifolius plants. To clarify the effects of the interaction environment of R. obtusifolius plants on their leaf traits and on resource utilisation by the leaf beetles, we compared leaf chemicals and preferences of adult leaf beetles among treatments where R. obtusifolius experienced intraspecific interaction, interspecific interaction or no interaction in cultivation experiments. Finally, we evaluated the independent and combined effects of patch size and intraspecific interaction on leaf beetle distribution in mesocosm experiments.
In the field, the presence of the specialist leaf beetle G. atrocyanea was positively correlated with the local population density (rosette overlap ratio) of R. obtusifolius plants; however, there was no correlation in the case of the generalist leaf beetle G. grisescens. In the cultivation experiments, plants in the intraspecific interaction treatment increased their leaf concentrations of condensed tannins and total phenolics, and G. atrocyanea consumed more of these leaves than leaves in other treatments. Similar results were observed in the field. In the mesocosm experiments, larger numbers of G. atrocyanea were distributed on R. obtusifolius plants exposed to below‐ground intraspecific interaction than on plants not exposed to intraspecific interaction.
Our results provide experimental evidence that leaf‐trait changes in response to intraspecific interaction between host plants influence specialist herbivore distribution. This highlights the need to integrate plant–plant interactions into our understanding of plant–animal interactions.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Summary
NADP is a key electron carrier for a broad spectrum of redox reactions, including photosynthesis. Hence, chloroplastic NADP status, as represented by redox status (ratio of NADPH to NADP+) ...and pool size (sum of NADPH and NADP+), is critical for homeostasis in photosynthetic cells. However, the mechanisms and molecules that regulate NADP status in chloroplasts remain largely unknown. We have now characterized an Arabidopsis mutant with imbalanced NADP status (inap1), which exhibits a high NADPH/NADP+ ratio and large NADP pool size. inap1 is a point mutation in At2g04700, which encodes the catalytic subunit of ferredoxin/thioredoxin reductase. Upon illumination, inap1 demonstrated earlier increases in NADP pool size than the wild type did. The mutated enzyme was also found in vitro to inefficiently reduce m‐type thioredoxin, which activates Calvin cycle enzymes, and NADP‐dependent malate dehydrogenase to export reducing power to the cytosol. Accordingly, Calvin cycle metabolites and amino acids diminished in inap1 plants. In addition, inap1 plants barely activate NADP‐malate dehydrogenase, and have an altered redox balance between the chloroplast and cytosol, resulting in inefficient nitrate reduction. Finally, mutants deficient in m‐type thioredoxin exhibited similar light‐dependent NADP dynamics as inap1. Collectively, the data suggest that defects in ferredoxin/thioredoxin reductase and m‐type thioredoxin decrease the consumption of NADPH, leading to a high NADPH/NADP+ ratio and large NADP pool size. The data also suggest that the fate of NADPH is an important influence on NADP pool size.
Significance statement
Coordination of NADP+ photoreduction with NADP+ production by NAD kinase in chloroplasts modulates redox status and prevents over‐reduction under fluctuating light. Thus, the discovery of a mechanism associated with chloroplastic NADP status advances our understanding of how plants optimize photosynthesis in response to variable light.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The high-yielding rice cultivar Takanari has fast photosynthetic induction owing to a high electron transport rate, stomatal conductance, and metabolic flux, leading to high daily carbon gain under ...fluctuating light.
Abstract
Leaves within crop canopies experience variable light over the course of a day, which greatly affects photosynthesis and crop productivity. Little is known about the mechanisms of the photosynthetic response to fluctuating light and their genetic control. Here, we examined gas exchange, metabolite levels, and chlorophyll fluorescence during the photosynthetic induction response in an Oryza sativa indica cultivar with high yield (Takanari) and a japonica cultivar with lower yield (Koshihikari). Takanari had a faster induction response to sudden increases in light intensity than Koshihikari, as demonstrated by faster increases in net CO2 assimilation rate, stomatal conductance, and electron transport rate. In a simulated light regime that mimicked a typical summer day, the faster induction response in Takanari increased daily CO2 assimilation by 10%. The faster response of Takanari was explained in part by its maintenance of a larger pool of Calvin–Benson cycle metabolites. Together, the rapid responses of electron transport rate, metabolic flux, and stomatal conductance in Takanari contributed to the greater daily carbon gain under fluctuating light typical of natural environments.
Arabidopsis NADK2 (NAD kinase 2) is a chloroplast-localized enzyme involved in NADP
+
synthesis, which acts as the final electron acceptor in the photosynthetic electron transfer chain. The ...NADK2-deficient mutant (nadk2) was used to analyze the effect of NAD(P)(H) unbalance in the dark-induced leaf senescence. During senescence, WT plants and nadk2 mutants showed a similar reduction in chlorophyll content. NAD(P)(H) quantification showed that the amount of total NAD(P)(H) decreased on the day 7 in WT but on the day 3 in nadk2. The phosphorylation ratio (i.e. NADP(H)/NAD(H)) decreased on day 1 in WT. In contrast, the nadk2 showed lower phosphorylation ratio at 0 day and no change throughout the aging process. Metabolome analysis showed that the metabolic profiles of both WT plants and nadk2 mutants subjected to dark-induced senescence adopted similar patterns as the senescence progressed. However, the changes in individual metabolites in the nadk2 mutants were different from those of the WT during dark-induced senescence.
Soluble oxalate accumulates in rice leaves, and it causes mineral deficiency and urinary syndrome in livestock that consume the leaves. In our previous study, we found that the oxalate content was ...higher in the leaves of Koshihikari (japonica type cultivar) than in those of Takanari (indica type cultivar). This difference was seen even when the two cultivars were grown under a high CO2 concentration, which inhibits oxalate synthesis via photorespiration, suggesting that the difference resulted from genetic factors rather than environmental factors. To clarify whether genetic factors affect the oxalate content of rice leaves, we measured the contents of oxalate and oxalate-related organic acids in the leaves of various rice cultivars the Rice Core Collection (WRC) and Japan Rice Core Collection (JRC) by capillary electrophoresis-mass spectrometry. Results showed that japonica type cultivars tended to accumulate more oxalate than aus or indica type cultivars. Correlation analysis revealed a positive correlation between oxalate accumulation and the citrate content, suggesting that the isocitrate pathway is involved in oxalate accumulation. On the other hand, a genome-wide association study for the oxalate content of the WRC and JRC cultivars did not reveal significant loci directly related to oxalate accumulation. This indicates that the combination of various loci may affect the oxalate contents of rice leaves.
Summary
Pyridine nucleotides (NAD(P)(H)) are electron carriers that are the driving forces in various metabolic pathways. Phosphorylation of NAD(H) to NADP(H) is performed by the enzyme NAD kinase ...(NADK). Synechocystis sp. PCC 6803 harbors two genes (sll1415 and slr0400) that encode proteins with NADK homology. When genetic mutants for sll1415 and slr0400 (Δ1415 and Δ0400, respectively) were cultured under photoheterotrophic growth conditions only the Δ1415 cells showed a growth defect. In wild‐type cells, the sll1415 transcript accumulated after the cells were transferred to photoheterotrophic conditions. Furthermore, NAD(P)(H) measurements demonstrated that a dynamic metabolic conversion was implemented during the adaptation from photoautotrophic to photoheterotrophic conditions. Electron microscopy observation and biochemistry quantification demonstrated the accumulation of glycogen in the Δ1415 cells under photoheterotrophic conditions at 96 h. Quantitative real‐time reverse transcription PCR (qRT‐PCR) demonstrated the accumulation of mRNAs that encoded glycogen biosynthesis‐related enzymes in photoheterotrophic Δ1415 cells. At 96 h, enzyme activity measurement in the photoheterotrophic Δ1415 cells demonstrated that the activities of glucose‐6‐phosphate dehydrogenase and 6‐phosphogluconate dehydrogenase were decreased, but the activities of glucose dehydrogenase were increased. Furthermore, metabolomics analysis demonstrated that the Δ1415 cells showed increased glucose‐6‐phosphate and 6‐phosphogluconate content at 96 h. Therefore, sll1415 has a significant function in the oxidative pentose phosphate (OPP) pathway for catabolism of glucose under photoheterotrophic conditions. Additionally, it is presumed that the slr0400 had a different role in glucose catabolism during growth. These results suggest that the two Synechocystis sp. PCC 6803 NADKs (Sll1415 and Slr0400) have distinct functions in photoheterotrophic cyanobacterial metabolism.
Significance Statement
Synechocystis sp. PCC 6803 contains two genes (sll1415 and slr0400) that encode proteins with NADK homology, but the biological function of these genes has not been revealed to date. Here, we show that the sll1415‐encoded NAD kinase is required to ensure an NADP+ supply sufficient for catabolism of glucose via the oxidative pentose phosphate pathway under photoheterotrophic conditions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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