Increased Rate of NAD Metabolism Shortens Plant Longevity by Accelerating Developmental Senescence in Arabidopsis

Adjustment of light-responsive NADP dynamics in chloroplasts by stromal pH

Cyclic electron transfer (CET) predominates when NADP+ is at basal levels, early in photosynthetic induction; however, the mechanism underlying the subsequent supply of NADP+ to fully drive steady-state linear electron transfer remains unclear. Here, we investigated whether CET is involved in de novo NADP+ supply in Arabidopsis thaliana and measured chloroplastic NADP dynamics to evaluate responsiveness to variable light, photochemical inhibitors, darkness, and CET activity. The sum of oxidized and reduced forms shows that levels of NADP and NAD increase and decrease, respectively, in response to light; levels of NADP and NAD decrease and increase in the dark, respectively. Moreover, consistent with the pH change in the stroma, the pH preference of chloroplast NAD+ phosphorylation and NADP+ dephosphorylation is alkaline and weakly acidic, respectively. Furthermore, CET is correlated with upregulation of light-responsive NADP level increases and downregulation of dark-responsive NADP level reductions. These findings are consistent with CET helping to regulate NADP pool size via stromal pH regulation under fluctuating light conditions.

Assessment of Greenhouse Tomato Anthesis Rate Through Metabolomics Using LASSO Regularized Linear Regression Model

While the high year-round production of tomatoes has been facilitated by solar greenhouse cultivation, these yields readily fluctuate in response to changing environmental conditions. Mathematic modeling has been applied to forecast phenotypes of tomatoes using environmental measurements (e.g., temperature) as indirect parameters. In this study, metabolome data, as direct parameters reflecting plant internal status, were used to construct a predictive model of the anthesis rate of greenhouse tomatoes. Metabolome data were obtained from tomato leaves and used as variables for linear regression with the least absolute shrinkage and selection operator (LASSO) for prediction. The constructed model accurately predicted the anthesis rate, with an R² value of 0.85. Twenty-nine of the 161 metabolites were selected as candidate markers. The selected metabolites were further validated for their association with anthesis rates using the different metabolome datasets. To assess the importance of the selected metabolites in cultivation, the relationships between the metabolites and cultivation conditions were analyzed via correspondence analysis. Trigonelline, whose content did not exhibit a diurnal rhythm, displayed major contributions to the cultivation, and is thus a potential metabolic marker for predicting the anthesis rate. This study demonstrates that machine learning can be applied to metabolome data to identify metabolites indicative of agricultural traits.

Overexpression of nicotinamidase 3 (NIC3) gene and the exogenous application of nicotinic acid (NA) enhance drought tolerance and increase biomass in Arabidopsis

Overexpressing Nicotinamidase 3 gene, and the exogenous application of its metabolite nicotinic acid (NA), enhance drought stress tolerance and increase biomass in Arabidopsis thaliana. With progressive global climatic changes, plant productivity is threatened severely by drought stress. Deciphering the molecular mechanisms regarding genes responsible for balancing plant growth and stress amelioration could imply multiple possibilities for future sustainable goals. Nicotinamide adenine dinucleotide (NAD) biosynthesis and recycling/ distribution is a crucial feature for plant growth. The current study focuses on the functional characterization of nicotinamidase 3 (NIC3) gene, which is involved in the biochemical conversion of nicotinamide (NAM) to nicotinic acid (NA) in the salvage pathway of NAD biosynthesis. Our data show that overexpression of NIC3 gene enhances drought stress tolerance and increases plant growth. NIC3-OX plants accumulated more NA as compared to WT plants. Moreover, the upregulation of several genes related to plant growth/stress tolerance indicates that regulating the NAD salvage pathway could significantly enhance plant growth and drought stress tolerance. The exogenous application of nicotinic acid (NA) showed a similar phenotype as the effect of overexpressing NIC3 gene. In short, we contemplated the role of NIC3 gene and NA application in drought stress tolerance and plant growth. Our results would be helpful in engineering plants with enhanced drought stress tolerance and increased growth potential.

Nicotinamide adenine dinucleotides are associated with distinct redox control of germination in Acer seeds with contrasting physiology

Seed germination is a complex process enabling plant reproduction. Germination was found to be regulated at the proteome, metabolome and hormonal levels as well as via discrete post-translational modification of proteins including phosphorylation and carbonylation. Redox balance is also involved but less studied. Acer seeds displaying orthodox and recalcitrant characteristics were investigated to determine the levels of redox couples of nicotinamide adenine dinucleotide (NAD) phosphate (NADP) and integrated with the levels of ascorbate and glutathione. NAD and NADP concentrations were higher in Norway maple seeds and exceptionally high at the germinated stage, being the most contrasting parameter between germinating Acer seeds. In contrast, NAD(P)H/NAD(P)+ ratios were higher in sycamore seeds, thus exhibiting higher reducing power. Despite distinct concentrations of ascorbate and glutathione, both seed types attained in embryonic axes and cotyledons had similar ratios of reduced/oxidized forms of ascorbate and half-cell reduction potential of glutathione at the germinated stage. Both species accomplished germination displaying different strategies to modulate redox status. Sycamore produced higher amounts of ascorbate and maintained pyridine nucleotides in reduced forms. Interestingly, lower NAD(P) concentrations limited the regeneration of ascorbate and glutathione but dynamically drove metabolic reactions, particularly in this species, and contributed to faster germination. We suggest that NAD(P) is an important player in regulating redox status during germination in a distinct manner in Norway maple and sycamore seeds.

Reciprocal regulation between nicotinamide adenine dinucleotide metabolism and abscisic acid and stress response pathways in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) is an essential coenzyme that has emerged as a central hub linking redox equilibrium and signal transduction in living organisms. The homeostasis of NAD is required for plant growth, development, and adaption to environmental cues. In this study, we isolated a chilling hypersensitive Arabidopsis thaliana mutant named qs-2 and identified the causal mutation in the gene encoding quinolinate synthase (QS) critical for NAD biosynthesis. The qs-2 mutant is also hypersensitive to salt stress and abscisic acid (ABA) but resistant to drought stress. The qs-2 mutant accumulates a reduced level of NAD and over-accumulates reactive oxygen species (ROS). The ABA-hypersensitivity of qs-2 can be rescued by supplementation of NAD precursors and by mutations in the ABA signaling components SnRK2s or RBOHF. Furthermore, ABA-induced over-accumulation of ROS in the qs-2 mutant is dependent on the SnRK2s and RBOHF. The expression of QS gene is repressed directly by ABI4, a transcription factor in the ABA response pathway. Together, our findings reveal an unexpected interplay between NAD biosynthesis and ABA and stress signaling, which is critical for our understanding of the regulation of plant growth and stress responses.

NAD(P)-Driven Redox Status Contributes to Desiccation Tolerance in Acer seeds

Desiccation tolerance is a developmental program enabling seed survival in a dry state and is common in seeds categorized as orthodox. We focused on NAD and its phosphorylated form (NADP) because their continual switching between reduced (NAD(P)H) and oxidized (NAD(P)+) forms is involved in the modulation of redox signaling and the determination of the reducing power and further antioxidant responses. Norway maple and sycamore seeds representing the orthodox and recalcitrant categories, respectively, were used as models in a comparison of responses to water loss. The process of desiccation up to 10% water content (WC) was monitored in Norway maple seeds, while dehydration up to 30% WC was monitored in desiccation-sensitive sycamore seeds. Norway maple and sycamore seeds, particularly their embryonic axes, exhibited a distinct redox status during dehydration and desiccation. High NADPH levels, NAD+ accumulation, low and stable NAD(P)H/NAD(P)+ ratios expressed as reducing power and high NADPH-dependent enzyme activity were reported in Norway maple seeds and were considered attributes of orthodox-type seeds. The contrasting results of sycamore seeds contributed to their low antioxidant capacity and high sensitivity to desiccation. NADPH deficiency, low NADPH-dependent enzyme activity and lack of NAD+ accumulation were primary features of sycamore seeds, with implications for their NAD(P)H/NAD(P)+ ratios and reducing power and with effects on many seed traits. Thus, we propose that the distinct levels of pyridine nucleotides and their redox status contribute to orthodox and recalcitrant phenotype differentiation in seeds by affecting cellular redox signaling, metabolism and the antioxidant system.

Measurement of Chloroplastic NAD Kinase Activity and Whole Tissue NAD Kinase Assay

Nicotinamide adenine dinucleotide phosphate (NADP) synthesis requires nicotinamide adenine dinucleotide (NAD) kinase activity, substrate NAD and ATP. The NAD kinase responds to various environmental stimuli and its activity is regulated via various regulatory pathways, such as Ca2+-dependent and redox-dependent signals. Conventional in vitro NAD kinase assay has been useful to evaluate enzyme activity; however, recent reports revealed a dynamics of NADP pool (the sum of NADP+ and NADPH) under fluctuating light condition, indicating that the rate of NADP synthesis is not always determined by NAD kinase activity. Here, we developed a novel method for the estimation of chloroplastic NAD kinase activity by quantifying the changes in the NADP amounts in response to illumination. As our approach does not involve protein extraction, it saves time (compared to the in vitro assay), thereby allowing for a sequence of assays, and provides several clues in the investigation of regulatory mechanisms behind NADP synthesis under various environmental conditions.

Inter-Organelle NAD Metabolism Underpinning Light Responsive NADP Dynamics in Plants

Upon illumination, photosystem I in chloroplasts catalyzes light-driven electron transport from plastocyanin to ferredoxin, followed by the reduction of NADP+ to NADPH by ferredoxin:NADP+ reductase for CO2 fixation. At the beginning of photosynthesis, NADP+ supply control is dominated by de novo NADP+ synthesis rather than being recycled from the Calvin cycle. Importantly, ferredoxin distributes electrons to NADP+ as well as to thioredoxins for light-dependent regulatory mechanisms, to cyclic electron flow for more adenosine triphosphate (ATP) production, and to several metabolites for reductive reactions. We previously demonstrated that the NADP+ synthesis activity and the amount of the NADP pool size, namely the sum of NADP+ and NADPH, varies depending on the light conditions and the ferredoxin-thioredoxin system. In addition, the regulatory mechanism of cytoplasmic NAD+ supply is also involved in the chloroplastic NADP+ supply control because NAD+ is an essential precursor for NADP+ synthesis. In this mini-review, we summarize the most recent advances on our understanding of the regulatory mechanisms of NADP+ production, focusing on the interactions, crosstalk, and co-regulation between chloroplasts and the cytoplasm at the level of NAD+ metabolism and molecular transport.

Physiological Significance of NAD Kinases in Cyanobacteria

Unicellular cyanobacteria are thought to be the evolutionary ancestors of plant chloroplasts and are widely used both for chemical production and as model organisms in studies of photosynthesis. Although most research focused on increasing reducing power (that is, NADPH) as target of metabolic engineering, the physiological roles of NAD(P)(H) in cyanobacteria poorly understood. In cyanobacteria such as the model species Synechocystis sp. PCC 6803, most metabolic pathways share a single compartment. This complex metabolism raises the question of how cyanobacteria control the amounts of the redox pairs NADH/NAD⁺ and NADPH/NADP⁺ in the cyanobacterial metabolic pathways. For example, photosynthetic and respiratory electron transport chains share several redox components in the thylakoid lumen, including plastoquinone, cytochrome b₆f (cyt b₆f), and the redox carriers plastocyanin and cytochrome c6. In the case of photosynthesis, NADP⁺ acts as an important electron mediator on the acceptor-side of photosystem I (PSI) in the linear electron chain as well as in the plant chloroplast. Meanwhile, in respiration, most electrons derived from NADPH and NADH are transferred by NAD(P)H dehydrogenases. Therefore, it is expected that Synechocystis employs unique NAD(P)(H) -pool control mechanisms to regulate the mixed metabolic systems involved in photosynthesis and respiration. This review article summarizes the current state of knowledge of NAD(P)(H) metabolism in Synechocystis. In particular, we focus on the physiological function in Synechocystis of NAD kinase, the enzyme that phosphorylates NAD⁺ to NADP⁺.

A hidden link between leaf development and senescence

Leaf senescence is the final step of leaf development and is usually accompanied by visible color changes from green to yellow or brown. Unlike the senescence of the whole body of animals and unicellular organisms, which is often associated with death, leaf senescence in plants requires highly integrative processes towards cell death with nutrient recycling and storage. Since leaf senescence plays pivotal roles in the production of plant biomass and grain yield, the mechanisms of degradation and relocation of macromolecules as well as the regulation of signaling and biosynthetic pathways have received much attention. The importance of the plant hormone ethylene in the onset of leaf senescence has been clearly documented. However, research has increasingly demonstrated that the function of ethylene in the regulation of leaf senescence is dependent on leaf development. This review raises the issue of how ethylene requires developmental regulators and focuses on the developmental aspect of leaf senescence. It also emphasizes the remarkable impact that developmental regulators have on regulating the onset of leaf senescence.

MeNA, Controlled by Reversible Methylation of Nicotinate, Is an NAD Precursor that Undergoes Long-Distance Transport in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) biosynthesis, including synthesis from aspartate via the de novo pathway and from nicotinate (NA) via the Preiss-Handler pathway, is conserved in land plants. Diverse species of NA conjugates, which are mainly involved in NA detoxification, were also found in all tested land plants. Among these conjugates, MeNA (NA methyl ester) has been widely detected in angiosperm plants, although its physiological function and the underlying mechanism for its production in planta remain largely unknown. Here, we show that MeNA is an NAD precursor undergoing more efficient long-distance transport between organs than NA and nicotinamide in Arabidopsis. We found that Arabidopsis has one methyltransferase (designated AtNaMT1) capable of catalyzing carboxyl methylation of NA to yield MeNA and one methyl esterase (MES2) predominantly hydrolyzing MeNA back to NA. We further uncovered that the transfer of [¹⁴C]MeNA from the root to leaf was significantly increased in both MES2 knockdown and NaMT1-overexpressing lines, suggesting that both NaMT1 and MES2 fine-tune the long-distance transport of MeNA, which is ultimately utilized for NAD production. Abiotic stress (salt, abscisic acid, and mannitol) treatments, which are known to exacerbate NAD degradation, induce the expression of NaMT1 but suppress MES2 expression, suggesting that MeNA may play a role in stress adaption. Collectively, our study indicates that reversible methylation of NA controls the biosynthesis of MeNA in Arabidopsis, which presumably functions as a detoxification form of free NA for efficient long-distance transport and eventually NAD production especially under abiotic stress, providing new insights into the relationship between NAD biosynthesis and NA conjugation in plants.

Ferredoxin/thioredoxin system plays an important role in the chloroplastic NADP status of Arabidopsis

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.

More to NAD+ than meets the eye: A regulator of metabolic pools and gene expression in Arabidopsis

Since its discovery more than a century ago, nicotinamide adenine dinucleotide (NAD⁺) is recognised as a fascinating cornerstone of cellular metabolism. This ubiquitous energy cofactor plays vital roles in metabolic pathways and regulatory processes, a fact emphasised by the essentiality of a balanced NAD⁺ metabolism for normal plant growth and development. Research on the role of NAD in plants has been predominantly carried out in the model plant Arabidopsis thaliana (Arabidopsis) with emphasis on the redox properties and cellular signalling functions of the metabolite. This review examines the current state of knowledge concerning how NAD can regulate both metabolic pools and gene expression in Arabidopsis. Particular focus is placed on recent studies highlighting the complexity of metabolic regulations involving NAD, more particularly in the mitochondrial compartment, and of signalling roles with respect to interactions with environmental fluctuations most specifically those involving plant immunity.

Characterization of l-aspartate oxidase from Arabidopsis thaliana

The flavoprotein l-aspartate oxidase (LASPO) is the first enzyme of the de novo biosynthetic pathway of NAD⁺ in plants. Although LASPO is considered pivotal to maintain NAD⁺ homeostasis, it has not been hitherto characterized in plants. Here, the cDNA encoding the LASPO from the model plant Arabidopsis thaliana (AtLASPO, At5g14760) has been cloned and expressed in Escherichia coli for subsequent enzyme characterization. The purified AtLASPO enzyme displayed a K_m of 0.79 mM for l-aspartate and a k_cat of 0.25 s^-1. We could further detect an l-aspartate: fumarate oxidoreductase activity of the recombinant plant enzyme. In addition, results indicated that NADP⁺ but not NAD⁺, and even more strongly NADH, inhibited AtLASPO at physiological concentrations by competing with the flavin for binding to the apoprotein. LASPO optimal pH and temperature, as well as plastidial pyridine nucleotide concentrations may contribute to an increased NAD⁺ production in planta. Moreover, in Arabidopsis thaliana AtLASPO gene expression exhibited a clear correlation between LASPO activity and NAD⁺ levels, thus demonstrating that plant LASPO catalyzes a key metabolic step of NAD⁺ synthesis.

The effector AvrRxo1 phosphorylates NAD in planta

Gram-negative bacterial pathogens of plants and animals employ type III secreted effectors to suppress innate immunity. Most characterized effectors work through modification of host proteins or transcriptional regulators, although a few are known to modify small molecule targets. The Xanthomonas type III secreted avirulence factor AvrRxo1 is a structural homolog of the zeta toxin family of sugar-nucleotide kinases that suppresses bacterial growth. AvrRxo1 was recently reported to phosphorylate the central metabolite and signaling molecule NAD in vitro, suggesting that the effector might enhance bacterial virulence on plants through manipulation of primary metabolic pathways. In this study, we determine that AvrRxo1 phosphorylates NAD in planta, and that its kinase catalytic sites are necessary for its toxic and resistance-triggering phenotypes. A global metabolomics approach was used to independently identify 3'-NADP as the sole detectable product of AvrRxo1 expression in yeast and bacteria, and NAD kinase activity was confirmed in vitro. 3'-NADP accumulated upon transient expression of AvrRxo1 in Nicotiana benthamiana and in rice leaves infected with avrRxo1-expressing strains of X. oryzae. Mutation of the catalytic aspartic acid residue D193 abolished AvrRxo1 kinase activity and several phenotypes of AvrRxo1, including toxicity in yeast, bacteria, and plants, suppression of the flg22-triggered ROS burst, and ability to trigger an R gene-mediated hypersensitive response. A mutation in the Walker A ATP-binding motif abolished the toxicity of AvrRxo1, but did not abolish the 3'-NADP production, virulence enhancement, ROS suppression, or HR-triggering phenotypes of AvrRxo1. These results demonstrate that a type III effector targets the central metabolite and redox carrier NAD in planta, and that this catalytic activity is required for toxicity and suppression of the ROS burst.