Versatile physiological functions of the Nudix hydrolase family in Arabidopsis

Nudix hydrolase WvNUDX24 is involved in borneol biosynthesis in Wurfbainia villosa

Borneol, camphor, and bornyl acetate are highly promising monoterpenoids widely used in medicine, flavor, food, and chemical applications. Bornyl diphosphate (BPP) serves as a common precursor for the biosynthesis of these monoterpenoids. Although bornyl diphosphate synthase (BPPS) that catalyzes the cyclization of geranyl diphosphate (GPP) to BPP has been identified in multiple plants, the enzyme responsible for the hydrolysis of BPP to produce borneol has not been reported. Here, we conducted in vitro and in vivo functional characterization to identify the Nudix hydrolase WvNUDX24 from W. villosa, which specifically catalyzes the hydrolysis of BPP to generate bornyl phosphate (BP), and then BP forms borneol under the action of phosphatase. Subcellular localization experiments indicated that the hydrolysis of BPP likely occurs in the cytoplasm. Furthermore, site-directed mutagenesis experiments revealed that four critical residues (R84, S96, P98, and G99) for the hydrolysis activity of WvNUDX24. Additionally, the functional identification of phosphatidic acid phosphatase (PAP) demonstrated that WvPAP5 and WvPAP10 were able to hydrolyze geranylgeranyl diphosphate (GGPP) and farnesyl diphosphate (FPP) to generate geranylgeranyl phosphate (GGP) and farnesyl phosphate (FP), respectively, but could not hydrolyze BPP, GPP, and neryl diphosphate (NPP) to produce corresponding monophosphate products. These findings highlight the essential role of WvNUDX24 in the first step of BPP hydrolysis to produce borneol and provide genetic elements for the production of BPP-related terpenoids through plant metabolic engineering and synthetic biology.

Nudix hydrolase 23 post-translationally regulates carotenoid biosynthesis in plants

Carotenoids are essential for photosynthesis and photoprotection. Plants must evolve multifaceted regulatory mechanisms to control carotenoid biosynthesis. However, the regulatory mechanisms and the regulators conserved among plant species remain elusive. Phytoene synthase (PSY) catalyzes the highly regulated step of carotenogenesis and geranylgeranyl diphosphate synthase (GGPPS) acts as a hub to interact with GGPP-utilizing enzymes for the synthesis of specific downstream isoprenoids. Here, we report a function of Nudix hydrolase 23 (NUDX23), a Nudix domain-containing protein, in post-translational regulation of PSY and GGPPS for carotenoid biosynthesis. NUDX23 expresses highly in Arabidopsis (Arabidopsis thaliana) leaves. Overexpression of NUDX23 significantly increases PSY and GGPPS protein levels and carotenoid production, whereas knockout of NUDX23 dramatically reduces their abundances and carotenoid accumulation in Arabidopsis. NUDX23 regulates carotenoid biosynthesis via direct interactions with PSY and GGPPS in chloroplasts, which enhances PSY and GGPPS protein stability in a large PSY-GGPPS enzyme complex. NUDX23 was found to co-migrate with PSY and GGPPS proteins and to be required for the enzyme complex assembly. Our findings uncover a regulatory mechanism underlying carotenoid biosynthesis in plants and offer promising genetic tools for developing carotenoid-enriched food crops.

Screening of genes involved in phage-resistance of Escherichia coli and effects of substances interacting with primosomal protein A on the resistant bacteria

AIMS

The study was to identify the genes involved in phage resistance and to develop an effective biocontrol method to improve the lytic activity of phages against foodborne pathogens.

METHODS AND RESULTS

A total of 3,909 single gene-deletion mutants of Escherichia coli BW25113 from the Keio collection were individually screened for genes involved in phage resistance. Phage S127BCL3 isolated from chicken liver, infecting both E. coli BW25113 and O157: H7, was characterized and used for screening. The 10 gene-deletion mutants showed increased susceptibility to phage S127BCL3. Among them, priA gene-deletion mutant strain showed significant susceptibility to the phages S127BCL3 and T7. Furthermore, we investigated the substances that have been reported to inhibit the function of primosomal protein A (PriA) and were used to confirm increased phage susceptibility in E. coli BW25113 (Parent strain) and O157: H7.

CONCLUSION

PriA inhibitors at a low concentration showed combined effects with phage against E. coli O157: H7 and delayed the regrowth rate of phage-resistant cells.

Arabidopsis thaliana NudiXes have RNA-decapping activity

Recent discoveries of various noncanonical RNA caps, such as dinucleoside polyphosphates (Np _n N), coenzyme A (CoA), and nicotinamide adenine dinucleotide (NAD) in all domains of life have led to a revision of views on RNA cap function and metabolism. Enzymes from the NudiX family capable of hydrolyzing a polyphosphate backbone attached to a nucleoside are the strongest candidates for degradation of noncanonically capped RNA. The model plant organism Arabidopsis thaliana encodes as many as 28 NudiX enzymes. For most of them, only in vitro substrates in the form of small molecules are known. In our study, we focused on four A. thaliana NudiX enzymes (AtNUDT6, AtNUDT7, AtNUDT19 and AtNUDT27), and we studied whether these enzymes can cleave RNA capped with Np _n Ns (Ap_2-5A, Gp_3-4G, Ap_3-5G, m⁷Gp₃G, m⁷Gp₃A), CoA, ADP-ribose, or NAD(H). While AtNUDT19 preferred NADH-RNA over other types of capped RNA, AtNUDT6 and AtNUDT7 preferentially cleaved Ap₄A-RNA. The most powerful decapping enzyme was AtNUDT27, which cleaved almost all types of capped RNA at a tenfold lower concentration than the other enzymes. We also compared cleavage efficiency of each enzyme on free small molecules with RNA capped with corresponding molecules. We found that AtNUDT6 prefers free Ap₄A, while AtNUDT7 preferentially cleaved Ap₄A-RNA. These findings show that NudiX enzymes may act as RNA-decapping enzymes in A. thaliana and that other noncanonical RNA caps such as Ap₄A and NADH should be searched for in plant RNA.

ADP-Ribosylation and Antiviral Resistance in Plants

ADP-ribosylation (ADPRylation) is a versatile posttranslational modification in eukaryotic cells which is involved in the regulation of a wide range of key biological processes, including DNA repair, cell signalling, programmed cell death, growth and development and responses to biotic and abiotic stresses. Members of the poly(ADP-ribosyl) polymerase (PARP) family play a central role in the process of ADPRylation. Protein targets can be modified by adding either a single ADP-ribose moiety (mono(ADP-ribosyl)ation; MARylation), which is catalysed by mono(ADP-ribosyl) transferases (MARTs or PARP "monoenzymes"), or targets may be decorated with chains of multiple ADP-ribose moieties (PARylation), via the activities of PARP "polyenzymes". Studies have revealed crosstalk between PARylation (and to a lesser extent, MARylation) processes in plants and plant-virus interactions, suggesting that these tight links may represent a novel factor regulating plant antiviral immunity. From this perspective, we go through the literature linking PARylation-associated processes with other plant regulation pathways controlling virus resistance. Once unraveled, these links may serve as the basis of innovative strategies to improve crop resistance to viruses under challenging environmental conditions which could mitigate yield losses.

Nudix hydrolase 14 influences plant development and grain chalkiness in rice

Nudix hydrolases (NUDX) can hydrolyze a wide range of organic pyrophosphates and are widely distributed in various organisms. Previous studies have shown that NUDXs are extensively involved in biotic and abiotic stress responses in different plant species; however, the role of NUDXs in plant growth and development remains largely unknown. In the present study, we identified and characterized OsNUDX14 localized in the mitochondria in rice. Results showed that OsNUDX14 is constitutively expressed in various tissues and most strongly expressed in mature leaves. We used CRISPR/Cas9 introducing mutations that editing OsNUDX14 and its encoding product. OsNUDX14-Cas9 (nudx14) lines presented early flowering and a larger flag leaf angle during the reproductive stage. In addition, OsNUDX14 affected grain chalkiness in rice. Furthermore, transcript profile analysis indicated that OsNUDX14 is associated with lignin biosynthesis in rice. Six major haplotypes were identified by six OsNUDX14 missense mutations, including Hap_1 to Hap_6. Accessions having the Hap_5 allele were geographically located mainly in South and Southeast Asia with a low frequency in the Xian/indica subspecies. This study revealed that OsNUDX14 is associated with plant development and grain chalkiness, providing a potential opportunity to optimize plant architecture and quality for crop breeding.

IbTLD modulates reactive oxygen species scavenging and DNA protection to confer salinity stress tolerance in tobacco

Plants accumulate reactive oxygen species (ROS) that may damage the cells under prolonged stress conditions. Reduction of the excessive ROS production can alleviate oxidative damage and enhance the survival rates under stress. TLDc-containing protein (TLD) was reported to confer tolerance to oxidative stress, but the regulatory mechanism of TLD remains unclear. In this study, we ectopically overexpressed the Ipomoea batatas TLDc gene (IbTLD) in tobacco and characterized its functions. RNA-sequencing analysis and Gene Ontology term enrichment analysis revealed that IbTLD up-regulates auxin-responsive genes in response to oxidative stress. Under salinity stress, the IbTLD transgenic lines showed higher germination rates, chlorophyll contents, and root lengths than wild type (W38). In addition, the IbTLD transgenic lines showed higher expression of ROS scavenging genes, nudix hydrolases, ROS scavenging enzyme activity, and lesser DNA damage compared to W38 under salinity stress. Therefore, our results suggest that IbTLD activates the expression of ROS scavenging genes and confers tolerance to salinity stress in planta.

Rice Nudix Hydrolase OsNUDX2 Sanitizes Oxidized Nucleotides

Nudix hydrolase (NUDX) hydrolyzes 8-oxo-(d)GTP to reduce the levels of oxidized nucleotides in the cells. 8-oxo-(d)GTP produced by reactive oxygen species (ROS) is incorporated into DNA/RNA and mispaired with adenine, causing replicational and transcriptional errors. Here, we identified a rice OsNUDX2 gene, whose expression level was increased 15-fold under UV-C irradiation. The open reading frame of the OsNUDX2 gene, which encodes 776 amino acid residues, was cloned into Escherichia coli cells to produce the protein of 100 kDa. The recombinant protein hydrolyzed 8-oxo-dGTP, in addition to dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP), as did Arabidopsis AtNUDX1; whereas the amino acid sequence of OsNUDX2 had 18% identity with AtNUDX1. OsNUDX2 had 14% identity with barley HvNUDX12, which hydrolyzes 8-oxo-dGTP and diadenosine tetraphosphates. Suppression of the lacZ amber mutation caused by the incorporation of 8-oxo-GTP into mRNA was prevented to a significant degree when the OsNUDX2 gene was expressed in mutT-deficient E. coli cells. These results suggest that the different substrate specificity and identity among plant 8-oxo-dGTP-hydrolyzing NUDXs and OsNUDX2 reduces UV stress by sanitizing the oxidized nucleotides.

PpNUDX8, a Peach NUDIX Hydrolase, Plays a Negative Regulator in Response to Drought Stress

Drought stress is a serious abiotic stress source that affects the growth and fruit quality of peach trees. However, the molecular mechanism of the NUDIX hydrolase family in peaches in response to drought stress is still unclear. Here, we isolated and identified the PpNUDX8 (Prupe.5G062300.1) gene from the peach NUDIX hydrolase family, and found that PpNUDX8 has a typical NUDIX hydrolase domain. In this study, we performed 15% PEG6000 drought treatment on peach seedlings, and qRT-PCR analysis showed that 15% PEG6000 induced the transcription level of PpNUDX8. Overexpression of PpNUDX8 reduced the tolerance of calli to 4% PEG6000 treatment. Compared with wild-type apple calli, PpNUDX8 transgenic apple calli had a lower fresh weight and higher MDA content. After 15% PEG6000 drought treatment, PpNUDX8 transgenic tobacco had a greater degree of wilting and shorter primary roots than Under control conditions. The chlorophyll, soluble protein, and proline contents in the transgenic tobacco decreased, and the MDA content and relative conductivity increased. At the same time, PpNUDX8 negatively regulated ABA signal transduction and reduced the transcriptional expression of stress response genes. In addition, PpNUDX8 was not sensitive to ABA, overexpression of PpNUDX8 reduced the expression of the ABA synthesis-related gene NCED6 and increases the expression of the ABA decomposition-related gene CYP1 in tobacco, which in turn leads to a decrease in the ABA content in tobacco. In addition, Under control conditions, overexpression of PpNUDX8 destroyed the homeostasis of NAD and reduced nicotinamide adenine dinucleotide (NADH) in tobacco. After 15% PEG6000 drought treatment, the changes in NAD and NADH in PpNUDX8 transgenic tobacco were more severe than those in WT tobacco. In addition, PpNUDX8 also interacted with PpSnRk1γ (Prupe.6G323700.1).

Duplication and specialization of NUDX1 in Rosaceae led to geraniol production in rose petals

Nudix hydrolases are conserved enzymes ubiquitously present in all kingdoms of life. Recent research revealed that several Nudix hydrolases are involved in terpenoid metabolism in plants. In modern roses, RhNUDX1 is responsible for formation of geraniol, a major compound of rose scent. Nevertheless, this compound is produced by monoterpene synthases in many geraniol-producing plants. As a consequence, this raised the question about the origin of RhNUDX1 function and the NUDX1 gene evolution in Rosaceae, in wild roses or/and during the domestication process. Here, we showed that three distinct clades of NUDX1 emerged in the Rosoidae subfamily (Nudx1-1 to Nudx1-3 clades), and two subclades evolved in the Rosa genus (Nudx1-1a and Nudx1-1b subclades). We also showed that the Nudx1-1b subclade was more ancient than the Nudx1-1a subclade, and that the NUDX1-1a gene emerged by a trans-duplication of the more ancient NUDX1-1b gene. After the transposition, NUDX1-1a was cis-duplicated, leading to a gene dosage effect on the production of geraniol in different species. Furthermore, the NUDX1-1a appearance was accompanied by the evolution of its promoter, most likely from a Copia retrotransposon origin, leading to its petal-specific expression. Thus, our data strongly suggest that the unique function of NUDX1-1a in geraniol formation was evolved naturally in the genus Rosa before domestication.

Isopentenyl diphosphate/dimethylallyl diphosphate-specific Nudix hydrolase from the methanogenic archaeon Methanosarcina mazei

Nudix hydrolases typically catalyze the hydrolysis of nucleoside diphosphate linked to moiety X and yield nucleoside monophosphate and X-phosphate, while some of them hydrolyze a terminal diphosphate group of non-nucleosidic compounds and convert it into a phosphate group. Although the number of Nudix hydrolases is usually limited in archaea comparing with those in bacteria and eukaryotes, the physiological functions of most archaeal Nudix hydrolases remain unknown. In this study, a Nudix hydrolase family protein, MM_2582, from the methanogenic archaeon Methanosarcina mazei was recombinantly expressed in Escherichia coli, purified, and characterized. This recombinant protein shows higher hydrolase activity toward isopentenyl diphosphate and short-chain prenyl diphosphates than that toward nucleosidic compounds. Kinetic studies demonstrated that the archaeal enzyme prefers isopentenyl diphosphate and dimethylallyl diphosphate, which suggests its role in the biosynthesis of prenylated flavin mononucleotide, a recently discovered coenzyme that is required, for example, in the archaea-specific modified mevalonate pathway.

Epigenetic and transcriptional responses underlying mangrove adaptation to UV-B

Tropical plants have adapted to strong solar ultraviolet (UV) radiation. Here we compare molecular responses of two tropical mangroves Avecennia marina and Rhizophora apiculata to high-dose UV-B. Whole-genome bisulfate sequencing indicates that high UV-B induced comparable hyper- or hypo-methylation in three sequence contexts (CG, CHG, and CHH, where H refers to A, T, or C) in A. marina but mainly CHG hypomethylation in R. apiculata. RNA and small RNA sequencing reveals UV-B induced relaxation of transposable element (TE) silencing together with up-regulation of TE-adjacent genes in R. apiculata but not in A. marina. Despite conserved upregulation of flavonoid biosynthesis and downregulation of photosynthesis genes caused by high UV-B, A. marina specifically upregulated ABC transporter and ubiquinone biosynthesis genes that are known to be protective against UV-B-induced damage. Our results point to divergent responses underlying plant UV-B adaptation at both the epigenetic and transcriptional level.

A genome-wide association study identifies Arabidopsis thaliana genes that contribute to differences in the outcome of infection with two Turnip mosaic potyvirus strains that differ in their evolutionary history and degree of host specialization

Viruses lie in a continuum between generalism and specialism depending on their ability to infect more or less hosts. While generalists are able to successfully infect a wide variety of hosts, specialists are limited to one or a few. Even though generalists seem to gain an advantage due to their wide host range, they usually pay a pleiotropic fitness cost within each host. On the contrary, a specialist has maximal fitness within its own host. A relevant yet poorly explored question is whether viruses differ in the way they interact with their hosts' gene expression depending on their degree of specialization. Using a genome-wide association study approach, we have identified host genes whose expression depends on whether hosts were infected with more or less specialized viral strains. Four hundred fifty natural accessions of Arabidopsis thaliana were inoculated with Turnip mosaic potyvirus strains with different past evolutionary histories and that shown different degrees of specialization. Three disease-related traits were measured and associated with different sets of host genes for each strain. The genetic architectures of these traits differed among viral strains and, in the case of the more specialized virus, also varied along the duration of infection. While most of the mapped loci were strain specific, one shared locus was mapped for both strains, a disease-resistance TIR-NBS-LRR class protein. Likewise, only putative cysteine-rich receptor-like protein kinases were involved in all three traits. The impact on disease progress of 10 selected genes was validated by studying the infection phenotypes of loss-of-function mutant plants. Nine of these mutants have altered the disease progress and/or symptoms intensity between both strains. Compared to wild-type plants six had an effect on both viral strains, three had an effect only on the more specialized, and two were significant during infection with the less specialized.

Overexpression of a Rosa rugosa Thunb. NUDX gene enhances biosynthesis of scent volatiles in petunia

Rosa rugosa is an important natural perfume plant in China. Rose essential oil is known as ‘liquid gold’ and has high economic and health values. Monoterpenes are the main fragrant components of R. rugosa flower and essential oil. In this study, a member of the hydrolase gene family RrNUDX1 was cloned from Chinese traditional R. rugosa ‘Tang Hong’. Combined analysis of RrNUDX1 gene expression and the aroma components in different development stages and different parts of flower organ, we found that the main aroma component content was consistent with the gene expression pattern. The RrNUDX1 overexpressed Petunia hybrida was acquired via Agrobacterium-mediated genetic transformation systems. The blades of the transgenic petunias became wider and its growth vigor became strong with stronger fragrance. Gas chromatography with mass spectrometry analysis showed that the contents of the main aroma components of the transgenic petunias including methyl benzoate significantly increased. These findings indicate that the RrNUDX1 gene plays a role in enhancing the fragrance of petunia flowers, and they could lay an important foundation for the homeotic transformation of RrNUDX1 in R. rugosa for cultivating new R. rugosa varieties of high-yield and -quality essential oil.

Messenger RNA 5' NAD+ Capping Is a Dynamic Regulatory Epitranscriptome Mark That Is Required for Proper Response to Abscisic Acid in Arabidopsis

Although eukaryotic messenger RNAs (mRNAs) normally possess a 5' end N⁷-methyl guanosine (m⁷G) cap, a non-canonical 5' nicotinamide adenine dinucleotide (NAD⁺) cap can tag certain transcripts for degradation mediated by the NAD⁺ decapping enzyme DXO1. Despite this importance, whether NAD⁺ capping dynamically responds to specific stimuli to regulate eukaryotic transcriptomes remains unknown. Here, we reveal a link between NAD⁺ capping and tissue- and hormone response-specific mRNA stability. In the absence of DXO1 function, transcripts displaying a high proportion of NAD⁺ capping are instead processed into RNA-dependent RNA polymerase 6-dependent small RNAs, resulting in their continued turnover likely to free the NAD⁺ molecules. Additionally, the NAD⁺-capped transcriptome is significantly remodeled in response to the essential plant hormone abscisic acid in a mechanism that is primarily independent of DXO1. Overall, our findings reveal a previously uncharacterized and essential role of NAD⁺ capping in dynamically regulating transcript stability during specific physiological responses.

Identity and functions of inorganic and inositol polyphosphates in plants

Inorganic polyphosphates (polyPs) and inositol pyrophosphates (PP-InsPs) form important stores of inorganic phosphate and can act as energy metabolites and signaling molecules. Here we review our current understanding of polyP and inositol phosphate (InsP) metabolism and physiology in plants. We outline methods for polyP and InsP detection, discuss the known plant enzymes involved in their synthesis and breakdown, and summarize the potential physiological and signaling functions for these enigmatic molecules in plants.

The first comprehensive phylogenetic and biochemical analysis of NADH diphosphatases reveals that the enzyme from Tuber melanosporum is highly active towards NAD

Nudix (for nucleoside diphosphatases linked to other moieties, X) hydrolases are a diverse family of proteins capable of cleaving an enormous variety of substrates, ranging from nucleotide sugars to NAD⁺-capped RNAs. Although all the members of this superfamily share a common conserved catalytic motif, the Nudix box, their substrate specificity lies in specific sequence traits, which give rise to different subfamilies. Among them, NADH pyrophosphatases or diphosphatases (NADDs) are poorly studied and nothing is known about their distribution. To address this, we designed a Prosite-compatible pattern to identify new NADDs sequences. In silico scanning of the UniProtKB database showed that 3% of Nudix proteins were NADDs and displayed 21 different domain architectures, the canonical architecture (NUDIX-like_zf-NADH-PPase_NUDIX) being the most abundant (53%). Interestingly, NADD fungal sequences were prominent among eukaryotes, and were distributed over several Classes, including Pezizomycetes. Unexpectedly, in this last fungal Class, NADDs were found to be present from the most common recent ancestor to Tuberaceae, following a molecular phylogeny distribution similar to that previously described using two thousand single concatenated genes. Finally, when truffle-forming ectomycorrhizal Tuber melanosporum NADD was biochemically characterized, it showed the highest NAD⁺/NADH catalytic efficiency ratio ever described.

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.

NUDIX hydrolases with inorganic polyphosphate exo- and endopolyphosphatase activities in the glycosome, cytosol and nucleus of Trypanosoma brucei

Trypanosoma brucei, a protist parasite that causes African trypanosomiasis or sleeping sickness, relies mainly on glycolysis for ATP production when in its mammalian host. Glycolysis occurs within a peroxisome-like organelle named the glycosome. Previous work from our laboratory reported the presence of significant amounts of inorganic polyphosphate (polyP), a polymer of three to hundreds of orthophosphate units, in the glycosomes and nucleoli of T. brucei In this work, we identified and characterized the activity of two Nudix hydrolases (NHs), T. brucei Nudix hydrolase (TbNH) 2 and TbNH4, one located in the glycosomes and the other in the cytosol and nucleus, respectively, which can degrade polyP. We found that TbNH2 is an exopolyphosphatase with higher activity on short chain polyP, while TbNH4 is an endo- and exopolyphosphatase that has similar activity on polyP of various chain sizes. Both enzymes have higher activity at around pH 8.0. We also found that only TbNH2 can dephosphorylate ATP and ADP but with lower affinity than for polyP. Our results suggest that NHs can participate in polyP homeostasis and therefore may help control polyP levels in glycosomes, cytosol and nuclei of T. brucei.

Arabidopsis DXO1 links RNA turnover and chloroplast function independently of its enzymatic activity

The DXO family of proteins participates in eukaryotic mRNA 5'-end quality control, removal of non-canonical NAD+ cap and maturation of fungal rRNA precursors. In this work, we characterize the Arabidopsis thaliana DXO homolog, DXO1. We demonstrate that the plant-specific modification within the active site negatively affects 5'-end capping surveillance properties of DXO1, but has only a minor impact on its strong deNADding activity. Unexpectedly, catalytic activity does not contribute to striking morphological and molecular aberrations observed upon DXO1 knockout in plants, which include growth and pigmentation deficiency, global transcriptomic changes and accumulation of RNA quality control siRNAs. Conversely, these phenotypes depend on the plant-specific N-terminal extension of DXO1. Pale-green coloration of DXO1-deficient plants and our RNA-seq data reveal that DXO1 affects chloroplast-localized processes. We propose that DXO1 mediates the connection between RNA turnover and retrograde chloroplast-to-nucleus signaling independently of its deNADding properties.

The Ralstonia solanacearum effector RipN suppresses plant PAMP-triggered immunity, localizes to the endoplasmic reticulum and nucleus, and alters the NADH/NAD+ ratio in Arabidopsis

Ralstonia solanacearum, one of the most destructive plant bacterial pathogens, delivers an array of effector proteins via its type III secretion system for pathogenesis. However, the biochemical functions of most of these proteins remain unclear. RipN is a type III effector with unknown function(s) from the pathogen R. solanacearum. Here, we demonstrate that RipN is a conserved type III effector found within the R. solanacearum species complex that contains a putative Nudix hydrolase domain and has ADP-ribose/NADH pyrophosphorylase activity in vitro. Further analysis shows that RipN localizes to the endoplasmic reticulum (ER) and nucleus in Nicotiana tabacum leaf cells and Arabidopsis protoplasts, and truncation of the C-terminus of RipN results in a loss of nuclear and ER targeting. Furthermore, the expression of RipN in Arabidopsis suppresses callose deposition and the transcription of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) marker genes under flg22 treatment, and promotes bacterial growth in planta. In addition, the expression of RipN in plant cells alters NADH/NAD+ , but not GSH/GSSG, ratios, and its Nudix hydrolase activity is indispensable for such biochemical function. These results suggest that RipN acts as a Nudix hydrolase, alters the NADH/NAD+ ratio of the plant and contributes to R. solanacearum virulence by suppression of PTI of the host.

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.

Peroxisomal plant metabolism - an update on nitric oxide, Ca2+ and the NADPH recycling network

Plant peroxisomes are recognized organelles that - with their capacity to generate greater amounts of H₂O₂ than other subcellular compartments - have a remarkable oxidative metabolism. However, over the last 15 years, new information has shown that plant peroxisomes contain other important molecules and enzymes, including nitric oxide (NO), peroxynitrite, a NADPH-recycling system, Ca²⁺ and lipid-derived signals, such as jasmonic acid (JA) and nitro-fatty acid (NO₂-FA). This highlights the potential for complex interactions within the peroxisomal nitro-oxidative metabolism, which also affects the status of the cell and consequently its physiological processes. In this review, we provide an update on the peroxisomal interactions between all these molecules. Particular emphasis will be placed on the generation of the free-radical NO, which requires the presence of Ca²⁺, calmodulin and NADPH redox power. Peroxisomes possess several NADPH regeneration mechanisms, such as those mediated by glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) proteins, which are involved in the oxidative phase of the pentose phosphate pathway, as well as that mediated by NADP-isocitrate dehydrogenase (ICDH). The generated NADPH is also an essential cofactor across other peroxisomal pathways, including the antioxidant ascorbate-glutathione cycle and unsaturated fatty acid β-oxidation, the latter being a source of powerful signaling molecules such as JA and NO₂-FA.

A novel plant enzyme with dual activity: an atypical Nudix hydrolase and a dipeptidyl peptidase III

In a search for plant homologues of dipeptidyl peptidase III (DPP III) family, we found a predicted protein from the moss Physcomitrella patens (UniProt entry: A9TLP4), which shared 61% sequence identity with the Arabidopsis thaliana uncharacterized protein, designated Nudix hydrolase 3. Both proteins contained all conserved regions of the DPP III family, but instead of the characteristic hexapeptide HEXXGH zinc-binding motif, they possessed a pentapeptide HEXXH, and at the N-terminus, a Nudix box, a hallmark of Nudix hydrolases, known to act upon a variety of nucleoside diphosphate derivatives. To investigate their biochemical properties, we expressed heterologously and purified Physcomitrella (PpND) and Arabidopsis (AtND) protein. Both hydrolyzed, with comparable catalytic efficiency, the isopentenyl diphosphate (IPP), a universal precursor for the biosynthesis of isoprenoid compounds. In addition, PpND dephosphorylated four purine nucleotides (ADP, dGDP, dGTP, and 8-oxo-dATP) with strong preference for oxidized dATP. Furthermore, PpND and AtND showed DPP III activity against dipeptidyl-2-arylamide substrates, which they cleaved with different specificity. This is the first report of a dual activity enzyme, highly conserved in land plants, which catalyzes the hydrolysis of a peptide bond and of a phosphate bond, acting both as a dipeptidyl peptidase III and an atypical Nudix hydrolase.

Loss-of-function of an Arabidopsis NADPH pyrophosphohydrolase, AtNUDX19, impacts on the pyridine nucleotides status and confers photooxidative stress tolerance

The levels and redox states of pyridine nucleotides, such as NADP(H), regulate the cellular redox homeostasis, which is crucial for photooxidative stress response in plants. However, how they are controlled is poorly understood. An Arabidopsis Nudix hydrolase, AtNUDX19, was previously identified to have NADPH hydrolytic activity in vitro, suggesting this enzyme to be a regulator of the NADPH status. We herein examined the physiological role of AtNUDX19 using its loss-of-function mutants. NADPH levels were increased in nudx19 mutants under both normal and high light conditions, while NADP+ and NAD+ levels were decreased. Despite the high redox states of NADP(H), nudx19 mutants exhibited high tolerance to moderate light- or methylviologen-induced photooxidative stresses. This tolerance might be partially attributed to the activation of either or both photosynthesis and the antioxidant system. Furthermore, a microarray analysis suggested the role of ANUDX19 in regulation of the salicylic acid (SA) response in a negative manner. Indeed, nudx19 mutants accumulated SA and showed high sensitivity to the hormone. Our findings demonstrate that ANUDX19 acts as an NADPH pyrophosphohydrolase to modulate cellular levels and redox states of pyridine nucleotides and fine-tunes photooxidative stress response through the regulation of photosynthesis, antioxidant system, and possibly hormonal signaling.

Modulation of NADH Levels by Arabidopsis Nudix Hydrolases, AtNUDX6 and 7, and the Respective Proteins Themselves Play Distinct Roles in the Regulation of Various Cellular Responses Involved in Biotic/Abiotic Stresses

Arabidopsis Nudix hydrolases, AtNUDX6 and 7, exhibit pyrophosphohydrolase activities toward NADH and contribute to the modulation of various defense responses, such as the poly(ADP-ribosyl)ation (PAR) reaction and salicylic acid (SA)-induced Nonexpresser of Pathogenesis-Related genes 1 (NPR1)-dependent defense pathway, against biotic and abiotic stresses. However, the mechanisms by which these enzymes regulate such cellular responses remain unclear. To clarify the functional role(s) of AtNUDX6 and 7 and NADH metabolism, we examined the effects of the transient expression of the active and inactive forms of AtNUDX6 and 7 under the control of an estrogen (ES)-inducible system on various stress responses. The transient expression of active AtNUDX6 and 7 proteins suppressed NADH levels and induced PAR activity, whereas that of their inactive forms did not, indicating the involvement of NADH metabolism in the regulation of the PAR reaction. A transcriptome analysis using KO-nudx6, KO-nudx7 and double KO-nudx6/7 plants, in which intracellular NADH levels increased, identified genes (NADH-responsive genes, NRGs) whose expression levels positively and negatively correlated with NADH levels. Many NRGs did not overlap with the genes whose expression was reported to be responsive to various types of oxidants and reductants, suggesting a novel role for intracellular NADH levels as a redox signaling cue. The active and inactive AtNUDX6 proteins induced the expression of thioredoxin-h5, the activator of NPR1 and SA-induced NPR1-dependent defense genes, while the active and inactive AtNUDX7 proteins suppressed the accumulation of SA and subsequent gene expression, indicating that AtNUDX6 and 7 proteins themselves play distinct roles in stress responses.

Activation of NADPH-recycling systems in leaves and roots of Arabidopsis thaliana under arsenic-induced stress conditions is accelerated by knock-out of Nudix hydrolase 19 (AtNUDX19) gene

NADPH is an important cofactor in cell growth, proliferation and detoxification. Arabidopsis thaliana Nudix hydrolase 19 (AtNUDX19) belongs to a family of proteins defined by the conserved amino-acid sequence GX5-EX7REUXEEXGU which has the capacity to hydrolyze NADPH as a physiological substrate in vivo. Given the importance of NADPH in the cellular redox homeostasis of plants, the present study compares the responses of the main NADPH-recycling systems including NADP-isocitrate dehydrogenase (ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and NADP-malic enzyme (ME) in the leaves and roots of Arabidopsis wild-type (Wt) and knock-out (KO) AtNUDX19 mutant (Atnudx19) plants under physiological and arsenic-induced stress conditions. Two major features were observed in the behavior of the main NADPH-recycling systems: (i) under optimal conditions in both organs, the levels of these activities were higher in nudx19 mutants than in Wt plants; and, (ii) under 500μM AsV conditions, these activities increase, especially in nudx19 mutant plants. Moreover, G6PDH activity in roots was the most affected enzyme in both Wt and nudx19 mutant plants, with a 4.6-fold and 5.0-fold increase, respectively. In summary, the data reveals a connection between the absence of chloroplastic AtNUDX19 and the rise in all NADP-dehydrogenase activities under physiological and arsenic-induced stress conditions, particularly in roots. This suggests that AtNUDX19 could be a key factor in modulating the NADPH pool in plants and consequently in redox homeostasis.

Identification and characterization of Arabidopsis AtNUDX9 as a GDP-d-mannose pyrophosphohydrolase: its involvement in root growth inhibition in response to ammonium

GDP-d-mannose (GDP-d-Man) is an important intermediate in ascorbic acid (AsA) synthesis, cell wall synthesis, protein N-glycosylation, and glycosylphosphatidylinositol-anchoring in plants. Thus, the modulation of intracellular levels of GDP-d-Man could be important for maintaining various cellular processes. Here an Arabidopsis GDP-d-Man pyrophosphohydrolase, AtNUDX9 (AtNUDT9; At3g46200), which hydrolysed GDP-d-Man to GMP and mannose 1-phosphate, was identified. The K m and V max values for GDP-d-Man of AtNUDX9 were 376±24 μM and 1.61±0.15 μmol min(-1) mg(-1) protein, respectively. Among various tissues, the expression levels of AtNUDX9 and the total activity of GDP-d-Man pyrophosphohydrolase were the highest in the roots. The GDP-d-Man pyrophosphohydrolase activity was increased in the root of plants grown in the presence of ammonium. No difference was observed in the levels of AsA in the leaf and root tissues of the wild-type and knockout-nudx9 (KO-nudx9) plants, whereas a marked increase in N-glycoprotein levels and enhanced growth were detected in the roots of KO-nudx9 plants in the presence of ammonium. These results suggest that AtNUDX9 is involved in the regulation of GDP-d-Man levels affecting ammonium sensitivity via modulation of protein N-glycosylation in the roots.

PLANT VOLATILES. Biosynthesis of monoterpene scent compounds in roses

The scent of roses (Rosa x hybrida) is composed of hundreds of volatile molecules. Monoterpenes represent up to 70% percent of the scent content in some cultivars, such as the Papa Meilland rose. Monoterpene biosynthesis in plants relies on plastid-localized terpene synthases. Combining transcriptomic and genetic approaches, we show that the Nudix hydrolase RhNUDX1, localized in the cytoplasm, is part of a pathway for the biosynthesis of free monoterpene alcohols that contribute to fragrance in roses. The RhNUDX1 protein shows geranyl diphosphate diphosphohydrolase activity in vitro and supports geraniol biosynthesis in planta.