UPS1 and UPS2 from Arabidopsis mediate high affinity transport of uracil and 5-fluorouracil

A gene cluster for polyamine transport and modification improves salt tolerance in tomato

Polyamines act as protective compounds directly protecting plants from stress-related damage, while also acting as signaling molecules to participate in serious abiotic stresses. However, the molecular mechanisms underlying these effects are poorly understood. Here, we utilized metabolome genome-wide association study to investigate the polyamine content of wild and cultivated tomato accessions, and we discovered a new gene cluster that drove polyamine content during tomato domestication. The gene cluster contains two polyphenol oxidases (SlPPOE and SlPPOF), two BAHD acyltransferases (SlAT4 and SlAT5), a coumaroyl-CoA ligase (Sl4CL6), and a polyamine uptake transporter (SlPUT3). SlPUT3 mediates polyamine uptake and transport, while the five other genes are involved in polyamine modification. Further salt tolerance assays demonstrated that SlPPOE, SlPPOF, and SlAT5 overexpression lines showed greater phenolamide accumulation and salt tolerance as compared with wild-type (WT). Meanwhile, the exogenous application of Spm to SlPUT3-OE lines displayed salt tolerance compared with WT, while having the opposite effect in slput3 lines, confirms that the polyamine and phenolamide can play a protective role by alleviating cell damage. SlPUT3 interacted with SlPIP2;4, a H2O2 transport protein, to maintain H2O2 homeostasis. Polyamine-derived H2O2 linked Spm to stress responses, suggesting that Spm signaling activates stress response pathways. Collectively, our finding reveals that the H2O2-polyamine-phenolamide module coordinately enhanced tomato salt stress tolerance and provide a foundation for tomato stress-resistance breeding.

Identification and Expression Analysis of UPS Gene Family in Potato

Ureide permeases (UPSs) mediate the transport of ureides, including allantoin and allantoate, which act as nitrogen-transporting compounds in plants and have recently been found to play a role in cellular signaling. To date, UPSs have not been reported in potato, and their identification is important for further function studies and for understanding molecular mechanisms of plant adverse responses. Based on potato genomic data, we identified 10 StUPS genes in potato (Solanum tuberosum L.). Then, we conducted a comprehensive study of the identified StUPS genes using bioinformatics methods. Genome phylogenetic and genomic localization analyses revealed that StUPSs can be classified into four categories, are highly homologous to Arabidopsis thaliana UPS members, and are distributed on three chromosomes. The six StUPS genes were investigated by RT-qPCR, and the findings indicated that all of these genes are involved in the response to several stresses, including low nitrogen, cold, ABA, salt, H2O2, and drought. This study establishes a strong theoretical framework for investigating the function of potato UPS genes, as well as the molecular mechanisms underlying the responses of these genes to various environmental stresses.

Deciphering the Diversity in Bacterial Transporters That Salvage Queuosine Precursors

Queuosine (Q) is a modification of the wobble base of tRNA harboring GUN anticodons with roles in decoding accuracy and efficiency. Its synthesis is complex with multiple enzymatic steps, and several pathway intermediates can be salvaged. The only two transporter families known to salvage Q precursors are QPTR/COG1738 and QrtT/QueT. Analyses of the distribution of known Q synthesis and salvage genes in human gut and oral microbiota genomes have suggested that more transporter families remain to be found and that Q precursor exchanges must occur within the structured microenvironments of the mammalian host. Using physical clustering and fusion-based association with Q salvage genes, candidate genes for missing transporters were identified and five were tested experimentally by complementation assays in Escherichia coli. Three genes encoding transporters from three different Pfam families, a ureide permease (PF07168) from Acidobacteriota bacterium, a hemolysin III family protein (PF03006) from Bifidobacterium breve, and a Major Facilitator Superfamily protein (PF07690) from Bartonella henselae, were found to allow the transport of both preQ0 and preQ1 in this heterologous system. This work suggests that many transporter families can evolve to transport Q precursors, reinforcing the concept of transporter plasticity.

Multi-scale analysis provides insights into the roles of ureide permeases in wheat nitrogen use efficiency

The ureides allantoin and allantoate serve as nitrogen (N) transport compounds in plants, and more recently, allantoin has been shown to play a role in signaling. In planta, tissue ureide levels are controlled by the activity of enzymes of the purine degradation pathway and by ureide transporters called ureide permeases (UPS). Little is known about the physiological function of UPS proteins in crop plants, and especially in monocotyledon species. Here, we identified 13 TaUPS genes in the wheat (Triticum aestivum L.) genome. Phylogenetic and genome location analyses revealed a close relationship of wheat UPSs to orthologues in other grasses and a division into TaUPS1, TaUPS2.1, and TaUPS2.2 groups, each consisting of three homeologs, with a total of four tandem duplications. Expression, localization, and biochemical analyses resolved spatio-temporal expression patterns of TaUPS genes, transporter localization at the plasma membrane, and a role for TaUPS2.1 proteins in cellular import of ureides and phloem and seed loading. In addition, positive correlations between TaUPS1 and TaUPS2.1 transcripts and ureide levels were found. Together the data support that TaUPSs function in regulating ureide pools at source and sink, along with source-to-sink transport. Moreover, comparative studies between wheat cultivars grown at low and high N strengthened a role for TaUPS1 and TaUPS2.1 transporters in efficient N use and in controlling primary metabolism. Co-expression, protein-protein interaction, and haplotype analyses further support TaUPS involvement in N partitioning, N use efficiency, and domestication. Overall, this work provides a new understanding on UPS transporters in grasses as well as insights for breeding resilient wheat varieties with improved N use efficiency.

Combined Metabolomic and Transcriptomic Analysis Reveals Allantoin Enhances Drought Tolerance in Rice

Drought is a misfortune for agriculture and human beings. The annual crop yield reduction caused by drought exceeds the sum of all pathogens. As one of the gatekeepers of China's "granary", rice is the most important to reveal the key drought tolerance factors in rice. Rice seedlings of Nipponbare (Oryza sativa L. ssp. Japonica) were subjected to simulated drought stress, and their root systems were analyzed for the non-targeted metabolome and strand-specific transcriptome. We found that both DEGs and metabolites were enriched in purine metabolism, and allantoin accumulated significantly in roots under drought stress. However, few studies on drought tolerance of exogenous allantoin in rice have been reported. We aimed to further determine whether allantoin can improve the drought tolerance of rice. Under the treatment of exogenous allantoin at different concentrations, the drought resistant metabolites of plants accumulated significantly, including proline and soluble sugar, and reactive oxygen species (ROS) decreased and reached a significant level in 100 μmol L^-1. To this end, a follow-up study was identified in 100 μmol L^-1 exogenous allantoin and found that exogenous allantoin improved the drought resistance of rice. At the gene level, under allantoin drought treatment, we found that genes of scavenge reactive oxygen species were significantly expressed, including peroxidase (POD), catalase (CATA), ascorbate peroxidase 8 (APX8) and respiratory burst oxidase homolog protein F (RbohF). This indicates that plants treated by allantoin have better ability to scavenge reactive oxygen species to resist drought. Alternative splicing analysis revealed a total of 427 differentially expressed alternative splicing events across 320 genes. The analysis of splicing factors showed that gene alternative splicing could be divided into many different subgroups and play a regulatory role in many aspects. Through further analysis, we restated the key genes and enzymes in the allantoin synthesis and catabolism pathway, and found that the expression of synthetase and hydrolase showed a downward trend. The pathway of uric acid to allantoin is completed by uric acid oxidase (UOX). To find out the key transcription factors that regulate the expression of this gene, we identified two highly related transcription factors OsERF059 and ONAC007 through correlation analysis. They may be the key for allantoin to enhance the drought resistance of rice.

Response of Multiple Tissues to Drought Revealed by a Weighted Gene Co-Expression Network Analysis in Foxtail Millet [Setaria italica (L.) P. Beauv.]

Characterization of drought-tolerance mechanisms during the jointing stage in foxtail millet under water-limited conditions is essential for improving the grain yield of this C₄ crop species. In this trial, two drought-tolerant and two drought-sensitive cultivars were examined using transcriptomic dissections of three tissues (root, stem, and leaf) under naturally occurring water-limited conditions. We detected a total of 32,170 expressed genes and characterized 13,552 differentially expressed genes (DEGs) correlated with drought treatment. The majority of DEGs were identified in the root tissue, followed by leaf and stem tissues, and the number of DEGs identified in the stems of drought-sensitive cultivars was about two times higher than the drought-tolerant ones. A total of 127 differentially expressed transcription factors (DETFs) with different drought-responsive patterns were identified between drought-tolerant and drought-sensitive genotypes (including MYB, b-ZIP, ERF, and WRKY). Furthermore, a total of 34 modules were constructed for all expressed genes using a weighted gene co-expression network analysis (WGCNA), and seven modules were closely related to the drought treatment. A total of 1,343 hub genes (including RAB18, LEA14, and RD22) were detected in the drought-related module, and cell cycle and DNA replication-related transcriptional pathways were identified as vital regulators of drought tolerance in foxtail millet. The results of this study provide a comprehensive overview of how Setaria italica copes with drought-inflicted environments during the jointing stage through transcriptional regulating strategies in different organs and lays a foundation for the improvement of drought-tolerant cereal cultivars through genomic editing approaches in the future.

Xanthine-derived metabolites enhance chlorophyll degradation in cotyledons and seedling growth during nitrogen deficient condition in Brassica rapa

Nitrogen (N) deficiency is a main environmental factor that induces early senescence. Cotyledons provide an important N source during germination and early seedling development. In this study, we observed that N deficient condition enhanced gene expression involved in purine catabolism in cotyledons of Chinese cabbage (Brassica rapa ssp. Pekinensis). Seedlings grown with added allopurinol, an inhibitor of xanthine dehydrogenase, in the growth medium showed reduced chlorophyll degradation in cotyledons and lower fresh weight, compared with seedlings grown on normal medium. On the basis of these results, we speculated that xanthine-derived metabolites might affect both seedling growth and early senescence in cotyledons. To confirm this, seedlings were grown with exogenous xanthine to analyze the role of xanthine-derived metabolites under N deficient condition. Seedlings with xanthine as the sole N-source grew faster, and more cotyledon chlorophyll was broken down, compared with seedlings grown without xanthine. The expression levels of senescence- and purine metabolism-related genes in cotyledons were higher than those in seedlings grown without xanthine. These results indicate the possibility that xanthine plays a role as an activator in both purine catabolism and chlorophyll degradation in cotyledons under N deficient condition.

Allantoin accumulation through overexpression of ureide permease1 improves rice growth under limited nitrogen conditions

In legumes, nitrogen (N) can be stored as ureide allantoin and transported by ureide permease (UPS) from nodules to leaves where it is catabolized to release ammonium and assimilation to amino acids. In non-leguminous plants especially rice, information on its roles in N metabolism is scarce. Here, we show that OsUPS1 is localized in plasma membranes and are highly expressed in vascular tissues of rice. We further evaluated an activation tagging rice overexpressing OsUPS1 (OsUPS1OX ) under several N regimes. Under normal field conditions, panicles from OsUPS1OX plants (14 days after flowering (DAF)) showed significant allantoin accumulation. Under hydroponic system at the vegetative stage, plants were exposed to N-starvation and measured the ammonium in roots after resupplying with ammonium sulphate. OsUPS1OX plants displayed higher ammonium uptake in roots compared to wild type (WT). When grown under low-N soil supplemented with different N-concentrations, OsUPS1OX exhibited better growth at 50% N showing higher chlorophyll, tiller number and at least 20% increase in shoot and root biomass relative to WT. To further confirm the effects of regulating the expression of OsUPS1, we evaluated whole-body-overexpressing plants driven by the GOS2 promoter (OsUPS1GOS2 ) as well as silencing plants (OsUPS1RNAi ). We found significant accumulation of allantoin in leaves, stems and roots of OsUPS1GOS2 while in OsUPS1RNAi allantoin was significantly accumulated in roots. We propose that OsUPS1 is responsible for allantoin partitioning in rice and its overexpression can support plant growth through accumulation of allantoin in sink tissues which can be utilized when N is limiting.

Disruption of an amino acid transporter LHT1 leads to growth inhibition and low yields in rice

BACKGROUND

Research on plant amino acid transporters was mainly performed in Arabidopsis, while our understanding of them is generally scant in rice. OsLHT1 (Lysine/Histidine transporter) has been previously reported as a histidine transporter in yeast, but its substrate profile and function in planta are unclear. The aims of this study are to analyze the substrate selectivity of OsLHT1 and influence of its disruption on rice growth and fecundity.

RESULTS

Substrate selectivity of OsLHT1 was analyzed in Xenopus oocytes using the two-electrode voltage clamp technique. The results showed that OsLHT1 could transport a broad spectrum of amino acids, including basic, neutral and acidic amino acids, and exhibited a preference for neutral and acidic amino acids. Two oslht1 mutants were generated using CRISPR/Cas9 genome-editing technology, and the loss-of-function of OsLHT1 inhibited rice root and shoot growth, thereby markedly reducing grain yields. QRT-PCR analysis indicated that OsLHT1 was expressed in various rice organs, including root, stem, flag leaf, flag leaf sheath and young panicle. Transient expression in rice protoplast suggested OsLHT1 was localized to the plasma membrane, which is consistent with its function as an amino acid transporter.

CONCLUSIONS

Our results indicated that OsLHT1 is an amino acid transporter with wide substrate specificity and with preference for neutral and acidic amino acids, and disruption of OsLHT1 function markedly inhibited rice growth and fecundity.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Disruption of ureide degradation affects plant growth and development during and after transition from vegetative to reproductive stages

BACKGROUND

The ureides allantoin and allantoate are major metabolic intermediates of purine catabolism with high nitrogen-to-carbon ratios. Ureides play a key role in nitrogen utilization in ureide-type legumes, but their effects on growth and development in non-legume plants are poorly understood. Here, we examined the effects of knocking out genes encoding ureide-degrading enzymes, allantoinase (ALN) and allantoate amidohydrolase (AAH), on the vegetative-to-reproductive transition and subsequent growth of Arabidopsis plants.

RESULTS

The ureide-degradation mutants (aln and aah) showed symptoms similar to those of nitrogen deficiency: early flowering, reduced size at maturity, and decreased fertility. Consistent with these phenotypes, carbon-to-nitrogen ratios and nitrogen-use efficiencies were significantly decreased in ureide-degradation mutants; however, adding nitrogen to irrigation water did not alleviate the reduced growth of these mutants. In addition to nitrogen status, levels of indole-3-acetic acid and gibberellin in five-week-old plants were also affected by the aln mutations. To test the possibility that ureides are remobilized from source to sink organs, we measured ureide levels in various organs. In wild-type plants, allantoate accumulated predominantly in inflorescence stems and siliques; this accumulation was augmented by disruption of its catabolism. Mutants lacking ureide transporters, ureide permeases 1 and 2 (UPS1 and UPS2), exhibited phenotypes similar to those of the ureide-degradation mutants, but had decreased allantoate levels in the reproductive organs. Transcript analysis in wild-type plants suggested that genes involved in allantoate synthesis and ureide transport were coordinately upregulated in senescing leaves.

CONCLUSIONS

This study demonstrates that ureide degradation plays an important role in supporting healthy growth and development in non-legume Arabidopsis during and after transition from vegetative to reproductive stages.

RNA Catabolites Contribute to the Nitrogen Pool and Support Growth Recovery of Wheat

Turn-over of RNA and catabolism of nucleotides releases one to four ammonia molecules; the released nutrients being reassimilated into primary metabolism. Preliminary evidence indicates that monocots store high levels of free nucleotides and nucleosides but their potential as a source of internal organic nitrogen for use and remobilization is uncharted. Early tillering wheat plants were therefore starved of N over a 5-day time-course with examination of nucleic acid yields in whole shoots, young and old leaves and roots. Nucleic acids constituted ∼4% of the total N pool of N starved wheat plants, which was comparable with the N available from nitrate (NO3 -) and greater than that available from the sum of 20 proteinogenic amino acids. Methods were optimized to detect nucleotide (purine and pyrimidine) metabolites, and wheat orthologs of RNA degradation (TaRNS), nucleoside transport (TaENT1, TaENT3) and salvage (TaADK) were identified. It was found that N starved wheat roots actively catabolised RNA and specific purines but accumulated pyrimidines. Reduced levels of RNA corresponded with induction of TaRNS2, TaENT1, TaENT3, and TaADK in the roots. Reduced levels of GMP, guanine, xanthine, allantoin, allantoate and glyoxylate in N starved roots correlated with accumulation of allantoate and glyoxylate in the oldest leaf, suggesting translocation of allantoin. Furthermore, N starved wheat plants exogenously supplied with N in the form of purine catabolites grew and photosynthesized as well as those plants re-supplied with NO3 -. These results support the hypothesis that the nitrogen and carbon recovered from purine metabolism can support wheat growth.

The solute transport and binding profile of a novel nucleobase cation symporter 2 from the honeybee pathogen Paenibacillus larvae

Here, we report that a novel nucleobase cation symporter 2 encoded in the genome of the honeybee bacterial pathogen Paenibacillus larvae reveals high levels of amino acid sequence similarity to the Escherichia coli and Bacillus subtilis uric acid and xanthine transporters. This transporter is named P. larvae uric acid permease-like protein (PlUacP). Even though PlUacP displays overall amino acid sequence similarities, has common secondary structures, and shares functional motifs and functionally important amino acids with E. coli xanthine and uric acid transporters, these commonalities are insufficient to assign transport function to PlUacP. The solute transport and binding profile of PlUacP was determined by radiolabeled uptake experiments via heterologous expression in nucleobase transporter-deficient Saccharomyces cerevisiae strains. PlUacP transports the purines adenine and guanine and the pyrimidine uracil. Hypoxanthine, xanthine, and cytosine are not transported by PlUacP, but, along with uric acid, bind in a competitive manner. PlUacP has strong affinity for adenine Km 7.04 ± 0.18 μm, and as with other bacterial and plant NCS2 proteins, PlUacP function is inhibited by the proton disruptor carbonyl cyanide m-chlorophenylhydrazone. The solute transport and binding profile identifies PlUacP as a novel nucleobase transporter.

Allantoin contributes to the stress response in cadmium-treated Arabidopsis roots

Ureides are nitrogen-rich compounds, derived from purine catabolism. A dual role for ureides, and for allantoin in particular, in both nitrogen recycling and the abiotic stress response has been recently identified. Previous work on the effect of allantoin on cadmium (Cd)-exposed Arabidopsis revealed that high concentration of allantoin in allantoinase-negative mutant (aln-3) leaves alleviates Cd toxicity via inducing antioxidant mechanisms in these plants. In the present study, we evaluate whether allantoin has a similar protective role in roots. Both wild type and aln-3 roots contain higher amounts of internal Cd compared to leaves. Likewise, aln-3 roots are more resistant to Cd, reflected in fresh and dry weight, and stimulated antioxidant enzyme activity, including superoxide dismutase (SOD) and catalase (CAT), resulting in lower reactive oxygen species concentration. In contrast with wild-type leaves, high levels of Cd in Col-0 roots reduces transcript abundance of uricase, leading to a significant decline in allantoin level of treated roots at 1000 and 1500 μM CdCl₂. This metabolite change is also accompanied by decreasing the activity of antioxidant enzymes (SOD and CAT). Additionally, contrary to wild-type leaves, root genotype has a significant effect on CAT activity under Cd treatment, suggesting the possible different sources of damage and oxidative stress response in these two tissues.

Uric acid in plants and microorganisms: Biological applications and genetics - A review

Uric acid increased accumulation and/or reduced excretion in human bodies is closely related to pathogenesis of gout and hyperuricemia. It is highly affected by the high intake of food rich in purine. Uric acid is present in both higher plants and microorganisms with species dependent concentration. Urate-degrading enzymes are found both in plants and microorganisms but the mechanisms by which plant degrade uric acid was found to be different among them. Higher plants produce various metabolites which could inhibit xanthine oxidase and xanthine oxidoreductase, so prohibit the oxidation of hypoxanthine to xanthine then to uric acid in the purine metabolism. However, microorganisms produce group of degrading enzymes uricase, allantoinase, allantoicase and urease, which catalyze the degradation of uric acid to the ammonia. In humans, researchers found that several mutations caused a pseudogenization (silencing) of the uricase gene in ancestral apes which exist as an insoluble crystalloid in peroxisomes. This is in contrast to microorganisms in which uricases are soluble and exist either in cytoplasm or peroxisomes. Moreover, many recombinant uricases with higher activity than the wild type uricases could be induced successfully in many microorganisms. The present review deals with the occurrence of uric acid in plants and other organisms specially microorganisms in addition to the mechanisms by which plant extracts, metabolites and enzymes could reduce uric acid in blood. The genetic and genes encoding for uric acid in plants and microorganisms are also presented.

Allantoin accumulation mediated by allantoinase downregulation and transport by Ureide Permease 5 confers salt stress tolerance to Arabidopsis plants

Allantoin, a metabolite generated in the purine degradation pathway, was primarily considered an intermediate for recycling of the abundant nitrogen assimilated in plant purines. More specifically, tropical legumes utilize allantoin and allantoic acid as major nodule-to-shoot nitrogen transport compounds. In other species, an increase in allantoin content was observed under different stress conditions, but the underlying molecular mechanisms remain poorly understood. In this work, Arabidopsis thaliana was used as a model system to investigate the effects of salt stress on allantoin metabolism and to know whether its accumulation results in plant protection. Plant seedlings treated with NaCl at different concentrations showed higher allantoin and lower allantoic acid contents. Treatments with NaCl favored the expression of genes involved in allantoin synthesis, but strongly repressed the unique gene encoding allantoinase (AtALN). Due to the potential regulatory role of this gene for allantoin accumulation, AtALN promoter activity was studied using a reporter system. GUS mediated coloration was found in specific plant tissues and was diminished with increasing salt concentrations. Phenotypic analysis of knockout, knockdown and stress-inducible mutants for AtALN revealed that allantoin accumulation is essential for salt stress tolerance. In addition, the possible role of allantoin transport was investigated. The Ureide Permease 5 (UPS5) is expressed in the cortex and endodermis of roots and its transcription is enhanced by salt treatment. Ups5 knockout plants under salt stress presented a susceptible phenotype and altered allantoin root-to-shoot content ratios. Possible roles of allantoin as a protectant compound in oxidative events or signaling are discussed.

Early signaling, synthesis, transport and metabolism of ureides

The symbiosis between α nitrogen (N2)-fixing Proteobacteria (family Rhizobiaceae) and legumes belonging to the Fabaceae (a single phylogenetic group comprising three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae) results in the formation of a novel root structure called a nodule, where atmospheric N2 is fixed into NH3(+). In the determinate type of nodules harbored by Rhizobium-nodulated Fabaceae species, newly synthesized NH3(+) is finally converted into allantoin (C4H6N4O3) and allantoic acid (C4H8N4O4) (ureides) through complex pathways involving at least 20 different enzymes that act synchronously in two types of nodule cells with contrasting ultrastructure, including the tree nodule cell organelles. Newly synthesized ureides are loaded into the network of nodule-root xylem vessels and transported to aerial organs by the transpirational water current. Once inside the leaves, ureides undergo an enzymatically driven reverse process to yield NH4(+) that is used for growth. This supports the role of ureides as key nitrogen (N)-compounds for the growth and yield of legumes nodulated by Rhizobium that grow in soils with a low N content. Thus, a concrete understanding of the mechanisms underlying ureide biogenesis and catabolism in legumes may help agrobiologists to achieve greater agricultural discoveries. In this review we focus on the transmembranal and transorganellar symplastic and apoplastic movement of N-precursors within the nodules, as well as on the occurrence, localization and properties of enzymes and genes involved in the biogenesis and catabolism of ureides. The synthesis and transport of ureides are not unique events in Rhizobium-nodulated N2-fixing legumes. Thus, a brief description of the synthesis and catabolism of ureides in non-legumes was included for comparison. The establishment of the symbiosis, nodule organogenesis and the plant's control of nodule number, synthesis and translocation of ureides via feed-back inhibition mechanisms are also reviewed.

The solute specificity profiles of nucleobase cation symporter 1 (NCS1) from Zea mays and Setaria viridis illustrate functional flexibility

The solute specificity profiles (transport and binding) for the nucleobase cation symporter 1 (NCS1) proteins, from the closely related C4 grasses Zea mays and Setaria viridis, differ from that of Arabidopsis thaliana and Chlamydomonas reinhardtii NCS1. Solute specificity profiles for NCS1 from Z. mays (ZmNCS1) and S. viridis (SvNCS1) were determined through heterologous complementation studies in NCS1-deficient Saccharomyces cerevisiae strains. The four Viridiplantae NCS1 proteins transport the purines adenine and guanine, but unlike the dicot and algal NCS1, grass NCS1 proteins fail to transport the pyrimidine uracil. Despite the high level of amino acid sequence similarity, ZmNCS1 and SvNCS1 display distinct solute transport and recognition profiles. SvNCS1 transports adenine, guanine, hypoxanthine, cytosine, and allantoin and competitively binds xanthine and uric acid. ZmNCS1 transports adenine, guanine, and cytosine and competitively binds, 5-fluorocytosine, hypoxanthine, xanthine, and uric acid. The differences in grass NCS1 profiles are due to a limited number of amino acid alterations. These amino acid residues do not correspond to amino acids essential for overall solute and cation binding or solute transport, as previously identified in bacterial and fungal NCS1, but rather may represent residues involved in subtle solute discrimination. The data presented here reveal that within Viridiplantae, NCS1 proteins transport a broad range of nucleobase compounds and that the solute specificity profile varies with species.

Heterologous complementation studies reveal the solute transport profiles of a two-member nucleobase cation symporter 1 (NCS1) family in Physcomitrella patens

As part of an evolution-function analysis, two nucleobase cation symporter 1 (NCS1) from the moss Physcomitrella patens (PpNCS1A and PpNCS1B) are examined--the first such analysis of nucleobase transporters from early land plants. The solute specificity profiles for the moss NCS1 were determined through heterologous expression, growth and radiolabeled uptake experiments in NCS1-deficient Saccharomyces cerevisiae. Both PpNCS1A and 1B, share the same profiles as high affinity transporters of adenine and transport uracil, guanine, 8-azaguanine, 8-azaadenine, cytosine, 5-fluorocytosine, hypoxanthine, and xanthine. Despite sharing the same solute specificity profile, PpNCS1A and PpNCS1B move nucleobase compounds with different efficiencies. The broad nucleobase transport profile of PpNCS1A and 1B differs from the recently-characterized Viridiplantae NCS1 in breadth, revealing a flexibility in solute interactions with NCS1 across plant evolution.

Root Uptake of Pharmaceuticals and Personal Care Product Ingredients

Crops irrigated with reclaimed wastewater or grown in biosolids-amended soils may take up pharmaceuticals and personal care product ingredients (PPCPs) through their roots. The uptake pathways followed by PPCPs and the propensity for these compounds to bioaccumulate in food crops are still not well understood. In this critical review, we discuss processes expected to influence root uptake of PPCPs, evaluate current literature on uptake of PPCPs, assess models for predicting plant uptake of these compounds, and provide recommendations for future research, highlighting processes warranting study that hold promise for improving mechanistic understanding of plant uptake of PPCPs. We find that many processes that are expected to influence PPCP uptake and accumulation have received little study, particularly rhizosphere interactions, in planta transformations, and physicochemical properties beyond lipophilicity (as measured by Kow). Data gaps and discrepancies in methodology and reporting have so far hindered development of models that accurately predict plant uptake of PPCPs. Topics warranting investigation in future research include the influence of rhizosphere processes on uptake, determining mechanisms of uptake and accumulation, in planta transformations, the effects of PPCPs on plants, and the development of predictive models.

Complex interactions among residues within pore region determine the K+ dependence of a KAT1-type potassium channel AmKAT1

KAT1-type channels mediate K(+) influx into guard cells that enables stomatal opening. In this study, a KAT1-type channel AmKAT1 was cloned from the xerophyte Ammopiptanthus mongolicus. In contrast to most KAT1-type channels, its activation is strongly dependent on external K(+) concentration, so it can be used as a model to explore the mechanism for the K(+) -dependent gating of KAT1-type channels. Domain swapping between AmKAT1 and KAT1 reveals that the S5-pore-S6 region controls the K(+) dependence of AmKAT1, and residue substitutions show that multiple residues within the S5-Pore linker and Pore are involved in its K(+) -dependent gating. Importantly, complex interactions occur among these residues, and it is these interactions that determine its K(+) dependence. Finally, we analyzed the potential mechanism for the K(+) dependence of AmKAT1, which could originate from the requirement of K(+) occupancy in the selectivity filter to maintain its conductive conformation. These results provide new insights into the molecular basis of the K(+) -dependent gating of KAT1-type channels.

Nucleobase and nucleoside transport and integration into plant metabolism

Nucleotide metabolism is an essential process in all living organisms. Besides newly synthesized nucleotides, the recycling (salvage) of partially degraded nucleotides, i.e., nucleosides and nucleobases serves to keep the homeostasis of the nucleotide pool. Both types of metabolites are substrates of at least six families of transport proteins in Arabidopsis thaliana (Arabidopsis) with a total of 49 members. In the last years several members of such transport proteins have been analyzed allowing to present a more detailed picture of nucleoside and nucleobase transport and the physiological function of these processes. Besides functioning in nucleotide metabolism it turned out that individual members of the before named transporters exhibit the capacity to transport a wide range of different substrates including vitamins and phytohormones. The aim of this review is to summarize the current knowledge on nucleobase and nucleoside transport processes in plants and integrate this into nucleotide metabolism in general. Thereby, we will focus on those proteins which have been characterized at the biochemical level.

The nucleobase cation symporter 1 of Chlamydomonas reinhardtii and that of the evolutionarily distant Arabidopsis thaliana display parallel function and establish a plant-specific solute transport profile

The single cell alga Chlamydomonas reinhardtii is capable of importing purines as nitrogen sources. An analysis of the annotated C. reinhardtii genome reveals at least three distinct gene families encoding for known nucleobase transporters. In this study the solute transport and binding properties for the lone C. reinhardtii nucleobase cation symporter 1 (CrNCS1) are determined through heterologous expression in Saccharomyces cerevisiae. CrNCS1 acts as a transporter of adenine, guanine, uracil and allantoin, sharing similar - but not identical - solute recognition specificity with the evolutionary distant NCS1 from Arabidopsis thaliana. The results suggest that the solute specificity for plant NCS1 occurred early in plant evolution and are distinct from solute transport specificities of single cell fungal NCS1 proteins.

Soybean ureide transporters play a critical role in nodule development, function and nitrogen export

Legumes can access atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing bacteroids that reside in root nodules. In soybean, the products of fixation are the ureides allantoin and allantoic acid, which are also the dominant long-distance transport forms of nitrogen from nodules to the shoot. Movement of nitrogen assimilates out of the nodules occurs via the nodule vasculature; however, the molecular mechanisms for ureide export and the importance of nitrogen transport processes for nodule physiology have not been resolved. Here, we demonstrate the function of two soybean proteins - GmUPS1-1 (XP_003516366) and GmUPS1-2 (XP_003518768) - in allantoin and allantoic acid transport out of the nodule. Localization studies revealed the presence of both transporters in the plasma membrane, and expression in nodule cortex cells and vascular endodermis. Functional analysis in soybean showed that repression of GmUPS1-1 and GmUPS1-2 in nodules leads to an accumulation of ureides and decreased nitrogen partitioning to roots and shoot. It was further demonstrated that nodule development, nitrogen fixation and nodule metabolism were negatively affected in RNAi UPS1 plants. Together, we conclude that export of ureides from nodules is mediated by UPS1 proteins, and that activity of the transporters is not only essential for shoot nitrogen supply but also for nodule development and function.

Nucleoside transport across the plasma membrane mediated by equilibrative nucleoside transporter 3 influences metabolism of Arabidopsis seedlings

The metabolism of nitrogen-rich nucleosides in Arabidopsis seedlings was investigated at the level of import and subsequent salvage or degradation. Uptake and fate of nucleosides imported by equilibrative nucleoside transporter 3 (ENT3) was analysed and, furthermore, a comprehensive analysis of the effect of exogenously fed nucleosides at the level of metabolic as well as transcriptomic alterations was performed. Expression of nucleoside transporters ENT1 and ENT3, together with nucleoside import, was increased upon nitrogen limitation. Thereby a role for ENT3, which is expressed mainly in the vasculature of roots and leaves, as a major import route for nucleosides was supported. Exogenously fed nucleosides were able to attenuate nitrogen starvation effects such as chlorophyll breakdown, anthocyanin accumulation, RNA breakdown and reduced levels of amino acids. In response to nucleoside supply, up-regulation of genes involved in nitrogen distribution in plants was observed. In addition, genes involved in nucleoside metabolism were identified as regulated upon nitrogen limitation. In summary, an overall beneficial effect of nucleoside supply to Arabidopsis seedlings, especially under limiting nitrogen conditions, was observed.

Genetic and molecular characterization reveals a unique nucleobase cation symporter 1 in Arabidopsis

Locus At5g03555 encodes a nucleobase cation symporter 1 (AtNCS1) in the Arabidopsis genome. Arabidopsis insertion mutants, AtNcs1-1 and AtNcs1-3, were used for in planta toxic nucleobase analog growth studies and radio-labeled nucleobase uptake assays to characterize solute transport specificities. These results correlate with similar growth and uptake studies of AtNCS1 expressed in Saccharomyces cerevisiae. Both in planta and heterologous expression studies in yeast revealed a unique solute transport profile for AtNCS1 in moving adenine, guanine and uracil. This is in stark contrast to the canonical transport profiles determined for the well-characterized S. cerevisiae NCS1 proteins FUR4 (uracil transport) or FCY2 (adenine, guanine, and cytosine transport).

Pyrimidine degradation influences germination seedling growth and production of Arabidopsis seeds

PYD1 (dihydropyrimidine dehydogenase) initiates the degradation of pyrimidine nucleobases and is located in plastids. In this study, a physiological analysis of PYD1 employing T-DNA knockout mutants and overexpressors was carried out. PYD1 knockout mutants were restricted in degradation of exogenously provided uracil and accumulated high uracil levels in plant organs throughout development, especially in dry seeds. Moreover, PYD1 knockout mutants showed delayed germination which was accompanied by low invertase activity and decreased monosaccharide levels. Abscisic acid (ABA) is an important regulator of seed germination, and ABA-responsive genes were deregulated in PYD1 knockout mutants. Together with an observed increased PYD1 expression in wild-type seedlings upon ABA treatment, an interference of PYD1 with ABA signalling can be assumed. Constitutive PYD1 overexpression mutants showed increased growth and higher seed number compared with wild-type and knockout mutant plants. During senescence PYD1 expression increased to allow uracil catabolism. From this it is concluded that early in development and during seed production PYD1 is needed to balance pyrimidine catabolism versus salvage.

Transcriptome analysis of the Arabidopsis megaspore mother cell uncovers the importance of RNA helicases for plant germline development

Germ line specification is a crucial step in the life cycle of all organisms. For sexual plant reproduction, the megaspore mother cell (MMC) is of crucial importance: it marks the first cell of the plant "germline" lineage that gets committed to undergo meiosis. One of the meiotic products, the functional megaspore, subsequently gives rise to the haploid, multicellular female gametophyte that harbours the female gametes. The MMC is formed by selection and differentiation of a single somatic, sub-epidermal cell in the ovule. The transcriptional network underlying MMC specification and differentiation is largely unknown. We provide the first transcriptome analysis of an MMC using the model plant Arabidopsis thaliana with a combination of laser-assisted microdissection and microarray hybridizations. Statistical analyses identified an over-representation of translational regulation control pathways and a significant enrichment of DEAD/DEAH-box helicases in the MMC transcriptome, paralleling important features of the animal germline. Analysis of two independent T-DNA insertion lines suggests an important role of an enriched helicase, MNEME (MEM), in MMC differentiation and the restriction of the germline fate to only one cell per ovule primordium. In heterozygous mem mutants, additional enlarged MMC-like cells, which sometimes initiate female gametophyte development, were observed at higher frequencies than in the wild type. This closely resembles the phenotype of mutants affected in the small RNA and DNA-methylation pathways important for epigenetic regulation. Importantly, the mem phenotype shows features of apospory, as female gametophytes initiate from two non-sister cells in these mutants. Moreover, in mem gametophytic nuclei, both higher order chromatin structure and the distribution of LIKE HETEROCHROMATIN PROTEIN1 were affected, indicating epigenetic perturbations. In summary, the MMC transcriptome sets the stage for future functional characterization as illustrated by the identification of MEM, a novel gene involved in the restriction of germline fate.

Equilibrative nucleoside transporter 1 (ENT1) is critical for pollen germination and vegetative growth in Arabidopsis

ENT1 of Arabidopsis thaliana was the first member of the equilibrative nucleoside transporter (ENT) family to be identified in plants and characterized as a cellular, high-affinity nucleoside importer. Evidence is presented here for a tonoplast localization of ENT1 based on proteome data and Western blot analyses. Increased export of adenosine from reconstituted tonoplast preparations from 35S:ENT1 mutants compared with those from the wild type and ENT1-RNAi mutants support this view. Furthermore, increased vacuolar adenosine and vacuolar 2'3'-cAMP (an intermediate of RNA catabolism) contents in ENT1-RNAi mutants, but decreased contents of these metabolites in 35S:ENT1 over-expresser mutants, were observed. An up-regulation of the salvage pathway was detected in the latter mutants, leading to the conclusion that draining the vacuolar adenosine storage by ENT1 over-expression interferes with cellular nucleotide metabolism. As a consequence of the observed metabolic alterations 35S:ENT1 over-expresser mutants exhibited a smaller phenotypic appearance compared with wild-type plants. In addition, ENT1:RNAi mutants exhibited significantly lower in vitro germination of pollen and contained reduced internal and external ATP levels. This indicates that ENT1-mediated nucleosides, especially adenosine transport, is important for nucleotide metabolism, thus influencing growth and pollen germination.

Nucleoside transport and associated metabolism

Nucleosides are intermediates of nucleotide metabolism. Nucleotide de novo synthesis generates the nucleoside monophosphates AMP and UMP, which are further processed to all purine and pyrimidine nucleotides involved in multiple cellular reactions, including the synthesis of nucleic acids. Catabolism of these substances results in the formation of nucleosides, which are further degraded by nucleoside hydrolase to nucleobases. Both nucleosides and nucleobases can be exchanged between cells and tissues through multiple isoforms of corresponding transport proteins. After uptake into a cell, nucleosides and nucleobases can undergo salvage reactions or catabolism. Whereas energy is preserved by salvage pathway reactions, catabolism liberates ammonia, which is then incorporated into amino acids. Keeping the balance between nitrogen consumption during nucleotide de novo synthesis and ammonia liberation by nucleotide catabolism is essential for correct plant development. Senescence and seed germination represent situations in plant development where marked fluctuations in nucleotide pools occur. Furthermore, extracellular nucleotide metabolism has become an immensely interesting research topic. In addition, selected aspects of nucleoside transport in yeast, protists and humans are discussed.

Regulation of expression and kinetic modeling of substrate interactions of a uracil transporter inAspergillus nidulans

Early genetic evidence suggested that A. nidulans possesses at least one uracil transporter. A gene, named furD, was recently identified by reverse genetics and in silico approaches and we confirm here that it encodes a high-affinity, high-capacity, uracil transporter. In this work, we study the regulation of expression of FurD and develop a kinetic model describing transporter-substrate interactions. The furD gene is not expressed in resting conidiospores, is transcriptionally activated and reaches a peak during the isotropic growth phase of conidiospore germination, and stays at a basic low level in mycelium. Transcriptional expression is correlated to uracil transport activity. Expression in a strain blocked in uracil biosynthesis (pyrG-) is moderately increased and extended to later stages of germination. The presence of excess uracil in the medium leads to down-regulation of furD expression and FurD activity. A detailed kinetic analysis using a number of pyrimidine and purine analogues showed that FurD is able to recognize with high-affinity uracil (Km 0.45 microM), thymine (Ki 3.3 microM) and several 5-substituted analogues of uracil, and with moderate affinity uric acid and xanthine (Ki 94-99 microM). Kinetic evidence supports a model in which the positions N1-H, =O2, N3-H, =O4, as well as planarity play a central role for the substrate binding. This model, which rationalizes the unique specificity of FurD for uracil, is compared to and found to be very similar to analogous models for protozoan uracil transporters.

A functional analysis of the pyrimidine catabolic pathway in Arabidopsis

* Reductive catabolism of pyrimidine nucleotides occurs via a three-step pathway in which uracil is degraded to beta-alanine, CO(2) and NH(3) through sequential activities of dihydropyrimidine dehydrogenase (EC 1.3.1.2, PYD1), dihydropyrimidinase (EC 3.5.2.2, PYD2) and beta-ureidopropionase (EC 3.5.1.6, PYD3). * A proposed function of this pathway, in addition to the maintenance of pyrimidine homeostasis, is the recycling of pyrimidine nitrogen to general nitrogen metabolism. PYD expression and catabolism of [2-(14)C]-uracil are markedly elevated in response to nitrogen limitation in plants, which can utilize uracil as a nitrogen source. * PYD1, PYD2 and PYD3 knockout mutants were used for functional analysis of this pathway in Arabidopsis. pyd mutants exhibited no obvious phenotype under optimal growing conditions. pyd2 and pyd3 mutants were unable to catabolize [2-(14)C]-uracil or to grow on uracil as the sole nitrogen source. By contrast, catabolism of uracil was reduced by only 40% in pyd1 mutants, and pyd1 seedlings grew nearly as well as wild-type seedlings with a uracil nitrogen source. These results confirm PYD1 function and suggest the possible existence of another, as yet unknown, activity for uracil degradation to dihydrouracil in this plant. * The localization of PYD-green fluorescent protein fusions in the plastid (PYD1), secretory system (PYD2) and cytosol (PYD3) suggests potentially complex metabolic regulation.

Uridine-ribohydrolase is a key regulator in the uridine degradation pathway of Arabidopsis

Nucleoside degradation and salvage are important metabolic pathways but hardly understood in plants. Recent work on human pathogenic protozoans like Leishmania and Trypanosoma substantiates an essential function of nucleosidase activity. Plant nucleosidases are related to those from protozoans and connect the pathways of nucleoside degradation and salvage. Here, we describe the cloning of such an enzyme from Arabidopsis thaliana, Uridine-Ribohydrolase 1 (URH1) and the characterization by complementation of a yeast mutant. Furthermore, URH1 was synthesized as a recombinant protein in Escherichia coli. The pure recombinant protein exhibited highest hydrolase activity for uridine, followed by inosine and adenosine, the corresponding K(m) values were 0.8, 1.4, and 0.7 mM, respectively. In addition, URH1 was able to cleave the cytokinin derivative isopentenyladenine-riboside. Promoter beta-glucuronidase fusion studies revealed that URH1 is mainly transcribed in the vascular cells of roots and in root tips, guard cells, and pollen. Mutants expressing the Arabidopsis enzyme or the homolog from rice (Oryza sativa) exhibit resistance toward toxic fluorouridine, fluorouracil, and fluoroorotic acid, providing clear evidence for a pivotal function of URH1 as regulative in pyrimidine degradation. Moreover, mutants with increased and decreased nucleosidase activity are delayed in germination, indicating that this enzyme activity must be well balanced in the early phase of plant development.

AtAzg1 and AtAzg2 comprise a novel family of purine transporters in Arabidopsis

In plants, nucleobase biochemistry is highly compartmented relying upon a well-regulated and selective membrane transport system. In Arabidopsis two proteins, AtAzg1 and AtAzg2, show substantial amino acid sequence similarity to the adenine-guanine-hypoxanthine transporter AzgA of Aspergillus nidulans. Analysis of single and double mutant lines harboring T-DNA insertion alleles AtAzg1-1 and AtAzg2-1 reveal a marked resistance to growth in the presence of 8-azaadenine and 8-azaguanine but not to other toxic nucleobase analogues. Conversely, yeast strains expressing AtAzg1 and AtAzg2 gain heightened sensitivity to growth on 8-azaadenine and 8-azaguanine. Radio-labeled purine uptake experiments in yeast and in planta confirm the function of AtAzg1 and AtAzg2 as plant adenine-guanine transporters.

Transporters for uptake and allocation of organic nitrogen compounds in plants

Nitrogen is an essential macronutrient for plant growth. Following uptake from the soil or assimilation within the plant, organic nitrogen compounds are transported between organelles, from cell to cell and over long distances in support of plant metabolism and development. These translocation processes require the function of integral membrane transporters. The review summarizes our current understanding of the molecular mechanisms of organic nitrogen transport processes, with a focus on amino acid, ureide and peptide transporters.

The fluorouridine insensitive 1 (fur1) mutant is defective in equilibrative nucleoside transporter 3 (ENT3), and thus represents an important pyrimidine nucleoside uptake system in Arabidopsis thaliana

The fluorouridine insensitive 1 (fur1) locus in Arabidopsis thaliana (L.) Heynh. has previously been identified in a screen for growth resistance towards the toxic compound fluorouridine. Mutation of this locus by ethylmethane sulfonate (EMS) allows mutants to grow on this uridine analogue. We identified that the A. thaliana equilibrative nucleoside transporter (AtENT3) was encoded by the fur1 locus. T-DNA insertional mutant plants for AtENT3 resemble the fur1 mutant phenotype: i.e. they grow on fluorouridine, and seedlings as well as leaf discs exhibit a markedly reduced uptake capacity for uridine and cytidine, but a less pronounced reduced uptake for adenosine and guanosine. These results indicate that AtENT3 is an important pyrimidine nucleoside transporter in Arabidopsis. In addition, we identified the mutation in fur1 as a single base-pair exchange, guanine --> adenine, leading to an amino acid exchange G --> R at position 281. Furthermore, we showed that this mutation is indeed responsible for the observed alterations in nucleoside transport in the fur1-1 line, because the introduction of this mutation in AtENT3 promoted fluorouridine resistance in yeast cells expressing this mutated protein. The biochemical characterization of AtENT3 expressed in Xenopus oocytes identified a proton-coupled concentrative mode of nucleoside transport, although this carrier possesses structural features characteristic for equilibrative nucleoside carriers.

Molecular and functional analysis of hypoxanthine-guanine phosphoribosyltransferase from Arabidopsis thaliana

Hypoxanthine-guanine phosphoribosyltransferase (HGPT) occurs in both eukaryotic and prokaryotic organisms. However, the molecular and functional properties of plant HGPT are not well understood. In this study, it was found that the putative HGPT proteins from dicot and monocot plant species exhibited significant identities to their homologs from other cellular organisms. Ectopic expression of the HGPTs from Arabidopsis, soybean or wheat complemented HGPT deficiency in the hpt1 mutant of Saccharomyces cerevisiae. Recombinant Arabidopsis HGPT (AtHGPT) catalyzed both forward and reverse reactions in in vitro biochemical assays. The relative catalytic efficiency for the synthesis of guanosine monophosphate (GMP) was significantly greater than that for the production of guanine from GMP. Further investigations led to identification of the candidate residues that may form the pyrophosphate (PPi) binding loop in AtHGPT. AtHGPT expression level was dynamically regulated in Arabidopsis organs and during leaf development and senescence and seed germination. AtHGPT knockout mutant germinated more slowly than wild type control, whereas its overexpression mutant exhibited accelerated germination. Collectively, the data suggest that functional HGPTs are expressed in higher plants. In Arabidopsis, HGPT plays an active role in the salvage of purine bases and its activity is required for efficient seed germination.

Comparative studies on Ureide Permeases in Arabidopsis thaliana and analysis of two alternative splice variants of AtUPS5

The recovery of free purine and pyrimidine bases and their degradation products represent alternative pathways in plant cells either to synthesize nucleotides (salvage pathways) by low energy consumption or to reuse organic nitrogen. Such recycling of metabolites often requires their uptake into the cell by specialized transport systems residing in the plasma membrane. In plants, it has been suggested that several protein families are involved in this process, but only a few transporters have so far been characterized. In this work, gene expression, substrate specificities, and transport mechanisms of members of the Ureide Permease family in Arabidopsis (AtUPS) were analyzed and compared. Promoter-GUS studies indicated that the members of the family have distinct and partially overlapping expression patterns with regard to developmental stages or tissue specific localization. In addition, two alternative splice variants of AtUPS5, a novel member of the transporter family, were identified and investigated. The abundance of both alternative mRNAs varied in different organs, while the relative amounts were comparable. AtUPS5l (longer isoform) shares similar structural prediction with AtUPS1 and AtUPS2. In contrast, AtUPS5s (shorter isoform) lacks two transmembrane domains as structural consequence of the additional splice event. When expressed in yeast, AtUPS5l mediates cellular import of cyclic purine degradation products and pyrimidines similarly to AtUPS1 and AtUPS2, but differences in transport efficiencies were observed. AtUPS5s, however, could not be shown to mediate uptake of these compounds into yeast cells and might therefore be defective or have a different function.

Identification of AtENT3 as the main transporter for uridine uptake in Arabidopsis roots

Previous studies have shown that Arabidopsis equilibrative nucleoside transporters (AtENTs) possess transport activities when produced in yeast cells and are differentially expressed in Arabidopsis organs. Herein, we report further analysis on the nucleoside transport activities and transcriptional patterns of AtENT members. The recombinant proteins of AtENTs 3, 6, and 7, but not those of AtENTs 1, 2, 4, and 8, were found to transport thymidine with high affinity. Contrary to previous suggestion that AtENT1 may not transport uridine, this work showed that recombinant AtENT1 was a pH-dependent and high-affinity transporter of uridine. When grown on MS plates, the AtENT3 knockout plants were more tolerant to the cytotoxic uridine analog 5-fluorouridine than wild-type plants and the knockout plants of AtENT1 or AtENT6. Consistent with this observation, the AtENT3 knockout line exhibited a significantly decreased ability to take up [(3)H]uridine via the roots when compared with wild-type plants and the plants with mutated AtENT1 or AtENT6. This indicates that AtENT3, but not AtENTs 1 and 6, is the main transporter for uridine uptake in Arabidopsis roots. The transcription of AtENTs 1, 3, 4, 6, 7, and 8 was regulated in a complex manner during leaf development and senescence. In contrast, the six AtENT members were coordinately induced during seed germination. This work provides new information on the transport properties of recombinant AtENT proteins and new clues for future studies of the in vivo transport activities and physiological functions of the different ENT proteins in Arabidopsis plants.

Heterologous expression of a plant uracil transporter in yeast: Improvement of plasma membrane targeting in mutants of the Rsp5p ubiquitin protein ligase

Plasma membrane proteins involved in transport processes play a crucial role in cell physiology. On account of these properties, these molecules are ideal targets for development of new therapeutic and agronomic agents. However, these proteins are of low abundance, which limits their study. Although yeast seems ideal for expressing heterologous transporters, plasma membrane proteins are often retained in intracellular compartments. We tried to find yeast mutants potentially able to improve functional expression of a whole set of heterologous transporters. We focused on Arabidopsis thaliana ureide transporter 1 (AtUPS1), previously cloned by functional complementation in yeast. Tagged versions of AtUPS1 remain mostly trapped in the endoplasmic reticulum and were able to reach slowly the plasma membrane. In contrast, untagged AtUPS1 is rapidly delivered to plasma membrane, where it remains in stable form. Tagged and untagged versions of AtUPS1 were expressed in cells deficient in the ubiquitin ligase Rsp5p, involved in various stages of the intracellular trafficking of membrane-bound proteins. rsp5 mutants displayed improved steady state amounts of untagged and tagged versions of AtUPS1. rsp5 cells are thus powerful tools to solve the many problems inherent to heterologous expression of membrane proteins in yeast, including ER retention.

Pyrimidine and purine biosynthesis and degradation in plants

Nucleotide metabolism operates in all living organisms, embodies an evolutionarily ancient and indispensable complex of metabolic pathways and is of utmost importance for plant metabolism and development. In plants, nucleotides can be synthesized de novo from 5-phosphoribosyl-1-pyrophosphate and simple molecules (e.g., CO(2), amino acids, and tetrahydrofolate), or be derived from preformed nucleosides and nucleobases via salvage reactions. Nucleotides are degraded to simple metabolites, and this process permits the recycling of phosphate, nitrogen, and carbon into central metabolic pools. Despite extensive biochemical knowledge about purine and pyrimidine metabolism, comprehensive studies of the regulation of this metabolism in plants are only starting to emerge. Here we review progress in molecular aspects and recent studies on the regulation and manipulation of nucleotide metabolism in plants.

Chemically Mediated Host-Plant Selection by the Milfoil Weevil: A Freshwater Insect–Plant Interaction

The milfoil weevil Euhrychiopsis lecontei is a specialist aquatic herbivore that feeds, oviposits, and mates on the invasive freshwater macrophyte Myriophyllum spicatum. We characterized the weevil's preference for M. spicatum, and through bioassay-driven fractionation, isolated and identified two chemicals released by M. spicatum that attract E. lecontei. Mass spectrometry and nuclear magnetic resonance spectroscopy were used to identify the attractive compounds as glycerol and uracil. Dose-response curves for glycerol and uracil indicated that weevil preference increased as sample concentration increased. Weevils were attracted to a crude sample of M. spicatum-released chemicals from 0.17 to 17 mg/l, to glycerol from 18 to 1800 microM (0.0017-0.17 mg/l), and to uracil from 0.015 to 15 microM (0.00014-1.4 mg/l). Although glycerol and uracil are ubiquitous, weevils are likely responding to high concentrations that are released as a result of the rapid growth of M. spicatum. Uracil concentration was greater in the exudates of M. spicatum than other Myriophyllum spp. E. lecontei was attracted to glycerol at a concentration similar to that at which terrestrial insects are attracted to sugar alcohols. This is the first example of a freshwater specialist insect being attracted to chemicals released by its host plant. Analysis of the water milfoil-weevil interaction provides further understanding as to how insects locate their host plants in aquatic systems.