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

AZGs: a new family of cytokinin transporters

Cytokinins (CKs) are phytohormones structurally similar to purines that play important roles in various aspects of plant physiology and development. The local and long-distance distribution of CKs is very important to control their action throughout the plant body. Over the past decade, several novel CK transporters have been described, many of which have been linked to a physiological function rather than simply their ability to transport the hormone in vitro. Purine permeases, equilibrative nucleotide transporters and ATP-binding cassette transporters are involved in the local and long-range distribution of CK. In addition, members of the Arabidopsis AZA-GUANINE RESISTANT (AZG) protein family, AZG1 and AZG2, have recently been shown to mediate CK uptake at the plasma membrane and endoplasmic reticulum. Despite sharing ∼50% homology, AZG1 and AZG2 have unique transport mechanisms, tissue-specific expression patterns, and subcellular localizations that underlie their distinct physiological functions. AZG2 is expressed in a small group of cells in the overlying tissue around the lateral root primordia, where its expression is induced by auxins and it is involved in the regulation of lateral root growth. AZG1 is ubiquitously expressed, with high levels in the division zone of the root apical meristem. Here, it binds and stabilises the auxin efflux carrier PIN1, thereby shaping root architecture, particularly under salt stress. This review highlights the latest findings on the protein properties, transport mechanisms and cellular functions of this new family of CK transporters and discusses perspectives for future research in this field.

Structures and mechanisms of the Arabidopsis cytokinin transporter AZG1

Cytokinins are essential for plant growth and development, and their tissue distributions are regulated by transmembrane transport. Recent studies have revealed that members of the 'Aza-Guanine Resistant' (AZG) protein family from Arabidopsis thaliana can mediate cytokinin uptake in roots. Here we present 2.7 to 3.3 Å cryo-electron microscopy structures of Arabidopsis AZG1 in the apo state and in complex with its substrates trans-zeatin (tZ), 6-benzyleaminopurine (6-BAP) or kinetin. AZG1 forms a homodimer and each subunit shares a similar topology and domain arrangement with the proteins of the nucleobase/ascorbate transporter (NAT) family. These structures, along with functional analyses, reveal the molecular basis for cytokinin recognition. Comparison of the AZG1 structures determined in inward-facing conformations and predicted by AlphaFold2 in the occluded conformation allowed us to propose that AZG1 may carry cytokinins across the membrane through an elevator mechanism.

AZG1 is a cytokinin transporter that interacts with auxin transporter PIN1 and regulates the root stress response

An environmentally responsive root system is crucial for plant growth and crop yield, especially in suboptimal soil conditions. This responsiveness enables the plant to exploit regions of high nutrient density while simultaneously minimizing abiotic stress. Despite the vital importance of root systems in regulating plant growth, significant gaps of knowledge exist in the mechanisms that regulate their architecture. Auxin defines both the frequency of lateral root (LR) initiation and the rate of LR outgrowth. Here, we describe a search for proteins that regulate root system architecture (RSA) by interacting directly with a key auxin transporter, PIN1. The native separation of Arabidopsis plasma membrane protein complexes identified several PIN1 co-purifying proteins. Among them, AZG1 was subsequently confirmed as a PIN1 interactor. Here, we show that, in Arabidopsis, AZG1 is a cytokinin (CK) import protein that co-localizes with and stabilizes PIN1, linking auxin and CK transport streams. AZG1 expression in LR primordia is sensitive to NaCl, and the frequency of LRs is AZG1-dependent under salt stress. This report therefore identifies a potential point for auxin:cytokinin crosstalk, which shapes RSA in response to NaCl.

Cytokinin activity - transport and homeostasis at the whole plant, cell, and subcellular levels

Cytokinins (CKs) are important plant hormones that regulate a variety of biological processes implicated in plant development and stress responses. Here, we summarize the most recent advances in discovering and characterizing the membrane transporters involved in long- and short-distance translocation of CKs and their significance in CK signal activity. We highlight the discovery of PUP7 and PUP21 tonoplast-localized transporters and propose potential mechanisms for CK subcellular homeostasis. Finally, we discuss the importance of subcellular hormone transport in light of the localization of histidine kinase receptors of CKs at the endoplasmic reticulum and plasma membrane.

Fine Mapping and Candidate Gene Analysis of Pm36, a Wild Emmer-Derived Powdery Mildew Resistance Locus in Durum Wheat

Powdery mildew (PM) is an economically important foliar disease of cultivated cereals worldwide. The cultivation of disease-resistant varieties is considered the most efficient, sustainable and economical strategy for disease management. The objectives of the current study were to fine map the chromosomal region harboring the wild emmer PM resistance locus Pm36 and to identify candidate genes by exploiting the improved tetraploid wheat genomic resources. A set of backcross inbred lines (BILs) of durum wheat were genotyped with the SNP 25K chip array and comparison of the PM-resistant and susceptible lines defined a 1.5 cM region (physical interval of 1.08 Mb) harboring Pm36. The genetic map constructed with F_2:3 progenies derived by crossing the PM resistant line 5BIL-42 and the durum parent Latino, restricted to 0.3 cM the genetic distance between Pm36 and the SNP marker IWB22904 (physical distance 0.515 Mb). The distribution of the marker interval including Pm36 in a tetraploid wheat collection indicated that the positive allele was largely present in the domesticated and wild emmer Triticum turgidum spp. dicoccum and ssp. dicoccoides. Ten high-confidence protein coding genes were identified in the Pm36 region of the emmer, durum and bread wheat reference genomes, while three added genes showed no homologous in the emmer genome. The tightly linked markers can be used for marker-assisted selection in wheat breeding programs, and as starting point for the Pm36 map-based cloning.

Transcriptomic Insights on the Preventive Action of Apple (cv Granny Smith) Skin Wounding on Superficial Scald Development

Superficial scald is a post-harvest chilling storage injury leading to browning of the surface of the susceptible cv Granny Smith apples. Wounding of skins has been reported to play a preventive role on scald development however its underlying molecular factors are unknown. We have artificially wounded the epidermal and sub-epidermal layers of apple skins consistently obtaining the prevention of superficial scald in the surroundings of the wounds during two independent vintages. Time course RNA-Seq analyses of the transcriptional changes in wounded versus unwounded skins revealed that two transcriptional waves occurred. An early wave included genes up-regulated by wounding already after 6 h, highlighting a specific transcriptional rearrangement of genes connected to the biosynthesis and signalling of JA, ethylene and ABA. A later transcriptional wave, occurring after three months of cold storage, included genes up-regulated exclusively in unwounded skins and was prevented from its occurrence in wounded skins. A significant portion of these genes was related to decay of tissues and to the senescence hormones ABA, JA and ethylene. Such changes suggest a wound-inducible reversed hormonal balance during post-harvest storage which may explain the local inhibition of scald in wounded tissues, an aspect that will need further studies for its mechanistic explanation.

Identification of New Specificity Determinants in Bacterial Purine Nucleobase Transporters based on an Ancestral Sequence Reconstruction Approach

The relation of sequence with specificity in membrane transporters is challenging to explore. Most relevant studies until now rely on comparisons of present-day homologs. In this work, we study a set of closely related transporters by employing an evolutionary, ancestral-reconstruction approach and reveal unexpected new specificity determinants. We analyze a monophyletic group represented by the xanthine-specific XanQ of Escherichia coli in the Nucleobase-Ascorbate Transporter/Nucleobase-Cation Symporter-2 (NAT/NCS2) family. We reconstructed AncXanQ, the putative common ancestor of this clade, expressed it in E. coli K-12, and found that, in contrast to XanQ, it encodes a high-affinity permease for both xanthine and guanine, which also recognizes adenine, hypoxanthine, and a range of analogs. AncXanQ conserves all binding-site residues of XanQ and differs substantially in only five intramembrane residues outside the binding site. We subjected both homologs to rationally designed mutagenesis and present evidence that these five residues are linked with the specificity change. In particular, we reveal Ser377 of XanQ (Gly in AncXanQ) as a major determinant. Replacement of this Ser with Gly enlarges the specificity of XanQ towards an AncXanQ-phenotype. The ortholog from Neisseria meningitidis retaining Gly at this position is also a xanthine/guanine transporter with extended substrate profile like AncXanQ. Molecular Dynamics shows that the S377G replacement tilts transmembrane helix 12 resulting in rearrangement of Phe376 relative to Phe94 in the XanQ binding pocket. This effect may rationalize the enlarged specificity. On the other hand, the specificity effect of S377G can be masked by G27S or other mutations through epistatic interactions.

Integrating the Roles for Cytokinin and Auxin in De Novo Shoot Organogenesis: From Hormone Uptake to Signaling Outputs

De novo shoot organogenesis (DNSO) is a procedure commonly used for the in vitro regeneration of shoots from a variety of plant tissues. Shoot regeneration occurs on nutrient media supplemented with the plant hormones cytokinin (CK) and auxin, which play essential roles in this process, and genes involved in their signaling cascades act as master regulators of the different phases of shoot regeneration. In the last 20 years, the genetic regulation of DNSO has been characterized in detail. However, as of today, the CK and auxin signaling events associated with shoot regeneration are often interpreted as a consequence of these hormones simply being present in the regeneration media, whereas the roles for their prior uptake and transport into the cultivated plant tissues are generally overlooked. Additionally, sucrose, commonly added to the regeneration media as a carbon source, plays a signaling role and has been recently shown to interact with CK and auxin and to affect the efficiency of shoot regeneration. In this review, we provide an integrative interpretation of the roles for CK and auxin in the process of DNSO, adding emphasis on their uptake from the regeneration media and their interaction with sucrose present in the media to their complex signaling outputs that mediate shoot regeneration.

Functional characterization of the adenine transporter EaAdeP from the fire blight pathogen Erwinia amylovora and its effect on disease establishment in apples and pears

Erwinia amylovora is the causal agent of fire blight, an economically important disease of apples and pears. As part of the infection process, Er. amylovora propagates on different plant tissues each with distinct nutrient environments. Here, the biochemical properties of the Er. amylovora adenine permease (EaAdeP) are investigated. Heterologous expression of EaAdeP in nucleobase transporter-deficient Escherichia coli strains, coupled with radiolabel uptake studies, revealed that EaAdeP is a high affinity adenine transporter with a Km of 0.43 ± 0.09 μM. Both Es. coli and Er. amylovora carrying extra copies of EaAdeP are sensitive to growth on the toxic analog 8-azaadenine. EaAdeP is expressed during immature pear fruit infection. Immature pear and apple fruit virulence assays reveal that an E. amylovora ΔadeP::Camr mutant is still able to cause disease symptoms, however, with growth at a lower level, indicating that external adenine is utilized in disease establishment.

Differential Subcellular Distribution of Cytokinins: How Does Membrane Transport Fit into the Big Picture?

Cytokinins are a class of phytohormones, signalling molecules specific to plants. They act as regulators of diverse physiological processes in complex signalling pathways. It is necessary for plants to continuously regulate cytokinin distribution among different organs, tissues, cells, and compartments. Such regulatory mechanisms include cytokinin biosynthesis, metabolic conversions and degradation, as well as cytokinin membrane transport. In our review, we aim to provide a thorough picture of the latter. We begin by summarizing cytokinin structures and physicochemical properties. Then, we revise the elementary thermodynamic and kinetic aspects of cytokinin membrane transport. Next, we review which membrane-bound carrier proteins and protein families recognize cytokinins as their substrates. Namely, we discuss the families of "equilibrative nucleoside transporters" and "purine permeases", which translocate diverse purine-related compounds, and proteins AtPUP14, AtABCG14, AtAZG1, and AtAZG2, which are specific to cytokinins. We also address long-distance cytokinin transport. Putting all these pieces together, we finally discuss cytokinin distribution as a net result of these processes, diverse in their physicochemical nature but acting together to promote plant fitness.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

Arabidopsis AZG2 transports cytokinins in vivo and regulates lateral root emergence

Cytokinin and auxin are key regulators of plant growth and development. During the last decade transport mechanisms have turned out to be the key for the control of local and long-distance hormone distributions. In contrast with auxin, cytokinin transport is poorly understood. Here, we show that Arabidopsis thaliana AZG2, a member of the AZG purine transporter family, acts as cytokinin transporter involved in root system architecture determination. Even though purines are substrates for both AZG1 and AZG2, we found distinct transport mechanisms. The expression of AZG2 is restricted to a small group of cells surrounding the lateral root (LR) primordia and induced by auxins. Compared to the wild-type (WT), mutants carrying loss-of-function alleles of AZG2 have higher LR density, suggesting that AZG2 is part of a regulatory pathway in LR emergence. Moreover, azg2 is partially insensitive to exogenous cytokinin, which is consistent with the observation that the cytokinin reporter TCSn_pro :GFP showed lower fluorescence signal in the roots of azg2 compared to the WT. These results indicate a defective cytokinin signalling pathway in the region of LR primordia. The integration of AZG2 subcellular localization and cytokinin transport capacity data allowed us to propose a local cytokinin : auxin signalling model for the regulation of LR emergence.

The guanine-hypoxanthine permease GhxP of Erwinia amylovora facilitates the influx of the toxic guanine derivative 6-thioguanine

AIM

Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. This study determines whether the E. amylovora guanine-hypoxanthine transporter (EaGhxP) is required for virulence and if it can import the E. amylovora produced toxic analogue 6-thioguanine (6TG) into cells.

METHODS AND RESULTS

Characterization of EaGhxP in guanine transport deficient Escherichia coli reveals that it can transport guanine, hypoxanthine and the toxic analogues 8-azaguanine (8AG) and 6TG. Similarly, EaGhxP transports 8AG and 6TG into E. amylovora cells. EaGhxP has a high affinity for 6TG with a K_i of 3·7 µmol l^-1 . An E. amylovora ⊿ghxP::Cam^r strain shows resistance to growth on 8AG and 6TG. Although EaGhxP is expressed during active disease propagation, it is not necessary for virulence as determined on immature apple and pear assays.

CONCLUSIONS

EaGhxP is not required for virulence, but it does import 6TG into E. amylovora cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

As part of the disease establishment process, E. amylovora synthesizes and exports a toxic guanine derivative 6TG. Our results are counter intuitive and show that EaGhxP, an influx transporter, can move 6TG into cells raising questions regarding the role of 6TG in disease establishment.

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.

Comparative Genomics, Whole-Genome Re-sequencing and Expression Profile Analysis of Nucleobase:Cation Symporter 2 (NCS2) Genes in Maize

Nucleobase:cation symporter 2 (NCS2) proteins are important for the transport of free nucleobases, participating in diverse plant growth and developmental processes, as well as response to abiotic stress. To date, a comprehensive analysis of the NCS2 gene family has not been performed in maize. In this study, we conducted a comparative genomics analysis of NCS2 genes in 28 plant species, ranging from aquatic algae to land plants, concentrating mainly on maize. Gene duplication events contributed to the expansion of NCS2 genes from lower aquatic plants to higher angiosperms, and whole-genome/segmental and single-gene duplication events were responsible for the expansion of the maize NCS2 gene family. Phylogenetic construction showed three NCS2 subfamilies, I, II, and III. According to homology-based relationships, members of subfamily I are NCS2/AzgA-like genes, whereas those in subfamilies II and III are NCS2/NATs. Moreover, subfamily I exhibited ancient origins. A motif compositional analysis showed that one symbolic motif (motif 4) of the NCS2/NAT genes was absent in subfamily I. In maize, three NCS2/AzgA-like and 21 NCS2/NAT genes were identified, and purifying selection influenced the duplication of maize NCS2 genes. Additionally, a population genetic analysis of NCS2 genes revealed that ZmNCS2-21 showed the greatest diversity between the 78 inbred and 22 wild surveyed maize populations. An expression profile analysis using transcriptome data and quantitative real-time PCR revealed that NCS2 genes in maize are involved in diverse developmental processes and responses to abiotic stresses, including abscisic acid, salt (NaCl), polyethylene glycol, and low (4°C) and high (42°C) temperatures. ZmNCS2 genes with relatively close relationships had similar expression patterns, strongly indicating functional redundancy. Finally, ZmNCS2-16 and ZmNCS2-23 localize in the plasma membrane, which confirmed their predicted membrane structures. These results provide a foundation for future studies regarding the functions of ZmNCS2 proteins, particularly those with potentially important roles in plant responses to abiotic stresses.

Purine salvage in plants

Purine bases and nucleosides are produced by turnover of nucleotides and nucleic acids as well as from some cellular metabolic pathways. Adenosine released from the S-adenosyl-L-methionine cycle is linked to many methyltransferase reactions, such as the biosynthesis of caffeine and glycine betaine. Adenine is produced by the methionine cycles, which is related to other biosynthesis pathways, such those for the production of ethylene, nicotianamine and polyamines. These purine compounds are recycled for nucleotide biosynthesis by so-called "salvage pathways". However, the salvage pathways are not merely supplementary routes for nucleotide biosynthesis, but have essential functions in many plant processes. In plants, the major salvage enzymes are adenine phosphoribosyltransferase (EC 2.4.2.7) and adenosine kinase (EC 2.7.1.20). AMP produced by these enzymes is converted to ATP and utilised as an energy source as well as for nucleic acid synthesis. Hypoxanthine, guanine, inosine and guanosine are salvaged to IMP and GMP by hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) and inosine/guanosine kinase (EC 2.7.1.73). In contrast to de novo purine nucleotide biosynthesis, synthesis by the salvage pathways is extremely favourable, energetically, for cells. In addition, operation of the salvage pathway reduces the intracellular levels of purine bases and nucleosides which inhibit other metabolic reactions. The purine salvage enzymes also catalyse the respective formation of cytokinin ribotides, from cytokinin bases, and cytokinin ribosides. Since cytokinin bases are the active form of cytokinin hormones, these enzymes act to maintain homeostasis of cellular cytokinin bioactivity. This article summarises current knowledge of purine salvage pathways and their possible function in plants and purine salvage activities associated with various physiological phenomena are reviewed.

Functional characterization of two novel purine transporters from the Basidiomycota Phanerochaete chrysosporium

Purine transporters as substrate entry points in organisms, are involved in a number of cellular processes such as nitrogen source uptake, energy metabolism and synthesis of nucleic acids. In this study, two nucleobase transporter genes (phZ, phU) from Phanerochaete chrysosporium were cloned, identified, and functionally characterized. Our results show that PhZ is a transporter of adenine and hypoxanthine, and a protein belonging to the AzgA-like family, whilst PhU belongs to the NAT/NCS2 family, transporting xanthine and uric acid. No other sequences belonging to these families were detected in P. chrysosporium's genome. Phylogenetic analyses show that AzgA-like sequences form monophyletic groups for each major lineage (Ascomycota, Basidiomycota and Zygomycota). In contrast, Ascomycota and Basidiomycota NAT/NCS2 sequences do not form monophyletic groups and several copies of this protein are distributed across the tree. Expression of phU was significantly downregulated in the presence of a primary source like ammonium, and enhanced if purines were present or if the mycelium was nitrogen starved. phZ was clearly induced by its substrates (hypoxanthine, adenine), very lightly induced by xanthine, suppressed by urea and amino acids and expressed at a basal level when uric acid or ammonium was the nitrogen source or when the mycelium was starved for nitrogen. In order to perform substrate analyses, both P. chrysosporium proteins (PhZ, PhU) were expressed in Aspergillus nidulans. Epifluorescent microscopy showed that under inducing conditions, PhZ-GFP and PhU-GFP were present at the plasma membrane of A. nidulans transformed strains, and were internalized in repressed conditions. Our results suggest that in the white-rot fungus P. chrysosporium, phU has a catabolic role and phZ, (less dependent of the nitrogen source), plays a key role in purine acquisition to provide biosynthetic components. These are the first purine transporters characterized in Basidiomycota.

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.

Differential transcriptional networks associated with key phases of ingrowth wall construction in trans-differentiating epidermal transfer cells of Vicia faba cotyledons

BACKGROUND

Transfer cells are characterized by intricate ingrowth walls, comprising an uniform wall upon which wall ingrowths are deposited. The ingrowth wall forms a scaffold to support an amplified plasma membrane surface area enriched in membrane transporters that collectively confers transfer cells with an enhanced capacity for membrane transport at bottlenecks for apo-/symplasmic exchange of nutrients. However, the underlying molecular mechanisms regulating polarized construction of the ingrowth wall and membrane transporter profile are poorly understood.

RESULTS

An RNAseq study of an inducible epidermal transfer cell system in cultured Vicia faba cotyledons identified transfer cell specific transcriptomes associated with uniform wall and wall ingrowth deposition. All functional groups of genes examined were expressed before and following transition to a transfer cell fate. What changed were the isoform profiles of expressed genes within functional groups. Genes encoding ethylene and Ca(2+) signal generation and transduction pathways were enriched during uniform wall construction. Auxin-and reactive oxygen species-related genes dominated during wall ingrowth formation and ABA genes were evenly expressed across ingrowth wall construction. Expression of genes encoding kinesins, formins and villins was consistent with reorganization of cytoskeletal components. Uniform wall and wall ingrowth specific expression of exocyst complex components and SNAREs suggested specific patterns of exocytosis while dynamin mediated endocytotic activity was consistent with establishing wall ingrowth loci. Key regulatory genes of biosynthetic pathways for sphingolipids and sterols were expressed across ingrowth wall construction. Transfer cell specific expression of cellulose synthases was absent. Rather xyloglucan, xylan and pectin biosynthetic genes were selectively expressed during uniform wall construction. More striking was expression of genes encoding enzymes for re-modelling/degradation of cellulose, xyloglucans, pectins and callose. Extensins dominated the cohort of expressed wall structural proteins and particularly so across wall ingrowth development. Ion transporters were selectively expressed throughout ingrowth wall development along with organic nitrogen transporters and a large group of ABC transporters. Sugar transporters were less represented.

CONCLUSIONS

Pathways regulating signalling and intracellular organization were fine tuned whilst cell wall construction and membrane transporter profiles were altered substantially upon transiting to a transfer cell fate. Each phase of ingrowth wall construction was linked with unique cohorts of expressed genes.

Molecular dissection of a Borrelia burgdorferi in vivo essential purine transport system

The Lyme disease spirochete Borrelia burgdorferi is dependent on purine salvage from the host environment for survival. The genes bbb22 and bbb23 encode purine permeases that are essential for B. burgdorferi mouse infectivity. We now demonstrate the unique contributions of each of these genes to purine transport and murine infection. The affinities of spirochetes carrying bbb22 alone for hypoxanthine and adenine were similar to those of spirochetes carrying both genes. Spirochetes carrying bbb22 alone were able to achieve wild-type levels of adenine saturation but not hypoxanthine saturation, suggesting that maximal hypoxanthine uptake requires the presence of bbb23. Moreover, the purine transport activity conferred by bbb22 was dependent on an additional distal transcriptional start site located within the bbb23 open reading frame. The initial rates of uptake of hypoxanthine and adenine by spirochetes carrying bbb23 alone were below the level of detection. However, these spirochetes demonstrated a measurable increase in hypoxanthine uptake over a 30-min time course. Our findings indicate that bbb22-dependent adenine transport is essential for B. burgdorferi survival in mice. The bbb23 gene was dispensable for B. burgdorferi mouse infectivity, yet its presence was required along with that of bbb22 for B. burgdorferi to achieve maximal spirochete loads in infected mouse tissues. These data demonstrate that both genes, bbb22 and bbb23, are critical for B. burgdorferi to achieve wild-type infection of mice and that the differences in the capabilities of the two transporters may reflect distinct purine salvage needs that the spirochete encounters throughout its natural infectious cycle.

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.

Functional identification of the hypoxanthine/guanine transporters YjcD and YgfQ and the adenine transporters PurP and YicO of Escherichia coli K-12

The evolutionarily broad family nucleobase-cation symporter-2 (NCS2) encompasses transporters that are conserved in binding site architecture but diverse in substrate selectivity. Putative purine transporters of this family fall into one of two homology clusters: COG2233, represented by well studied xanthine and/or uric acid permeases, and COG2252, consisting of transporters for adenine, guanine, and/or hypoxanthine that remain unknown with respect to structure-function relationships. We analyzed the COG2252 genes of Escherichia coli K-12 with homology modeling, functional overexpression, and mutagenesis and showed that they encode high affinity permeases for the uptake of adenine (PurP and YicO) or guanine and hypoxanthine (YjcD and YgfQ). The two pairs of paralogs differ clearly in their substrate and ligand preferences. Of 25 putative inhibitors tested, PurP and YicO recognize with low micromolar affinity N(6)-benzoyladenine, 2,6-diaminopurine, and purine, whereas YjcD and YgfQ recognize 1-methylguanine, 8-azaguanine, 6-thioguanine, and 6-mercaptopurine and do not recognize any of the PurP ligands. Furthermore, the permeases PurP and YjcD were subjected to site-directed mutagenesis at highly conserved sites of transmembrane segments 1, 3, 8, 9, and 10, which have been studied also in COG2233 homologs. Residues irreplaceable for uptake activity or crucial for substrate selectivity were found at positions occupied by similar role amino acids in the Escherichia coli xanthine- and uric acid-transporting homologs (XanQ and UacT, respectively) and predicted to be at or around the binding site. Our results support the contention that the distantly related transporters of COG2233 and COG2252 use topologically similar side chain determinants to dictate their function and the distinct purine selectivity profiles.

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.

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).

Substrate selectivity of YgfU, a uric acid transporter from Escherichia coli

The ubiquitous nucleobase-ascorbate transporter (NAT/NCS2) family includes more than 2,000 members, but only 15 have been characterized experimentally. Escherichia coli has 10 members, of which the uracil permease UraA and the xanthine permeases XanQ and XanP are functionally known. Of the remaining members, YgfU is closely related in sequence and genomic locus with XanQ. We analyzed YgfU and showed that it is a proton-gradient dependent, low-affinity (K(m) 0.5 mM), and high-capacity transporter for uric acid. It also shows a low capacity for transport of xanthine at 37 °C but not at 25 °C. Based on the set of positions delineated as important from our previous Cys-scanning analysis of permease XanQ, we subjected YgfU to rationally designed site-directed mutagenesis. The results show that the conserved His-37 (TM1), Glu-270 (TM8), Asp-298 (TM9), and Gln-318 and Asn-319 (TM10) are functionally irreplaceable, and Thr-100 (TM3) is essential for the uric acid selectivity because its replacement with Ala allows efficient uptake of xanthine. The key role of these residues is corroborated by the conservation pattern and homology modeling on the recently described x-ray structure of permease UraA. In addition, site-specific replacements at TM8 (S271A, M274D, V282S) impair expression in the membrane, and V320N (TM10) inactivates the permease, whereas R327G (TM10) or S426N (TM14) reduces the affinity for uric acid (4-fold increased K(m)). Our study shows that comprehensive analysis of structure-function relationships in a newly characterized transporter can be accomplished with relatively few site-directed replacements, based on the knowledge available from Cys-scanning mutagenesis of a prototypic homolog.

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.