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

Soil-borne fungi alter the apoplastic purinergic signaling in plants by deregulating the homeostasis of extracellular ATP and its metabolite adenosine

Purinergic signaling activated by extracellular nucleotides and their derivative nucleosides trigger sophisticated signaling networks. The outcome of these pathways determine the capacity of the organism to survive under challenging conditions. Both extracellular ATP (eATP) and Adenosine (eAdo) act as primary messengers in mammals, essential for immunosuppressive responses. Despite the clear role of eATP as a plant damage-associated molecular pattern, the function of its nucleoside, eAdo, and of the eAdo/eATP balance in plant stress response remain to be fully elucidated. This is particularly relevant in the context of plant-microbe interaction, where the intruder manipulates the extracellular matrix. Here, we identify Ado as a main molecule secreted by the vascular fungus Fusarium oxysporum. We show that eAdo modulates the plant's susceptibility to fungal colonization by altering the eATP-mediated apoplastic pH homeostasis, an essential physiological player during the infection of this pathogen. Our work indicates that plant pathogens actively imbalance the apoplastic eAdo/eATP levels as a virulence mechanism.

Uncovering transcriptional reprogramming during callus development in soybean: insights and implications

Callus, a valuable tool in plant genetic engineering, originates from dedifferentiated cells. While transcriptional reprogramming during callus formation has been extensively studied in Arabidopsis thaliana, our knowledge of this process in other species, such as Glycine max, remains limited. To bridge this gap, our study focused on conducting a time-series transcriptome analysis of soybean callus cultured for various durations (0, 1, 7, 14, 28, and 42 days) on a callus induction medium following wounding with the attempt of identifying genes that play key roles during callus formation. As the result, we detected a total of 27,639 alterations in gene expression during callus formation, which could be categorized into eight distinct clusters. Gene ontology analysis revealed that genes associated with hormones, cell wall modification, and cell cycle underwent transcriptional reprogramming throughout callus formation. Furthermore, by scrutinizing the expression patterns of genes related to hormones, cell cycle, cell wall, and transcription factors, we discovered that auxin, cytokinin, and brassinosteroid signaling pathways activate genes involved in both root and shoot meristem development during callus formation. In summary, our transcriptome analysis provides significant insights into the molecular mechanisms governing callus formation in soybean. The information obtained from this study contributes to a deeper understanding of this intricate process and paves the way for further investigation in the field.

TOR mediates the autophagy response to altered nucleotide homeostasis in an RNase mutant

The Arabidopsis thaliana T2 family endoribonuclease RNS2 localizes to the vacuole and functions in rRNA degradation. Loss of RNS2 activity impairs rRNA turnover and leads to constitutive autophagy, a process for degradation of cellular components. Autophagy is normally activated during environmental stress and is important for stress tolerance and homeostasis. Here we show that restoration of cytosolic purine nucleotide levels rescues the constitutive autophagy phenotype of rns2-2 seedlings, whereas inhibition of purine synthesis induces autophagy in wild-type seedlings. rns2-2 seedlings have reduced activity of the target of rapamycin (TOR) kinase complex, a negative regulator of autophagy, and this phenotype is rescued by addition of inosine to increase purine levels. Activation of TOR in rns2-2 by exogenous auxin blocks the enhanced autophagy, indicating a possible involvement of the TOR signaling pathway in the activation of autophagy in the rns2-2 mutant. Our data suggest a model in which loss of rRNA degradation in rns2-2 leads to a reduction in cytoplasmic nucleotide concentrations, which in turn inhibits TOR activity, leading to activation of autophagy to restore homeostasis.

More Transporters, More Substrates: The Arabidopsis Major Facilitator Superfamily Revisited

The Major Facilitator Superfamily (MFS) is ubiquitous in living organisms and represents the largest group of secondary active membrane transporters. In plants, significant research efforts have focused on the role of specific families within the MFS, particularly those transporting macronutrients (C, N, and P) that constitute the vast majority of the members of this superfamily. Other MFS families remain less explored, although a plethora of additional substrates and physiological functions have been uncovered. Nevertheless, the lack of a systematic approach to analyzing the MFS as a whole has obscured the high diversity and versatility of these transporters. Here, we present a phylogenetic analysis of all annotated MFS domain-containing proteins encoded in the Arabidopsis thaliana genome and propose that this superfamily of transporters consists of 218 members, clustered in 22 families. In reviewing the available information regarding the diversity in biological functions and substrates of Arabidopsis MFS members, we provide arguments for intensified research on these membrane transporters to unveil the breadth of their physiological relevance, disclose the molecular mechanisms underlying their mode of action, and explore their biotechnological potential.

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.

Botrytis cinerea can import and utilize nucleosides in salvage and catabolism and BcENT functions as high affinity nucleoside transporter

Nucleotide de novo synthesis is an essential pathway in nearly all organisms. Transport processes as well as salvage and catabolism of nucleotides and pathway intermediates are required to balance nucleotide pools. We have analysed the genome of the fungal plant pathogen Botrytis cinerea for genes involved in nucleotide metabolism and found a complete set of genes necessary for purine and pyrimidine uptake and salvage based on homology of the gene products to corresponding proteins from Aspergillus nidulans. Candidate genes required for a complete purine catabolic sequence were identified in addition. These analyses were complemented by growth tests showing functional transport and salvage activity for pyrimidines. Growth of B. cinerea mycelium in nitrogen free medium could be restored by addition of purines, indicating the presence of a functional purine catabolism, whereas pyrimidines did not support growth. Bcin07g05490 (BcENT) was identified as sole member of the equilibrative nucleoside transporter (ENT) family. The protein synthesized in Saccharomyces cerevisiae revealed high affinity transport of adenosine (KM = 6.81 μM) and uridine (KM=9.04 μM). Furthermore, a BcENT knockout mutant was generated and tested in a range of growth and infection assays. These results provide detailed insight in the use of externally supplied nucleobases and nucleosides by B. cinerea.

High yield expression and purification of equilibrative nucleoside transporter 7 (ENT7) from Arabidopsis thaliana

BACKGROUND

Equilibrative nucleoside transporters (ENTs) facilitate the import of nucleosides and their analogs into cells in a bidirectional, non-concentrative manner. However, in contrast to their name, most characterized plant ENTs act in a concentrative manner. A direct characterization of any ENT protein has been hindered due to difficulties in overexpression and obtaining pure recombinant protein.

METHODS

The equilibrative nucleoside transporter 7 from Arabidopsis thaliana (AtENT7) was expressed in Xenopus laevis oocytes to assess mechanism of substrate uptake. Recombinant protein fused to enhanced green fluorescent protein (eGFP) was expressed in Pichia pastoris to characterize its oligomeric state by gel filtration and substrate binding by microscale thermophoresis (MST).

RESULTS

AtENT7 expressed in X. laevis oocytes works as a classic equilibrative transporter. The expression of AtENT7-eGFP in the P. pastoris system yielded milligram amounts of pure protein that exists as stable homodimers. The concentration dependent binding of purine and pyrimidine nucleosides to the purified recombinant protein, assessed by MST, confirmed that AtENT7-eGFP is properly folded. For the first time the binding of nucleobases was observed for AtENT7.

SIGNIFICANCE

The availability of pure recombinant AtENT7 will permit detailed kinetic and structural studies of this unique member of the ENT family and, given the functional similarity to mammalian ENTs, will serve as a good model for understanding the structural basis of translocation mechanism for the family.

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.

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.

Arabidopsis nucleoside hydrolases involved in intracellular and extracellular degradation of purines

Recently, the first plant nucleoside hydrolase, NSH1 (former designation URH1), was identified at the molecular level. This enzyme's highest hydrolysis capacity is for uridine, thereby balancing pyrimidine salvage and catabolism. NSH1 was found to be less efficient in the hydrolysis of further nucleosides. However, it remained unclear whether purine nucleosides are processed by NSH1. Moreover, the biochemical and physiological functions of further NSH isoforms in Arabidopsis has not been analyzed. Here we show that NSH1 is also able to hydrolyze xanthosine with high efficiency, and thus represents the leading activity in purine and pyrimidine breakdown in a cell. A knockout mutant for NSH1 showed symptoms of accelerated senescence, accompanied by marked accumulation of uridine and xanthosine under conditions of prolonged darkness. The closest, so far uncharacterized, homolog of NSH1, NSH2, was found to act during the late phase of senescence and may support inosine breakdown. NSH3, another NSH isoform, surprisingly functions as an extracellular, purine-specific hydrolase that is involved in degradation of extracellular nucleosides and may participate in wound and pathogen responses.

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.