Profiling the aminopropyltransferases in plants: their structure, expression and manipulation

Genome-wide identification of SAMS gene family in Cucurbitaceae and the role of ClSAMS1 in watermelon tolerance to abiotic stress

S-Adenosyl-L-methionine (SAM) is widely involved in plant growth, development, and abiotic stress response. SAM synthetase (SAMS) is the key enzyme that catalyzes the synthesis of SAM from methionine and ATP. However, the SAMS gene family has not been identified and their functions have not been characterized in most Cucurbitaceae plants. Here, a total of 30 SAMS genes were identified in nine Cucurbitaceae species and they were categorized into 3 subfamilies. Physicochemical properties and gene structure analysis showed that the SAMS protein members are tightly conserved. Further analysis of the cis-regulatory elements (CREs) of SAMS genes' promoter implied their potential roles in stress tolerance. To further understand the molecular functions of SAMS genes, watermelon SAMSs (ClSAMSs) were chosen to analyze the expression patterns in different tissues and under various abiotic stress and hormone responses. Among the investigated genes, ClSAMS1 expression was observed in all tissues and found to be up-regulated by abiotic stresses including salt, cold and drought treatments as well as exogenous hormone treatments including ETH, SA, MeJA and ABA. Furthermore, knockdown of ClSAMS1 via virus-induced gene silencing (VIGS) decreased SAM contents in watermelon seedings. The pTRSV2-ClSAMS1 plants showed reduced susceptibility to drought, cold and NaCl stress, indicating a positive role of ClSAMS1 in abiotic stresses tolerance. Those results provided candidate SAMS genes to regulate plant resistance against abiotic stresses in Cucurbitaceae plants.

Genome-wide identification of polyamine metabolism and ethylene synthesis genes in Chenopodium quinoa Willd. and their responses to low-temperature stress

BACKGROUND

Quinoa (Chenopodium quinoa Willd.) is valued for its nutritional richness. However, pre-harvest sprouting poses a significant threat to yield and grain quality. This study aims to enhance our understanding of pre-harvest sprouting mitigation strategies, specifically through delayed sowing and avoiding rainy seasons during quinoa maturation. The overarching goal is to identify cold-resistant varieties and unravel the molecular mechanisms behind the low-temperature response of quinoa. We employed bioinformatics and genomics tools for a comprehensive genome-wide analysis of polyamines (PAs) and ethylene synthesis gene families in quinoa under low-temperature stress.

RESULTS

This involved the identification of 37 PA biosynthesis and 30 PA catabolism genes, alongside 227 ethylene synthesis. Structural and phylogenetic analyses showcased conserved patterns, and subcellular localization predictions indicated diverse cellular distributions. The results indicate that the PA metabolism of quinoa is closely linked to ethylene synthesis, with multiple genes showing an upregulation in response to cold stress. However, differential expression within gene families suggests a nuanced regulatory network.

CONCLUSIONS

Overall, this study contributes valuable insights for the functional characterization of the PA metabolism and ethylene synthesis of quinoa, which emphasize their roles in plant low-temperature tolerance and providing a foundation for future research in this domain.

Proline Biosynthesis Enzyme Genes Confer Salt Tolerance to Switchgrass (Panicum virgatum L.) in Cooperation With Polyamines Metabolism

Understanding the regulation of proline metabolism necessitates the suppression of two Δ₁-pyrroline-5-carboxylate synthetase enzyme (P5CS) genes performed in switchgrass (Panicum virgatum L.). The results reveal that overexpressing PvP5CS1 and PvP5CS2 increased salt tolerance. Additionally, transcript levels of spermidine (Spd) and spermine (Spm) synthesis and metabolism related genes were upregulated in PvP5CS OE-transgenic plants and downregulated in the PvP5CS RNAi transformants. According to salt stress assay and the measurement of transcript levels of Polyamines (PAs) metabolism-related genes, P5CS enzyme may not only be the key regulator of proline biosynthesis in switchgrass, but it may also indirectly affect the entire subset of pathway for ornithine to proline or to putrescine (Put). Furthermore, application of proline prompted expression levels of Spd and Spm synthesis and metabolism-related genes in both PvP5CS-RNAi and WT plants, but transcript levels were even lower in PvP5CS-RNAi compared to WT plants under salt stress condition. These results suggested that exogenous proline could accelerate polyamines metabolisms under salt stress. Nevertheless, the enzymes involved in this process and the potential functions remain poorly understood. Thus, the aim of this study is to reveal how proline functions with PAs metabolism under salt stress in switchgrass.

Thermospermine Synthase (ACL5) and Diamine Oxidase (DAO) Expression Is Needed for Zygotic Embryogenesis and Vascular Development in Scots Pine

Unlike in flowering plants, the detailed roles of the enzymes in the polyamine (PA) pathway in conifers are poorly known. We explored the sequence conservation of the PA biosynthetic genes and diamine oxidase (DAO) in conifers and flowering plants to reveal the potential functional diversification of the enzymes between the plant lineages. The expression of the genes showing different selective constraints was studied in Scots pine zygotic embryogenesis and early seedling development. We found that the arginine decarboxylase pathway is strongly preferred in putrescine production in the Scots pine as well as generally in conifers and that the reduced use of ornithine decarboxylase (ODC) has led to relaxed purifying selection in ODC genes. Thermospermine synthase (ACL5) genes evolve under strong purifying selection in conifers and the DAO gene is also highly conserved in pines. In developing Scots pine seeds, the expression of both ACL5 and DAO increased as embryogenesis proceeded. Strong ACL5 expression was present in the procambial cells of the embryo and in the megagametophyte cells destined to die via morphologically necrotic cell death. Thus, the high sequence conservation of ACL5 genes in conifers may indicate the necessity of ACL5 for both embryogenesis and vascular development. Moreover, the result suggests the involvement of ACL5 in morphologically necrotic cell death and supports the view of the genetic regulation of necrosis in Scots pine embryogenesis and in plant development. DAO transcripts were located close to the cell walls and between the walls of adjacent cells in Scots pine zygotic embryos and in the roots of young seedlings. We propose that DAO, in addition to the role in Put oxidation for providing H2O2 during the cell-wall structural processes, may also participate in cell-to-cell communication at the mRNA level. To conclude, our findings indicate that the PA pathway of Scots pines possesses several special functional characteristics which differ from those of flowering plants.

Spermidine Synthase (SPDS) Undergoes Concerted Structural Rearrangements Upon Ligand Binding - A Case Study of the Two SPDS Isoforms From Arabidopsis thaliana

Spermidine synthases (SPDSs) catalyze the production of the linear triamine, spermidine, from putrescine. They utilize decarboxylated S-adenosylmethionine (dc-SAM), a universal cofactor of aminopropyltransferases, as a donor of the aminopropyl moiety. In this work, we describe crystal structures of two SPDS isoforms from Arabidopsis thaliana (AtSPDS1 and AtSPDS2). AtSPDS1 and AtSPDS2 are dimeric enzymes that share the fold of the polyamine biosynthesis proteins. Subunits of both isoforms present the characteristic two-domain structure. Smaller, N-terminal domain is built of the two β-sheets, while the C-terminal domain has a Rossmann fold-like topology. The catalytic cleft composed of two main compartments, the dc-SAM binding site and the polyamine groove, is created independently in each AtSPDS subunits at the domain interface. We also provide the structural details about the dc-SAM binding mode and the inhibition of SPDS by a potent competitive inhibitor, cyclohexylamine (CHA). CHA occupies the polyamine binding site of AtSPDS where it is bound at the bottom of the active site with the amine group placed analogously to the substrate. The crystallographic snapshots show in detail the structural rearrangements of AtSPDS1 and AtSPDS2 that are required to stabilize ligands within the active site. The concerted movements are observed in both compartments of the catalytic cleft, where three major parts significantly change their conformation. These are (i) the neighborhood of the glycine-rich region where aminopropyl moiety of dc-SAM is bound, (ii) the very flexible gate region with helix η6, which interacts with both, the adenine moiety of dc-SAM and the bound polyamine or inhibitor, and (iii) the N-terminal β-hairpin, that limits the putrescine binding grove at the bottom of the catalytic site.

Scots pine aminopropyltransferases shed new light on evolution of the polyamine biosynthesis pathway in seed plants

Background and Aims

Polyamines are small metabolites present in all living cells and play fundamental roles in numerous physiological events in plants. The aminopropyltransferases (APTs), spermidine synthase (SPDS), spermine synthase (SPMS) and thermospermine synthase (ACL5), are essential enzymes in the polyamine biosynthesis pathway. In angiosperms, SPMS has evolved from SPDS via gene duplication, whereas in gymnosperms APTs are mostly unexplored and no SPMS gene has been reported. The present study aimed to investigate the functional properties of the SPDS and ACL5 proteins of Scots pine (Pinus sylvestris L.) in order to elucidate the role and evolution of APTs in higher plants.

Methods

Germinating Scots pine seeds and seedlings were analysed for polyamines by high-performance liquid chromatography (HPLC) and the expression of PsSPDS and PsACL5 genes by in situ hybridization. Recombinant proteins of PsSPDS and PsACL5 were produced and investigated for functional properties. Also gene structures, promoter regions and phylogenetic relationships of PsSPDS and PsACL5 genes were analysed.

Key Results

Scots pine tissues were found to contain spermidine, spermine and thermospermine. PsSPDS enzyme catalysed synthesis of both spermidine and spermine. PsACL5 was found to produce thermospermine, and PsACL5 gene expression was localized in the developing procambium in embryos and tracheary elements in seedlings.

Conclusions

Contrary to previous views, our results demonstrate that SPMS activity is not a novel feature developed solely in the angiosperm lineage of seed plants but also exists as a secondary property in the Scots pine SPDS enzyme. The discovery of bifunctional SPDS from an evolutionarily old conifer reveals the missing link in the evolution of the polyamine biosynthesis pathway. The finding emphasizes the importance of pre-existing secondary functions in the evolution of new enzyme activities via gene duplication. Our results also associate PsACL5 with the development of vascular structures in Scots pine.

Diminution of arsenic accumulation in rice seedlings co-cultured with Anabaena sp.: Modulation in the expression of lower silicon transporters, two nitrogen dependent genes and lowering of antioxidants activity

The present study was intended to investigate the role of algae, Anabaena sp. in the amelioration of As toxicity, when co-cultured with rice seedlings. The reduction of growth in rice seedlings against As(III) and As(V) was recovered with Anabaena sp. The Anabaena sp. also reduced the accumulation of As, where it was more efficient against 60µM As(III) (49%) than As(V) (23%) in rice shoot. Similarly, with reduction of As accumulation, lower silicon transporters (Lsi-1 and Lsi-2) was found to be suppressed against As treatments. However, the expression of two nitrogen dependent genes i.e., NR and SAMT were found to be enhanced with the Anabaena sp. Likewise, the activity of antioxidant enzyme, GST, was enhanced, whereas, the activity of other enzymes such as SOD, APX, GPX, GR and DHAR were decreased with As+Algae combinations. Overall, the result suggested that the Anabaena sp. reduces As accumulation, modulates gene expressions and antioxidants to ameliorate the As toxicity in Oryza sativa L.

Polyamines in the Context of Metabolic Networks

Polyamines (PAs) are essential biomolecules that are known to be involved in the regulation of many plant developmental and growth processes as well as their response to different environmental stimuli. Maintaining the cellular pools of PAs or their metabolic precursors and by-products is critical to accomplish their normal functions. Therefore, the titre of PAs in the cells must be under tight regulation to enable cellular PA homeostasis. Polyamine homeostasis is hence achieved by the regulation of their input into the cellular PA pool, their conversion into secondary metabolites, their transport to other issues/organs, and their catabolism or turnover. The major contributors of input to the PA pools are their in vivo biosynthesis, interconversion between different PAs, and transport from other tissues/organs; while the output or turnover of PAs is facilitated by transport, conjugation and catabolism. Polyamine metabolic pathways including the biosynthesis, catabolism/turnover and conjugation with various organic molecules have been widely studied in all kingdoms. Discoveries on the molecular transporters facilitating the intracellular and intercellular translocation of PAs have also been reported. Numerous recent studies using transgenic approaches and mutagenesis have shown that plants can tolerate quite large concentrations of PAs in the cells; even though, at times, high cellular accumulation of PAs is quite detrimental, and so is high rate of catabolism. The mechanism by which plants tolerate such large quantities of PAs is still unclear. Interestingly, enhanced PA biosynthesis via manipulation of the PA metabolic networks has been suggested to contribute directly to increased growth and improvements in plant abiotic and biotic stress responses; hence greater biomass and productivity. Genetic manipulation of the PA metabolic networks has also been shown to improve plant nitrogen assimilation capacity, which may in turn lead to enhanced carbon assimilation. These potential benefits on top of the widely accepted role of PAs in improving plants' tolerance to biotic and abiotic stressors are invaluable tools for future plant improvement strategies.

The Aspergillus flavus Spermidine Synthase (spds) Gene, Is Required for Normal Development, Aflatoxin Production, and Pathogenesis During Infection of Maize Kernels

Aspergillus flavus is a soil-borne saprophyte and an opportunistic pathogen of both humans and plants. This fungus not only causes disease in important food and feed crops such as maize, peanut, cottonseed, and tree nuts but also produces the toxic and carcinogenic secondary metabolites (SMs) known as aflatoxins. Polyamines (PAs) are ubiquitous polycations that influence normal growth, development, and stress responses in living organisms and have been shown to play a significant role in fungal pathogenesis. Biosynthesis of spermidine (Spd) is critical for cell growth as it is required for hypusination-mediated activation of eukaryotic translation initiation factor 5A (eIF5A), and other biochemical functions. The tri-amine Spd is synthesized from the diamine putrescine (Put) by the enzyme spermidine synthase (Spds). Inactivation of spds resulted in a total loss of growth and sporulation in vitro which could be partially restored by addition of exogenous Spd. Complementation of the Δspds mutant with a wild type (WT) A. flavus spds gene restored the WT phenotype. In WT A. flavus, exogenous supply of Spd (in vitro) significantly increased the production of sclerotia and SMs. Infection of maize kernels with the Δspds mutant resulted in a significant reduction in fungal growth, sporulation, and aflatoxin production compared to controls. Quantitative PCR of Δspds mutant infected seeds showed down-regulation of aflatoxin biosynthetic genes in the mutant compared to WT A. flavus infected seeds. Expression analyses of PA metabolism/transport genes during A. flavus-maize interaction showed significant increase in the expression of arginine decarboxylase (Adc) and S-adenosylmethionine decarboxylase (Samdc) genes in the maize host and PA uptake transporters in the fungus. The results presented here demonstrate that Spd biosynthesis is critical for normal development and pathogenesis of A. flavus and pre-treatment of a Δspds mutant with Spd or Spd uptake from the host plant, are insufficient to restore WT levels of pathogenesis and aflatoxin production during seed infection. The data presented here suggest that future studies targeting spermidine biosynthesis in A. flavus, using RNA interference-based host-induced gene silencing approaches, may be an effective strategy to reduce aflatoxin contamination in maize and possibly in other susceptible crops.

Genetic manipulation of putrescine biosynthesis reprograms the cellular transcriptome and the metabolome

BACKGROUND

With the increasing interest in metabolic engineering of plants using genetic manipulation and gene editing technologies to enhance growth, nutritional value and environmental adaptation, a major concern is the potential of undesirable broad and distant effects of manipulating the target gene or metabolic step in the resulting plant. A comprehensive transcriptomic and metabolomic analysis of the product may shed some useful light in this regard. The present study used these two techniques with plant cell cultures to analyze the effects of genetic manipulation of a single step in the biosynthesis of polyamines because of their well-known roles in plant growth, development and stress responses.

RESULTS

The transcriptomes and metabolomes of a control and a high putrescine (HP) producing cell line of poplar (Populus nigra x maximowiczii) were compared using microarrays and GC/MS. The HP cells expressed an ornithine decarboxylase transgene and accumulated several-fold higher concentrations of putrescine, with only small changes in spermidine and spermine. The results show that up-regulation of a single step in the polyamine biosynthetic pathway (i.e. ornithine → putrescine) altered the expression of a broad spectrum of genes; many of which were involved in transcription, translation, membrane transport, osmoregulation, shock/stress/wounding, and cell wall metabolism. More than half of the 200 detected metabolites were significantly altered (p ≤ 0.05) in the HP cells irrespective of sampling date. The most noteworthy differences were in organic acids, carbohydrates and nitrogen-containing metabolites.

CONCLUSIONS

The results provide valuable information about the role of polyamines in regulating nitrogen and carbon use pathways in cell cultures of high putrescine producing transgenic cells of poplar vs. their low putrescine counterparts. The results underscore the complexity of cellular responses to genetic perturbation of a single metabolic step related to nitrogen metabolism in plants. Combined with recent studies from our lab, where we showed that higher putrescine production caused an increased flux of glutamate into ornithine concurrent with enhancement in glutamate production via additional nitrogen and carbon assimilation, the results from this study provide guidance in designing transgenic plants with increased nitrogen use efficiency, especially in plants intended for non-food/feed applications (e.g. increased biomass production for biofuels).

FERONIA receptor kinase interacts with S-adenosylmethionine synthetase and suppresses S-adenosylmethionine production and ethylene biosynthesis in Arabidopsis

Environmental inputs such as stress can modulate plant cell metabolism, but the detailed mechanism remains unclear. We report here that FERONIA (FER), a plasma membrane receptor-like kinase, may negatively regulate the S-adenosylmethionine (SAM) synthesis by interacting with two S-adenosylmethionine synthases (SAM1 and SAM2). SAM participates in ethylene, nicotianamine and polyamine biosynthetic pathways and provides the methyl group for protein and DNA methylation reactions. The Arabidopsis fer mutants contained a higher level of SAM and ethylene in plant tissues and displayed a dwarf phenotype. Such phenotype in the fer mutants was mimicked by over-expressing the S-adenosylmethionine synthetase in transgenic plants, whereas sam1/2 double mutant showed an opposite phenotype. We propose that FER receptor kinase, in response to environmental stress and plant hormones such as auxin and BR, interacts with SAM synthases and down-regulates ethylene biosynthesis.

Physiological implications of arginine metabolism in plants

Nitrogen is a limiting resource for plant growth in most terrestrial habitats since large amounts of nitrogen are needed to synthesize nucleic acids and proteins. Among the 21 proteinogenic amino acids, arginine has the highest nitrogen to carbon ratio, which makes it especially suitable as a storage form of organic nitrogen. Synthesis in chloroplasts via ornithine is apparently the only operational pathway to provide arginine in plants, and the rate of arginine synthesis is tightly regulated by various feedback mechanisms in accordance with the overall nutritional status. While several steps of arginine biosynthesis still remain poorly characterized in plants, much wider attention has been paid to inter- and intracellular arginine transport as well as arginine-derived metabolites. A role of arginine as alternative source besides glutamate for proline biosynthesis is still discussed controversially and may be prevented by differential subcellular localization of enzymes. Apparently, arginine is a precursor for nitric oxide (NO), although the molecular mechanism of NO production from arginine remains unclear in higher plants. In contrast, conversion of arginine to polyamines is well documented, and in several plant species also ornithine can serve as a precursor for polyamines. Both NO and polyamines play crucial roles in regulating developmental processes as well as responses to biotic and abiotic stress. It is thus conceivable that arginine catabolism serves on the one hand to mobilize nitrogen storages, while on the other hand it may be used to fine-tune development and defense mechanisms against stress. This review summarizes the recent advances in our knowledge about arginine metabolism, with a special focus on the model plant Arabidopsis thaliana, and pinpoints still unresolved critical questions.

The roles of polyamines during the lifespan of plants: from development to stress

Compelling evidence indicates that free polyamines (PAs) (mainly putrescine, spermidine, spermine, and its isomer thermospermine), some PA conjugates to hydroxycinnamic acids, and the products of PA oxidation (hydrogen peroxide and γ-aminobutyric acid) are required for different processes in plant development and participate in abiotic and biotic stress responses. A tight regulation of PA homeostasis is required, since depletion or overaccumulation of PAs can be detrimental for cell viability in many organisms. In plants, homeostasis is achieved by modulation of PA biosynthesis, conjugation, catabolism, and transport. However, recent data indicate that such mechanisms are not mere modulators of PA pools but actively participate in PA functions. Examples are found in the spermidine-dependent eiF5A hypusination required for cell division, PA hydroxycinnamic acid conjugates required for pollen development, and the involvement of thermospermine in cell specification. Recent advances also point to implications of PA transport in stress tolerance, PA-dependent transcriptional and translational modulation of genes and transcripts, and posttranslational modifications of proteins. Overall, the molecular mechanisms identified suggest that PAs are intricately coordinated and/or mediate different stress and developmental pathways during the lifespan of plants.

A maize spermine synthase 1 PEST sequence fused to the GUS reporter protein facilitates proteolytic degradation

Polyamines are low molecular weight aliphatic compounds involved in various biochemical, cellular and physiological processes in all organisms. In plants, genes involved in polyamine biosynthesis and catabolism are regulated at transcriptional, translational, and posttranslational level. In this research, we focused on the characterization of a PEST sequence (rich in proline, glutamic acid, serine, and threonine) of the maize spermine synthase 1 (ZmSPMS1). To this aim, 123 bp encoding 40 amino acids of the C-terminal region of the ZmSPMS1 enzyme containing the PEST sequence were fused to the GUS reporter gene. This fusion was evaluated in Arabidopsis thaliana transgenic lines and onion monolayers transient expression system. The ZmSPMS1 PEST sequence leads to specific degradation of the GUS reporter protein. It is suggested that the 26S proteasome may be involved in GUS::PEST fusion degradation in both onion and Arabidopsis. The PEST sequences appear to be present in plant spermine synthases, mainly in monocots.

Enhanced flux of substrates into polyamine biosynthesis but not ethylene in tomato fruit engineered with yeast S-adenosylmethionine decarboxylase gene

S-adenosylmethionine (SAM), a major substrate in 1-C metabolism is a common precursor in the biosynthetic pathways of polyamines and ethylene, two important plant growth regulators, which exhibit opposing developmental effects, especially during fruit ripening. However, the flux of various substrates including SAM into the two competing pathways in plants has not yet been characterized. We used radiolabeled (14)C-Arg, (14)C-Orn, L-[U-(14)C]Met, (14)C-SAM and (14)C-Put to quantify flux through these pathways in tomato fruit and evaluate the effects of perturbing these pathways via transgenic expression of a yeast SAM decarboxylase (ySAMDC) gene using the fruit ripening-specific promoter E8. We show that polyamines in tomato fruit are synthesized both from Arg and Orn; however, the relative contribution of Orn pathway declines in the later stages of ripening. Expression of ySAMDC reversed the ripening associated decline in spermidine (Spd) and spermine (Spm) levels observed in the azygous control fruit. About 2- to 3-fold higher levels of labeled-Spd in transgenic fruit (556HO and 579HO lines) expressing ySAMDC confirmed the enzymatic function of the introduced gene. The incorporation of L-[U-(14)C]Met into Spd, Spm, ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC) was used to determine Met-flux into these metabolites. The incorporation of (14)C-Met into Spd/Spm declined during ripening of the control azygous fruit but this was reversed in fruits expressing ySAMDC. However, incorporation of (14)C-Met into ethylene or ACC during ripening was not altered by the expression of ySAMDC in the fruit. Taken together these results show that: (1) There is an inverse relationship between the production of higher polyamines and ethylene during fruit ripening, (2) the inverse relationship between higher polyamines and ethylene is modulated by ySAMDC expression in that the decline in Spd/Spm during fruit ripening can be reversed without significantly altering ethylene biosynthesis, and (3) cellular flux of SAM in plants is homeostatically regulated based on its demand for competing pathways.

Ornithine: the overlooked molecule in the regulation of polyamine metabolism

We overexpressed a mouse ornithine decarboxylase gene under the control of a constitutive and an estradiol-inducible promoter in Arabidopsis thaliana to increase our understanding of the regulation of polyamine metabolism. Of particular interest was the role of the substrate ornithine not only in the regulation of polyamine biosynthesis, but also in the accumulation of related amino acids in response to short-term induction of this enzyme. We hypothesized that the inducible expression of the transgene would mimic the natural responses of plants to changing conditions, e.g. under stress conditions and during rapid growth. Our results reveal that ornithine, even though present in relatively small quantities (compared with other amino acids of the glutamate-arginine-proline pathway), may not only be the key regulator of polyamine biosynthesis in Arabidopsis, but it may also regulate the entire subset of pathways for glutamate to arginine and to proline. Indirectly, it could also regulate putrescine catabolism, therefore contributing to the γ-aminobutyric acid content of the cells. Furthermore, the induction of mouse ornithine decarboxylase resulted in up- and down-regulation of several amino acids in the transgenic plants. It was learned that the turnover of putrescine in both the wild type and the transgenic plants occurs rapidly, with a half-life of 6-8 h.

Methionine salvage and S-adenosylmethionine: essential links between sulfur, ethylene and polyamine biosynthesis

Both Met (methionine) and SAM (S-adenosylmethionine), the activated form of Met, participate in a number of essential metabolic pathways in plants. The subcellular compartmentalization of Met fluxes will be discussed in the present review with respect to regulation and communication with the sulfur assimilation pathway, the network of the aspartate-derived amino acids and the demand for production of SAM. SAM enters the ethylene, nicotianamine and polyamine biosynthetic pathways and provides the methyl group for the majority of methylation reactions required for plant growth and development. The multiple essential roles of SAM require regulation of its synthesis, recycling and distribution to sustain these different pathways. A particular focus of the present review will be on the function of recently identified genes of the Met salvage cycle or Yang cycle and the importance of the Met salvage cycle in the metabolism of MTA (5'-methylthioadenosine). MTA has the potential for product inhibition of ethylene, nicotianamine and polyamine biosynthesis which provides an additional link between these pathways. Interestingly, regulation of Met cycle genes was found to differ between plant species as shown for Arabidopsis thaliana and Oryza sativa.

Aminopropyltransferases involved in polyamine biosynthesis localize preferentially in the nucleus of plant cells

Plant aminopropyltransferases consist of a group of enzymes that transfer aminopropyl groups derived from decarboxylated S-adenosyl-methionine (dcAdoMet or dcSAM) to propylamine acceptors to produce polyamines, ubiquitous metabolites with positive charge at physiological pH. Spermidine synthase (SPDS) uses putrescine as amino acceptor to form spermidine, whereas spermine synthase (SPMS) and thermospermine synthase (TSPMS) use spermidine as acceptor to synthesize the isomers spermine and thermospermine respectively. In previous work it was shown that both SPDS1 and SPDS2 can physically interact with SPMS although no data concerning the subcellular localization was reported. Here we study the subcellular localization of these enzymes and their protein dimer complexes with gateway-based Bimolecular Fluorescence Complementation (BiFC) binary vectors. In addition, we have characterized the molecular weight of the enzyme complexes by gel filtration chromatography with in vitro assembled recombinant enzymes and with endogenous plant protein extracts. Our data suggest that aminopropyltransferases display a dual subcellular localization both in the cytosol and nuclear enriched fractions, and they assemble preferably as dimers. The BiFC transient expression data suggest that aminopropyltransferase heterodimer complexes take place preferentially inside the nucleus.

Regulation of polyamine metabolism and biosynthetic gene expression during olive mature-fruit abscission

Exogenous ethylene and some inhibitors of polyamine biosynthesis can induce mature-fruit abscission in olive, which could be associated with decreased nitric oxide production as a signaling molecule. Whether H₂O₂ also plays a signaling role in mature-fruit abscission is unknown. The possible involvement of H₂O₂ and polyamine in ethylene-induced mature-fruit abscission was examined in the abscission zone and adjacent cells of two olive cultivars. Endogenous H₂O₂ showed an increase in the abscission zone during mature-fruit abscission, suggesting that accumulated H₂O₂ may participate in abscission signaling. On the other hand, we followed the expression of two genes involved in the polyamine biosynthesis pathway during mature-fruit abscission and in response to ethylene or inhibitors of ethylene and polyamine. OeSAMDC1 and OeSPDS1 were expressed differentially within and between the abscission zones of the two cultivars. OeSAMDC1 showed slightly lower expression in association with mature-fruit abscission. Furthermore, our data show that exogenous ethylene or inhibitors of polyamine encourage the free putrescine pool and decrease the soluble-conjugated spermidine, spermine, homospermidine, and cadaverine in the olive abscission zone, while ethylene inhibition by CoCl₂ increases these soluble conjugates, but does not affect free putrescine. Although the impact of these treatments on polyamine metabolism depends on the cultivar, the results confirm that the mature-fruit abscission may be accompanied by an inhibition of S-adenosyl methionine decarboxylase activity, and the promotion of putrescine synthesis in olive abscission zone, suggesting that endogenous putrescine may play a complementary role to ethylene in the normal course of mature-fruit abscission.