Metabolism and function of polyamines in plants: recent development (new approaches)

A review is presented of the recent developments in the metabolism andfunction of polyamines in plants. Polyamines appear to be involved in a widerange of plant processes so their exact role is not completely understood. Inthis review, the metabolic pathways involved in polyamine biosynthesis anddegradation are explained, along with the transport and conjugation of thesecompounds. The methodologies involved in the analysis of polyamine functionusing metabolic inhibitors and genetic and molecular approaches are described.The occurrence and distribution of polyamine-derived alkaloids are also dealtwith. The direction of future research in the study of plant polyamines isindicated.     

Ornithine decarboxylase transgene in tobacco affects polyamines,in vitro-morphogenesis and response to salt stress

Transgenic tobacco plants expressing the putrescine synthesis gene ornithine decarboxylase from mouse were raised to study the effects of up-regulation of a metabolic pathway as critical as the polyamine biosynthesis on the plant growth and development, in vitro-morphogenesis and their response to salt stress. Further, the response of the alternate pathway (arginine decarboxylase) for putrescine synthesis to the modulation of the ornithine decarboxylase pathway has also been investigated. The over-expression of the odc gene and increased levels of putrescine in tobacco led to a delay in plant regeneration on selection medium which could be overcome by the exogenous application of polyamine biosynthesis inhibitors and spermidine. Further, the lines generated had a variable in vitro morphogenic potential, which could be correlated to the shifts in their polyamine metabolism. These studies have brought forward the critical role played by polyamines in the normal development of plants and also their role in plant regeneration. Since polyamines are known to accumulate in cells under abiotic stress conditions, the tolerance of the transgenics to salt stress was also investigated and the transgenics with their polyamine metabolism up-graded showed increased tolerance to salt stress.     

Polyamine catabolism: target for antiproliferative therapies in animals and stress tolerance strategies in plants

Metabolism of polyamines spermidine and spermine, and their diamine precursor, putrescine, has been a target for antineoplastic therapy since these naturally occurring alkyl amines were found essential for normal mammalian cell growth. Intracellular polyamine concentrations are maintained at a cell type-specific set point through the coordinated and highly regulated interplay between biosynthesis, transport, and catabolism. A correlation between regulation of cell proliferation and polyamine metabolism is described. In particular, polyamine catabolism involves copper-containing amine oxidases and FAD-dependent polyamine oxidases. Several studies showed an important role of these enzymes in several developmental and disease-related processes in both animals and plants through a control on polyamine homeostasis in response to normal cellular signals, drug treatment, environmental and/or cellular stressors. The production of toxic aldehydes and reactive oxygen species, H(2)O(2) in particular, by these oxidases using extracellular and intracellular polyamines as substrates, suggests a mechanism by which the oxidases can be exploited as antineoplastic drug targets. This minireview summarizes recent advances on the physiological roles of polyamine catabolism in animals and plants in an attempt to highlight differences and similarities that may contribute to determine in detail the underlined mechanisms involved. This information could be useful in evaluating the possibility of this metabolic pathway as a target for new antiproliferative therapies in animals and stress tolerance strategies in plants.     

植物多胺代谢途径研究进展

多胺是一类小分子生物活性物质,广泛存在于生物体内,与植物的生长发育、衰老及抗逆性都有着密切的联系。目前,在植物中的多胺合成途径已经基本揭示,其生理作用在分子水平上逐步得到阐明。对多胺合成突变体和各种转基因植物的研究也使得人们更深入地了解了多胺以及其合成代谢相关酶在植物生长发育等生理过程中的重要作用。以下概述了植物多胺代谢途径,重点综述了代谢途径中各基因的功能及遗传操作的最新进展,并对将来的研究方向尤其是相关基因在植物抗逆境 (包括生物和非生物逆境) 基因工程方面的应用作了讨论。     

Improvement of plant abiotic stress tolerance through modulation of the polyamine pathway

Polyamines(mainly putrescine(Put),spermidine(Spd),and spermine(Spm))have been widely found in a range of physiological processes and in almost all diverse environmental stresses.In various plant species,abiotic stresses modulated the accumulation of polyamines and related gene expression.Studies using loss-of-function mutants and transgenic overexpression plants modulating polyamine metabolic pathways confirmed protective roles of polyamines during plant abiotic stress responses,and indicated the possibility to improve plant tolerance through genetic manipulation of the polyamine pathway.Additionally,putative mechanisms of polyamines involved in plant abiotic stress tolerance were thoroughly discussed and crosstalks among polyamine,abscisic acid,and nitric oxide in plant responses to abiotic stress were emphasized.Special attention was paid to the interaction between polyamine and reactive oxygen species,ion channels,amino acid and carbon metabolism,and other adaptive responses.Further studies are needed to elucidate the polyamine signaling pathway,especially polyamine-regulated downstream targets and the connections between polyamines and other stress responsive molecules.     

Functional diversity inside the Arabidopsis polyamine oxidase gene family

Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. All so far characterized PAOs from monocotyledonous plants, such as the apoplastic maize PAO, oxidize spermine (Spm) and spermidine (Spd) to produce 1,3-diaminopropane, H(2)O(2), and an aminoaldehyde, and are thus considered to be involved in a terminal catabolic pathway. Mammalian PAOs oxidize Spm or Spd (and/or their acetyl derivatives) differently from monocotyledonous PAOs, producing Spd or putrescine, respectively, in addition to H(2)O(2) and an aminoaldehyde, and are therefore involved in a polyamine back-conversion pathway. In Arabidopsis thaliana, five PAOs (AtPAO1-AtPAO5) are present with cytosolic or peroxisomal localization and three of them (the peroxisomal AtPAO2, AtPAO3, and AtPAO4) form a distinct PAO subfamily. Here, a comparative study of the catalytic properties of recombinant AtPAO1, AtPAO2, AtPAO3, and AtPAO4 is presented, which shows that all four enzymes strongly resemble their mammalian counterparts, being able to oxidize the common polyamines Spd and/or Spm through a polyamine back-conversion pathway. The existence of this pathway in Arabidopsis plants is also evidenced in vivo. These enzymes are also able to oxidize the naturally occurring uncommon polyamines norspermine and thermospermine, the latter being involved in important plant developmental processes. Furthermore, data herein reveal some important differences in substrate specificity among the various AtPAOs, which suggest functional diversity inside the AtPAO gene family. These results represent a new starting point for further understanding of the physiological role(s) of the polyamine catabolic pathways in plants.     

多胺与环境胁迫关系研究进展

多胺是植物对胁迫响应的重要物质,可以抵消胁迫引起的负效应.多胺预处理可缓解胁迫引起的伤害,通过转基因技术过量表达多胺可提高植物胁迫耐性.本文综述了生物胁迫和非生物胁迫条件下,多胺的合成、代谢、功能及其作为抗氧化剂减少胁迫诱导氧化损伤的研究现状.重点综述了病虫胁迫、盐胁迫、重金属胁迫、渗透胁迫,也简要的综述了其它胁迫如低氧胁迫、冷胁迫、酸胁迫、辐射胁迫等条件下植物体内多胺合成的变化及功能.     

Involvement of polyamines in plant response to abiotic stress

Environmental stresses are the major cause of crop loss worldwide. Polyamines are involved in plant stress responses. However, the precise role(s) of polyamine metabolism in these processes remain ill-defined. Transgenic approaches demonstrate that polyamines play essential roles in stress tolerance and open up the possibility to exploit this strategy to improve plant tolerance to multiple environmental stresses. The use of Arabidopsis as a model plant enables us to carry out global expression studies of the polyamine metabolic genes under different stress conditions, as well as genome-wide expression analyses of insertional-mutants and plants over-expressing these genes. These studies are essential to dissect the polyamine mechanism of action in order to design new strategies to increase plant survival in adverse environments.     

Improvement of plant abiotic stress tolerance through modulation of the polyamine pathway FA简

Polyamines （mainly putrescine （Put）, spermidine （Spd）, and spermine （Spin）） have been widely found in a range of physiological processes and in almost all diverse environ- mental stresses. In various plant species, abiotic stresses modulated the accumulation of polyamines and related gene expression. Studies using loss-of-function mutants and transgenic overexpression plants modulating polyamine metabolic pathways confirmed protective roles of polyamines during plant abiotic stress responses, and indicated the possibility to improve plant tolerance through genetic manipulation of the polyamine pathway. Additionally, puta- tive mechanisms of polyamines involved in plant abiotic stress tolerance were thoroughly discussed and crosstalks among polyamine, abscisic acid, and nitric oxide in plant responses to abiotic stress were emphasized. Special attention was paid to the interaction between polyamine and reactive oxygen species, ion channels, amino acid and carbon metabolism, and other adaptive responses. Further studies are needed to elucidate the polyamine signaling pathway, especially polyamine-regulated downstream tar- gets and the connections between polyamines and other stress responsive molecules.     

The roles of polyamines in microorganisms

Polyamines are small polycations that are well conserved in all the living organisms except Archae, Methanobacteriales and Halobacteriales. The most common polyamines are putrescine, spermidine and spermine, which exist in varying concentrations in different organisms. They are involved in a variety of cellular processes such as gene expression, cell growth, survival, stress response and proliferation. Therefore, diverse regulatory pathways are evolved to ensure strict regulation of polyamine concentration in the cells. Polyamine levels are kept under strict control by biosynthetic pathways as well as cellular uptake driven by specific transporters. Reverse genetic studies in microorganisms showed that deletion of the genes in polyamine metabolic pathways or depletion of polyamines have negative effects on cell survival and proliferation. The protein products of these genes are also used as drug targets against pathogenic protozoa. These altogether confirm the significant roles of polyamines in the cells. This mini-review focuses on the differential concentrations of polyamines and their cellular functions in different microorganisms. This will provide an insight about the diverse evolution of polyamine metabolism and function based on the physiology and the ecological context of the microorganisms.     

Animal disease models generated by genetic engineering of polyamine metabolism

The polyamines putrescine, spermidine and spermine are natural components of all living cells. Although their exact cellular functions are still largely unknown, a constant supply of these compounds is required for mammalian cell proliferation to occur. Studies with animals displaying genetically altered polyamine metabolism have shown that polyamines are intimately involved in the development of diverse tumors, putrescine apparently has specific role in skin physiology and neuroprotection and the higher polyamines spermidine and spermine are required for the maintenance of pancreatic integrity and liver regeneration. In the absence of ongoing polyamine biosynthesis, murine embryogenesis does not proceed beyond the blastocyst stage. The last years have also witnessed the appearance of the first reports linking genetically altered polyamine metabolism to human diseases.     

Essential,deadly, enigmatic: Polyamine metabolism and roles in fungal cells

Polyamines are essential metabolites found in all organisms. Intracellular polyamine levels are tightly maintained by biosynthesis, degradation, uptake and excretion processes that involve regulatory mechanisms – such as the antizyme inhibitory protein – that are conserved across the kingdoms of life, indicating that polyamine levels are critical to cell function. Nonetheless, the biochemical roles of polyamines and their involvement in numerous fundamental cellular processes including aging, cell cycle progression and growth only become apparent when polyamine homeostasis is perturbed. Thus, while polyamines are present in most cells and essential for cell growth, their biochemical functions are largely enigmatic. Studies in fungi have contributed to our basic understanding of polyamines, and might continue to bridge knowledge gaps regarding polyamine metabolism and cell function. Moreover, when considering the impact of fungi – directly or indirectly, for good or for ill – on human society, closing gaps in our understanding of polyamine functions in fungal physiology is an important goal in itself that might lead to the discovery of new targets for enhancing beneficial fungal interactions and diminishing those detrimental to crop and human health. To facilitate progress towards this prospect, here we appraise what is known about polyamine metabolism in fungi, how prevalent polyamines impact fungal physiology and metabolism, and how the levels of each polyamine are maintained in the fungal cell – thus pointing to how they might be perturbed.     

Inflammation,carcinogenesis and neurodegeneration studies in transgenic animal models for polyamine research

Natural polyamines (PA) are cationic molecules affecting cell growth and proliferation. An association between increased polyamine biosynthesis and inflammation-induced carcinogenesis has been recognised. On the other hand, there are indications that inflammatory stimuli can up-regulate polyamine catabolism and that altered polyamine metabolism could affect pro- and anti-inflammatory cytokines. Since the polyamine content is strictly related to cell growth, a consistent number of evidences relate polyamine metabolism dysfunction with cancer. The increase of polyamine levels in malignant and proliferating cells attracted the interest of scientists during last decades, addressing polyamine depletion as a new strategy to inhibit carcinogenesis. Several studies suggest that PA also play an important role in neurodegeneration, but the mechanisms by which they participate in neuronal death are still unclear. Furthermore, the role of endogenous PA in normal brain functioning is yet to be elucidated. The consequences of an alteration of polyamine metabolism have also been approached in vivo with the use of transgenic animals overexpressing or devoid of some enzymes involved in polyamine metabolism. In the present work we review the experimental investigation carried out on inflammation, cancerogenesis and neurodegeneration using transgenic animals engineered as models for polyamine research.     

Selective regulation of polyamine metabolism with methylated polyamine analogues

Polyamine metabolism is intimately linked to the physiological state of the cell. Low polyamines levels promote growth cessation, while increased concentrations are often associated with rapid proliferation or cancer. Delicately balanced biosynthesis, catabolism, uptake and excretion are very important for maintaining the intracellular polyamine homeostasis, and deregulated polyamine metabolism is associated with imbalanced metabolic red/ox state. Although many cellular targets of polyamines have been described, the precise molecular mechanisms in these interactions are largely unknown. Polyamines are readily interconvertible which complicate studies on the functions of the individual polyamines. Thus, non-metabolizable polyamine analogues, like carbon-methylated analogues, are needed to circumvent that problem. This review focuses on methylated putrescine, spermidine and spermine analogues in which at least one hydrogen atom attached to polyamine carbon backbone has been replaced by a methyl group. These analogues allow the regulation of both metabolic and catabolic fates of the parent molecule. Substituting the natural polyamines with methylated analogue(s) offers means to study either the functions of an individual polyamine or the effects of altered polyamine metabolism on cell physiology. In general, gem-dimethylated analogues are considered to be non-metabolizable by polyamine catabolizing enzymes spermidine/spermine-N ¹-acetyltransferase and acetylpolyamine oxidase and they support short-term cellular proliferation in many experimental models. Monomethylation renders the analogues chiral, offering some advantage over gem-dimethylated analogues in the specific regulation of polyamine metabolism. Thus, methylated polyamine analogues are practical tools to meet existing biological challenges in solving the physiological functions of polyamines.     

Genetic approaches to the cellular functions of polyamines in mammals.

The polyamines putrescine, spermidine and spermine are organic cations shown to participate in a bewildering number of cellular reactions, yet their exact functions in intermediary metabolism and specific interactions with cellular components remain largely elusive. Pharmacological interventions have demonstrated convincingly that a steady supply of these compounds is a prerequisite for cell proliferation to occur. The last decade has witnessed the appearance of a substantial number of studies, in which genetic engineering of polyamine metabolism in transgenic rodents has been employed to unravel their cellular functions. Transgenic activation of polyamine biosynthesis through an overexpression of their biosynthetic enzymes has assigned specific roles for these compounds in spermatogenesis, skin physiology, promotion of tumorigenesis and organ hypertrophy as well as neuronal protection. Transgenic activation of polyamine catabolism not only profoundly disturbs polyamine homeostasis in most tissues, but also creates a complex phenotype affecting skin, female fertility, fat depots, pancreatic integrity and regenerative growth. Transgenic expression of ornithine decarboxylase antizyme has suggested that this unique protein may act as a general tumor suppressor. Homozygous deficiency of the key biosynthetic enzymes of the polyamines, ornithine and S-adenosylmethionine decarboxylase, as achieved through targeted disruption of their genes, is not compatible with murine embryogenesis. Finally, the first reports of human diseases apparently caused by mutations or rearrangements of the genes involved in polyamine metabolism have appeared.     

Enigmas of biosyntheses of unusual polyamines in an extreme thermophile,Thermus thermophilus

Thermus thermophilus, an extreme thermophile belonging to Domain Bacteria, produces unusual polyamines in addition to standard polyamines. To understand mechanisms of changes of polyamine compositions of the thermophile upon change of growth conditions such as environmental temperature, metabolic pathways of polyamine biosyntheses of T. thermophilus have been studied and a new polyamine metabolic pathway was proposed. However, many enigmas remain to be solved in future studies. In this paper, biosyntheses of two non-standard polyamines, thermospermine and sym-homospermidine which are also produced and play important roles in plant cells, of the extreme thermophile are discussed in relation to the biosynthetic reactions in plants.     

Molecular Genetic Analyses of Plant Polyamines

Polyamines are small, positively charged aliphatic amines that play a variety of roles in plant physiology. Putrescine, spermidine, and spermine are usually what are collectively meant by the term polyamines, although plants also have a variety of other related compounds and secondary product conjugates to polyamines. Organisms synthesize putrescine, spermidine, and spermine by pathways leading from ornithine, arginine, and SAM, with three of the important enzymes being amino acid decarboxylases. There has been recent progress in understanding plant polyamines, both their function and the regulation of their synthesis, as a result of molecular genetic investigations. The cDNAs for many of the key enzymes have been cloned and se-quenced, and studies on regulation of the enzymes have begun. Mutational and transgenic approaches are being used to perturb the pathway. Some of the phenotypes observed suggest interactions between polyamines and either ethylene or cytokinin, consistent with some of the correlations observed many years previously by polyamine physiologists. These studies, while still in their early stages, should improve our understanding of polyamine synthesis, but difficult problems remain to be solved before we can answer the question: What are the biological functions and associated mechanisms of action of polyamines?