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This reaction is carried out by an enzyme, glutamine synthetase, whose occurrence is not restricted to microorganisms. It is esential not only for the synthesis of glutamine, a building block of proteins. Glutamine contributes also through its amide group to the synthesis of many nitrogen containing substances; in addition, it is an obligatory intermediate in ammonia assimilation in ammonia-limited media.

The use of stem cells in cell replacement therapy for neurodegenerative diseases has received a great deal of scientific and public interest in recent years. Oxidative stress, toxic byproducts, which prevails in the microenvironment during the diseased condition, may limit the survival of the transplanted stem cells affecting tissue regeneration and even longevity. Recently, it has become clear that balanced regulation of reactive oxygen species is of critical significance for cell fate determination as well as for stem cell differentiation, function, and survival following transplantation in injured tissues. Recent evidences suggested that genetically modified stem cells with antioxidant genes could exert stem cell protection and their potential for central nervous system (CNS) maladies treatment. Incorporating this knowledge into designs of novel approaches for stem cell protection is a major issue now. Polyamines are ubiquitous components in all living organisms that play essential roles in growth and signal transduction. The polyamine biosynthetic pathway is established as a valid target for the synthesis of drugs. Several comprehensive review articles have described arginine decarboxylase (ADC) as a component of the polyamine pathways metabolizing arginine and the presence of ADC as a means of generating agmatine which is an endogenous primary amine and a novel neuromodulator and possess neuroprotective properties. The presence of an ADC-regulated pathway to agmatine synthesis would also need to be considered in the interpretation of studies in which gene transfer approaches to increase or decrease agmatine levels both in in vitro and in vivo studies. Agmatine synthesized from the decarboxylation of l-arginine catalysed by arginine decarboxylase (ADC). Recently it was demonstrated that retrovirus containing ADC gene transfer which can synthesize agmatine endogenously conferred protection against toxic insult in neural stem cells. This article summarizes the results demonstrating the various gene transfer effects which can allow for stem cell protection. This review unravels the impact for development and design of genetically engineered stem cells for future cell therapies.

Plant metabolic engineering is quite a young science in plants, however, in the past decade it has generated a lot of interest. The reason being that plant secondary metabolites are economically very important as they find applications in pharmaceuticals, insecticides, flavours, fragrances and dyes but their production in plants are in very low quantities. With the development of basic molecular biology and genetic engineering techniques a lot of strategies and tools have become available for targeted improvement of quality and quantity of secondary metabolites. Plant alkaloids constitute the second largest group of secondary metabolites and provide many pharmacologically active compounds. The knowledge of secondary metabolite pathways is however very limited and is the major constraint for successful application of metabolic engineering. Polyamines are low molecular weight polycationic molecules, ubiquitous in nature and are known to play an important role in the regulation of plant growth and development. They also act as precursors of many of the economically important secondary metabolites. Polyamine biosynthetic pathway provides an attractive model for such studies as the various steps in the pathway are very well worked out and most of the genes involved in the pathway have been cloned. Further, polyamine biosynthesis has also been engineered for enhanced alkaloid content by over-expression of ornithine decarboxylase and arginine decarboxylase genes. The present review highlights the prospects of engineering of polyamine biosynthesis for enhanced alkaloid content and also provides a forward looking perspective of this exciting field over the coming years

The effect of polyamines, based on their charge, has been observed as binding to macromolecules. The binding of polyamines to macromolecules may indicate their role in the regulation of basic processes in plants such as cell division, morphogenesis, senescence and response to stress [1]. The enzyme transglutaminase is capable to bind polyamines covalently between two carboxyl groups of two glutamic acid residues in the same protein [2]. The apoproteins of the chlorophyll-a/b antenna complex, LHCP II, (CP24, CP26, CP29) [3], the proteins D1, D2, Cyt f and the large subunit of ribulose-1,5bisphosphate carboxylase/oxygenase [4] are some of the substrates of the plastidic transglutaminase. Kotzabasis and coworkers [5] reported that Put, Spd, and Spm are present in intact chloroplasts, particulary in thylakoid membranes, in PS II membranes, in the LHC-proteins and in the PS II-complex of spinach. However, when further fractionation of the photosynthetic membranes was performed, only Spm was detected in the photochemically active PS II-core complex and the PS II-reaction center. Furthermore, diamines and polyamines, exogenously supplied to osmotically stressed oat leaves, are reported to retard protein degradation, to inhibit chlorophyll (Chl) loss and to stabilize thylakoid membranes [4]. Isolated intact etioplasts from Zea mays contained great amounts of polyamines and exhibited high activities of the main polyamine biosynthetic enzymes, arginine decarboxylase (ADC) and ornithine decarboxylase (ODC), but conversly very low activity of the main catabolic enzyme, diamine oxidase (DAO), compared to the chloroplast enzyme levels [6].

It is well accepted that auxin delays and ethylene accelerates abscission (5). During the abscission process cells in the abscission zone (AZ) pass from a juvenile stage, characterized by cell division and enlargement, to a senescence stage leading to cell wall degradation and organ separation. Extensive evidence links polyamines (PA) to the control of cell division perceives them as “antisenescence” agents (1,4,8), while ethylene, is considered a senescence-promoting hormone (1). The antisenescence properties of exogenously applied PA may result from both an inhibition of ethylene biosynthesis and promotion of endogenous PA biosynthesis (8). Many studies dealt with the interactions between auxin and ethylene in the control of abscission (5). However, the biosynthetic linkage between PA and ethylene via S-adenosylmethionine as a common intermediate, and the fact that PA directly antagonize many ethylene-mediated responses (1,8), raised the question whether PA are involved in the control of abscission. The linkage between PA and ethylene was previously studied both at the biosynthetic and at the physiological levels, and mainly in relation to senescence and fruit ripening (1). Most of the reports dealt with the effects of exogenous PA on several aspects of ethylene production and biosynthesis, and only a few examined how exogenous ethylene interferes with PA levels and biosynthesis (1,8). The interactions between plant hormones and PA can be summarized by saying that in general, plant hormones that promote growth induce an increase in the biosynthesis of PA while plant hormones that retard growth decrease PA biosynthesis (for detailed references see reviews 1,4,8). Exogenous ethylene was reported to decrease the activity of arginine decarboxylase (ADC) and S-adenosylmethionine decarboxylase, and to increase the activity of ornithine decarboxylase (ODC) and lysine decarboxylase (2,6). In the following we refer to a study on the effects of ethylene on abscission of citrus leaf explants; PA biosynthesis was examined to establish the kind of relationships existing between the two regulators in the process of abscission.

This review on polyamines attempts to present evidence in favour of their role as plant growth substances. Many results suggest that polyamines are implicated in a number of growth and developmental processes in plants, ranging from cell division, embryogenesis, senescence and flowering. However, due to their cationic nature and low molecular weight, polyamines also display effects similar to those of inorganic cations that will be only marginally considered in this short review Finally, their changes in response to stress phenomena and their utilization for alkaloid synthesis are not dealt with and the reader should refer to recent reviews covering these aspects in considerable detail (Bachrach and Heimer, 1989; Flores et al., 1990).

Putrescine is an important precursor of alkaloids such as nicotine in Nicotiana and scopolamine and hyoscyamine in Datura. It acts at the interface between primary and secondary metabolism and its primary role is as an intermediate in the formation of polyamines which have important roles in the growth of animal and microbial as well as plant cells. In animals and microbes the formation of putrescine and polyamines is regulated by effects on the expression of the enzymes ornithine decarboxylase (ODC) and S-adenosyl methionine decarboxylase (SAM dC)1 both at the nucleic acid and protein levels. In plants, the situation is complicated by the occurrence of an alternative route of formation of putrescine from arginine and arginine decarboxylase (ADC) as well as from ornithine and ornithine decarboxylase and the presence of large buffering pools of putrescine and polyamine conjugates. Nevertheless, the role of polyamines in growth-related processes in plants is well established2,3,4. In tissues where putrescine-derived alkaloids are found, there is clearly competition for putrescine as substrate between the polyamine and alkaloid pathways. Feeding low levels of putrescine (1–5mM) can have a stimulatory effect upon the production of nicotine in N. rustica transformed root cultures5. This suggests that increasing the levels of this intermediate, by genetic manipulation techniques, may allow a greater flux down the metabolic pathway leading to nicotine and thus increase the secondary product concentrations of these cultures. We have investigated these interactions by attempting to over-express a yeast ornithine decarboxylase (yODC) gene in transformed roots of Nicotiana rustica to increase internal pools of putrescine and to study its partitioning between polyamines and alkaloids.

Polyamines are widely distributed throughout microbial, animal and plant cells. They are universally associated with plant growth and cell division (1,2) and recently the role of a “second messenger” has been proposed for them (3). In microbial and plant cells, putrescine is synthesized either from arginine via agmatine or directly from ornithine. The putrescine formed serves as an acceptor for a propylami no moiety to form spermidine, and spermidine in turn serves as an acceptor of a second propylamino moiety to form spermine (4). The origin of the propylamino transferred is S-adenosylmethylthiopropylamine, formed from S-adenosylmethionine (SAM), through the action of S-adenosylmethionine decarboxylase (E.C.4.1.1.50) (SAMDC) (5).

The apparent involvement of ornithine decarboxylase (ODC) and putrescine in the early stages of fruit growth in tomato (Lycopersicon esculentum Mill.) has been previously described. Further evidence presented here supports the direct involvement of ODC and putrescine in the cell division process in tomato fruits. In tomato fruits grown in vitro, in which basic growth processes are inhibited, the activity of ODC and arginine decarboxylase (ADC) and the level of free polyamines were reduced. While ODC and ADC activity was correlated with the period of cell division in the tomato fruit, the free polyamine content was correlated with the DNA content, cell size, and fruit fresh weight. The addition of exogenous putrescine, however, did not restore the basic growth processes in the fruits grown in vitro.

A mutant carrot callus line resistant to a high concentration (1 mM) of putrescine has been isolated. A high level of endogenous putrescine, about 13-fold higher than in the controls grown in the absence of putrescine, characterized this resistant mutant. Ornithine-, arginine- and S-adenosylmethionine decarboxylase activities were similar in the resistant line and in the controls by the fifth subculture. The uptake of putrescine when supplied to the medium at high concentration (1 mM), was similar in both the putrescine-treated calli and in the untreated controls. At low concentrations (0.64 μM) however the putrescine absorbed by the resistant calli was less than that absorbed by the controls. Putrescine uptake took place almost always against a concentration gradient and might be due to an active mechanism.