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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 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.

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.

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.