The physiological relationship between abiotic stress in plants and polyamines was reported more than 40 years ago. Ever since there has been a debate as to whether increased polyamines protect plants against abiotic stress (e.g., due to their ability to deal with oxidative radicals) or cause damage to them (perhaps due to hydrogen peroxide produced by their catabolism). The observation that cellular polyamines are typically elevated in plants under both short-term as well as long-term abiotic stress conditions is consistent with the possibility of their dual effects, i.e., being protectors from as well as perpetrators of stress damage to the cells. The observed increase in tolerance of plants to abiotic stress when their cellular contents are elevated by either exogenous treatment with polyamines or through genetic engineering with genes encoding polyamine biosynthetic enzymes is indicative of a protective role for them. However, through their catabolic production of hydrogen peroxide and acrolein, both strong oxidizers, they can potentially be the cause of cellular harm during stress. In fact, somewhat enigmatic but strong positive relationship between abiotic stress and foliar polyamines has been proposed as a potential biochemical marker of persistent environmental stress in forest trees in which phenotypic symptoms of stress are not yet visible. Such markers may help forewarn forest managers to undertake amelioration strategies before the appearance of visual symptoms of stress and damage at which stage it is often too late for implementing strategies for stress remediation and reversal of damage. This review provides a comprehensive and critical evaluation of the published literature on interactions between abiotic stress and polyamines in plants, and examines the experimental strategies used to understand the functional significance of this relationship with the aim of improving plant productivity, especially under conditions of abiotic stress.

The metabolism of glutamate into ornithine, arginine, proline, and polyamines is a major network of nitrogen-metabolizing pathways in plants, which also produces intermediates like nitric oxide, and γ-aminobutyric acid (GABA) that play critical roles in plant development and stress. While the accumulations of intermediates and the products of this network depend primarily on nitrogen assimilation, the overall regulation of the interacting sub-pathways is not well understood. We tested the hypothesis that diversion of ornithine into polyamine biosynthesis (by transgenic approach) not only plays a role in regulating its own biosynthesis from glutamate but also affects arginine and proline biosynthesis. Using two high putrescine producing lines of Arabidopsis thaliana (containing a transgenic mouse ornithine decarboxylase gene), we studied the: (1) effects of exogenous supply of carbon and nitrogen on polyamines and pools of soluble amino acids; and, (2) expression of genes encoding key enzymes in the interactive pathways of arginine, proline and GABA biosynthesis as well as the catabolism of polyamines. Our findings suggest that: (1) the overall conversion of glutamate to arginine and polyamines is enhanced by increased utilization of ornithine for polyamine biosynthesis by the transgene product; (2) proline and arginine biosynthesis are regulated independently of polyamines and GABA biosynthesis; (3) the expression of most genes (28 that were studied) that encode enzymes of the interacting sub-pathways of arginine and GABA biosynthesis does not change even though overall biosynthesis of Orn from glutamate is increased several fold; and (4) increased polyamine biosynthesis results in increased assimilation of both nitrogen and carbon by the cells.

The metabolism of polyamines (putrescine, spermidine, and spermine) has become the target of genetic manipulation because of their significance in plant development and possibly stress tolerance. We studied the polyamine metabolism in non-transgenic (NT) and transgenic cells of poplar (Populus nigra ϫ maximowiczii) expressing a mouse Orn decarboxylase (odc) cDNA. The transgenic cells showed elevated levels of mouse ODC enzyme activity, severalfold higher amounts of putrescine, a small increase in spermidine, and a small reduction in spermine as compared with NT cells. The conversion of labeled ornithine (Orn) into putrescine was significantly higher in the transgenic than the NT cells. Whereas exogenously supplied Orn caused an increase in cellular putrescine in both cell lines, arginine at high concentrations was inhibitory to putrescine accumulation. The addition of urea and glutamine had no effect on polyamines in either of the cell lines. Inhibition of glutamine synthetase by methionine sulfoximine led to a substantial reduction in putrescine and spermidine in both cell lines. The results show that: (a) Transgenic expression of a heterologous odc gene can be used to modulate putrescine metabolism in plant cells, (b) accumulation of putrescine in high amounts does not affect the native arginine decarboxylase activity, (c) Orn biosynthesis occurs primarily from glutamine/glutamate and not from catabolic breakdown of arginine, (d) Orn biosynthesis may become a limiting factor for putrescine production in the odc transgenic cells, and (e) assimilation of nitrogen into glutamine keeps pace with an increased demand for its use for putrescine production.Polyamines (putrescine, spermidine, and spermine) are low M r polycations found in all living organisms. At the cellular level, polyamines are involved in DNA and protein synthesis, stabilization of membranes, scavenging of free radicals, and modulation of enzyme activities (Minocha and

Changes in the cellular content of three polyamines (putrescine, spermidine and spermine) were compared at different stages of development in zygotic and somatic embryos of Pinus radiata D. Don. During embryo development, both the zygotic and the somatic embryos showed a steady increase in spermidine content, with either a small decrease or no significant change in putrescine. This led to a several‐fold increase in spermidine/putrescine ratios during development of both types of embryos. Cell cultures of plant‐forming and non‐plant‐forming lines derived from the same clone and growing on proliferation (maintenance) medium differed significantly in their polyamine levels. Mature, cotyledonary stage somatic embryos capable of germination and formation of plants could be distinguished by their higher spermidine/putrescine ratios from abnormal cotyledonary stage somatic embryos which were incapable of forming plants.

Distribution of biogenic amines-the diamine putrescine (Put), triamine spermidine (Spd), and tetraamine spermine (Spm)-differs between species with Put and Spd being particularly abundant and Spm the least abundant in plant cells. These amines are important for cell viability and their intracellular levels are tightly regulated, which have made it difficult to characterize individual effects of Put, Spd and Spm on plant growth and developmental processes. The recent transgenic intervention and mutational genetics have made it possible to stably alter levels of naturally occurring polyamines and study their biological effects. We bring together an analysis of certain metabolic changes, particularly in amino acids, to infer the responsive regulation brought about by increased diamine or polyamine levels in actively growing poplar cell cultures (transformed with mouse ornithine decarboxylase gene to accumulate high Put levels) and ripening tomato pericarp (transformed with yeast S-adenosylmethionine decarboxylase gene to accumulate high Spd and Spm levels at the cost of Put). Our analysis indicates that increased Put has little effect on increasing the levels of Spd and Spm, while Spd and Spm levels are inter-dependent. Further, Put levels were positively associated with Ala (alpha and beta), Ile and GABA and negatively correlated with Gln and Glu in both actively growing poplar cell cultures and non-dividing tomato pericarp tissue. Most amino acids showed positive correlations with Spd and Spm levels in actively growing cells. Collectively these results suggest that Put is a negative regulator while Spd-Spm are positive regulators of cellular amino acid 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.

We investigated the catabolism of putrescine (Put) in a non-transgenic (NT) and a transgenic cell line of poplar (Populus nigra × maximowiczii) expressing a mouse (Mus musculus) ornithine (Orn) decarboxylase (odc) cDNA. The transgenic cells produce 3- to 4-fold higher amounts of Put than the NT cells. The rate of loss of Put from the cells and the initial half-life of cellular Put were determined by feeding the cells with [U-14C]Orn and [1,4-14C]Put as precursors and following the loss of [14C]Put in the cells at various times after transfer to label-free medium. The amount of Put converted into spermidine as well as the loss of Put per gram fresh weight were significantly higher in the transgenic cells than the NT cells. The initial half-life of exogenously supplied [14C]Put was not significantly different in the two cell lines. The activity of diamine oxidase, the major enzyme involved in Put catabolism, was comparable in the two cell lines even though the Put content of the transgenic cells was severalfold higher than the NT cells. It is concluded that in poplar cells: (a) exogenously supplied Orn enters the cells and is rapidly converted into Put, (b) the rate of Put catabolism is proportional to the rate of its biosynthesis, and (c) the increased Put degradation occurs without significant changes in the activity of diamine oxidase.

Plants in nutrient-poor environments typically have low foliar nitrogen (N) concentrations, long-lived tissues with leaf traits designed to use nutrients efficiently, and low rates of photosynthesis. We postulated that increasing N availability due to atmospheric deposition would increase photosynthetic capacity, foliar N, and specific leaf area (SLA) of bog shrubs. We measured photosynthesis, foliar chemistry and leaf morphology in three ericaceous shrubs (Vaccinium myrtilloides, Ledum groenlandicum and Chamaedaphne calyculata) in a long-term fertilization experiment at Mer Bleue bog, Ontario, Canada, with a background deposition of 0.8 g N m(-2) a(-1). While biomass and chlorophyll concentrations increased in the highest nutrient treatment for C. calyculata, we found no change in the rates of light-saturated photosynthesis (A(max)), carboxylation (V(cmax)), or SLA with nutrient (N with and without PK) addition, with the exception of a weak positive correlation between foliar N and A(max) for C. calyculata, and higher V(cmax) in L. groenlandicum with low nutrient addition. We found negative correlations between photosynthetic N use efficiency (PNUE) and foliar N, accompanied by a species-specific increase in one or more amino acids, which may be a sign of excess N availability and/or a mechanism to reduce ammonium (NH(4)) toxicity. We also observed a decrease in foliar soluble Ca and Mg concentrations, essential minerals for plant growth, but no change in polyamines, indicators of physiological stress under conditions of high N accumulation. These results suggest that plants adapted to low-nutrient environments do not shift their resource allocation to photosynthetic processes, even after reaching N sufficiency, but instead store the excess N in organic compounds for future use. In the long term, bog species may not be able to take advantage of elevated nutrients, resulting in them being replaced by species that are better adapted to a higher nutrient environment.