Nitric oxide, polyamines and Cd-induced phytotoxicity in wheat roots.

To further explore the biochemical basis of Cd toxicity in developing wheat seedlings, we studied the possible role of nitric oxide (NO) and polyamines as signaling molecules involved in metal-induced root growth inhibition. When used at 0.1 mM, sodium nitroprusside, a NO-releasing compound, inhibited root growth to a similar extent as Cd and enhanced the polyamine contents as Cd also did. Putrescine and spermidine treatments caused significant decreases in root growth with spermine giving the greatest level of inhibition (77% reduction). The simultaneous addition of Cd and inhibitors of putrescine biosynthesis (DFMA and DFMO) prevented increases in putrescine levels but did not restore normal root growth. NO content, as evidenced by the fluorescent probe DAF-FM diacetate, was found to be significantly increased in the roots of both Cd and polyamine treated plants, especially in those exposed to spermine. The effect was specific for NO since the NO scavenger cPTIO almost suppressed the fluorescent signal. Concerning the oxidative status of the root system, only Cd and spermine enhanced lipid peroxidation in roots. At the same time, all treatments led to a significant increase in levels of the non-enzymatic antioxidant defense glutathione. Our results strongly suggest that Cd and spermine treatments induce NO formation in wheat roots which, in turn, is involved in root growth inhibition.

Polyamines and abiotic stress: recent advances.

In this review we will concentrate in the results published the last years regarding the involvement of polyamines in the plant responses to abiotic stresses, most remarkably on salt and drought stress. We will also turn to other types of abiotic stresses, less studied in relation to polyamine metabolism, such as mineral deficiencies, chilling, wounding, heavy metals, UV, ozone and paraquat, where polyamine metabolism is also modified. There is a great amount of data demonstrating that under many types of abiotic stresses, an accumulation of the three main polyamines putrescine, spermidine and spermine does occur. However, there are still many doubts concerning the role that polyamines play in stress tolerance. Several environmental challenges (osmotic stress, salinity, ozone, UV) are shown to induce ADC activity more than ODC. The rise in Put is mainly attributed to the increase in ADC activity as a consequence of the activation of ADC genes and their mRNA levels. On the other hand, free radicals are now accepted as important mediators of tissue injury and cell death. The polycationic nature of polyamines, positively charged at physiological pH, has attracted the attention of researchers and has led to the hypothesis that polyamines could affect physiological systems by binding to anionic sites, such as those associated with nucleic acids and membrane phospholipids. These amines, involved with the control of numerous cellular functions, including free radical scavenger and antioxidant activity, have been found to confer protection from abiotic stresses but their mode of action is not fully understood yet. In this review, we will also summarize information about the involvement of polyamines as antioxidants against the potential abiotic stress-derived oxidative damage.

Polyamine metabolism in sunflower plants under long-term cadmium or copper stress.

The effect of different doses of cadmium and copper was studied in relation to growth and polyamine (Pas) metabolism in shoots of sunflower plants. Cadmium accumulated to higher levels than copper and shoot length was reduced by 0.5 and 1 mM Cd, but only by 1 mM Cu. At 1 mM of Cd or Cu, Put content increased 270% and 160% with Cd2+ and Cu2+, respectively. Spermidine (Spd) was modified only by 1 mM Cd, while spermine (Spm) declined after seeds germinated, increasing thereafter but only with 1 mM Cd or Cu (273% over the controls for Cd and 230% for Cu at day 16). Both ADC and ODC activities were increased by 1 mM Cd, whereas 1 mM Cu enhanced ADC activity, but reduced ODC activity at every concentration used. The role of Pas as markers of Cd or Cu toxicity is discussed.

Relationship between oxidative stress and heme oxygenase induction by copper sulfate.

The effect of copper sulfate (CuSO4) on both hepatic oxidative stress and heme oxygenase induction was studied. A strong increase in in vivo rat liver chemiluminescence was observed 1 h after Cu(II) administration. To evaluate liver antioxidant enzymatic defenses, superoxide dismutase, catalase, and glutathione peroxidase activities were determined. Catalase and glutathione peroxidase were found to be significantly decreased 5 h after CuSO4 injection. In contrast, superoxide dismutase activity was increased. Heme oxygenase activity appeared 5 h after treatment, reaching a maximum value 18 h after CuSO4 administration. This induction was preceded by a decrease in the intrahepatic GSH pool and an increase in the generation of thiobarbituric acid reactive substances, both effects taking place a number of hours before induction of heme oxygenase. Administration of bilirubin, the end product of heme catabolism in mammals, and alpha-tocopherol, a widely employed antioxidant, completely prevented heme oxygenase induction as well as the decrease in hepatic GSH and the increase in chemiluminescence when administered 2 h before CuSO4 treatment. Under the same experimental conditions, beta-carotene showed a moderate preventive effect on both heme oxygenase induction and oxidative stress parameters. These data obtained with Cu(II) treatment are in agreement with our previous reports suggesting a correlation between heme oxygenase induction and oxidative stress.