Abdelilah Chaoui

Cadmium and zinc induction of lipid peroxidation and effects on antioxidant enzyme activities in bean (Phaseolus vulgaris L.)

The relationship between Cd and Zn phytotoxicities and oxidative reactions in bean plants was studied. Ten-day-old bean (Phaseolus vulgaris L. cv. Morgan) plants treated for 96 h by 5 μM Cd and 100 μM Zn, separately, showed the same reduction of growth. In response to each metal, lipid peroxidation was enhanced in all plant organs, and catalase (CAT) activity was decreased in both roots and leaves, but not in stems. However, Cd and Zn stimulated the activity of guaiacol-dependent peroxidase only in stems, where native electrophoresis revealed, at least, two new anionic isozymes. The induction of one of these iso-guaiacol peroxidase (iso-GPX) was Zn-specific. The exposure to metals did not modify the activity of ascorbate-specific peroxidase either in roots or in stems. In leaves, where the glutathione reductase (GR) activity was assayed, increases of ascorbate peroxidase (APX) and glutathione reductase (GR) activities were recorded. These results suggest that some antioxidant enzymes can be activated, notably in upper plant parts, in response to oxidative stress induced by Cd and Zn.

Response of antioxidant enzymes to excess copper in tomato (Lycopersicon esculentum, Mill.)

Abstract 15-day-old seedlings of tomato (Lycopersicon esculentum, Mill., cv. Ibiza F1) were treated, for 7 days, by addition of 50 μM Cu to the nutrient medium. Under these conditions, a decrease in growth, more pronounced in leaves and in stems than in roots, was observed, as compared to controls. Copper accumulation was markedly higher in roots as compared to shoots. In all plant parts, excess copper induced an increase in the rate of lipid peroxidation. Catalase (CAT) activity was not modified in leaves and in stems, but it was decreased in roots. The activity of ascorbate peroxidase (APX) was unaltered in roots and in stems, while it was diminished in leaves. However, guaiacol peroxidase (GPX) activity was increased only in roots and stems. Enhancement of this activity was more important in stems than in roots. Analysis of the profile of anionic isoenzymes of GPX revealed quantitative and qualitative changes in roots and stems of stressed plants. These results, together, indicate that a toxic concentration of copper (50 μM) induces oxidative stress and differential responses of antioxidant enzymes in plant parts. Activity level of GPX, but not APX nor CAT, increased in response to damage.

Nickel-induced oxidative damage and antioxidant responses in Zea mays shoots

Abstract Zea mays (cv. LG 23/01) seedlings precultured in hydroponic culture for seven days were treated with 250 μM NiCl 2 for five days. Several metabolic parameters representative of oxidative damage and antioxidant activity in shoots were regularly studied after the metal treatment. Two days after Ni-application, membrane lipid peroxidation intensified. A stimulation of guaiacol peroxidase (EC 1.11.1.7) activity was recorded. Analysis of the profile of anionic isoenzymes of guaiacol peroxidase revealed quantitative changes occurring during Ni-exposure. Superoxide dismutase (EC 1.15.1.1) activity was stimulated early and transiently. Later, the activities of ascorbate peroxidase (EC 1.11.1.11), catalase (EC 1.11.1.6) and glutathione reductase (EC 1.6.4.2) increased. The results are discussed with regard to nickel-induced oxidative stress and potential antioxidant reactions.

NICKEL TOXICITY INDUCES OXIDATIVE DAMAGE IN ZEA MAYS ROOTS

Seven day-old maize (Zea mays cv. LG 23/01) plants were treated for five days with 250 μM NiCl2. The relationship between nickel (Ni) toxicity and oxidative reactions were studied in roots during metal accumulation. Membrane lipid peroxidation enhanced only 6 h after metal treatment before roots revealed a decrease in growth. The activities of superoxide dismutase (SOD), EC 1.15.1.1, guaiacol peroxidase (GPX), EC 1.11.1.7, were unaffected by Ni stress. However, catalase (CAT), EC 1.11.1.6, activity was increased from 24 h after metal treatment. In the ascorbate gluthatione cycle, only two enzymes, the ascorbate peroxidase (APX), EC 1.11.1.11, and monodehydroascorbate reductase (MDHAR), EC 1.6.5.4, were stimulated, while the activities of dehydroascorbate reductase (DHAR), EC 1.8.5.1, and glutathione reductase (GR), EC 1.6.4.2, remained around the control values. The results suggest that oxidative disorder is part of the overall expression of Ni toxicity in roots of Zea mays and that enhanced lipid peroxidation could be a consequence of primary effects of Ni stress.

Nickel toxicity: Effects on growth and metabolism of maize

Abstract Effects of graded concentrations of nickel on maize (Zea mays L.) growth and certain metabolic parameters were studied. Under nickel (Ni) stress conditions, leaves become chlorotic at low concentrations and necrotic at, relatively, high ones. Concerning its effect on plant growth, nickel reduces dry matter production, more significantly in root system which accumulates large amounts of Ni. In leaves, chlorophyll content was severely decreased. Furthermore, this metal induces leaf accumulation of soluble phenolics, starch and reducing sugars. The accumulation of carbohydrates in shoots might be, at least in part, the cause of root growth inhibition.

Respiratory metabolism in the embryonic axis of germinating pea seed exposed to cadmium.

Seeds of pea (Pisum sativum L.) were germinated for 5d by soaking in distilled water or 5mM cadmium nitrate. The relationships among cadmium stress, germination rate, changes in respiratory enzyme activities and carbohydrates mobilization were studied. Two cell fractions were obtained from embryonic axis: (1) mitochondria, used to determine enzyme activities of citric acid cycle and electron transport chain, and (2) soluble, to measure some enzyme activities involved in fermentation and pentose phosphate pathway. Activities of malate- and succinate-dehydrogenases (MDH, SDH) and NADH- and succinate-cytochrome c reductases (NCCR, SCCR) were rapidly inhibited, while cytochrome c oxidase (CCO) was unaltered by cadmium treatment. However, this stimulated the NADPH-generating enzyme activities of the pentose phosphate pathway, glucose-6-phosphate- and 6-phosphogluconate-dehydrogenases (G6PDH, 6PGDH), as well as enzyme activity of fermentation, alcohol dehydrogenase (ADH), with concomitant inhibition in the capacity of enzyme inactivator (INADH). Moreover, Cd restricted carbohydrate mobilization in the embryonic axis. Almost no glucose and less than 7% of control fructose and total soluble sugars were available in the embryo tissues after 5d of exposure to cadmium. Cotyledonary invertase isoenzyme activity was also inhibited by Cd. The results indicate that cadmium induces disorder in the resumption of respiration in germinating pea seeds. The contribution of Cd-stimulated alternative metabolic pathways to compensate for the failure in mitochondrial respiration is discussed in relation to the delay in seed germination and embryonic axis growth.

Cadmium impairs mineral and carbohydrate mobilization during the germination of bean seeds.

The germination rate, mineral (Ca, Fe, K, Mn) and carbohydrate (starch, soluble sugars, sucrose, glucose, fructose) contents and hydrolase activities in cotyledons and embryonic axes of bean seeds subjected to cadmium stress were investigated. Compared to the control, Cd caused a reduction in germination percent, embryo growth and in distribution of biomass, mineral and sugars between cotyledon and embryonic axis and inhibited the activities of alpha-amylase and invertases: soluble acid (INV-AS), soluble neutral (INV-NS), cell wall bound acid (INV-AW). Moreover, the solute leakage into the germination medium was also used as bioindicator parameter to evaluate the toxicity of cadmium accumulation, which increased in different tissues of germinating seeds in the duration of treatment and provoked nutrient loss and, thereby, electrical conductivity enhancement in the imbibition medium. This was correlated with an impairment of membrane integrity, as evidenced by high malondialdehyde (MDA) content and lipoxygenase (LOX) activity in Cd-poisoned embryo. The contribution of solute loss at the expense of growing embryonic axis to failure in reserve mobilization after Cd exposure is emphasized in association with the delay in seed germination.

Membrane damage and solute leakage from germinating pea seed under cadmium stress.

Seed germination represents a limiting stage of plant life cycle under heavy metal stress situation. Delay in germination can be associated with disorders in the event chain of germinative metabolism which is a highly complex multistage process, but one of underlying metabolic activities following imbibition of seed is the storage mobilization. The influence of cadmium on carbohydrates and aminoacids export from cotyledon to embryonic axis during germination of pea seed was investigated. Compared to the control, Cd caused a restriction in reserve mobilization as evidenced by the pronounced increase in cotyledon/embryo ratios of total soluble sugars, glucose, fructose and aminoacids. Moreover, the nutrient concentrations, as well as the electrical conductivity of germination medium were determined to quantify the extent of solute leakage. Such nutrients were lost into the imbibition medium at the expense of suitable mobilization to the growing embryonic axis. This was concomitant with an over-accumulation of lipid peroxidation products in Cd-poisoned embryonic tissues. However, the impairment of membrane integrity cannot be due to a stimulation in lipoxygenase activity, since the later was markedly inhibited after Cd exposure.

Role of the ubiquitin-proteasome pathway and some peptidases during seed germination and copper stress in bean cotyledons.

The role of the ubiquitin (Ub)-proteasome pathway and some endo- and aminopeptidases (EPs and APs, respectively) was studied in cotyledons of germinating bean seeds (Phaseolus vulgaris L.). The Ub system appeared to be important both in the early (3 days) and late (9 days) phases of germination. In the presence of copper, an increase in protein carbonylation and a decrease in reduced -SH pool occurred, indicating protein damage. This was associated with an enhancement in accumulation of malondialdehyde, a major product of lipid peroxidation, and an increase in content of hydrogen peroxide (H2O2), showing oxidative stress generation. Moreover, copper induced inactivation of the Ub-proteasome (EC 3.4.25) pathway and inhibition of leucine and proline aminopeptidase activities (EC 3.4.11.1 and EC 3.4.11.5, respectively), thus limiting their role in modulating essential metabolic processes, such as the removal of regulatory and oxidatively-damaged proteins. By contrast, total trypsin and chymotrypsin-like activities (EC 3.4.21.4 and EC 3.4.21.1, respectively) increased after copper exposure, in parallel with a decrease in their inhibitor capacities (i.e. trypsin inhibitor and chymotrypsin inhibitor activity), suggesting that these endoproteases are part of the protective mechanisms against copper stress.

RESERVE MOBILIZATION DISORDER IN GERMINATING SEEDS OF VICIA FABA L. EXPOSED TO CADMIUM

The experiment was performed to study the germination of Vicia faba L. under the influence of cadmium (Cd) stress. Seeds were germinated for ten days by soaking in distilled water or cadmium nitrate solution. Germination rate, embryo radicle growth and water, Cd, and amino acid and sugar contents in cotyledons were investigated. Nutrient concentrations were also determined in the germination medium to quantify the solute leakage. Results showed that Cd-induced delay in seed germination can be associated to disturbance due to a failure in sugars and amino acids freeing. Such nutrients were lost in the imbibition medium at the expense of suitable mobilization to the growing embryonic axis. To explore the mechanism of cadmium toxicity, the solute leakage provides an indication of the way in which Cd might affect the germinative metabolism.