Mohamed Habib Ghorbel

Tissue-specific cadmium accumulation and its effects on nitrogen metabolism in tobacco (Nicotiana tabaccum, Bureley v. Fb9).

Tobacco (Nicotiana Tabaccum, Bureley v. Fb9) seedlings were grown for 30 days on control medium, and then treated for seven days with different concentrations (0, 10, 20, 50 and 100 muM) of CdCl(2). Cadmium (Cd) was mostly accumulated in the leaves. However, nitrate reductase and nitrite reductase activities (NR, EC 1.6.1.6 and NiR, EC 1.7.7.1) were more inhibited by Cd stress in the roots than in leaves. Glutamine synthetase activity (GS, EC 6.3.1.2) was inhibited by Cd treatment in roots and leaves. In both organs, aminating activity of glutamate dehydrogenase (GDH, EC 1.4.1.2) and protease activity were significantly stimulated in the leaves and roots of stressed plants. The lesser extents of Cd stress effects on leaves, despite their high Cd accumulation, suggest that: (i) tobacco leaves may evolve adaptive process to partially inactivate Cd ions; and (ii) tobacco is useful for phytoremediation.

Expression pattern of genes encoding nitrate and ammonium assimilating enzymes in Arabidopsis thaliana exposed to short term NaCl stress

Key steps in nitrate nutrition and assimilation were assessed over two weeks in control and 100mM NaCl-exposed Arabidopsis thaliana (Columbia) plants. The data showed that NaCl stress lowered nitrate contents in both leaves and roots. While NaCl stress decreased ammonium contents in leaves, it increased the contents in roots at the end of treatment. A survey of transcript levels of NIA1 (At1g77760) and NIA2 (At1g37130) and nitrate reductase (NR, EC 1.6.1.6) activity in the leaves and roots suggested a major role of NIA2 rather than NIA1 in the regulation of NR by salt stress. A drop in mRNA levels for GLN2 (At5g35630) and GLN1;2 (At1g66200) by salt was associated with a similar inhibition of glutamine synthetase (GS, EC 6.3.1.2) activity in the leaves. In the roots, NaCl stress was found to enhance mRNA levels of GLN2 and cytosolic-encoding genes (GLN1;1 (At5g37600) and GLN1;2).

Cadmium and copper genotoxicity in plants

Heavy metal contamination in soils is easily transmitted to human through plants via the food chain. A major concern is to understand the plant response to heavy metal soil contamination to develop phytoremediation. Two plant models have been investigated in our study, the tomato, which is of agronomical importance, Arabidopsis, which is used as a model for molecular genetics. Heavy metal toxicity is described to induce oxidative stress linked to oxidation of proteins and membrane lipids but also to alterations of DNA damage response. We have investigated the metabolic response of cadmium and copper in parallel in both plant models and analyzed the transcriptional response of Arabidopsis RNR genes coding for isoforms of ribonucleotide reductase, an essential enzyme involved in DNA synthesis. Both Cd and Cu had a dose-dependent effect on plant growth. We also observed a rapid increase of catalase activity upon Cd or Cu treatments in tomato and Arabidopsis. At the transcriptional level, treatment with Cd resulted in a biphasic induction of two RNR genes in Arabidopsis; the first induction peak could be paralleled to the increase of the catalase activity.

Physiological and biochemical effects of cadmium toxicity in enzymes involved in nitrogen and amino—acid metabolism in tomato plants

Abstract The activities of several enzymes related to amino—acid metabolism were investigated in tomato plants (Lycopersicon esculentum Mill. 63/5F1) in control and treated leaves and roots. Glutamine synthetase and glutamate synthase ferredoxine dependent activities rapidly declined in stress condition. In presence of cadmium, the decline of glutamine synthetase activity was retarded, while the activity of glutamate synthase significantly and more rapid declined. The activity of NADH—dependent glutamate dehydrogenase was markedly increased in presence of cadmium. Western blots analysis from cadmium treated plants, showed a decrease in chloroplastic GS (GS2) protein and an increase in cytosolic GS (GS1) and glutamate dehydrogenase (GDH) proteins. The activities of phosphoenol pyruvate carboxylase and NADP—dependent isocitrate dehydrogenase also increased in both organs of plantes treated. The activity of NAD and NADP—dependent malate dehydrogenase were decreased especially in leaves.

Effects of nitrate concentration on growth and nitrogen metabolism of tomato (Lycopersicon esculentum Mill.) under cadmic stress

Abstract Nitrate assimilation in higher plants is the main biosynthetic pathway leading to glutamate, required for synthesis of some metabolites that participate in mechanisms of biochemical adaptation to heavy metal such as cadmium. Seven day—old seedlings were grown for 14 days on hydroponic culture medium containing 2 mM or 5 mM of nitrate. Cadmium was added at 25 μM concentration. The supply of nitrate at an important concentration (5 mM) enhanced growth activity and nitrate content in leaves and roots. In parallel, nitrate and nitrite reductase activities were induced. Regardless the nitrate concentration, presence of cadmium (25 μM) exerted an inhibitory effect on plant growth, nitrate content, nitrate and nitrite reductase activities. This negative effect was more important when nitrate was added at 2 mM concentration. Aminating GDH activity was enhanced by the presence of cadmium in order to detoxify ammonium. Our results suggest that tomato plant was more sensitive to cadmium toxicity when received a low nitrogen concentration (nitrate).

Cadmium stress effects on photosynthesis and PSII efficiency in tomato grown on NO3 − or NH4 + as nitrogen source

Abstract This work investigates the combined effects of N form and Cd treatment on photosynthesis of tomato (Solanum lycopersicum Mill. cv Ibiza F1). Our study shows that, compared to plants fed with NO3“, dry weight, fresh weight and leaf area decrease when plants received NH4 + as N form. Regardless the N form used, Cd inhibits this parameters. In this report we show that, when grown in NH4 +, Cd—stressed seedlings accumulate higher levels of chlorophyll. Under Cd stress, water content, transpiration rate (E), stomatal conductance (Gs) and photosynthetic activity (Amax) decreased in plants fed with NO3 − and increased in NH4 +- grown-plants. The Fv/Fm ratio of dark adapted leaves was lower in NH4+- grown tomato compared to in NO3“-grown seedlings. Upon exposure to metal, the Fv/Fm ratio and the fraction of open PSII reaction centers (qp) were depressed by Cd in NO3“−grown plants and enhanced in NH4 +-grown ones. NPQ stimulated by Cd in NO3 −-grown plants was inhibits in NH4 +-grown ones.

Effets du NaCl et de la déficience en azote sur la fluorescence chlorophyllienne du photosystème II chez la tomate (Solanum lycopersicon, Chibli F1)

Abstract Tomato seedlings (Solanum lycopersicon cv. Chibli F1) were cultivated on a low (LN = 0.1 mM NO3“) or rich (HN = 5 mM NO3“) nitrogen medium. At two fully expanded leaves, plants were divided into control and salt—treated plants. NaCl (100 mM) was applied in the medium during 10 days. Tomato dry weigh (DW) production and chlorophyll contents were more affected by salinity in HN than in LN medium. Analysis of photosystem II (PSII) fluorescence showed that maximum quantum yield Fv/Fm was not significantly affected by NaCl, indicating that PSII was not damaged under salinity. The quantum yield of the transport of electrons φPSII was less affected by salt in LN than in HN medium. This was due to a sustain of sufficient value of photochemical quenching and efficiency of the open centers φexcin LN medium. Salt stress as well as nitrogen deficiency induced a significant increase in the non—photochemical quenching, which seems to constitute an adaptive reaction in response to the excess of energy. The obtained results suggested that PSII was not directly affected by NaCl, thus the plant growth decrease may be related to NaCl—induced effects within CO2 reduction and assimilation.

Alteration in membrane tomato lipids (Solanum esculentum) induced by nitrite stress

Abstract Plants of tomato (Solanum esculentum Mill. cv. Ibiza F1) are exposed to 0.25, 1, 2.5, 5 and 10 mM NO3 - or NO2 - through rooting medium during 7 days with the aim to study KNO2 - induces changes in biomass production and membrane lipid composition. Cultivated on the same doses of KNO3 plants serve as indicators. The accumulation of NO2 - increases with external KNO2 concentrations, and is considerably higher in roots than in primary leaves. Biomass production of the growing leaves and roots is strongly depressed at higher KNO2 levels, which is associated with a significant decrease in content of total lipids and changes in fatty acid composition. Total lipids are more decreased in roots than in primary leaves. Also, significant decreases in content of global lipid fractions are recorded in elevated nitrite-stressed plants in comparison with controls. The contents of lipid components decline in all plant organs under NO2 - treatment. However, amounts of phospholipid molecules are more affected in roots than in old leaves. Moreover, in all lipid classes, levels polyunsaturated acids decrease, and that of saturated (C16:0) one increase.

Foliar nitrogen and changes in free amino acid composition of Solanum lycopersicum under cadmium toxicity: kinetics of 15NH4+

Glutamate metabolism and amino acid translocation were investigated in the control and cadmium stressed shoots of tomato (Solanum lycopersicum - 63/5 F1), using (15N) ammonium and (15N) glutamate tracers. Regardless of organ type, [15N] ammonium assimilation occurred via glutamine synthetase (EC 6.3.1.2), in the control and stressed plants, and it did not depend on glutamate dehydrogenase (EC 1.4.1.2). The [15N] ammonium and ammonium accumulation patterns support the role of glutamate dehydrogenase in the deamination of [15N] glutamate to provide 2-oxoglutarate and [15N] ammonium. In the presence of cadmium, excess [15N] ammonium was incorporated into asparagine, which served as an additional detoxification molecule. In the presence both of methionine sulfoximine and cadmium, glutamate, alanine, and γ-amino butyrate of leaf tissue continued to become labelled with 15N. The labelling kinetics of amino acids in leaves of tomato plants in the presence of cadmium show that continued assimilation of [15N] ammonium can occur when the glutamine synthetase-glutamate synthase cycle is inhibited. The data provided evidence that the glutamine synthetase pathway and glutamate dehydrogenase play distinct roles in the source-sink nitrogen cycle of tomato leaves under cadmium stress conditions.

La réversibilité des effets du cadmium sur les différents paramètres de la croissance chez la tomate cultivée en présence de NO3 − ou NH4 +

Abstract In order to understand the effects of heavy metals in morphological and physiological parameters affecting the growth of a plant, we followed the evolution of these same parameters in Cd pre—stressed tomato seedlings after Cd had been removed from nutrient solution. Seven day—old seedlings were grown for one week on hydroponic culture medium containing 5 mM NO3 − or NH4 + and enriched by 25 μM Cd for 8 days. After that, plants were transferred on the same media without Cd. Results obtained show that regardless the nitrogen form used Cd suppression results in resumption of growth activity in all organs. Parallel, we observe a resumption of foliar area, roots growth and increasing contents in Ca, ammonium and soluble total proteins in all plants issued from either nitric or ammoniacal nutrition. The resumption of the different parameters was shown important in plants received ammonium nutrient.