Mohamed Debouba

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.

An Arabidopsis mutant disrupted in ASN2 encoding asparagine synthetase 2 exhibits low salt stress tolerance

Salt tolerance of Arabidopsis knockout mutant with T-DNA insertion in ASN2 gene encoding asparagine synthetase (AS, EC 6.3.5.4) (asn2-1) was investigated. Wild-type Arabidopsis Col0 and asn2-1 mutant were grown for one month by hydroponic culture and subjected to 100 mM NaCl stress for a short time from 6 to 24 h. The salt treatment decreased chlorophyll and soluble protein contents, and increased ammonium level in the asn2-1 leaves. The salinity induced ASN1 mRNA level in the wild-type and asn2-1 leaves. By contrast, the salt treatment inhibited the transcript and protein levels of chloroplastic glutamine synthetase 2 (GS2, EC 6.3.1.2) in the wild-type and asn2-1 leaves. Increase in asparagine and proline contents in response to the salt treatment provides evidence for the role of asparagine as a prevailing stress responding amino acid. Glutamate dehydrogenase (NADH-GDH, EC 1.4.1.2) exhibited a slight increase in the α-subunit and β-subunit in the wild-type line and the asn2-1 line, respectively under the salinity, whereas its in vitro aminating activity in the wild-type leaves was not affected. The results indicate that the asn2-1 mutant was impaired in nitrogen assimilation and translocation under salt treatment.

Glutamate metabolism on Solanum lycopersicon grown under cadmium stress conditions

Abstract Glutamate occupies a central position in aminoacid metabolism in plants. However, it is also the substrate for the synthesis of glutamine from ammonia, catalysed by glutamine synthetase. The α-amino group of glutamate may be transferred to other aminoacids. In addition, both the carbon skeleton and α-amino group of glutamate form the basis for the synthesis of γ-aminobu- tyric acid, arginine and proline. Finally, glutamate may be deaminated by glutamate dehydrogenase to form ammonia and 2-oxoglutarate. The concentrations of glutamate within the plant are homeostatically regulated by the combined action of these pathways is examined. Glutamine represented the major aminoacid transported through xylem sap of cadmium treated and control plants. Cadmium treatment increased the total aminoacid content in the phloem, maintaining Gln/Glu ratios. Glutamate signalling is examined from an evolutionary perspective, and the roles it might play in plants.

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.

NaCl effects growth, ions and water status of Tomato (Lycopersicon esculentum) seedlings

Abstract Tomato seedlings (Lycopersicon esculentum Mill, cv Chibli F1) were subjected to different NaCl treatments (0, 25, 50 and 100 mM) during 10 days. Under low salinity (25 mM NaCl), tomato preserved a normal growth activity, associated with maintain of the tissues water status and a high selectivity for K+ over Na+. Under high salinity (50 and 100 mM), endogenous Na+ and CI− accumulations were contradicted by a decrease of K+ and NO3 − contents. Our results support the existence of a competition between K+/Na+ and NO3 −CI− at the level of uptake and storage process. The high salt stress (50 and 100 mM) emphasized a typical glycophytic behavior; displaying i) a severe decline in plant growth activity, ii) the low Na+ amounts compared to CI− in the leaves, make an evidence for Na+ extrusion from the leaves to the stems-petioles and roots, and iii) an inaptitude to use Na+ instead of K+ to carry out osmotic adjustments at high salinity (100 mM NaCl).

Aminoacid changes in leaves and roots of tomato (Solanum lycopersicum) during salt stress

Abstract Tomato seedlings were exposed to 100 mM NaCl during 10 days. NaCl stress led to about 40% and 20% decrease in leaf and root biomass, respectively. Accumulation of Na+ and Cl− ions was concomitant with NH4 + contents increase in stressed leaves and roots. While total aminoacids contents rapidly increased by salt in both organs, they recovered control value in the leaves at the end of treatment. In leaves, NaCl stress increased proline and serine contents, lately associated with accumulation of y—aminobutyric and asparagine. Slat—treated roots showed 2-fold higher contents of serine, glycine, glutamine and GABA. Asparagine became the most abundant aminoacid, accounting for 26% of total aminoacids in the treated roots. These disturbances in the aminoacids composition reflect a new distribution of internal nitrogen compounds, building up an aminoacid pool with high N to C ratio and avoiding thus an ammoniacal intoxication. Proline, Asn and GABA are useful for nitrogen transport to developing tissues and protect against osmotic and oxidative stresses during salt stress.