Cadmium Toxicity Induced Changes in Nitrogen Management in Lycopersicon esculentum Leading to a Metabolic Safeguard Through an Amino Acid Storage Strategy

Abstract

Tomato (Lycopersicon esculentum) seedlings were grown in the presence of cadmium. After 1 week of Cd treatment, a sharp decline in biomass accumulation in the leaves and roots was observed, together with a decrease in the rate of photosynthetic activity due to both Rubisco and chlorophyll degradation and stomata closure. Cadmium induced a significant decrease in nitrate content and inhibition of the activities of nitrate reductase, nitrite reductase, glutamine synthetase (GS) and ferredoxin-glutamate synthase. An increase in NADH-glutamate synthase and NADH-glutamate dehydrogenase activity was observed in parallel. The accumulation of ammonium into the tissues of treated plants was accompanied by a loss of total protein and the accumulation of amino acids. Gln represented the major amino acid transported through xylem sap of Cd-treated and control plants. Cadmium treatment increased the total amino acid content in the phloem, maintaining Gln/Glu ratios. Western and Northern blot analysis of Cd-treated plants showed a decrease in chloroplastic GS protein and mRNA and an increase in cytosolic GS and glutamate dehydrogenase transcripts and proteins. An increase in asparagine synthetase mRNA was observed in roots, in parallel with a strong increase in asparagine. Taken together, these results suggest that the plant response to Cd stress involved newly induced enzymes dedicated to coordinated leaf nitrogen remobilization and root nitrogen storage.

TheArabidopsisNitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance

“…The Role of NRT1.8-Regulated Nitrate Distribution under Cadmium Stress Cd 2+ , a nonessential toxic heavy metal, has long been observed to severely inhibit nitrate uptake, transport, and assimilation (Sanita di Toppi and Gabbrielli, 1999), resulting in biomass and protein content reduction (Chaffei et al, 2004). Furthermore, Cd 2+ was also observed to affect nitrogen distribution in various plant species.…”

“…Furthermore, Cd 2+ was also observed to affect nitrogen distribution in various plant species. In Cd 2+ -treated tomato (Solanum lycopersicum) seedlings, the total amino acid content increased in phloem and roots, together with an increase of glutamate dehydrogenase, cytosolic glutamine synthetase, and asparagine synthetase expression, suggesting that nitrogen was recycled and translocated from shoots to roots as a Cd 2+ protection and storage strategy that may preserve roots (Chaffei et al, 2004).…”

Glutamine Synthetase-Glutamate Synthase Pathway and Glutamate Dehydrogenase Play Distinct Roles in the Sink-Source Nitrogen Cycle in Tobacco

“…Equal total amounts of 18S ribosomal RNA were used as the internal standard for RT (data Table I. Amino acid determination (% and total content) in phloem sap collected from tobacco leaves of different ages Amino acids were pooled prior to analysis, which was carried out according to Chaffei et al (2004). Numbers in parentheses correspond to the amino acid determination (% and total content) in leaf blades, determined as described by Masclaux et al (2000).…”

“…Phloem exudates were collected from 10-week-old wild-type tobacco plants in the light period as described previously (Chaffei et al, 2004). Leaf petioles were excised and recut under water and rapidly immersed in 1 mL of collection buffer consisting of 10 mM HEPES, pH 7.5, and 1 mM EDTA.…”

Prolonged root hypoxia effects on enzymes involved in nitrogen assimilation pathway in tomato plants

“…The GS activity decrease in leaves was due to a lower content of the chloroplastic GS isoform (GS2) protein and transcripts, whereas the increase in GS activity in roots was related to the induction of the cytosolic GS isoform (GS1) protein and transcripts. 61 The significant increase in NH 4 + content in roots and leaves of HT relative to NT plants (Fig. 1C) was concomitant to a significant increase in the GDH activities (Fig.…”