How does silicon alleviate Cd-induced phytotoxicity in barley, Hordeum vulgare L.?

Toxic heavy metal accumulation in edible plants has become a problem worth worrying about for human health. Cadmium is one of the most toxic metals presenting high bioavailability in the environment. The main route of transfer of Cd to humans is the consumption of contaminated food which suggests that reducing of Cd absorption by plants could reduce this risk. In this context, it was suggested that silicon supply would be able to limit the transfer of Cd to the plants. Thus, this work evaluated the effects of 0.5 mM Si on Cd absorption and accumulation in barley (Hordeum vulgare L.). Plants were grown hydroponically for 21 days in the presence of 0 and 15 µM Cd2+ combined or not with 0.5 mM Si. Analyses were related to growth and photosynthesis parameters, Cd accumulation in organs and Cd subcellular distribution in the shoots. Results showed that, under Cd alone, plants showed severe toxicity symptoms as chlorosis and necrosis and produced significantly less biomass as compared to control. 0.5 mM Si in the medium culture significantly reduced Cd-induced toxicity by mitigating symptoms and restoring growth, photosynthesis, and nutrition. Si also induced a significant reduction of Cd concentration in plants and changed its sub-cellular compartmentalization by enhancing fixation to cell walls and reducing the Cd concentration in the cytoplasmic and organelles fractions. These data suggest that the application of Si could significantly increase Cd tolerance and reduce the risk of the Cd accumulation in edible plants.

NaCl effect on Cd accumulation and cell compartmentalization in barley.

The effect of sodium chloride (NaCl) on cadmium (Cd) tolerance, uptake, translocation, and compartmentation was investigated in 3 barley genotypes. Seedlings were cultivated hydroponically in the absence of NaCl and Cd (control), in the presence of 50 mM NaCl alone, in the presence of 10 µM Cd alone, and in the combined addition of NaCl (50 mM) and Cd (10 µM). Plants were cultivated during one month under 16 h light period at a minimal light intensity of 250 µmol m-2 s-1, a temperature of 25 ± 3 °C, and 70-80% of relative humidity. Results showed that NaCl alone did not significantly affect plant development and biomass production; however, Cd alone reduced plant development rate leading to a decline in biomass production in Raihane and Giza 127 but did not affect that in Amalou. NaCl addition in Cd-treated plants accentuated the Cd effect on plant growth. NaCl limited Cd accumulation in the roots and in the shoots in all tested barley varieties by reducing Cd-absorption efficiency and the translocation of Cd from the root to the shoot. In all Cd-treated plants, cell Cd compartmentalization showed the following gradient: organelles < cell wall < vacuole. NaCl in the medium increased Cd accumulation in the soluble fraction and reduced that in organelle and cell wall fractions. Globally our results showed that, although NaCl reduces Cd accumulation in barley, it accentuates the Cd toxic effects, hence limiting the plant yield. We advise farmers to avoid barley cultivation near mine sites and its irrigation with moderately salty water, although this plant is considered as salt tolerant.

Cd tolerance and accumulation in barley: screening of 36 North African cultivars on Cd-contaminated soil.

In North Africa, barley (Hordeum vulgare L) is the second most cultivated cereal. In Tunisia, barley is cultivated in mining areas with possible Cd soil contamination. The accumulation of Cd was studied in the 36 most cultivated North African barley cultivars cultured during 6 months on control soil and on soil containing 10 ppm of Cd. Cadmium did not affect germination and morphology in any cultivar. However, Cd induced variable effects on the biomass according to the cultivar. The cultivar Lemsi was the most sensitive one and Gisa 127 the most tolerant to Cd. The spike morphology did not show any differences between control and Cd-treated plants. The number of grains per spike and the weight of kernels were differently affected by Cd. On this basis, we identified Manel, Temassine, Giza 130, and Firdaws as the most tolerant cultivars and Raihane, Giza 123, Adrar, and Amira as the most sensitive ones. Cd accumulated at a higher concentration in straw than in the grains, but for both organs, we observed a significant intraspecific variability. In the straw, Lemsi and Massine showed the highest Cd concentration, while the lowest concentration was recorded in Temassine. In the kernels, Amalou showed the highest Cd concentration, 14 µgg-1 of dry weight (DW), but the lowest Cd concentration was 1.7 µg g-1 DW in Kebelli. Based on the official allowable limit of Cd in the grain, all cultivars represent a potential risk when cultivated on soil contaminated with 10 ppm Cd. The molecular and physiological basis responsible for the differences in Cd tolerance and accumulation among barley cultivars will require more investigations.

Why Does the Halophyte Mesembryanthemum crystallinum Better Tolerate Ni Toxicity than Brassica juncea: Implication of Antioxidant Defense Systems.

The implication of enzymatic and non-enzymatic antioxidative systems in response to Ni was evaluated in the halophyte Mesembryanthemum crystallinum in comparison with the metal tolerant glycophyte species Brassica juncea. Seedlings of both species were hydroponically subjected during 21 days to 0, 25, 50, and 100 µM NiCl2. Growth parameters showed that the halophyte M. crystallinum was more tolerant to Ni than B. juncea. Malondialdehyde (MDA) content increased to a higher extent in B. juncea than in M. crystallinum. Antioxidant enzymesactivities were differently affected by Ni in both species. Nickel increased shoot superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in B. juncea, whereas these activities were reduced in M. crystallinum when exposed to metal stress. The root SOD, APX and guaiacol peroxidase (GPX) activities increased upon Ni treatments for both species. The content of non-enzymatic antioxidative molecules such as glutathione, non-protein thiols and proline increased in Ni-treated plants, except for GSH content in the shoot of B. juncea. Based on the oxidative balance, our findings confirm the higher tolerance of the halophyte M. crystallinum to Ni-induced oxidative stress comparatively to B. juncea. We suggest that M. crystallinum is able to overcome the produced ROS using the non-enzymatic system, while Ni-induced oxidative stress was more acute in B. juncea, leading this species to mainly use the enzymatic system to protect against reactive oxygen species.

Polyamine and tyramine involvement in NaCl-induced improvement of Cd resistance in the halophyte Inula chrithmoides L.

The aim of the present work was to analyze the impact of salinity on the plant response to Cd toxicity in the Mediterranean halophyte species Inula crithmoides. For this purpose, cuttings were cultivated hydroponically during 21d in the presence of 0, 25 or 50µM CdCl2 combined or not with 0, 100, 200 and 400mM NaCl. The obtained data demonstrated that, in the absence of Cd, NaCl strongly increased plant growth (the maximal dry weight being observed at 100mM) and enhanced the Na+/K+ ratio in the shoot. Cd alone strongly affected plant growth in this halophyte. However, in Cd-treated plants, NaCl protected Inula crithmoides from Cd toxicity and contributed to reduce Cd absorption and translocation. Small aliphatic polyamine (putrescine, spermidine, spermine) increased in response to both NaCl and CdCl2, the highest concentration in plants being observed when both agents are present in the medium. The recorded increase preferentially concerned the polyamine bound fraction, which might be related to their involvement in the protection of endogenous cellular structures. The aromatic monoamine tyramine also strongly increased in response to Cd toxicity and its putative role is discussed in relation to conjugation processes. Salinity and Cd increased ammonium/nitrate ratio in leaves and roots and the involvement of stress-induced modification of N nutrition on polyamine oversynthesis is also discussed.

Nodulation by Sinorhizobium meliloti originated from a mining soil alleviates Cd toxicity and increases Cd-phytoextraction in Medicago sativa L.

Besides their role in nitrogen supply to the host plants as a result of symbiotic N fixation, the association between legumes and Rhizobium could be useful for the rehabilitation of metal-contaminated soils by phytoextraction. A major limitation presents the metal-sensitivity of the bacterial strains. The aim of this work was to explore the usefulness of Sinorhizobium meliloti originated from a mining site for Cd phytoextraction by Medicago sativa. Inoculated and non-inoculated plants were cultivated for 60 d on soils containing 50 and/or 100 mg Cd kg(-1) soil. The inoculation hindered the occurrence of Cd- induced toxicity symptoms that appeared in the shoots of non-inoculated plants. This positive effect of S. meliloti colonization was accompanied by an increase in biomass production and improved nutrient acquisition comparatively to non-inoculated plants. Nodulation enhanced Cd absorption by the roots and Cd translocation to the shoots. The increase of plant biomass concomitantly with the increase of Cd shoot concentration in inoculated plants led to higher potential of Cd-phytoextraction in these plants. In the presence of 50 mg Cd kg(-1) in the soil, the amounts of Cd extracted in the shoots were 58 and 178 µg plant(-1) in non-inoculated and inoculated plants, respectively. This study demonstrates that this association M. sativa-S. meliloti may be an efficient biological system to extract Cd from contaminated soils.

Cd and Ni transport and accumulation in the halophyte Sesuvium portulacastrum: implication of organic acids in these processes.

The implication of organic acids in Cd and Ni translocation was studied in the halophyte species Sesuvium portulacastrum. Citric, fumaric, malic, and ascorbic acids were separated and quantified by HPLC technique in shoots, roots and xylem saps of plants grown on nutrient solutions added with 50 µM Cd, 100 µM Ni and the combination of 50 µM Cd + 100 µM Ni. Results showed that Cd had no significant impact on biomass production while Ni and the combination of both metals drastically affected plant development. Cadmium and Ni concentrations in tissues and xylem sap were higher in plants subjected to individual metal application than those subjected to the combined effect of Cd and Ni suggesting a possible competition between these metals for absorption. Both metals applied separately or in combination induced an increase in citrate concentration in shoots and xylem sap but a decrease of this concentration in the roots. However, a minor relationship was observed between metal application and fumaric, malic, and ascorbic acids. Both observations suggest the implication of citric acid in Cd, Ni translocation and shoot accumulation in S. portulacastrum. The relatively high accumulation of citric acid in xylem sap and shoot of S. portulacastrum could be involved in metal chelation and thus contributes to heavy metal tolerance in this species.

NaCl alleviates Cd toxicity by changing its chemical forms of accumulation in the halophyte Sesuvium portulacastrum.

It has previously been shown that certain halophytes can grow and produce biomass despite of the contamination of their saline biotopes with toxic metals. This suggests that these plants are able to cope with both salinity and heavy metal constraints. NaCl is well tolerated by halophytes and apparently can modulate their responses to Cd. However, the underlying mechanisms remain unclear. This study explores the impact of NaCl on growth, Cd accumulation, and Cd speciation in tissues of the halophyte Sesuvium portulacastrum. Seedlings of S. portulacastrum were exposed during 1 month to 0, 25, and 50 µM Cd combined with low salinity (LS, 0.09 mM NaCl) or high salinity (HS, 200 mM NaCl) levels. Growth parameters and total tissue Cd concentrations were determined, in leaves, stems, and root. Moreover, Cd speciation in these organs was assessed by specific extraction procedures. Results showed that, at LS, Cd induced chlorosis and necrosis and drastically reduced plant growth. However, addition of 200 mM NaCl to Cd containing medium alleviated significantly Cd toxicity symptoms and restored plant growth. NaCl reduced the concentration of Cd in the shoots; nevertheless, due to maintenance of higher biomass under HS, the quantity of accumulated Cd was not modified. NaCl modified the chemical form of Cd in the tissues by increasing the proportion of Cd bound to pectates, proteins, and chloride suggesting that this change in speciation is involved in the positive impact of NaCl on Cd tolerance. We concluded that the tolerance of S. portulacastrum to Cd was enhanced by NaCl. This effect is rather governed by the modification of the speciation of the accumulated Cd than by the reduction of Cd absorption and translocation.

EDTA-enhanced phytoremediation of lead-contaminated soil by the halophyte Sesuvium portulacastrum.

The low bioavailability of Pb and low number of Pb-tolerant plant species represent an important limitation for Pb phytoextraction. It was recently suggested that halophyte plant species may be a promising material for this purpose, especially in polluted salt areas while Pb mobility may be improved by synthetic chelating agents. This study aims to evaluate Pb extraction by the halophyte Sesuvium portulacastrum in relation to the impact of EDTA application. Seedling were cultivated during 60 days on Pb artificially contaminated soil (200, 400, and 800 ppm Pb) in the presence or in the absence of EDTA (3 g kg(-1) soil). Results showed that upon to 400 ppm, Pb had no impact on plant growth. However, exogenous Pb induce a decrease in shoot K(+) while it increased shoot Mg(2+) and had no impact on shoot Ca(2+) concentrations. Lead concentration in the shoots increased with increasing external Pb doses reaching 1,390 ppm in the presence of 800 ppm lead in soil. EDTA addition had no effect on plant growth but strongly increased Pb accumulation in the shoot which increased from 1,390 ppm in the absence of EDTA to 3,772 ppm in EDTA-amended plants exposed to 800 ppm exogenous Pb. Both Pb absorption and translocation from roots to shoots were significantly enhanced by EDTA application, leading to an increase in the total amounts of extracted Pb per plant. These data suggest that S. portulacastrum is very promising species for decontamination of Pb(2+)-contaminated soil and that its phytoextraction potential was significantly enhanced by addition of EDTA to the polluted soil.

Comparative study of Pb-phytoextraction potential in Sesuvium portulacastrum and Brassica juncea: tolerance and accumulation.

Lead phytoextraction from salty soils is a difficult task because this process needs the use of plants which are able to tolerate salt and accumulate Pb(2+) within in their shoots. It has recently been suggested that salt-tolerant plants are more suitable for heavy metals extraction than salt-sensitive ones commonly used in this approach. The aim of this study was to investigate Pb-phytoextraction potential of the halophyte Sesuvium portulacastrum in comparison with Brassica juncea commonly used in Pb-phytoextraction. Seedlings of both species were exposed in nutrient solution to 0, 200, 400, 800 and 1000 µM Pb(2+) for 21 days. Lead strongly inhibited growth in B. juncea but had no impact on S. portulacastrum. Exogenous Pb(2+) reduced nutrients uptake mainly in B. juncea as compared to S. portulacastrum. Lead was preferentially accumulated in roots in both species. S. portulacastrum accumulated more Pb(2+) in the shoot than B. juncea. Hence, the amounts of Pb(2+) translocated at 1000 µM Pb(2+) were 3400 µg g(-1) DW and 2200 µg g(-1) DW in S. portulacastrum and B. juncea, respectively. These results suggest that S. portulacastrum is more efficient to extract Pb(2+) than B. juncea.