Silicon amelioration of aluminium toxicity in teosinte (Zea mays L. ssp. mexicana)

The influence of different Al concentrations, (0, 60 and 120 M Al) on growth and internal concentrations of Al, Si and selected organic acids was analysed in plants of teosinte (Zea mays L. ssp. mexicana), a wild form of maize from acid soils from Mexico. The plants were grown in nutrient solutions (pH 4.0) with or without 4 M silicon. Analysis with the GEOCHEM speciation program did not reveal differences between free activities of Al³⁺ in solutions with and without 4 M Si, but solutions with Si yielded lower concentrations of monomeric Al species, [Al]_mono, when analysed by a modified aluminon method. Plants grown on solutions with similar [Al]_mono, but differing in silicon, showed highly significant differences in growth and tissue concentrations of Al and organic acids. Silicon prevented growth inhibition at [Al]_mono concentrations as high as 35 M, while plants grown without Si suffered severe growth reductions with 33 M [Al]_mono. In solutions with similar [Al]_mono concentrations plants with Si had lower tissue Al concentrations and higher concentrations of malic acid than plants without Si. In view of both the significant influence of Si on the response of plants to Al toxicity and the fact that some soluble Si is always present in soil solutions, the addition of low Si concentrations to nutrient solutions used for Al-tolerance screening is recommended.     

Cadmium exclusion a key factor in differential Cd-resistance in Thlaspi arvense ecotypes

Differences in Cd accumulation and Cd tolerance between Thlaspi arvense ecotype Aigues Vives (AV) from a commercial grower in South France and ecotype Jena collected in the polluted urban area of Jena (Germany) were reported here. Ecotype Jena exhibited considerable Cd-tolerance. Shoot and root masses were unaffected and root elongation was even enhanced by exposure to 50 μM Cd. In contrast, growth of ecotype AV was severely affected by this Cd treatment. Ecotype Jena was much more efficient in excluding Cd from both roots and shoots than ecotype AV. Despite the efficient restriction of Cd transport from roots to shoots in Jena, this ecotype maintained high root to shoot transport of Zn and Fe under Cd exposure. Cd supply strongly decreased the activities of antioxidant enzymes in AV, while in the Cd resistant Jena these activities either remained unaffected (SOD, APX) or were increased (CAT) by Cd supply. In conclusion, naturally selected Cd-tolerance in Thlaspi arvense is due to efficient Cd exclusion. The mechanisms underlying exclusion of Cd from the shoots seem Cd-specific yet they did not affect the homeostasis of Fe and Zn in the shoots.     

Evolution of salt tolerance in Arabidopsis thaliana on siliceous soils does not confer tolerance to saline calcareous soils

Plant and Soil - Alkaline salinity constrains crop yield. Previously, we observed local adaptation of Arabidopsis thaliana to saline-siliceous soils (pH?≤?7) and to non-saline...     

Different mechanisms account for enhanced copper resistance in Silene armeria ecotypes from mine spoil and serpentine sites

The resistance to excess Cu was evaluated in solution culture in three ecotypes of Silene armeria from different origin, a garden soil (Cadriano), a serpentine site (Prinzera) and a Cu mine spoil (Vigonzano). Root elongation and viability staining of root tip cells were used as indicators for Cu resistance. The Cu resistance increased in the order Cadriano <Prinzera<Vigonzano. Renewal of the root cap in Prinzera and enhanced border cell production in Vigonzano in response to excess Cu provided a more efficient protection of the root tip meristem than in Cu sensitive Cadriano. The enhanced Cu resistance in Prinzeracould not be attributed to high soil Cu acting as a natural selection factor at the serpentine site. In PrinzeraCu exclusion from roots and shoots probably was a consequence of root impermeabilization causing reduced radial water and ion flux in roots. In contrast, the high Cu resistance in the mine spoil ecotype, Vigonzano, was due to both reduced Cu uptake and higher tissue tolerance of Cu.     

Copper in plant species in a copper gradient in Catalonia (North East Spain) and their potential for phytoremediation

The accumulation of Cu in roots and shoots of 32 plant species growing on soils with a wide range of Cu concentrations (30–18 500 g g^–1 total soil Cu) located in Collserola Mountain (Barcelona, Spain) was analysed. High Cu availability decreased the species diversity in the Hyparrhenietum hirto-pubescentis, the natural plant association at the study sites. Shoot and root Cu concentrations in relation to extractable soil Cu concentrations were used for the evaluation of the Cu resistance strategy in the different species. Saturation of Cu accumulation in roots was observed in most species. Hyparrhenia hirta was the most efficient shoot excluder, while the linear increase of shoot Cu with the Cu soil concentration exhibited the highest slope in Hirschfeldia incana. Most species accumulated more Cu in roots than in shoots. High shoot/root Cu ratios were only found in the highly Cu-resistant Hirschfeldia incana, in the resistant or moderately resistant Spartium junceum and Reseda sp. (R. lutea and R. phyteuma), and in the much less resistant Ononis natrix. Only two species, Hirschfeldia incana and Sedum sediforme were able to support the extreme Cu-toxicity conditions on soils with 5000–16 800 g g^–1 extractable Cu. Among the grass species tested Hyparrhenia hirta was the most Cu-resistant species (up to 1950 g g^–1 extractable soil Cu). The potential usefulness of these pseudometallophytes for phytoremediation of Cu-contaminated soils is discussed.     

Signal cross talk in Arabidopsis exposed to cadmium, silicon, and Botrytis cinerea

The role of defence gene expression triggered by Cd toxicity in the plant’s response to Botrytis cinerea was investigated in Arabidopsis thaliana Columbia 0. Silicon (0 or 1.5 mM) and Cd (0, 1 or 10 μM) were supplied to 3-month-old solution-cultured plants. After 3 days, half of the plants of each treatment were inoculated with Botrytis. Supplied Cd concentrations were below the toxicity threshold and did not cause shoot growth inhibition or evidence of oxidative stress, while Botrytis infection severely decreased plant growth in all treatments. The expression of marker genes PR1 and BGL2 for the salicylic acid (SA) and the PDF1.2 for the jasmonic acid–ethylene (JA–ET) signalling pathways was enhanced in 10 μM Cd-treated non-infected plants. Twenty hours after inoculation, PDF1.2 expression showed a strong increase in all treatments, while enhanced PR1, BGL2, and CHIB expression was only found 7 days after infection. A great synergistic effect of Cd and Botrytis on PDF1.2 expression was found in 10 μM Cd-treated plants. Silicon decreased PR1, BGL2, and CHIB, while increasing PDF1.2 expression, which indicates its role as a modulator of the signalling pathways involved in the plant’s response to fungal infection. Botrytis growth decreased in 10 μM Cd-treated plants, which could be due to the combined effects of Cd and Botrytis activating the SA and JA–ET-mediated signalling pathways. Taken together, our results provide support for the view that Cd concentrations close to the toxicity threshold induce defence signalling pathways which potentiate the plant’s response against fungal infection.     

Role of sodium in the ABA‐mediated long‐term growth response of bean to salt stress

Long‐term salt effects on plant growth have often been related to direct ion toxicity due to the accumulation of high ion concentrations in plant tissue. This work examines the relative importance of endogenous ABA, as well as Na⁺ and Cl⁻ toxicity, in the inhibition of leaf growth and photosynthesis, in bean plants grown at 1, 25, 50 and 75 m M NaCl until the fruit‐bearing stage. All salt‐treated plants showed very high leaf Cl⁻ concentrations, with little difference between plants exposed to 50 or 75 m M NaCl. The 25 and 50 mM salt‐treated plants were able to successfully exclude Na⁺ from their leaves, and only suffered an initial decline in the rate of leaf growth. Plants exposed to 75 m M NaCl showed an increase in Na⁺ leaf concentrations with an accompanying decrease in growth and photosynthesis as salt exposure progressed. A high correlation was found between leaf Na⁺ and leaf growth. Leaf ABA significantly increased with salt supply, and was highly correlated with both leaf Na⁺ and leaf growth. Our results suggest that in bean plants under long‐term salt stress, leaf ABA may participate in the regulation of leaf growth, and leaf Na⁺ would be at least partly responsible for increased ABA levels.     

Can metals defend plants against biotic stress?

Farmers have used metal compounds in phytosanitary treatments for more than a century; however, it has recently been suggested that plants absorb high concentrations of metals from the substrate as a self-defense mechanism against pathogens and herbivores. This metal defense hypothesis is among the most attractive proposals for the 'reason to be' of metal hyperaccumulator species. On a molecular basis, metal defense against biotic stress seems to imply common and/or complementary pathways of signal perception, signal transduction and metabolism. This does not imply a broad band of co-resistance to different stress types but reflects a continuous cross talk during the coevolution of plants, pathogens and herbivores competing in an environment where efficient metal ion acquisition and ion homeostasis are essential for survival.     

Ion allocation in two different salt-tolerant Mediterranean Medicago species

The relationship between Na+, major cation concentrations and salt tolerance under long-term saline conditions of Medicago arborea and Medicago citrina was studied. Plants were grown in solution culture in 1, 50, 100, or 200 mmol/L NaCl for 30 days in a climate-controlled greenhouse. Stem and petiole growth was the most affected by salt in both species. Leaf growth was inhibited in M. arborea, with increased salt, while only the 200 mmol/L NaCl-treated M. citrina plants were significantly affected. Both species had the highest Na+ concentrations in the shoots, however, the allocation pattern was different; M. arborea showed the highest concentrations in the leaf blades, whereas M. citrina distributed the salt into the petioles. K+/Na+ ratio decreased with salt in both species; however, leaf K+ use efficiency (g leaf DW mg-1 leaf K+) was higher in M. citrina. The difference in Na+ allocation and cation concentrations found in these medic species and their importance is discussed in relation to their response to NaCl salinity.