Circadian Life Without Micronutrients: Effects of Altered Micronutrient Supply on Clock Function in Arabidopsis

The plant circadian clock is formed by a number of interlocked feedback loops that control the expression of thousands of genes. Genetic and pharmacological approaches towards the study of the plant clock are routinely carried out on Murashige and Skoog growth medium, which is both Fe-replete and Cu-deficient. However, it has recently become clear that the plant clock responds to available iron (Fe) supply: circadian pace slows down under conditions of Fe deficiency; circadian period progressively shortens with increasing Fe supply. Here, we describe several growth media that may be used to study the effects of varying micronutrient supply on the circadian clock, in which deficiency in a given micronutrient are imposed by the addition of a specific chelator or, alternatively, by using EDTA-washed agar as gelling agent, thus minimizing micronutrient contamination

Metal complexes increase uptake of Zn and Cu by plants: implications for uptake and deficiency studies in chelator-buffered solutions

The uptake of trace metals by plants is commonly assumed to depend on the free metal-ion activity, rather than on the total concentration of dissolved metal. Although this free-ion hypothesis has proved to be useful for the interpretation and prediction of metal uptake, several exceptions have been reported where metal complexes also affected metal uptake by plants. In this study, we measured uptake of Zn and Cu by spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) in chelator-buffered or resin (Chelex)-buffered solutions, under Zn-deficient and non-deficient conditions. Several ligands, with differing dissociation rates, were used in the chelator-buffered solutions. At the same free-ion activity, Cu and Zn uptake was less in Chelex-buffered than in chelator-buffered solutions. In the chelator-buffered solution, uptake of Cu and Zn at same free-ion activity and same total concentration followed the order: NTA > HEDTA > EDTA > CDTA, i.e., the same order as the dissociation rate. These differences in metal uptake were also reflected in the deficiency symptoms and plant yield in the experiments where Zn deficiency was imposed. The critical Zn2+ activity for Zn deficiency varied by one order of magnitude depending on the buffer, and followed the order HEDTA < CDTA < resin-buffered (no soluble ligand). These results suggest that, when present, aqueous complexes can increase metal uptake by plants because uptake is rate-limited by diffusion of the free ion to the root or cell surface. Thus, the critical free-ion activity in chelator-buffered solutions depends on the type and concentration of the ligand employed. © 2006 Springer Science+Business Media B.V.F. Degryse, E. Smolders, D. R. Parke

Plant and rhizosphere processes involved in phytoremediation of metal-contaminated soils

This paper reviews the recent advances in understanding of metal removal from contaminated soils, using either hyperaccumulator plants, or high biomass crop species after soil treatment with chelating compounds. Progress has been made at the physiology and molecular level regarding Zn and Ni uptake and translocation in some hyperaccumulators. It is also known that natural hyperaccumulators do not use rhizosphere acidification to enhance their metal uptake. Recently, it has been found that some natural hyperaccumulators proliferate their roots positively in patches of high metal availability. In contrast, non-accumulators actively avoid these areas, and this is one of the mechanisms by which hyperaccumulators absorb more metals when grown in the same soil. However, there are few studies on the exudation and persistence of natural chelating compounds by these plants. It is thought that rhizosphere microorganisms are not important for the hyperaccumulation of metals from soil. Applications of chelates have been shown to induce large accumulations of metals like Pb, U and Au in the shoots of non-hyperaccumulators, by increasing metal solubility and root to shoot translocation. The efficiency of metal uptake does vary with soil properties, and a full understanding of the relative importance of mass flow and diffusion in the presence and absence of artificial chelates is not available. To successfully manipulate and optimise future phytoextraction technologies, it is argued that a fully combined understanding of soil supply and plant uptake is needed