Symplasmic and transmembrane zinc transport is modulated by cadmium in the Cd/Zn hyperaccumulator Sedum alfredii.

This study investigated the influence of Cd (25 µM) on Zn accumulation in a hyperaccumulating (HE) and a non-hyperaccumulating (NHE) ecotype of Sedum alfredii Hance at short-term supply of replete (Zn5, 5 µM) and excess (Zn400, 400 µM) Zn. Cd inhibited Zn accumulation in both ecotypes, especially under Zn400, in organs with active metal sequestration, i.e. roots of NHE and shoots of HE. Direct biochemical Cd/Zn competition at the metal-protein interaction and changes in transporter gene expression contributed to the observed accumulation patterns in the roots. Specifically, in HE, Cd stimulated SaZIP4 and SaPCR2 under Zn5, but downregulated SaIRT1 and SaZIP4 under Zn400. However, Cd downregulated related transporter genes, except for SaNRAMP1, in NHE, irrespective of Zn. Cadmium stimulated casparian strip (CSs) development in NHE, as part of the defense response, while it had a subtle effect on the (CS) in HE. Moreover, Cd delayed the initiation of the suberin lamellae (SL) in HE, but stimulated SL deposition in NHE under both Zn5 or Zn400. Changes in suberization were mainly ascribed to suberin-biosynthesis-related genes and hormonal signaling. Altogether, Cd regulated Zn accumulation mainly via symplasmic and transmembrane transport in HE, while Cd inhibited both symplasmic and apoplasmic Zn transport in NHE.

Naturally zinc-containing bacteriochlorophyll a ([Zn]-BChl a) protects the photosynthetic apparatus of Acidiphilium rubrum from copper toxicity damage.

In almost all photosynthetic organisms the photosynthetic pigments chlorophyll and bacteriochlorophyll (BChl) are Mg2+ containing complexes, but Mg2+ may be exchanged against other metal ions when these are present in toxic concentrations, leading to inactivation of photosynthesis. In this report we studied mechanisms of copper toxicity to the photosynthetic apparatus of Acidiphilium rubrum, an acidophilic purple bacterium that uses Zn2+ instead of Mg2+ as the central metal in the BChl molecules ([Zn]-BChl) of its reaction centres (RCs) and light harvesting proteins (LH1). We used a combination of in vivo measurements of photosynthetic activity (fast fluorescence and absorption kinetics) together with analysis of metal binding to pigments and pigment-protein complexes by HPLC-ICP-sfMS to monitor the effect of Cu2+ on photosynthesis of A. rubrum. Further, we found that its cytoplasmic pH is neutral. We compared these results with those obtained from Rhodospirillum rubrum, a purple bacterium for which we previously reported that the central Mg2+ of BChl can be replaced in vivo in the RCs by Cu2+ under environmentally realistic Cu2+ concentrations, leading to a strong inhibition of photosynthesis. Thus, we observed that A. rubrum is much more resistant to copper toxicity than R. rubrum. Only slight changes of photosynthetic parameters were observed in A. rubrum at copper concentrations that were severely inhibitory in R. rubrum and in A. rubrum no copper complexes of BChl were found. Altogether, the data suggest that [Zn]-BChl protects the photosynthetic apparatus of A. rubrum from detrimental insertion of Cu2+ (trans-metallation) into BChl molecules of its RCs.

Deletion of the N-terminal domain of the yeast vacuolar (Na+ ,K+ )/H+ antiporter Vnx1p improves salt tolerance in yeast and transgenic Arabidopsis.

Cation/proton antiporters play a major role in the control of cytosolic ion concentrations in prokaryotes and eukaryotes organisms. In yeast, we previously demonstrated that Vnx1p is a vacuolar monovalent cation/H+ exchanger showing Na+ /H+ and K+ /H+ antiporter activity. We have also shown that disruption of VNX1 results in an almost complete abolishment of vacuolar Na+ /H+ exchange, but yeast cells overexpressing the complete protein do not show improved salinity tolerance. In this study, we have identified an autoinhibitory N-terminal domain and have engineered a constitutively activated version of Vnx1p, by removing this domain. Contrary to the wild type protein, the activated protein has a pronounced effect on yeast salt tolerance and vacuolar pH. Expression of this truncated VNX1 gene also improves Arabidopsis salt tolerance and increases Na+ and K+ accumulation of salt grown plants thus suggesting a biotechnological potential of activated Vnx1p to improve salt tolerance of crop plants.

Ultratrace Metal Speciation Analysis by Coupling of Sector-Field ICP-MS to High-Resolution Size Exclusion and Reversed-Phase Liquid Chromatography.

Techniques for metal speciation analysis with subnanomolar (ppt) detection limits in complex matrices, with simultaneous quantification of matrix elements, have become a necessity for investigating targets of trace metal binding to macromolecules and pigments at environmentally relevant concentrations. In this work we optimized the analysis of such metal binding in a custom-built HPLC-ICP-MS system. Key elements of the optimization were the choice of components for the metal-free HPLC-DAD system and sector-field ICP-MS detection (ICF-sfMS) with desolvating injection and optimization of sample handling. Protein analysis was done using ammonium bicarbonate buffer and size exclusion chromatography (SEC-ICP-sfMS), with possible addition of anion exchange chromatography. Detection of metal exchange in pigments (chlorophylls and bacteriochlorophylls) was based on reversed-phase chromatography with a methanol-acetone gradient and coupling to the ICP-sfMS via a dedicated organic matrix interface (RPC-ICP-sfMS). The resulting HPLC-DAD-ICP-sfMS system has detection limits in the picomolar range in protein buffer, limited by the maximal achievable purity of buffers/solvents and not by system sensitivity. Tests for method optimization showed that sonication, meant to increase protein solubilization, leads to artifacts of metal loss from metalloproteins. Examples for Cd binding to soybean proteins and chlorophyll, Cr binding to Arabidopsis thaliana proteins, La binding to Desmodesmus quadricauda proteins, and Cu binding to Rhodospirillum rubrum proteins and pigments are shown. These application examples demonstrate that the system is sensitive enough to detect binding of metals to proteins and pigments at background concentration levels of typical nutrient solutions made from analytical grade chemicals, equivalent to ultratrace metal concentrations in nonpolluted environments.

Mechanisms of sublethal copper toxicity damage to the photosynthetic apparatus of Rhodospirillum rubrum.

Magnesium (Mg2+) is the ubiquitous metal ion present in chlorophyll and bacteriochlorophyll (BChl), involved in photosystems in photosynthetic organisms. In the present study we investigated targets of toxic copper binding to the photosynthetic apparatus of the anoxygenic purple bacterium Rhodospirillum rubrum. This was done by a combination of in vivo measurements of flash photolysis and fast fluorescence kinetics combined with the analysis of metal binding to pigments and pigment-protein complexes isolated from Cu-stressed cells by HPLC-ICPMS (ICP-sfMS). This work concludes that R. rubrum is highly sensitive to Cu2+, with a strong inhibition of the photosynthetic reaction centres (RCs) already at 2 µM Cu2+. The inhibition of growth and of RC activity was related to the formation of Cu-containing BChl degradation products that occurred much more in the RC than in LH1. These results suggest that the shift of metal centres in BChl from Mg2+ to Cu2+ can occur in vivo in the RCs of R. rubrum under environmentally realistic Cu2+ concentrations, leading to a strong inhibition of the function of these RCs.

The sodium transporter encoded by the HKT1;2 gene modulates sodium/potassium homeostasis in tomato shoots under salinity.

Excessive soil salinity diminishes crop yield and quality. In a previous study in tomato, we identified two closely linked genes encoding HKT1-like transporters, HKT1;1 and HKT1;2, as candidate genes for a major quantitative trait locus (kc7.1) related to shoot Na+ /K+ homeostasis - a major salt tolerance trait - using two populations of recombinant inbred lines (RILs). Here, we determine the effectiveness of these genes in conferring improved salt tolerance by using two near-isogenic lines (NILs) that were homozygous for either the Solanum lycopersicum allele (NIL17) or for the Solanum cheesmaniae allele (NIL14) at both HKT1 loci; transgenic lines derived from these NILs in which each HKT1;1 and HKT1;2 had been silenced by stable transformation were also used. Silencing of ScHKT1;2 and SlHKT1;2 altered the leaf Na+ /K+ ratio and caused hypersensitivity to salinity in plants cultivated under transpiring conditions, whereas silencing SlHKT1;1/ScHKT1;1 had a lesser effect. These results indicate that HKT1;2 has the more significant role in Na+ homeostasis and salinity tolerance in tomato.

Two closely linked tomato HKT coding genes are positional candidates for the major tomato QTL involved in Na+ /K+ homeostasis.

The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na(+) ] and [K(+) ] was previously reported in chromosome 7 using two connected populations of recombinant inbred lines (RILs) of tomato. HKT1;1 and HKT1;2, two tomato Na(+) -selective class I-HKT transporters, were found to be closely linked, where the maximum logarithm of odds (LOD) score for these QTLs located. When a chromosome 7 linkage map based on 278 single-nucleotide polymorphisms (SNPs) was used, the maximum LOD score position was only 35 kb from HKT1;1 and HKT1;2. Their expression patterns and phenotypic effects were further investigated in two near-isogenic lines (NILs): 157-14 (double homozygote for the cheesmaniae alleles) and 157-17 (double homozygote for the lycopersicum alleles). The expression pattern for the HKT1;1 and HKT1;2 alleles was complex, possibly because of differences in their promoter sequences. High salinity had very little effect on root dry and fresh weight and consequently on the plant dry weight of NIL 157-14 in comparison with 157-17. A significant difference between NILs was also found for [K(+) ] and the [Na(+) ]/[K(+) ] ratio in leaf and stem but not for [Na(+) ] arising a disagreement with the corresponding RIL population. Their association with leaf [Na(+) ] and salt tolerance in tomato is also discussed.