Lead (Pb) is a non-essential, highly toxic element. As it forms stable complexes with organic matter and adsorbs to Al-, Fe- and Mn-oxides it is highly persistent in soil, accumulating in the environment and posing a threat to human health. To prevent the potentially harmful effects of a long-term human Pb exposition by food consumption and to take measures for the elimination of Pb in the environment profound knowledge of the processes underlying Pb uptake, translocation and accumulation in plants is of great importance. At present our knowledge regarding these mechanisms lags far behind arsenic and cadmium. This can be mainly attributed to Pb having a low solubility at a pH greater than 5, as well as the formation of Pb-phosphate precipitates, making it challenging to analyze plant-Pb interactions in commonly used experimental settings. We have therefore developed a novel low-phosphate, low-pH assay system, enabling the analysis of plant Pb toxicity and tolerance under realistic Pb concentrations.
This is being used for two different approaches: (i) the search for relevant genes and proteins by studying induced or natural genetic variation; (ii) the physiological analysis of Pb uptake and accumulation in the model cereal barley.
Evidence is available for Pb uptake via hitch-hiking of transporters for essential elements, e.g. Ca2+ channels and Pb detoxification by the phytochelatin pathway. To find additional factors involved in Pb toxicity and tolerance we tested metal hypersensitive EMS mutants isolated in our lab for Pb hypersensitivity and identified one mutant. We will report the identification and functional characterization of the causal gene responsible for the phenotype.
Furthermore, we could detect pronounced natural variation in A. thaliana Pb tolerance. This enabled the identification of quantitative trait loci involved in Pb toxicity effects through a genome wide association study. For this over 300 ecotypes were screened regarding their Pb phenotype and the correlation between respective phenotype and single nucleotide polymorphisms (SNPs) was determined. Additionally, we characterized a Pb hypersensitive ecotype, which we discovered and collected ourselves. The SNPs we found converged into the same metabolic pathway that is affected by Pb stress and possibly involved in tolerance.
Considering the mobility and accumulation of Pb in crops in our second approach we use Hordeum vulgare as a model crop plant. We have analyzed three different ecotypes of H. vulgare, which display contrasting profiles of grain micronutrient accumulation. For the first time we could detect Pb in the grain of barley both after hydroponic and contaminated soil cultivation. The screening for ecotypes which accumulate an enhanced amount of essential elements combined with a decreased accumulation of toxic elements is an important tool towards diminishing the entry of pollutants into the food web.