DFG Priority Programme “Adaptomics” 1529

Natural variation, underlying molecular basis and ecological role of metal hyperaccumulation in Arabidopsis halleri

Project code: KR 1967/10-1

Heavy-metal contaminated site in Czech Republic. Arabidopsis halleri individuals can be found around the disturbed area.

So-called metal hyperaccumulator plants are capable of accumulating extraordinarily high concentrations of transition metals or metalloids, for example Ni, Zn, Se, As, Cd, Co or Cu, in their above-ground tissues^1,2. Discovered in ~500 angiosperm species to date, metal hyperaccumulation is a complex extreme trait of remarkably high occurrence in the Brassicaceae family. Arabidopsis halleri, a stoloniferous perennial and close relative of Arabidopsis thaliana, can hyperaccumulate both Zn and Cd and also shows hypertolerance to these metals. A. halleri was first reported as a colonizer of metalliferous soils and highly disturbed heavy-metal contaminated areas.

The objectives of this collaborative project are:

large-scale assessment of total leaf concentrations of multiple elements in A. halleri individuals at their natural sites, followed by laboratory-based phenotyping of a large number of individuals grown under controlled environmental conditions. We will thereby analyze the respective contributions of two hypothesized causative factors – genetic diversity and phenotypic plasticity – on natural variation of the leaf ionome in A. halleri, focusing primarily on Cd, Zn and Fe accumulation.
To work towards an understanding of the molecular mechanisms and genetic basis underlying this natural variation.
To examine the ecological role of natural variation in Cd and Zn hyperaccumulation in A. halleri with respect to the elemental defense hypothesis.

In order to access the natural variation of metal hyperaccumulation in A. halleri, we have sampled individuals from 85 sites in 8 countries (Germany, Belgium, France, Austria, Italy, Czech Republic, Poland and Switzerland), comprising both non-contaminated and contaminated soils. All field-collected plants accumulated high leaf Zn concentrations, whereas the dynamic range of leaf Cd concentrations was substantially larger, indicating that Cd hyperaccumulation is not a constitutive species-wide trait in A. halleri. There was no correlation between leaf concentrations of Cd and Zn, suggesting that, in contrast to other plants, A. halleri exerts a high level of control over the pathways of movement of each of these metals individually. Moreover, there was no correlation between leaf Cd concentrations and soil Cd concentrations or availability.

Field-collected individuals are now being phenotyped under controlled conditions in the laboratory. All follow-up experiments are taking advantage of previously generated transgenic and newly obtained field-collected A. halleri genotypes exhibiting divergent levels of metal accumulation. To determine the genetic basis underlying natural variation in metal accumulation, we are pursuing candidate gene approaches based on previous results and nucleotide sequence data from individuals at the extreme ends of the dynamic range of metal accumulation^3-5. Moreover, we will conduct first steps in preparation of a genetic approach to identify QTL underlying the observed variation. The project is using tools and biological resources which have been developed by our laboratory, including the stable transformation of A. halleri, the upcoming A. halleri genome sequence, an existing 454 transcriptome and RNA-Seq transcriptomic profiles. Laboratory experiments addressing the ecological role of metal accumulation are being complemented by reciprocal transplant experiments between field sites harboring natural A. halleri populations. We will focus on testing the "elemental defense hypothesis". This hypothesis postulates that the high concentrations of metals or metalloids present in leaves of metal hyperaccumulator plants are toxic to pathogens and herbivors and thus serve as an elemental defense, which complements other known defenses consisting of organic compounds and protein biochemical activities⁶.

To expand the methodological repertoire towards a more profound analysis of metal homeostasis in A. halleri, cell-type specific translatome profiling is being developed by the collaborating group of Professor Dr. Stephan Clemens, University of Bayreuth. Through the contributions of project partner Professor Dr. Caroline Müller, Bielefeld University , to the work addressing the ecological role of metal hyperaccumulation, we will employ local herbivores, feeding experiments with artificial diets, and the monitoring of metabolite fingerprints as well as of metal accumulation and plant herbivory resistance after a first incidence of herbivore attack.

Arabidopsis halleri growing in a heavy-metal contaminated site in Germany (Harz Mountains)

Publications related to the project

Kazemi-Dinan A, Thomaschky S, Stein RJ, Krämer U, Müller C (2014) Zn and Cd hyperaccumulation act as deterrents towards specialist herbivores and impede the performance of a generalist herbivore. New Phytologist 202(2):628-39.
Hanikenne M, Kroymann J, Trampczynska A, Bernal M, Motte P, Clemens S,
Krämer U (2013) Hard selective sweep and ectopic gene conversion in a gene cluster affording environmental adaptation, PLoS Genet 9(8): e1003707).
Deinlein U, Weber M, Schmidt H, Rensch S, Trampczynska A, Hansen TH, Husted S, Schjoerring JK, Talke IN, Krämer U, Clemens S (2012) Elevated
nicotianamine levels in Arabidopsis halleri roots play a key role in Zn
hyperaccumulation. Plant Cell 24:708-23.
Krämer, U. (2010) Metal hyperaccumulation in plants. Annu Rev Plant Biol 61, 517-534).
Boyd, R. (2010) Elemental Defenses of Plants by Metals. Nature Education Knowledge 1, 6.
Hanikenne, M. et al. (2008) Evolution of metal hyperaccumulation required cis-regulatory changes and triplication of HMA4. Nature 453, 391-395.
Talke, I. N., Hanikenne, M. & Krämer, U. (2006) Zinc-Dependent Global Transcriptional Control, Transcriptional Deregulation, and Higher Gene Copy Number for Genes in Metal Homeostasis of the Hyperaccumulator Arabidopsis halleri. Plant Physiol 142, 148-167.
Becher, M., Talke, I. N., Krall, L. & Krämer, U. (2004) Cross-species microarray transcript profiling reveals high constitutive expression of metal homeostasis genes in shoots of the zinc hyperaccumulator Arabidopsis halleri. Plant J 37, 251-268.
Krämer, U., Cotter-Howells, J. D., Charnock, J. M., Baker, A. J. M. & Smith, J. A. C. (1996) Free histidine as a metal chelator in plants that accumulate nickel. Nature 379, 635-638.