November 12 2009
12:00 LSB 2320

Abstract:

One of our research aims is to uncover the molecular basis underlying naturally selected, extreme traits in plants. The extremophile model species Arabidopsis halleri displays species-wide zinc and cadmium hyperaccumulation and associated hypertolerance, and occurs primarily on soils polluted with toxic levels of heavy metals. By contrast, its sister species A. lyrata, as well as A. thaliana, are metal excluders and possess merely basic metal tolerance. A number of candidate genes were identified through heterologous screening of cDNA libraries and cross-species transcriptome comparisons. Using stable genetic transformation of A. halleri for the silencing of candidate genes by RNA interference, we demonstrated that constitutively high transcript levels of HMA4 in A. halleri, when compared to A. thaliana or A. lyrata, are necessary for both Zn hyperaccumulation and Zn and Cd hypertolerance. HMA4 is a P-type Zn/Cd-pumping ATPase that functions in cellular metal export and metal release into the xylem for subsequent root-to-shoot transport. Sequencing of the genomic HMA4 region in A. halleri and promoter swap experiments between A. halleri and A. thaliana indicated that high transcript levels of HMA4 in A. halleri result from a combination of gene copy number expansion and promoter mutations causing high expression levels of all three HMA4 gene copies. Introduction of an A. halleri HMA4 mini-gene into the non-accumulator A. thaliana partially recapitulated the metal hyperaccumulation phenotype. The possibility of the occurrence of a selective sweep at the HMA4 locus of A. halleri will be discussed based on a scan of sequence diversity at and in the vicinity of this locus. Finally, the biological role of metal hyperaccumulation will be discussed. A second major area of interest in my lab is how plants maintain metal homeostasis, and how the plant metal homeostasis network interacts with metabolism, signalling pathways and development, using primarily Arabidopsis thaliana as a model.