The Molecular Basis for Zn Hyperaccumulation in Thlaspi caerulescens

Metal hyperaccumulating plant species are plants that are endemic to metalliferous soils and are able to tolerate and accumulate metals in their above ground tissues to very high concentrations. One such hyperaccumulator, Thlaspi caerulescens, has been widely studied for its remarkable properties to tolerate toxic levels of zinc (Zn) and cadmium (Cd) in the soil, and accumulate these metals to very high levels in the shoot. Thlaspi caerulescens has the ability to accumulate as much as 3% Zn and 1% Cd in the shoots of the plant with no signs of toxicity. Early molecular investigations in our lab into how this plant functions as a metal hyperaccumulator led to the identification of TcZNT1 as a gene encoding a high affinity Zn transporter and low affinity Cd transporter which is expressed at very high levels in the roots of T. caerulescens. Our lab and other labs have subsequently found that high expression of TcZNT1 and other metal transporters is a hallmark of metal hyperaccumulators. To begin to understand the molecular basis of this “hyperexpression”, we have identified two different members of the E2F family of transcription factors that complement a yeast mutant defective in Zn uptake through disruption of the yeast transcription factor that regulates expression of the yeast high affinity Zn transporter. We have also found that only one of these E2Fs is able to bind with high affinity directly to a putative E2F-binding motif in the promoter of TcZNT1 in T. caerulescens, possibly regulating its expression. Further characterization of the interactions between ZNT1 and the E2F family members is being conducted to more fully elucidate the role these transcription factors may play both in plant Zn homeostasis, and the hyperaccumulation of heavy metals