Submitted to: Annals Of Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/26/2008
Publication Date: 4/25/2008
Citation: Milner, M., Kochian, L.V. 2008. Investigating Heavy-metal Hyperaccumulation using Thlaspi caerulescens as a Model System. Annals Of Botany. 102(1):3-13. Interpretive Summary: Heavy metal contamination of soils poses serious problems worldwide, and the current technologies used to remediate soils are costly and disruptive. There is considerable interest in the use of terrestrial plants to clean up heavy metals from the soil. Several metal hyperaccumulating plant species have been identified that tolerate highly contaminated soils and accumulate these metals to high concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium hyperaccumulator. In this paper we detail the molecular and physiological findings from our lab regarding zinc and cadmium accumulation in T. caerulescens. We have found that the hyperaccumulator maintains much higher rates of Zn and cd uptake into and throughout the plant root and shoot compared to related non-accumulators. This increased metal transport is associated with much higher expression of a suite of metal transporter and metal metabolism genes. We are working to identify the molecular basis for this gene hyperexpression, and have identified some unique proteins that appear to regulate expression of metal transporter genes and may play a key role in gene hyperexpression that leads to metal hyperaccumulation.
Technical Abstract: Hyperaccumulation was a term first coined by Brooks for plants that are endemic to metalliferous soils and are able to tolerate and accumulate large amounts of metals in their above ground tissues. Of the nearly 90 metal hyperaccumulating species in the Brassicaceae family, two species in particular, Thlaspi caerulescens and Arabidopsis halleri, have been studied extensively for their ability to hyperaccumulate a number of various heavy metals, mainly Zn, Cd, Ni and Pb. In particular, certain ecotypes of T. caerulescens can accumulate as much as 30,000 ppm Zn and 10,000 ppm Cd in the shoot without any signs of toxicity (typical shoot levels are 100-200 ug/g DW Zn and 0.1 to 10 ug/g DW Cd). Contamination of soils with a number of various heavy metals poses both an environmental risk as well as a human health risk. Clean up of these sites based on more destructive methods using engineering methods have been estimated to cost 400 billion dollars in the US alone. The hope is that with the discovery of the metallophytes a less disruptive and cheaper way to decontaminate heavy metal contaminated soils can be developed. Thlaspi caerulescens has been at the forefront of these studies on metal hyperaccumulation. However due to T. caerulescens slow growth, the underlying mechanisms have been studied with the hope of using this information to transform higher biomass producing plants into hyperaccumulators. In this manuscript, we will present findings from our research as well as from other researchers on the physiological mechanisms of heavy metal tolerance and the underlying molecular and genetic basis for the extreme metal transport and storage exhibited by T. caerulescens.