Submitted to: Plant Physiology
Publication Type: Abstract Only
Publication Acceptance Date: 6/1/2005
Publication Date: 7/1/2005
Citation: Papoyan, A., Kochian, L.V. 2005. Characterization of a heavy metal transporting p-type atpase from thlaspi caerulescens: does it play a role in heavy metal hyperaccumulation?. Plant Physiology. p. 52. Interpretive Summary:
Technical Abstract: Thlaspi caerulescens is a heavy metal hyperaccumulator plant species that accumulates Zn and Cd to very high levels in the shoots (3 % Zn; 1.5% Cd). We are currently studying several genes that may be involved in the heavy metal hyperaccumulation process. Among these genes is TcHMA4, a P-Type heavy metal ATPase that is highly similar to the Arabidopsis thaliana homolog, AtHMA4. A detailed characterization of TcHMA4 transport function via heterologous expression in yeast (S. cerevisiae) as well as gene expression studies in Thlaspi caerulescens will be presented. When TcHMA4 is expressed in yeast, it confers a high degree of tolerance to potentially toxic levels of Cd. This heavy metal tolerance correlates with significantly lower accumulation of Cd, Pb, and Zn in transformed yeast cells. Comparative radiotracer (109Cd) flux analysis of wild type yeast and yeast expressing TcHMA4 indicated that there were no differences in unidirectional 109Cd2+ influx in the two yeast genotypes, while cells expressing TcHMA4 maintained a much larger 109Cd2+ efflux. These findings are consistent with this transporter residing in the plasma membrane and functioning to transport heavy metals (and possibly micronutrients) out of the cell. Northern analysis indicates that TcHMA4 is expressed primarily in the root and unlike its counterpart in Arabidopsis, TcHMA4 expression is strongly upregulated both by Cd exposure and Zn deficiency. These findings, along with information from the Arabidopsis root gene expression map of Birnbaum et al (Science  302: 1956) showing that AtHMA4 is expressed primarily within the stele, suggest that this transporter may function in the loading of micronutrients and heavy metals into the xylem, and the subsequent translocation of these metals to the shoot.