Minisymposium 13: Heavy metals

Abs # 26003: Characterization of a heavy metal transporting P-type ATPase from Thlaspi caerulescens: Does it play a role in heavy metal hyperaccumulation?

Presenter:	Papoyan, Ashot H, ap75@cornell.edu
Authors	Papoyan, Ashot H (A)   Kochian, Leon V (B)
Affiliations:	(A): Department of Plant Biology, Cornell University (B): US Plant, Soil and Nutrition Laboratory, USDA-ARS, Cornell University

Thlaspi caerulescens is a heavy metal hyperaccumulator plant species that is able to tolerate soils highly contaminated with zinc (Zn) and cadmium (Cd) and also will accumulate these metals to very high levels in the shoots (3 % Zn and 1.5% Cd per shoot dry wt.). We have isolated and are studying several genes that may be involved in the heavy metal hyperaccumulation process in Thlaspi caerulescens. 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 levels of Cd that are highly toxic to untransformed wild type yeast. 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 as well as by imposition of plant Zn deficiency. These findings, along with information from the Arabidopsis root gene expression map of Birnbaum et al (Science [2003] 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.