Elevated expression of metal transporter genes in three accessions of the metal hyperaccumulator Thlaspi caerulescens

Heavy metal hyperaccumulation in plants is an intriguing and poorly underst ood phenomenon. Transmembrane metal transporters are assumed to play a key role in this process. We describe the cloning and isolation of three zinc t ransporter cDNAs from the Zn hyperaccumulator Thlaspi caerulescens. The ZTP 1 gene is highly similar to the Arabidopsis ZAT gene. Of the other two, one is most probably an allele of the recently cloned ZNT1 gene from T. caerul escens (Pence et al; Proceedings of the National Academy of Science USA 97, 4956-4960, 2000). The second, called ZNT2, is a close homologue of ZNT1. A ll three zinc transporter genes show increased expression in T. caerulescen s compared with the non-hyperaccumulator congener T. arvense, suggesting an important role in heavy metal hyperaccumulation. ZNT1 and ZNT2 are predomi nantly expressed in roots and ZTP1 is mainly expressed in leaves but also i n roots. In T. arvense, ZNT1 and ZNT2 are exclusively expressed under condi tions of Zn deficiency. Their expression in T. caerulescens is barely Zn-re sponsive, suggesting that Zn hyperaccumulation might rely on a decreased Zn -induced transcriptional downregulation of these genes. ZTP1 expression was higher in plants from calamine soil than in plants from serpentine or norm al soil. The calamine plants were also the most Zn tolerant, suggesting tha t high ZTP1 expression might contribute to Zn tolerance.