Elevated expression of metal transporter genes in three accessions of the metal hyperaccumulatorThlaspi caerulescens

Heavy metal hyperaccumulation in plants is an intriguing and poorly understood phenomenon. Transmembrane metal transporters are assumed to play a key role in this process. We describe the cloning and isolation of three zinc transporter cDNAs from the Zn hyperaccumulatorThlaspi caerulescens. TheZTP1gene is highly similar to the ArabidopsisZATgene. Of the other two, one is most probably an allele of the recently clonedZNT1gene fromT. caerulescens( Penceet al ;Proceedings of the National Academy of Science USA97, 4956–4960, 2000). The second, calledZNT2, is a close homologue ofZNT1. All three zinc transporter genes show increased expression inT. caerulescenscompared with the non‐hyperaccumulator congenerT. arvense, suggesting an important role in heavy metal hyperaccumulation.ZNT1andZNT2are predominantly expressed in roots andZTP1is mainly expressed in leaves but also in roots. InT. arvense,ZNT1andZNT2are exclusively expressed under conditions of Zn deficiency. Their expression inT. caerulescensis barely Zn‐responsive, suggesting that Zn hyperaccumulation might rely on a decreased Zn‐induced transcriptional downregulation of these genes.ZTP1expression was higher in plants from calamine soil than in plants from serpentine or normal soil. The calamine plants were also the most Zn tolerant, suggesting that highZTP1expression might contribute to Zn tolerance.