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

Abstract

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 hyperaccumulator Thlaspi caerulescens. The ZTP1 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. caerulescens (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. All three zinc transporter genes show increased expression in T. caerulescens compared with the non‐hyperaccumulator congener T. arvense, suggesting an important role in heavy metal hyperaccumulation. ZNT1 and ZNT2 are predominantly expressed in roots and ZTP1 is mainly expressed in leaves but also in roots. In T. arvense, ZNT1 and ZNT2 are exclusively expressed under conditions of Zn deficiency. Their expression in T. caerulescens is barely Zn‐responsive, 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 normal soil. The calamine plants were also the most Zn tolerant, suggesting that high ZTP1 expression might contribute to Zn tolerance.

Transition metal transport

“…In both hyperaccumulators, A. halleri and T. caerulescens, the genes ZIP4, ZIP10 and IRT3 are highly expressed in roots under conditions of sufficient Zn supply [44][45][46][51][52][53]. Nevertheless, the expression of these genes remains responsive to changes in Zn supply and decreases with increasing external Zn in both species.…”

“…AhZIP4, AhZIP10 and AhIRT3 are most likely single copy genes [44]. The T. caerulescens homologues of AtZIP4 and AtIRT3 have previously been isolated in molecular approaches involving screens of cDNA libraries and were named ZNT1 and ZNT2, respectively [51][52][53]. ZNT1/TcZIP4 can mediate Zn and Cd influx when expressed in yeast [51,52].…”

“…Two of these loci account for the high AhMTP1 transcript levels and have been shown to co-segregate with Zn tolerance in a segregating backcross 1 population of an interspecific cross between A. halleri and A. lyrata [54]. Thlaspi caerulescens MTP1 has been cloned previously from a cDNA library and was named ZTP1 [53]. Compared to ZTP1 from T. arvense, ZTP1/TcMTP1 was found to be more highly expressed in leaves and roots of three different accessions of T. caerulescens, with the expression in leaves being slightly higher than in roots.…”

Isolation of Zn-responsive genes from two accessions of the hyperaccumulator plant Thlaspi caerulescens

“…Thlaspi caerulescens is a Zn and Cd hyperaccumulator with considerable within and between population variability in the hyperaccumulation trait (Meerts and Van Isacker 1997;Assunção et al 2001). Comparison of T. caerulescens accessions suggests that Zn tolerance, transport and accumulation are subject to independent variation (Assunção et al 2003a, b).…”

“…The ZNT1 gene, a member of the ZIP (ZRT/IRT-like proteins) transporter gene family, mediates high-aYnity Zn uptake in T. caerulescens roots (Pence et al 2000). The expression of ZNT1 is higher in T. caerulescens than in the non-accumulator Thlaspi arvense, and is not down-regulated by elevated Zn, thus potentially resulting in the increased Zn uptake (Pence et al 2000;Assunção et al 2001). …”