Elevated expression of TcHMA3 plays a key role in the extreme Cd tolerance in a Cd‐hyperaccumulating ecotype of Thlaspi caerulescens

Abstract

Cadmium (Cd) is a highly toxic heavy metal for plants, but several unique Cd-hyperaccumulating plant species are able to accumulate this metal to extraordinary concentrations in the aboveground tissues without showing any toxic symptoms. However, the molecular mechanisms underlying this hypertolerance to Cd are poorly understood. Here we have isolated and functionally characterized an allelic gene, TcHMA3 (heavy metal ATPase 3) from two ecotypes (Ganges and Prayon) of Thlaspi caerulescens contrasting in Cd accumulation and tolerance. The TcHMA3 alleles from the higher (Ganges) and lower Cd-accumulating ecotype (Prayon) share 97.8% identity, and encode a P(1B)-type ATPase. There were no differences in the expression pattern, cell-specificity of protein localization and transport substrate-specificity of TcHMA3 between the two ecotypes. Both alleles were characterized by constitutive expression in the shoot and root, a tonoplast localization of the protein in all leaf cells and specific transport activity for Cd. The only difference between the two ecotypes was the expression level of TcHMA3: Ganges showed a sevenfold higher expression than Prayon, partly caused by a higher copy number. Furthermore, the expression level and localization of TcHMA3 were different from AtHMA3 expression in Arabidopsis. Overexpression of TcHMA3 in Arabidopsis significantly enhanced tolerance to Cd and slightly increased tolerance to Zn, but did not change Co or Pb tolerance. These results indicate that TcHMA3 is a tonoplast-localized transporter highly specific for Cd, which is responsible for sequestration of Cd into the leaf vacuoles, and that a higher expression of this gene is required for Cd hypertolerance in the Cd-hyperaccumulating ecotype of T. caerulescens.

A Member of the Heavy Metal P-Type ATPase OsHMA5 Is Involved in Xylem Loading of Copper in Rice

“…Furthermore, AhHMA4 for Zn translocation showed a higher expression level (Chiang et al, 2006;Hanikenne et al, 2008). On the other hand, TcHMA3 from ecotype Ganges of T. caerulescens, a Cd hyperaccumulator, plays an important role in the detoxification of Cd by sequestering Cd into the vacuole of the leaves (Ueno et al, 2011). High expression of TcHMA4 (NcHMA4) was also reported in T. caerulescens (Bernard et al, 2004;Papoyan and Kochian, 2004;Craciun et al, 2012).…”

Noccaea caerulescens populations adapted to grow in metalliferous and non-metalliferous soils: Ni tolerance, accumulation and expression analysis of genes involved in metal homeostasis

“…High expressions of genes for ATP-hydrolising metal transporters such as HMA4 and HMA3 were found in N. caerulescens compared to related non-tolerant, non-accumulator species (Papoyan and Kochian, 2004;van de Mortel et al, 2006;Ueno et al, 2011). In addition, HMA3 and HMA4 varied in copy number among populations of N. caerulescens adapted to grow on different soil types.…”

“…In addition, HMA3 and HMA4 varied in copy number among populations of N. caerulescens adapted to grow on different soil types. Interestingly, both the copy number and the level of expression of these gene seem to be associated with N. caerulescens populations showing higher levels of Cd tolerance and accumulation (Ueno et al, 2011;Ó Lochlainn et al, 2011;Craciun et al, 2012).…”

Strategies to increase zinc deficiency tolerance and homeostasis in plants

“…Recently, HMA3 has been cloned from Thlaspi carulescens Alpine Penny-cress (Ueno et al, 2011), and rice (Ueno et al, 2010), therefore it is a vacuolar influx transporter, important for cadmium tolerance in both species. This suggests that different homologs of HMA3 may have different metal-substrate specificity.…”