Variation in root-to-shoot translocation of cadmium and zinc among different accessions of the hyperaccumulators Thlaspi caerulescens and Thlaspi praecox

• Efficient root-to-shoot translocation is a key trait of the zinc/cadmium hyperaccumulators Thlaspi caerulescens and Thlaspi praecox, but the extent of variation among different accessions and the underlying mechanisms remain unclear. • Root-to-shoot translocation of Cd and Zn and apoplastic bypass flow were determined in 10 accessions of T. caerulescens and one of T. praecox, using radiolabels ¹⁰⁹Cd and ⁶⁵Zn. Two contrasting accessions (Pr and Ga) of T. caerulescens were further characterized for TcHMA4 expression and metal compartmentation in roots. • Root-to-shoot translocation of ¹⁰⁹Cd and ⁶⁵Zn after 1 d exposure varied 4.4 to 5-fold among the 11 accessions, with a significant correlation between the two metals, but no significant correlation with uptake or the apoplastic bypass flow. The F₂ progeny from a cross between accessions from Prayon, Belgium (Pr) and Ganges, France (Ga) showed a continuous phenotype pattern and transgression. There was no significant difference in the TcHMA4 expression in roots between Pr and Ga. Compartmentation analysis showed a higher percentage of ¹⁰⁹Cd sequestered in the root vacuoles of Ga than Pr, the former being less efficient in translocation than the latter. • Substantial natural variation exists in the root-to-shoot translocation of Cd and Zn, and root vacuolar sequestration may be an important factor related to this variation.