Heavy metals are naturally present in soils, or can be brought by agricultural practices or spread out by human activities. Some of them are essential for plant development. Cadmium is a non-essential heavy metal which is classified in Group 1 (= known human carcinogen) by the International Agency for Research on Cancer (IARC). Nicotiana tabacum possesses the ability to highly translocate cadmium from roots to shoots. Therefore cadmium is accumulated at significant levels in leaves.

The molecular factors involved in cadmium distribution in-planta are well described in Arabidopsis thaliana. Among those, two Heavy Metal Atpases (HMA) are involved in the loading of cadmium into the xylem: AtHMA2 and AtHMA4. Thus, genes orthologous to AtHMA2 and AtHMA4 were looked for in the N. tabacum genome.

Two NtHMA genes were identified, originating from N. sylvestris and from N. tomentosiformis, the two ancestors of the amphidiploid N. tabacum. Mutants were identified for each gene in our collection of EMS mutagenised tobacco plants. N. tabacum lines expressing artificial micro RNA (amiRNA) constructs were also developed to silence their expression.

Together with the expression pattern of the two NtHMA genes, data suggest that the two genes are highly redundant.

The ability of mutant lines to translocate cadmium from roots to shoots was tested. Results with plants grown under hydroponic or greenhouse conditions show a significant reduction of cadmium accumulation in the shoots. However, this reduction could be dependent on the availability of cadmium in the environment.