Root-selective expression of AtCAX4 and AtCAX2 results in reduced lamina cadmium in field-grown Nicotiana tabacum L.

To assess the impact of enhanced root vacuole cadmium (Cd) sequestration on leaf Cd accumulation under a low Cd dose, as generally occurs in agriculture, leaf Cd accumulation was examined in field-grown tobacco plants expressing genes encoding the high-capacity-Cd, tonoplast-localized, divalent cation/H antiporters AtCAX4 and AtCAX2 (AtCAX, Arabidopsis cation exchanger). It has been shown previously that root tonoplast vesicles isolated from plants expressing these genes, directed by root-selective promoters, show enhanced Cd transport activity, and young plants show enhanced root Cd accumulation when grown in solution culture containing 0.02 mu m Cd, a moderate Cd dose. In this article, we present results which show that the lower leaves of mature plants expressing AtCAX2 or AtCAX4, under the control of two different root-selective promoters, accumulate 15%-25% less lamina Cd than control plants when grown in the field (3 years, three different collection methods). Reciprocal grafting experiments of AtCAX2 shoots onto control roots (and vice versa), grown in solution culture with 0.005 mu m Cd, indicated that the root controls Cd translocation and accumulation in the shoot in control and AtCAX2 and AtCAX4 tobacco plants exposed to low Cd concentration. The results are consistent with a model in which supplementation of Cd/H antiporter activity in root cell tonoplasts enhances root Cd sequestration, resulting in decreased translocation of Cd to the shoot of field-grown plants. These results suggest that human Cd intake from food and tobacco use could be reduced via the enhancement of root vacuolar sequestration of this pollutant.