Enhanced Cd2+-selective root-tonoplast-transport in tobaccos expressing Arabidopsis cation exchangers

Several Arabidopsis CAtion eXchangers (CAXs) encode tonoplast-localized transporters that appear to be major contributors to vacuolar accumulation/sequestration of cadmium (Cd2+), an undesirable pollutant ion that occurs in man largely as a result of dietary consumption of aerial tissues of food plants. But, ion-selectivity of individual CAX transporter types remains largely unknown. Here, we transformed Nicotiana tabacum with several CAX genes driven by the Cauliflower Mosaic Virus ( CaMV) 35S promoter and monitored divalent cation transport in root-tonoplast vesicles from these plants in order to select particular CAX genes directing high Cd2+ antiporter activity in root tonoplast. Comparison of seven different CAX genes indicated that all transported Cd2+, Ca2+, Zn2+, and Mn2+ to varying degrees, but that CAX4 and CAX2 had high Cd2+ transport and selectivity in tonoplast vesicles. CAX4 driven by the CaMV 35S and FS3 [figwort mosaic virus (FMV)] promoters increased the magnitude and initial rate of Cd2+/H+ exchange in root-tonoplast vesicles. Ion selectivity of transport in roottonoplast vesicles isolated from FS3:: CAX4-expressing plant lines having a range of gene expression was Cd2+> Zn2+ >> Ca2+ >> Mn2+ and the ratios of maximal Cd2+ (and Zn2+) versus maximal Ca2+ and Mn2+ transport were correlated with the levels of CAX4 expression. Root Cd accumulation in high CAX4 and CAX2 expressing lines was increased in seedlings grown with 0.02 mu M Cd. These observations are consistent with a model in which expression of an Arabidopsis-gene-encoded, Cd2+-efficient antiporter in host plant roots results in greater root vacuole Cd2+ transport activity, increased root Cd accumulation, and a shift in overall root tonoplast ion transport selectivity towards higher Cd2+ selectivity. Results support a model in which certain CAX antiporters are somewhat more selective for particular divalent cations.