Elevated Expression of Vacuolar Nickel Transporter Gene IREG2 Is Associated With Reduced Root-to-Shoot Nickel Translocation in Noccaea japonica

A number of metal hyperaccumulator plants, including nickel (Ni) hyperaccumulators, have been identified in the genusNoccaea. The ability to accumulate Ni in shoots varies widely among species and ecotypes in this genus; however, little is known about the molecular mechanisms underlying this intra- and inter-specific variation. Here, in hydroponic culture, we compared Ni accumulation patterns betweenNoccaea japonica, which originated in Ni-enriched serpentine soils in Mt. Yubari (Hokkaido, Japan), andNoccaea caerulescensecotype Ganges, which originated in zinc/lead-mine soils in Southern France. BothNoccaeaspecies showed extremely high Ni tolerance compared with that of the non-accumulatorArabidopsis thaliana. But, following treatment with 200 mu M Ni,N. caerulescensshowed leaf chlorosis, whereasN. japonicadid not show any stress symptoms. Shoot Ni concentration was higher inN. caerulescensthan inN. japonica; this difference was due to higher efficiency of root-to-shoot Ni translocation inN. caerulescensthanN. japonica. It is known that the vacuole Ni transporter IREG2 suppresses Ni translocation from roots to shoots by sequestering Ni in the root vacuoles. The expression level of theIREG2gene in the roots ofN. japonicawas 10-fold that in the roots ofN. caerulescens. Moreover, the copy number ofIREG2per genome was higher inN. japonicathan inN. caerulescens, suggesting thatIREG2expression is elevated by gene multiplication inN. japonica. The heterologous expression ofIREG2ofN. japonicaandN. caerulescensin yeast andA. thalianaconfirmed that bothIREG2genes encode functional vacuole Ni transporters. Taking these results together, we hypothesize that the elevation ofIREG2expression by gene multiplication causes the lower root-to-shoot Ni translocation inN. japonica.