AUTHOR=Nishida Sho , Tanikawa Ryoji , Ishida Shota , Yoshida Junko , Mizuno Takafumi , Nakanishi Hiromi , Furuta Naoki 

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

JOURNAL=Frontiers in Plant Science

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00610

DOI=10.3389/fpls.2020.00610

ISSN=1664-462X

ABSTRACT=<p>A number of metal hyperaccumulator plants, including nickel (Ni) hyperaccumulators, have been identified in the genus <italic>Noccaea</italic>. 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 between <italic>Noccaea japonica</italic>, which originated in Ni-enriched serpentine soils in Mt. Yubari (Hokkaido, Japan), and <italic>Noccaea caerulescens</italic> ecotype Ganges, which originated in zinc/lead-mine soils in Southern France. Both <italic>Noccaea</italic> species showed extremely high Ni tolerance compared with that of the non-accumulator <italic>Arabidopsis thaliana</italic>. But, following treatment with 200 μM Ni, <italic>N. caerulescens</italic> showed leaf chlorosis, whereas <italic>N. japonica</italic> did not show any stress symptoms. Shoot Ni concentration was higher in <italic>N. caerulescens</italic> than in <italic>N. japonica</italic>; this difference was due to higher efficiency of root-to-shoot Ni translocation in <italic>N. caerulescens</italic> than <italic>N. japonica</italic>. 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 the <italic>IREG2</italic> gene in the roots of <italic>N. japonica</italic> was 10-fold that in the roots of <italic>N. caerulescens</italic>. Moreover, the copy number of <italic>IREG2</italic> per genome was higher in <italic>N. japonica</italic> than in <italic>N. caerulescens</italic>, suggesting that <italic>IREG2</italic> expression is elevated by gene multiplication in <italic>N. japonica</italic>. The heterologous expression of <italic>IREG2</italic> of <italic>N. japonica</italic> and <italic>N. caerulescens</italic> in yeast and <italic>A. thaliana</italic> confirmed that both <italic>IREG2</italic> genes encode functional vacuole Ni transporters. Taking these results together, we hypothesize that the elevation of <italic>IREG2</italic> expression by gene multiplication causes the lower root-to-shoot Ni translocation in <italic>N. japonica</italic>.</p>