Nickel (Ni) is a necessary trace element for plants, but excessive amounts can be harmful. To mitigate Ni toxicity, plants have evolved distinct mechanisms, such as storing Ni in the vacuole or confining it to less sensitive root tissues to prevent its translocation to the vulnerable shoot tissues. Despite this, the exact mechanism of Ni immobilization remains unclear. In Arabidopsis, sequestration of excess Ni into root vacuoles is crucial for Ni immobilization, facilitated by distinct tonoplast-localized transporters. As some members of the aquaporin superfamily have been implicated in transporting both metal ions and polar, non-charged small molecules, we explored whether Arabidopsis thaliana tonoplast intrinsic proteins (TIPs) are involved in Ni immobilization and tolerance. We found that TIP3;2 helps retain excess Ni in the root, restricts its translocation to the shoot, and facilitates its accumulation in the leaf trichome. Furthermore, when TIP3;2 was expressed in yeast, it enhanced Ni resistance, suggesting that TIP3;2 plays a vital role in Ni detoxification. In addition, partial expression of TIP3;2 at the yeast plasma membrane demonstrated its capability to facilitate the uptake of Ni-PC complexes into yeast cells. The results reveal a dual function for TIP3;2 in Ni detoxification and underscore the expanded substrate specificity of aquaporins to include heavy-metal complexes, uncovering a new aspect of plant adaptive responses to metal stress.
Feng et al. (Tue,) studied this question.