Nicotianamine facilitates zinc translocation from roots to shoots in the nickel hyperaccumulator Odontarrhena chalcidica

Background and aims

Nickel (Ni) hyperaccumulator Odontarrhena chalcidica can absorb high levels of zinc (Zn) in its roots but fails to hyperaccumulate Zn in its shoots. The reasons behind the absence of this Zn hyperaccumulation trait in O. chalcidica, in contrast to known Zn hyperaccumulators, remain elusive. Nicotianamine (NA) is an organic ligand which can increase the mobility of metals in vivo by forming stable metals-NA complex. Thus, this study evaluated the influence of NA on root-shoot transport of Zn in O. chalcidica by comparison with the Zn hyperaccumulator Noccaea caerulescens.

Methods

Both species were cultivated under + Ni and + Zn treatments in hydroponic solutions. NA concentrations, the expression levels of NA synthesis related genes and Zn distribution in subcellular fractions of roots were evaluated. Additionally, the effect of exogenous NA supply on Zn transport was explored.

Results

NA concentrations in the roots of O. chalcidica declined from 2.30 to 0.600 mg kg⁻¹ under Zn exposure, whereas that of N. caerulescens significantly increased by 40.5% (to 3.09 mg kg⁻¹). Zn treatment suppressed the expression of OcNAS3 and OcSAMS2, which involved in NA biosynthesis, by 97.0% and 89.7%, respectively. Nevertheless, pretreatment with NA enhance soluble Zn fraction in roots, and increased root-shoot transport of Zn in O. chalcidica, raising the shoot-to-root Zn concentration ratio from 0.548 to 0.919.

Conclusions

The inhibition of NA synthesis by Zn is an important reason for the impaired root-shoot transfer for Zn in O. chalcidica. And NA plays a key role on the Zn mobility within plants.