Nicotianamine (NA) is among the most studied plant metal chelators. A large body of evidence supports its crucial role for Fe distribution in plants and as a precursor of phytosiderophore synthesis in grasses. NA forms stable complexes in vitro not only with Fe(II) and Fe(III) but also with various other divalent metal cations including Zn(II). Early observations indicated a possible contribution of NA to Zn trafficking in plants. Numerous studies on transgenic monocot and dicot plants with modulated NA levels have since then reported Zn accumulation phenotypes. NAS genes were shown to represent promising targets for biofortification efforts. For instance, NA was found to bind Zn in rice grains in a form bioavailable for humans. Recently, additional strong support for the existence of Zn-NA complexes in planta has been obtained in rice, Arabidopsis thaliana and the Zn hyperaccumulating plant A. halleri. We review the evidence for a role of NA in the intercellular and long-distance transport of Zn in plants and discuss open questions.

It is a plausible hypothesis that parallel adaptation events to the same environmental challenge should result in genetic changes of similar or identical effects, depending on the underlying fitness landscapes. However, systematic testing of this is scarce. Here we examine this hypothesis in two closely related plant species, Arabidopsis halleri and Arabidopsis arenosa , which co-occur at two calamine metalliferous (M) sites harbouring toxic levels of the heavy metals zinc and cadmium. We conduct individual genome resequencing alongside soil elemental analysis for 64 plants from eight populations on M and non-metalliferous (NM) soils, and identify genomic footprints of selection and local adaptation. Selective sweep and environmental association analyses indicate a modest degree of gene as well as functional network convergence, whereby the proximal molecular factors mediating this convergence mostly differ between site pairs and species. Notably, we observe repeated selection on identical single nucleotide polymorphisms in several A. halleri genes at two independently colonized M sites. Our data suggest that species-specific metal handling and other biological features could explain a low degree of convergence between species. The parallel establishment of plant populations on calamine M soils involves convergent evolution, which will probably be more pervasive across sites purposely chosen for maximal similarity in soil composition. This article is part of the theme issue ‘Convergent evolution in the genomics era: new insights and directions’.

Plants have the ability to colonize highly diverse environments. The zinc and cadmium hyperaccumulator Arabidopsis halleri has adapted to establish populations on soils covering an extreme range of metal availabilities. The A. halleri ZIP6 gene presents several hallmarks of hyperaccumulation candidate genes: it is constitutively highly expressed in roots and shoots and is associated with a zinc accumulation quantitative trait locus. Here, we show that AhZIP6 is duplicated in the A. halleri genome. The two copies are expressed mainly in the vasculature in both A. halleri and Arabidopsis thaliana, indicative of conserved cis regulation, and acquired partial organ specialization. Yeast complementation assays determined that AhZIP6 is a zinc and cadmium transporter. AhZIP6 silencing in A. halleri or expression in A. thaliana alters cadmium tolerance, but has no impact on zinc and cadmium accumulation. AhZIP6silenced plants display reduced cadmium uptake upon short-term exposure, adding AhZIP6 to the limited number of Cd transporters supported by in planta evidence. Altogether, our data suggest that AhZIP6 is key to fine-tune metal homeostasis in specific cell types. This study additionally highlights the distinct fates of duplicated genes in A. halleri.

The molecular analysis of metal hyperaccumulation in species such as Arabidopsis halleri offers the chance to gain insights into metal homeostasis and into the evolution of adaptation to extreme habitats. A prerequisite of metal hyperaccumulation is metal hypertolerance. Genetic analysis of a backcross population derived from Arabidopsis lyrata × A. halleri crosses revealed three quantitative trait loci for Cd hypertolerance. A candidate gene for Cdtol2 is AhCAX1, encoding a vacuolar Ca /H antiporter. We developed a method for the transformation of vegetatively propagated A. halleri plants and generated AhCAX1-silenced lines. Upon Cd exposure, several-fold higher accumulation of reactive oxygen species (ROS) was detectable in roots of AhCAX1-silenced plants. In accordance with the dependence of Cdtol2 on external Ca concentration, this phenotype was exclusively observed in low Ca conditions. The effects of external Ca on Cd accumulation cannot explain the phenotype as they were not influenced by the genotype. Our data strongly support the hypothesis that higher expression of CAX1 in A. halleri relative to other Arabidopsis species represents a Cd hypertolerance factor. We propose a function of AhCAX1 in preventing a positive feedback loop of Cd-elicited ROS production triggering further Ca -dependent ROS accumulation.

Clarifying the connection between metal exposure and anatomical changes represents an important challenge for a better understanding of plant phytoextraction potential. A hydroponic screening experiment was carried out to evaluate the effects of combined interactions of Cd and Zn on mineral uptake (Mg, K, Ca, Na) and on the physiological and anatomical characteristics of Brassica napus L cv. Cadeli, Viking, and Navajo. Plants were exposed to 5 μM Cd (CdCl), 10 μM Zn (ZnSO), or both Cd + Zn, for 14 days. Cadmium exposure led to a significant reduction in root growth, shoot biomass, and chlorophyll content. After Cd-only and Cd + Zn treatment, primary root tips became thicker and pericycle cells were enlarged compared to the control and Zn-only treatment. No differences between metals were observed under UV excitation, where all treatments showed more intensive autofluorescence connected with lignin/suberin accumulation compared to control conditions. The highest concentrations of Cd and Zn were found in the roots of all tested plants, and translocation factors did not exceed the threshold of 1.0. The root mineral composition was not affected by any treatment. In the shoots, the Mg concentration slightly increased after Cd-only and Cd + Zn treatments, whereas Zn-only treatment caused a sharp decrease in Ca content. Slight increases in K were seen after the addition of Zn. Significantly higher concentrations of Na were induced by Cd- or Zn-only treatment.