The N-terminal degenerated metal-binding domain is involved in the heavy metal transport activity of TaHMA2

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

The metal-binding domain of wheat heavy metal ATPase 2 (TaHMA2) is involved in zinc/cadmium tolerance and translocation in Arabidopsis

Cysteine in the N-terminal metal-binding domain (N-MBD) of TaHMA2 participates in Zn²⁺/Cd²⁺ binding and translocation in Arabidopsis. Wheat heavy metal ATPase 2 (TaHMA2) can transport Zn²⁺ and Cd²⁺ across membranes. A previous study showed that cysteine (Cys) and glutamate residues in the N-terminal metal-binding domain (N-MBD) were necessary for metal-binding and translocation of TaHMA2 in yeast. However, the function of TaHMA2 in plants was not fully revealed. In this study, we investigated the roles of the CCxxE and CPC motifs in the N-MBD and the N/C-terminal regions of TaHMA2 in Zn²⁺/Cd²⁺ translocation in root and shoot of Arabidopsis. Compared with the wild type, overexpression of TaHMA2 and the TaHMA2 derivative (glutamic substituted for alanine from CCxxE) in Arabidopsis increased root length, fresh weight and enhanced Zn²⁺/Cd²⁺ root-to-shoot translocation. The plants with a truncated N/C-terminal of TaHMA2 were impaired in Zn²⁺/Cd²⁺ tolerance and translocation, while mutagenesis of Cys in the N-MBD reduced the tolerance and transport activity of TaHMA2, suggesting the involvement of Cys in Zn²⁺/Cd²⁺ binding and translocation in Arabidopsis. This study therefore provides a theoretical possibility for the application of TaHMA2 in transgenic breeding to regulate metal element balance in crop plants.

A Functional Study Identifying Critical Residues Involving Metal Transport Activity and Selectivity in Natural Resistance-Associated Macrophage Protein 3 in Arabidopsis thaliana

Arabidopsis thaliana natural resistance-associated macrophage protein 3 (AtNRAMP3) is involved in the transport of cadmium (Cd), iron (Fe), and manganese (Mn). Here, we present a structure-function analysis of AtNRAMP3 based on site-directed mutagenesis and metal toxicity growth assays involving yeast mutants, combined with three-dimensional (3D) structure modeling based on the crystal structure of the Eremococcus coleocola NRAMP family transporter, EcoDMT. We demonstrated that two conservative sites, D72 and N75, are essential for the transport activity. The M248A mutation resulted in a decrease in Cd sensitivity, while maintaining Mn transport. The mutation involving G61 caused a significant impairment of Fe and Mn transport, thereby indicating the importance of the conserved residue for proper protein function. The mutation involving G171 disrupted Fe transport activity but not that of Mn and Cd, suggesting that G171 is essential to metal binding and selectivity. Two residues, E194 and R262, may play an important role in stabilizing outward-facing conformation, which is essential for transport activity. Deletion assays indicated that the N-terminus is necessary for the function of AtNRAMP3. The findings of the present study revealed the structure-function relationship of AtNRAMP3 and metal transport activity and selectivity, which may possibly be applied to other plant NRAMP proteins.

Isolation and characterization of a novel cadmium-regulated Yellow Stripe-Like transporter (SnYSL3) in Solanum nigrum

SnYSL3 encodes a plasma-localized transporter delivering various metal-nicotianamine complexes. The expression of SnYSL3 is up-regulated by excess Cd, suggesting an important role for SnYSL3 in response to Cd stress. The Yellow Stripe-Like (YSL) transporters have been proposed to participate in metal uptake and long-range transport in model plants. In this study, we isolated and characterized a novel member of the YSL gene family, SnYSL3, from the cadmium hyperaccumulator Solanum nigrum. SnYSL3 was constitutively expressed and encodes a plasma membrane-localized protein. In situ RNA hybridization localized the SnYSL3 transcripts predominantly in vascular tissues and epidermal cells of the roots and stems, while in leaves, the mRNA levels were high in the vasculature. The SnYSL3 expression level was up-regulated by excess Cd, excess Fe and Cu deficiency. Heterologous expression of SnYSL3 in yeast revealed that SnYSL3 transports nicotianamine complexes containing Fe(II), Cu, Zn and Cd. SnYSL3 overexpression in Arabidopsis thaliana decreased Fe and Mn concentrations in the roots and increased the root-to-shoot translocation ratios of Fe and Mn. Under Cd exposure, the transgenic plants showed increased translocation ratios of Fe and Cd, but no difference was observed in Mn translocation from roots to shoots between the transgenic and wild-type lines. Although the accurate function of SnYSL3 remains to be confirmed, these results suggest that SnYSL3 is a transporter delivering a broad range of metal-nicotianamine complexes and is potentially important for the response to heavy metal stress, especially due to Cd and Fe.