Genome-wide identification of NRAMP gene family in Brassica U’s triangle species and its response to Cadmium stress in B. napus

BACKGROUND: Soil cadmium contamination is a persistent environmental and agricultural problem worldwide, largely because it stays in the soil and can easily enter the food chain. In plants, the Natural Resistance-Associated Macrophage Protein (NRAMP) gene family plays key role in cadmium uptake, transport, and homeostasis. However, systematic investigation of NRAMP genes across all six Brassica U's triangle species has been limited, even though these plants produce high biomass, have well-established genetic resources, and are considered promising candidates for phytoremediation. RESULTS: This study presents a genome-wide analysis of NRAMP genes across the six Brassica U's triangle species, identifying 81 members in total (87 when including six Arabidopsis thaliana NRAMPs as an outgroup). Phylogenetic classification devided these genes into two distinct subfamilies: Subfamily I (NRAMP1/6) and Subfamily II (NRAMP2-5), which differ clearly in gene structure and conserved protein motifs. An evolutionary analysis revealed that the NRAMP family expanded mainly through whole-genome duplications followed by segmental duplications, with some gene loss later during diploidization. Nearly all duplicated and orthologous gene pairs had Ka/Ks ratios below 1, indicating functional conservation of these metal transporters. Expression profiling in Brassica napus under cadmium stress demonstrated clear tissue-specific functional specialization. Correlation analyses identified key genotype-phenotype relationships: BnNRAMP2c and BnNRAMP2d showed a strong positive correlation with root cadmium content, indicating potential roles in metal uptake or sequestration; BnNRAMP1c/1e/1f expression correlated with leaf cadmium levels, suggesting their involvement in shoot translocation; whereas BnNRAMP6a correlated negatively with root cadmium, hinting a possible exclusion mechanism. CONCLUSIONS: This study provides basic understanding of the evolutionary history and functional diversification of NRAMP transporters in Brassica species. We reveal a division of labor among specific members. BnNRAMP2d was identified as a candidate gene that may play an important role in root cadmium accumulation. The BnNRAMP1s cluster may be involved in shoot distribution, and BnNRAMP6a may contribute to cadmium exclusion. The identified genes serve as potential targets for developing rapeseed cultivars with reduced cadmium content to enhance food safety, or alternatively, for optimizing cadmium-efficient phytoremediation to alleviate environmental contamination.