Genome-wide characterization of the NRAMP gene family in sugar beet and functional analysis of BvNRAMP6 under cadmium stress

Abstract

Natural resistance-associated macrophage proteins (NRAMPs) are multifunctional metal transporters involved in plant biological responses to cadmium (Cd). Sugar beet ( Beta vulgaris L.) is a promising energy crop with potential for heavy metal remediation, making it essential to understand the function of BvNRAMPs in Cd regulation. In this study, four BvNRAMP family members ( BvNRAMP2 , BvNRAMP3 , BvNRAMP6 and BvNRAMP7 ) were identified through genome-wide analysis and phylogenetic methods. They are distributed across different chromosomes and share common motifs (1–8). Cis-element analysis revealed that BvNRAMPs contain 6 ( BvNRAMP2 ) to 32 ( BvNRAMP6 ) elements responsive to various biotic and abiotic stresses and hormones such as ABRE. Subcellular localization studies demonstrated that all four BvNRAMPs are localized in the plasma membrane. Transcriptomic data and qRT-PCR analyses suggested that BvNRAMP3, BvNRAMP6 , and BvNRAMP7 were significantly upregulated in both leaves and roots following 0.5 mmol/L CdCl 2 treatment ( p  < 0.05), whereas BvNRAMP2 was downregulated in leaves but upregulated in roots. Notably, BvNRAMP6 expression levels in leaves and roots were 14.9- and 17.5-fold higher, respectively, after 12 h of Cd exposure compared to the untreated control. Yeast heterologous expression assays confirmed that BvNRAMP6 and BvNRAMP7 enhanced Cd content and tolerance in Cd-sensitive ∆ycf1 yeast mutant. Furthermore, transgenic Arabidopsis overexpressing BvNRAMP6 (OE-1 and OE-2) exhibited improved growth under 10, 20, and 40 µmol/L CdCl 2 , evidenced by greater root length, fresh weight, and dry weight, as well as over 70% higher Cd accumulation compared to wild-type plants; the transgenic lines also exhibited increased SOD and POD activities and reduced MDA levels, indicating enhanced ROS scavenging, reduced Cd toxicity, and improved Cd tolerance. This study provides insights into the functional roles of BvNRAMPs , particularly BvNRAMP6 , in Cd accumulation and tolerance, providing a molecular basis for applying sugar beet to the phytoremediation of Cd-contaminated soils.