Genome-wide exploration of MTP gene family in mustard (Brassica juncea L.): evolution and expression patterns during heavy metal stress

Abstract

Members of the Metal Tolerance Protein (MTP) family are critical in mediating the transport and tolerance of divalent metal cations. Despite their significance, little is known about the MTP genes in mustard (Brassica juncea), particularly in relation to how they react to HM stress. In our study, we identified MTP gene sets in Brassica rapa (17 genes), Brassica nigra (18 genes), and B. juncea (33 genes) using the HMMER tool (Cation_efflux; PF01545) and BLAST analysis. Then, for the 33 BjMTPs, we carried out a detailed bioinformatics analysis covering the physicochemical properties, phylogenetic relationships, conserved motifs, protein structures, collinearity, spatiotemporal RNA-seq expression, GO enrichment, and expression profiling under six HM stresses (Mn²⁺, Fe²⁺, Zn²⁺, Cd²⁺, Sb³⁺, and Pb²⁺). According to the findings of physicochemical characteristics and phylogenetic tree, the allopolyploid B. juncea’s MTP genes were inherited from its progenitors, B. rapa and B. nigra, with minimal gene loss during polyploidization. The BjMTP gene family exhibited conserved motifs, promoter elements, and expression patterns that aligned with seven evolutionary branches (G1, G4-G9, and G12). Further, by co-expression analysis, the core and gene-specific expression modules of BjMTPs under six HM stresses were found. The HM treatments exhibited consistently upregulated of BjA04.MTP4, BjA09.MTP10, and BjB01.MTP5 genes, indicating their critical roles in enhancing HM tolerance in B. juncea. These discoveries may contribute to a genetic improvement in B. juncea's HM tolerance, which would facilitate the remediation of HM-contaminated areas.