Functional Characterization of Maize ZmMTP1-1 and ZmMTP1-2 Reveals Their Roles in Cd Tolerance

Cadmium (Cd) contamination severely threatens crop productivity and food safety, particularly in maize (Zea mays L.), which exhibits relatively high capacities for metal uptake and translocation. Metal tolerance proteins (MTPs) play essential roles in metal homeostasis and detoxification; however, the functions of maize MTP under Cd stress remain poorly understood. In this study, a comprehensive expression analysis of the maize MTP gene family revealed that two Zn-CDF members, ZmMTP1-1 and ZmMTP1-2, displayed the strongest and most consistent transcriptional induction in response to Cd stress, especially in roots. Phylogenetic and structural analyses confirmed that both genes are closely related to MTP1 homologs from other plant species, while exhibiting distinct gene structures and regulatory features. Functional characterization in transgenic Arabidopsis thaliana demonstrated that overexpression of ZmMTP1-1 or ZmMTP1-2 significantly enhanced tolerance to Cd and Zn stress, as reflected by improved seed germination, root growth, survival, and biomass accumulation. Enhanced metal tolerance was associated with elevated antioxidant enzyme activities, reduced oxidative damage, and coordinated upregulation of endogenous metal transporter genes. Moreover, heterologous expression of ZmMTP1-1 in yeast further supported its conserved role in Cd tolerance. Collectively, these findings indicate that ZmMTP1-1 and ZmMTP1-2 contribute to Cd detoxification through coordinated metal transport and stress-response pathways, providing potential genetic resources for improving heavy metal tolerance in maize.