Elucidating the Role of Rice Thioredoxin-m in Mitigating Copper Toxicity

The pervasive accumulation of elevated heavy metal concentrations in the environment exerts significant selective pressures, driving the progressive evolution of a comprehensive suite of adaptive strategies. These include both physiological tolerance and biochemical detoxification mechanisms. Our research identified a novel role of a rice-specific m-type thioredoxin (OsTrx-m) in conferring tolerance specifically to copper-induced heavy metal stress. Upon exposure to CuSO 4 , recombinant OsTrx-m underwent a rapid functional transformation from a disulfide reductase to a holdase chaperone, accompanied by a concurrent alteration in protein conformation from a monomeric form to oligomeric complexes. This transition was highly selective for Cu 2+ and did not occur with other metal cations, such as Fe 2+ , Zn 2+ , Cd 2+ , and Ni 2+ . The Cu 2+ -induced structural and functional transitions of OsTrx-m were reversibly mitigated by adding EDTA, a metal chelator. Comparative analysis of OsTrx-m with Arabidopsis m-type thioredoxins (AtTrx-m1, -m2, and -m3) revealed that this Cu 2+ -responsive property is a distinctive characteristic of OsTrx-m. Consequently, ectopic expression of OsTrx-m , but not AtTrx-m1 , in Arabidopsis ( OsTrx-m OE /Col-0 and AtTrx-m1 OE /Col-0, respectively) conferred robust tolerance to copper-induced toxicity, as evidenced by enhanced root growth and fresh weight. • Rice OsTrx-m confers rice tolerance to copper-induced heavy metal stress. • Cu 2+ exposure induces a structural transition of OsTrx-m from a disulfide reductase to a holdase chaperone. • Unique Cu 2+ -responsive OsTrx-m enhances copper tolerance and growth when expressed in Arabidopsis.