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 CuSO₄, 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 Cu²⁺ and did not occur with other metal cations, such as Fe²⁺, Zn²⁺, Cd²⁺, and Ni²⁺. The Cu²⁺-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 Cu²⁺-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.