Heterologous expression and biochemical characterization of an NAD(P)H:quinone oxidoreductase from the hemiparasitic plant Triphysaria versicolor

Quinones are widespread secondary metabolites that function as signal molecules between organisms in the rhizosphere. Quinones are particularly important in the exchange of chemical signals between plant roots, a phenomenon classically termed allelopathy. The bioactivity of quinones is due in large part to radical intermediates formed during redox cycling between quinone and hydroquinone states. In order to investigate the role of quinone oxidoreductases in processing quinone signals exchanged between plant roots, we characterized an NAD(P)H-dependent quinone reductase expressed in roots of the parasitic plant Triphysaria versicolor (TvQR2). The predicted amino acid sequence encoded by TvQR2 shares homology with quinone reductases from Archaea, Eubacteria and Eukaryota organisms. The complete TvQR2 cDNA was cloned into the fungus Pichia pastoris and the heterologous protein purified. The recombinant protein reduced a variety of quinones and napthoquinones, including several of allelopathic significance, using either NADH or NADPH as electron donors. The protein had an absorption spectrum consistent with it being a flavoprotein and was inhibited by the quinone reductase inhibitor dicumarol. We propose that the TvQR2 protein functions as a quinone reductase in plant roots to mitigate the toxicity of exogenous quinones in the rhizosphere.