Key enzyme involved in purine catabolism. Catalyzes the oxidation of hypoxanthine to xanthine and the oxidation of xanthine to urate. Regulates the level of ureides and plays an important role during plant growth and development, senescence and response to stresses. Possesses NADH oxidase activity and may contribute to the generation of superoxide anions in planta. H2O + NAD(+) + xanthine = H(+) + NADH + urate H2O + hypoxanthine + NAD(+) = H(+) + NADH + xanthine Binds 2 [2Fe-2S] clusters. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. Homodimer. Expressed in roots, leaves, stems, flowers and siliques. By salt and drought stresses, and abscisic (ABA) treatment. Down-regulated by cold and freezing stresses. Plants silencing simultaneously XDH1 and XDH2 show reduced growth, impaired silique development, increased seed sterility, precocious senescence of mature leaves and overaccumulation of xanthine. Belongs to the xanthine dehydrogenase family.