The Mo-hydroxylases xanthine dehydrogenase and aldehyde oxidase in ryegrass as affected by nitrogen and salinity

The influence of salinity and nitrogen source on xanthine dehydrogenase (XDH; EC 1.2.1.37) and aldehyde oxidase (AO; EC 1.2.3.1) was studied in annual ryegrass (Lolium multiflorum cv. Westerwoldicum). The activities of AO and XDH in the roots and shoots of ryegrass plants increased with salinity and NH₄/⁺ concentration. The salinity-enhanced activities of XDH and AO were more pronounced in the roots than in the shoots. Roots of NH₄/⁺-grown plants had higher AO and XDH activities than plants grown in NO₃/^-. Immunoblotting revealed a higher level of AO protein in roots than in shoots. Root AO protein increased with salinity and was the highest in roots of NH₄/⁺-grown plants. The assays of the molybdenum cofactor (MoCo) hydroxylases (XDH and AO) showed a similar response to salinity and nitrogen, and differed in molecular weight and substrate specificity. The concentration of ureides (allantoic acid and allantoin) increased with salinity and NH₄⁺, especially in the roots. The ureide contents of plants grown on NH₄/⁺ were higher than in plants receiving NO₃^-. The increase in Mo-hydroxylases with salinity and NH₄/⁺ may constitute part of the mechanisms of plant adaptation to stress by (1) enhancing the activity of AO, which catalyzes the final step in biosynthesis of phytohormones such as abscisic acid (ABA), and (2) increased XDH activity and the subsequent production of ureides allowing transport of organic nitrogen compounds with a low C/N ratio.