Effect of ammonium on the regulation of nitrate and nitrite transport systems in roots of intact barley (Hordeum vulgare L.) seedlings

Abstract

The effect of NH ⁺₄ on the regulation of NO ⁻₃ and NO ⁻₂ transport systems in roots of intact barley (Hordeum vulgareL.) seedlings grown in NO ⁻₃ or NO ⁻₂ was studied. Ammonium partially inhibited “induction” of both transport systems. The inhibition was less severe in NO ⁻₂ -fed than in NO ⁻₃ -fed seedlings, presumably due to lower uptake of NH ⁺₄ in the presence of NO ⁻₂ . In seedlings pretreated with NH ⁺₄ subsequent “induction” was inhibited only when NH ⁺₄ was also present during “induction”, even though pretreated roots accumulated high levels of NH ⁺₄ . This indicates that inhibition may be regulated by NH ⁺₄ concentration in the cytoplasm rather than its total accumulation in roots. L-Methionine sulfoximine did not relieve the inhibition by NH ⁺₄ , suggesting that inhibition is caused by NH ⁺₄ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO ⁻₃ transport activity similarly in roots grown in 0.1, 1.0, or 10 mM NO ⁻₃ for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. “Induction” of the NO ⁻₃ transport system was about twice as sensitive to NH ⁺₄ as compared to the NO ⁻₂ transport system. This may relate to higher turnover rates of membraneassociated NO ⁻₃ -transport proteins.