NITRATE REDUCTION, NITRITE REDUCTION AND AMMONIUM ASSIMILATION IN BARLEY ROOTS IN RESPONSE TO ANOXIA

摘要

Nitrate reduction, nitrate reductase (NR)-activity and activation state, nitrite reduction and ammonium assimilation were measured in barley roots (Hordeum vulgare cv. Gaulois) in response to anoxia, uncouplers or cellular acidification. Ammonium and nitrite accumulation were followed in root segments and in the suspending buffer solution hushed with air or nitrogen, in the presence of methionine sulfoximine (MSX). Catabolic ammonium formation was subtracted from nitrate-dependent ammonium formation for a correct estimation of nitrite reduction rates in vivo. In anoxic roots, nitrite was accumulated and partly released. This was due to both, a stimulation of nitrate reduction and a partial inhibition of nitrite reduction. Ammonium assimilation rates were also impaired by anoxia. Due to nitrite and ammonium leakage, anoxic roots lost a major proportion of their primary nitrate assimilation products. In aerobic roots, excess nitrite production could be also induced by treatment with uncoupler (CCCP) or by tissue acidification (propionic acid at pH 4.8). The same conditions caused an activation of (NADH)-nitrate reductase (NR, EC 1.6.6.1), a partial inhibition of nitrite reduction and of ammonium assimilation. Increased nitrite production and activation of NR were prevented by cantharidin, a protein phosphatase inhibitor. Our data indicate that nitrite release by anoxic roots was a consequence of both, an activation of NR by protein dephosphorylation and an inhibition of nitrite reduction. Both events may be causally related to cellular acidification under anoxia. [References: 22]