Inhibition of ammonium assimilation restores elongation of seminal rice roots repressed by high levels of exogenous ammonium

Abstract

Elongation of seminal and lateral roots of rice seedlings was markedly inhibited by high ammonium levels in growth medium. However, high exogenous nitrate concentrations had little inhibitory effect on root growth. The objective of this study was to elucidate the relationship between inhibition of rice root growth induced by high ammonium conditions and ammonium assimilation in the seedlings. Activity of glutamine synthetase (GS) was kept at a low level in the seminal roots of the seedlings grown under high nitrate levels. In contrast, high ammonium levels significantly enhanced the GS activity in the roots, so that Gln abundantly accumulated in the shoots. These results indicate that ammonium assimilation may be activated in the seminal roots under high ammonium conditions. Application of methionine sulfoximine (MSO), an inhibitor of GS, relieved the repression of the seminal root elongation induced by high ammonium concentrations. However, the elongation of lateral roots remained inhibited even under the same condition. Furthermore, MSO drastically increased ammonium level and remarkably decreased Gln level in the shoots grown under high ammonium conditions. These results show that, for rice seedlings, an assimilatory product of ammonium, and not ammonium itself, may serve as an endogenous indicator of the nitrogen status involved in the inhibition of seminal root elongation induced by high levels of exogenous ammonium.

γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings

“…). The root growth was inhibited by 1.5 m M NH 4 + , which is similar to the observation of Hirano et al () using the same culture system (gel).Ammonium toxicity threshold may depend on the culture systems, because at the same concentration in the different medium, the ammonium availability is not the same for the plant. Root architecture plays an essential role in the efficiency of nutrient and water acquisition from soil (Lynch ).…”

“…Previous studies reported that the seminal root elongation in rice was inhibited with increasing ammonium concentration in culture medium (Hirano et al ). Similarly, our data showed that excess NH 4 + significantly suppressed rice seedling growth, particularly in roots (Fig.…”

A mutation in GDP-mannose pyrophosphorylase causes conditional hypersensitivity to ammonium, resulting in Arabidopsis root growth inhibition, altered ammonium metabolism, and hormone homeostasis

“…GS activity is inhibited by methionine sulphoximine (MSO), resulting in elevated NH 4 + and reduced Gln levels (Lee et al , 1992; King et al , 1993; Hirano et al , 2008). This inhibitor was used to test whether NH 4 + metabolism is altered in vtc1-1 compared to the wild type when plants were grown in the presence or absence of NH 4 + .…”

“…Total glutamine synthetase activity was determined following a spectrophotometric assay (Mäck, 1995; Hirano et al , 2008). In brief, 30–60 mg tissue was extracted using 500 μl of an imidazole buffer consisting of 50 mM imidazole-HCl, 0.5 mM EDTA, 1 mM DTT, 10 mM MgSO 4 , and 1 mM phenylmethanesulphonyl fluoride.…”

Isolation and characterization of a novel ammonium overly sensitive mutant, amos2, in Arabidopsis thaliana

“…metabolism, when inhibited, has been shown to result in elevated NH 4 ? levels and higher toxicity (Lee et al 1992;Kronzucker et al 1995;Hirano et al 2008). Indeed, the general view has emerged that plants with higher GS activity are more tolerant to NH 4 ?…”