The specificity of methionine sulfoximine and azaserine inhibition in plant tissues

Abstract

According to the current paradigm, an inhibition of nitrate and ammonium assimilation by methionine sulfoximine (MSX) and azaserine indicates the importance of glutamine synthetase (GS) and glutamate synthase (GOGAT) in the assimilation of ammonia and the production of glutamate. Inhibitors of GS and GOGAT also result in altered levels of asparagine and glutamine. These amides have been shown to inhibit the induction of nitrate uptake and its reduction to ammonium ions. In the current experiments, the expected reduction in glutamine levels were found when we used MSX; with azaserine, however, levels of both amides were higher. In roots, MSX additions resulted in enhanced levels of nitrate reductase (NR) as expected. In shoot tissues, on the other hand, MSX additions resulted in an inhibition of the NADH dependent NR and a seven-fold enhancement of the NAD(P)H dependent NR. With azaserine there was a severe inhibition of NR as would be predicted with an increase in the levels of either of the amides. Thus although MSX and azaserine do have direct effects respectively on GS and GOGAT, there are other fundamental effects on the assimilation of nitrate caused by 1) altered levels of nitrogen metabolites, and 2) effect of the inhibitors themselves at other sites in the nitrate assimilation pathway.