Glycolytic flux is adjusted to nitrogenase activity in nodules of detoppedand argon-treated alfalfa plants

To investigate the short-term (30-240 min) interactions among nitrogenase a ctivity, NH4+ assimilation, and plant glycolysis, we measured the concentra tions of selected C and N metabolites in alfalfa (Medicago sativa L.) root nodules after detopping and during continuous exposure of the nodulated roo ts to Ar:O-2 (80:20, v/v). Both treatments caused an increase in the ratios of glucose-6-phosphate to fructose-1,6-bisphosphate, fructose-6-phosphate to fructose-1,6-bisphosphate, phosphoenolpyruvate (PEP) to pyruvate, and PE P to malate. This suggested that glycolytic flux was inhibited at the steps catalyzed by phosphofructokinase, pyruvate kinase, and PEP carboxylase. In the Ar:O-2-treated plants the apparent inhibition of glycolytic flux was r eversible, whereas in the detopped plants it was not. In both groups of pla nts the apparent inhibition of glycolytic flux was delayed relative to the decline in nitrogenase activity. The decline in nitrogenase activity was fo llowed by a dramatic increase in the nodular glutamate to glutamine ratio. In the detopped plants this was coincident with the apparent inhibition of glycolytic flux, whereas in the Ar:O-2-treated plants it preceded the appar ent inhibition of glycolytic flux. We propose that the increase in the nodu lar glutamate to glutamine ratio, which occurs as a result of the decline i n nitrogenase activity, may act as a signal to decrease plant glycolytic fl ux in legume root nodules.