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Glycolytic Flux Is Adjusted to Nitrogenase Activity in Nodules of Detopped and Argon-Treated Alfalfa Plants1

Paola M.G. Curioni, Ueli A. Hartwig*, Josef Nösberger, and Kathryn A. Schuller

Institute of Plant Sciences, Swiss Federal Institute of Technology, 8092 Zurich, Switzerland (P.M.G.C., U.A.H., J.N.); and School of Biological Sciences, Flinders University, Adelaide, S.A. 5001, Australia (K.A.S.)

To investigate the short-term (30-240 min) interactions among nitrogenase activity, NH4+ assimilation, and plant glycolysis, we measured the concentrations of selected C and N metabolites in alfalfa (Medicago sativa L.) root nodules after detopping and during continuous exposure of the nodulated roots to Ar:O2 (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 PEP to malate. This suggested that glycolytic flux was inhibited at the steps catalyzed by phosphofructokinase, pyruvate kinase, and PEP carboxylase. In the Ar:O2-treated plants the apparent inhibition of glycolytic flux was reversible, whereas in the detopped plants it was not. In both groups of plants the apparent inhibition of glycolytic flux was delayed relative to the decline in nitrogenase activity. The decline in nitrogenase activity was followed 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:O2-treated plants it preceded the apparent inhibition of glycolytic flux. We propose that the increase in the nodular glutamate to glutamine ratio, which occurs as a result of the decline in nitrogenase activity, may act as a signal to decrease plant glycolytic flux in legume root nodules.

1   This work was supported by a grant from the Swiss Federal Institute of Technology (to U.A.H.) and fellowships from the Human Frontier Science Program and the Organisation for Economic Cooperation and Development (to K.A.S.).
*   Corresponding author; e-mail ueli.hartwig{at}ipw.agrl.ethz.ch; fax 41-1-632-1153.

Plant Physiol. (1999) 119: 445-454
Copyright Clearance Center:   0032-0889/99/119//10
© 1999 American Society of Plant Physiologists




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