STRESS-INDUCED DECLINES IN SOYBEAN N-2 FIXATION ARE RELATED TO NODULESUCROSE SYNTHASE ACTIVITY

Soybean (Glycine max L.) plants were subjected to a number of treatmen ts (drought, 10 mM nitrate, 150 mM NaCl, shoot meristem removal, and r emoval of approximately 50% of the nodules) to test the hypothesis tha t metabolic responses contribute to the regulation of N-2 fixation. Ni trogenase activity was correlated with the activity of nodule sucrose synthase (SS), but not with that of glutamine oxoglutarate amino trans ferase. Leghemoglobin levels and other enzyme activities were not sign ificantly or consistently affected by the treatments. SS mRNA was grea tly reduced in nodules of drought-, salt-, and nitrate-treated plants; however, this was not correlated with changes in soluble carbohydrate , starch, amino acids, or ureides. Leghemoglobin mRNA was only slightl y affected by the treatments. The time course of drought stress showed a decline in the SS transcript level by 1 d, but levels of leghemoglo bin, glutamine synthetase, and ascorbate peroxidase mRNA were not mark edly affected by 4 d. SS activity at 4 d was reduced by 46%. We propos e that N-2 fixation in soybean nodules is mediated by both the oxygen- diffusion barrier and the potential to metabolize sucrose via SS. The response to environmental perturbation may involve down-regulation of the nodule SS gene.