Short-term metabolic responses of soybean root nodules to nitrate

Soybean (Glycine max L. Merr.) plants exposed to 10 mM KNO3 for a 4 d period were used to test the correlation between nitrogenase activity, gene expression and sucrose metabolism. Nitrate caused the down-regulation of sucrose synthase (SS) transcripts within 1 d, although a decline in nodule SS activity and an increase in nodule sucrose content only occurred after 3–4 d. In a second experiment, plants were exposed to 15N-labelled nitrate for 48 h to determine the time period during which nitrate was taken up, and to relate this to the decline in apparent nitrogenase activity (H2 production in air) and the reduction in SS gene transcript levels. The peak of nitrate uptake appeared to be between 8 h and 14 h whilst apparent nitrogenase activity began to decline at about 17.5 h. The SS mRNA signal declined markedly between 14 h and 24 h. The correlative association of these factors is clear. However, SS activity per se does not appear to be related to the initial decline in apparent nitrogenase activity as a result of nitrate uptake. These findings, therefore, do not support the hypothesis that the regulation of nodule function is mediated by the regulation of SS activity.