Coordinate control of sucrose formation in soybean leaves by sucrose-phosphate synthase and fructose-2,6-bisphosphate.

Net photosynthesis (CER), assimilate-export rate, sucrose-phosphate-synthase (EC 2.4.1.14) activity, fructose-2,6-bisphosphate content, and 6-phosphofructo-2-kinase (EC 2.7.1.105) activity were monitored in leaves of soybean (Glycine max (L.) Merr.) plants during a 12:12 h day-night cycle, and in plants transferred, at regular intervals throughout the diurnal cycle, to an illuminated chamber for 3 h. In the control plants, assimilate-export rate decreased progressively during the day whereas in transferred plants, a strongly rhythmic fluctuation in both CER and export rate was observed over the 24-h test period. Two maxima during the 24-h period for both processes were observed: one when plants were transferred during the middle of the normal light period, and a second when plants were transferred during the middle of the normal dark period. Overall, the results indicated that export rate was correlated positively with photosynthetic rate and sucrose-phosphate-synthase activity, and correlated negatively with fructose-2,6-bisphosphate levels, and that coarse control and fine control of the sucrose-formation pathway are coordinated during the diurnal cycle. Diurnal changes in sucrose-phosphate-synthase activity were not associated with changes in regulatory properties (phosphate inhibition) or substrate affinities. The biochemical basis for the diurnal rhythm in sucrose-phosphate-synthase activity in the soybean leaf thus appears to involve changes in the amount of the enzyme or a post-translational modification that affects only the maximum velocity.