Plant biochemists are confronted with two path- ways leading to sucrose, both of which involve UDP- glucose as glucosyl donor. Sucrose synthetase [l] (UDP-glucose: D-fructose-2glucosyltransferase, EC 2.4.2.13) and sucrose phosphate synthetase [2] (UDP-glucose: D-fructose-6-phosphate-Zglucosyl- transferase, EC 2.4.2.14) with its associate sucrose phosphate phosphatase [3,4] (sucrose8-phosphate phosphohydrolase, EC 3.1.3 .OO) catalyze the follow- ing reactions : UDP-glucose t fructose ti UDP t sucrose UDP-glucose + fructose-6-P 2 UDP t sucrose-6-P sucrose-6-P 4 sucrose + Pi The existence of these two separate mechanisms for the synthesis of sucrose raises the question of their respective roles in vivo [ 51. Present views tend to stress that the physiological role of sucrose synthetase would be sucrose cleavage, while the role of sucrose phosphate synthetase coupled to sucrose phosphate phosphatase would correspondingly be that of sucrose synthesis [6]. Thus, sucrose synthesis would be the * It