Sucrose metabolism during cotyledon development of Vicia faba L. is controlled by the concerted action of both sucrose-phosphate synthase and sucrose synthase: expression patterns, metabolic regulation and implications for seed development.

The roles of sucrose-phosphate synthase (Sps) and sucrose synthase (Sus) in developing embryos of Vicia faba have been characterized. In the cotyledons the expression of both Sps and Sus is initiated in cells differentiating into storage tissue. This stage is characterized by a switch in the carbohydrate state from a high to a low hexoses to sucrose ratio. The carbohydrate state was found earlier to be controlled by seed coat-associated invertase. During cotyledon development the Sps-enzyme undergoes a cycle of deactivation and reactivation: the activated state is associated with the prestorage phase, desiccation and germination and the deactivated state with the storage phase. Sus activity is associated with the storage phase. Sps and Sus are differentially influenced by free sugars. Feeding hexoses to storage phase cotyledons increases levels of Sps-mRNA but not Sus-mRNA, Sps activity and Sps activation state and impairs storage functions evidenced by an increased sucrose to starch ratio and a downregulation of storage protein legumin B-mRNA. Sus enzyme activity is inhibited by free hexoses in vitro. It is proposed that the changing carbohydrate state during cotyledon development controls the ratio of Sps to Sus. Sps may have some significance for the initiation of the storage process possibly decreasing hexoses and/or increasing sucrose. The relevance of the changing carbohydrate state with respect to development and storage processes is discussed.