Can Sucrose Cleavage Enzymes Serve as Markers for Sink Strength and Is Sucrose a Signal Molecule During Plant Sink Development?

Sucrose cleavage is an essential reaction for higher plant cells to initiate intermediary metabolism and to direct its carbon into the host of essential compounds derived therefrom for maintaining the cells of intact plants. Indeed critical cellular events during their lifetime, such as cell division or growth, are not initiated in the absence of sucrose. As the primary form of systemically translocated carbon in plants, sucrose, via its sweetness, has attracted attention since antiquity. In addition, a large literature exists on the roles of sucrose and measurements of sucrose concentration in various plant tissues (1,19,30,51). Hence, we can readily appreciate the attention that the pioneers of sucrose biochemistry, Drs. Luis F. Leloir and Carlos E. Cardini, gave to understanding \ both the synthesis and the cleavage of sucrose. Though immense progress has been made subsequently in understanding the roles of sucrose in plants, their pioneering biochemical discoveries are foundation stones for all sucrose research today! Knowing about the pivotal roles of sucrose in supporting so many activities of plants, about a decade ago we set out to study plant sucrose metabolism stimulated by the discovery of a substrate level pool of pyrophosphate (PPi) (13,36) and the identification of FIX-2,6-P, (11,33,48) as a potent regulator of plant intermediary metabolism particularly with the PPidependent phosphofructokinase (PPi-PFK) (4,5). A first focal point was to characterize the three enzyme activities that may cleave sucrose at a given cell because we reasoned