The regulation of sugar uptake and accumulation in bean pod tissue

The identity, localization and physiological significance of enzymes involved in sugar uptake and accumulation were determined for endocarp tissue of pods of Kentucky Wonder pole beans (Phaseolus vulgaris). An intracellular, alkaline invertase (pH optimum, 8) was assayed in extracted protein, as well as enzymes involved in sucrose synthesis, namely, uridinediphosphate (UDP-glucose pyrophosphorylase and UDP-glucose-fructose transglucosylase). Indirect evidence indicated the presence also of hexokinase, phosphohexoseisomerase and phosphoglucomutase. The data suggested that sucrose synthesis occurred in the cytoplasm, and that both sugar storage and an alkaline invertase occurred in the vacuole. The latter functions to hydrolyze accumulated sucrose. An outer space invertase (pH optimum, 4.0) was detected, but was variable in occurrence. Although its activity at the cell surface enhanced sucrose uptake, sucrose may be taken up unaltered.

Over a wide range of concentrations of exogenous glucose the sucrose/reducing sugar ratio of accumulated sugars remained unchanged at about 20. Synthesis of sucrose appears to be requisite to initial accumulation from glucose or fructose, as free hexoses do not increase at the apparent saturating concentration for uptake. Sucrose accumulation from exogenous hexose represents a steady-state value, in which sucrose is transported across the tonoplast into the vacuole at a rate equivalent to its rate of synthesis. Evidence indicates that this component of the accumulation process involves active transport of sucrose against a concentration gradient. The ratio of sucrose/reducing sugars in the accumulated sugars immediately after a period of uptake was inversely related to the level of inner space invertase. Within 16 hours after a period of accumulation, practically all of the sugar occurs as glucose and fructose.

The absence of competition among hexoses and sucrose indicated that a common carrier was not involved in their uptake. From a series of studies on the kinetics of uptake of glucose and fructose, including competition studies, the effects of inhibitors, radioactive assay of accumulated sugars and the distribution of label in accumulated sucrose it appeared that rate limitation for glucose or fructose uptake resides in the sequence of reactions leading to sucrose synthesis, rather than in a process mediated by a carrier protein.