The Regulation of Sugar Uptake and Accumulation in Bean Pod Tissue

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Abstract
The identity, localization, and physiological significance of enzymes involved in sugar uptake and accumulation were determined for endocarp tissue of pod of Kentucky Wonder pole beans (Phaseolus vulgaris). The data suggested that sucrose synthesis occurred in 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 could be taken up unaltered. Synthesis of sucrose appeared to be requisite to initial accumulation from glucose or 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 resided in the sequence of reactions leading to sucrose synthesis, rather than in a process mediated by a carrier-protein.