Characterization of the active sucrose transport system of immature soybean embryos

Immature soybean embryos were isolated from soybean [Glycine max (L.) Merr.] seeds at various stages of development to study their accumulation of [(14)C]sucrose in vitro. Isolated embryos accumulate sucrose at a constant rate over several hours, the label entering large, endogenous pools of sucrose from which starch, protein, and lipid storage products are formed. Accumulation is without extracellular sucrose hydrolysis and occurs predominantly by active transport at physiological sucrose concentrations. A nonsaturable diffusion component, apparently superimposed upon the active saturable component, dominates overall uptake at exogenous concentrations greater than approximately 50 millimolar sucrose. Active transport is sensitive to uncoupling agents and the sulfhydryl-modifying reagent p-chloromecuribenzene sulfonate, is dependent on more than one energy source, and exhibits well-defined requirements for incubation temperature, pH, and oxygen availability. Under optimal incubation conditions of 35 degrees C, saturating illumination (pH 6), and 21% oxygen, the apparent K(m) for sucrose is approximately 8 millimolar and V(max) is approximately 0.6 micromoles per hour per 100 milligrams fresh weight. Embryos readily accumulate sucrose from dilute exogenous solutions and, when preloaded with large amounts of sucrose, maintain the internal sucrose pool against steep outward gradients. These and other observations indicate that, although perhaps fully saturated in vivo, active sucrose transport is a significant component of photosynthate uptake in developing soybean embryos, enhancing uptake at physiological sucrose concentrations 2- to 5-fold over diffusion alone.