Co-transport of Potassium and Sugars across the Plasmalemma of Mesophyll Protoplasts

Abstract

Sugars (sucrose + hexoses) produced photosynthetically by isolated mesophyll protoplasts of wheat and tobacco were effluxed across the plasma membrane (3 to 10 micromoles hexose equivalents per milligram chlorophyll per hour). The effiux was sensitive to uncouplers and oligomycin which indicated a requirement for energy. A proton gradient was probably not coupled directly to the transport because changing the proton gradient across the plasma membrane by varying the pH of the medium or by adding sodium acetate had no significant effect on the rate of sugar release.A release of K+ was associated with sugar efflux from the protoplasts. The molar ratio of K' to sugar varied between 1.5 and 2. In most higher plants, sucrose is a major end product of photosynthesis and is also the principal form in which photosynthetically derived carbohydrates are translocated within the plant. When translocation of assimilates from the leaf is reduced, photosynthetic sucrose formation is reduced, whereas starch formation is increased (38). Studies such as these indicate an influence of end-product accumulation (e.g., sucrose) on the partitioning of carbon between starch and sucrose in the leaf cell.Sucrose is also a principal end product of photosynthesis by isolated leaf cells and protoplasts (5,18,35 Previous studies (19,23,33) showed that less that 10% of the '4C02 fixed over 60 min by isolated leaf preparations is released to the medium, which has been taken to suggest that the products of photosynthesis are retained within the protoplast. Recently, we (19) reported an efflux of sucrose and hexoses, measured enzymically, from isolated mesophyll protoplasts at a rate similar to that of photosynthetic sugar formation in vitro. Release of radioactivity to the medium did not quantitatively reflect the sugar released because of dilution with internal pools. The results provided the first evidence for a significant transport of photosynthetic end products across the mesophyll plasma membrane.Characterization of the mechanism involved in transport of sugars across the mesophyll plasma membrane was the subject of the study reported herein. Specific objectives of the study presented here were to determine whether sugar efflux required an energized membrane and, if so, whether it involved co-transport (symport) of H+ or alkali cations. MATERIALS AND METHODSProtoplast Isolation. Protoplasts were isolated from fully expanded leaves of greenhouse-grown 10-to 14-day-old wheat (Triticum aestivum L.) and 4-to 6-week-old tobacco (Nicotiana tabacum L.) plants. Wheat protoplasts were isolated and purified as previously described (6). The purified protoplasts were washed three times in 0.5 M sorbitol and I mm CaCl2 after flotation on sucrose. Tobacco leaf segments (0.5 mm) were incubated in a medium containing 0.5 M sorbitol, I mm CaCl2, 2% (w/v) Cellulysin2 (Calbiochem), 0.4% (w/v) Macerozyme (All Japan Biochemicals), 0.5% (w/v) BSA, and 0.5% (w/v) PVP-40 at pH 5.5 Protoplasts were released after a 1-h incubation at 30 C into a solutio...

Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

“…Similarly, Anderson (1) observed that the rate of sugar efflux from leaf discs was not affected by external pH, even though a dramatic effect on sugar uptake was noted. Extracellular pH also had no effect on sugar efflux from isolated mesophyll protoplasts (12). MOPS, Mes, and Tes, nonpermeant buffers, gave virtually the same results as citrate phosphate And K-phosphate buffers, which are considered to be permeant buffers (Table II).…”

“…MOPS, Mes, and Tes, nonpermeant buffers, gave virtually the same results as citrate phosphate And K-phosphate buffers, which are considered to be permeant buffers (Table II). Several studies have demonstrated that sugar unloading from plant cells may be an ion-mediated process and that external potassium often influences the observed rate of unloading (1,12,22). Wolswinkel and Ammerlaan (27), however, observed that 25 mm K+ had no consistent effect on phloem unloading in Viciafaba.…”

Release of Sucrose from Vicia faba L. Leaf Discs

“…In the first study, Ca2+ was shown to inhibit flow of metabolites, including sucrose, out of isolated cotton leaf cells (28). In the second study, Huber and Moreland (16,17) demonstrated that sugar release from tobacco and wheat protoplasts was an energy-dependent process where sucrose release was coupled to K+ exit. However, data taken from isolated cells or protoplasts might not be representative of the in vivo state of the mesophyll.…”

“…Sugar release from isolated leaf cells and protoplasts has also been studied (16,17,28). In the first study, Ca2+ was shown to inhibit flow of metabolites, including sucrose, out of isolated cotton leaf cells (28).…”

TRANSPORT OF SUGAR IN PHYTOPHTHORA PALMIVORA (BUTL.) BUTL.

“…In healthy green leaves, sucrose is commonly at mean concentrations of 5 to 70 mol m-3 (Huber & Moreland, 1981;Gerhardt & Heldt, 1984;Farrar & Farrar, 1985), and so both low-and high-affinity carrier systems may be important in vivo. Since fungal growth will to a great extent depend on the activity of these carriers, their importance in pathogenicity deserves investigation.…”