Proton Co‐transport of Organic Solutes by Plant Cells

The mechanism of sucrose transport into the vacuole of root parenchyma cells of sugar beet was investigated using discs of intact tissue. Active sucrose uptake was evident only at the tonoplast. Sucrose caused a transient 8.3 millivolts depolarization of the membrane potential, suggesting an ion co-transport mechanism. Sucrose also stimulated net proton efflux. Active (net) uptake of sucrose was strongly affected by factors that influence the alkali cation and proton gradients across biological membranes. Alkali cations (Na+ and K+) at 95 millimolar activity stimulated active uptake of sucrose 2.1-to 4-fold, whereas membrane-permeating anions inhibited active sucrose uptake. The pH optima for uptake was between 6.5 and 7.0, pH values slightly higher than those of the vacuole. The ionophores valinomycin, gramicidin D, and carbonyl cyanide m-chlorophenylhydrazone at 10 micromolar concentrations strongly inhibited active sucrose uptake. These data are consistent with the hypothesis that an alkali cation influx/proton efflux reaction is coupled to the active uptake of sucrose into the vacuole of parenchyma cells in the root sink of sugar beets.The transport and accumulation of sugars in sink tissues of economically important crops is poorly understood. In sugar beet, sucrose is translocated and stored in the vacuole of parenchyma cells in the root sink without hydrolysis (4, 19). However, the mechanism of phloem unloading and the subsequent uptake and storage of sucrose is not understood (19). The important role of the taproot sink in controlling photosynthate partitioning was evident in grafting and photosynthate (sugar) supply studies (18). These studies showed that the root sink of sugar beet, independent of photosynthate supply, controls partitioning between sucrose and non-sucrose organic dry matter. The elucidation of cellular events during sucrose accumulation in the sugar beet root would provide valuable information about photosynthate transport and partitioning in sink tissues and, possibly, about source-sink communication.

Section: Preparationmentioning

confidence: 62%

Alkali Cation/Sucrose Co-transport in the Root Sink of Sugar Beet

Saftner¹,

Wyse²

1980

“…Axes were excised from seeds which were either (a) imbibed for 6 h; (b) imbibed for 6 h, dehydrated, and reimbibed for 2 h; (c) imbibed for 36 h; (d) _ _. 1 imbibed for 36 h, dehydrated, and reimbibed for 2 h.…”

Section: Methodsmentioning

confidence: 99%

Characterization of Solute Efflux from Dehydration Injured Soybean (Glycine max L. Merr) Seeds

Senaratna¹,

McKersie²

1983

A seed's tolerance of dehydration is lost at a specific stage in germination (1 1). For example, soybean seeds can be imbibed for 6 h, and dehydrated to 10%7o moisture without loss of seed viability or vigor. If the seed is imbibed for 36 h, at which time the radicle is beginning to emerge from the seed coat, and then dehydrated to 10%o moisture, seed viability is lost (17). The ability of plant tissues to tolerate dehydration is thought to reflect the inherent protoplasmic properties of these tissues (2) and in seeds, the protoplasmic properties which impart tolerance are presumably lost as the seed germinates. The loss of tolerance has been asso-'

“…Evidence has been accumulating which suggests that nutrient uptake in plants is carrier-mediated and is coupled to proton flux (1) in a process similar to that which operates during f8-galactoside transport in Escherichia coli. In addition to this carrier-mediated process, some tissues exhibit a passive nonsaturable component of nutrient uptake, especially at high extracellular concentrations (8,15).…”

mentioning

confidence: 99%

Sucrose Uptake by Developing Soybean Cotyledons

Lichtner¹,

Spanswick²

1981

Sucrose uptake by excised developing soybean cotyledons shows a biphasic dependence on sucrose concentration. At concentrations less than about 50 millimolar external sucrose, uptake can be described as a carriermediated process, with a Km of 8 milimolar. At higher external sucrose concentrations, a linear dependence becomes apparent, which suggests the participation of a nonsaturable component in total uptake. Sucrose absorption is dependent on the presence of an electrochemical potential gradient for protons since agents interfering with the generation or maintenance of this gradient (NaN3 or carbonylcyanide-m-chlorophenyl hydrazone) decrease sucrose transport to a level at or below that predicted from the operation of the noncarrier-mediated process alone. redistribution of photoassimilate during seed-fill (27), either sucrose itself or its invert products (glucose and fructose) should be transported into the developing cotyledons. Starch is transiently accumulated in the plastids of the cotyledonary mesophyll, disappearing by maturation and seed desiccation as protein and lipid bodies are formed. Besides utilizing carbohydrates as carbon sources ior amino acids and lipids, accumulated sugars are a means of osmotic adjustment, providing a driving force for water uptake, turgor maintenance, cellular enlargement, and substrate for cell wall components.Soybean fruits act as principal sinks for photoassimilate during reproductive growth. A recent investigation describes sucrose uptake by the cotyledons as a necessary step in the maintenance of a source-sink concentration gradient (27).An understanding of the mechanisms of sucrose movement into the developing fruit also is necessary to evaluate limitations to seed productivity and yield. The final weight of the seed, that is, the yield, is in part determined by its ability to draw nutrients from supply sources in the plant, i.e., by its sink strength. Sink strength, the product of sink size and sink activity, is defined as the net gain in dry weight per unit time per unit dry weight and necessarily includes factors relating to transport and metabolism of the assimilates. In source/sink relationships, an evaluation of the specific conductances at points along the pathway from the region of synthesis to that of accumulation can indicate which site(s) limit the movement of assimilate, and thus, may limit yield. This type of analysis has been applied to wheat (12, 13) where assimilate movement has been extensively studied. In wheat, distinct reductions in sucrose concentration were noted between the vascular bundle in the grain furrow and the endosperm cells.Such concentration gradients imply that movement of sugar from sites of phloem unloading to the endosperm cavity (across the plasmalemma ofthe endosperm cell) meets considerable resistance and hence, may be limiting sucrose accumulation. Such a sharp profile in sugar concentration also exists between the liquid endosperm and the cotyledons in developing Phaseolus vulgaris seeds (26). Recent experiments (27...