Release of Sucrose from Vicia faba L. Leaf Discs

Abstract

The release of sucrose from leaf discs of Vicia faba L. to a bathing medium was studied for evidence of a relationship between this release and mesophyUl export of photosynthate in vivo. Sucrose was released specificaily over hexoses and represented over 85% of total photosynthate released. The sucrose appeared to be derived from the mesophyUl tissue directly and release did not require concurrent photosynthesis. The data indicated two separate channels for sucrose release. The first was sensitive to inhibition by 1 milimolar p-chioromercuribenzenesulfonic acid and the second was promoted by lowering the Ca2" concentration below 0.1 miZlimolar. Flow through both channels was about equal when tissue that had been actively photosynthesizing for several hours was used. The rate of release was not dependent on the extraceilular pH, but was inhibited by 10 micromolar carbonylcyanide p-trifluromethoxyphenylhydrazone. Lowering the Ca'+ concentration below 0.1 millimolar or raising the K+ concentration above 100 mi_llmolar stimulated sucrose release. The stimulation by high K+ was not reversed by adding Ca"+. Ihe data supported the postulate that Ca2" removal or K+ addition changed the permeability of the mesophyll plasma membrane to sucrose.Central to an understanding of how plants utilize photosynthate is an understanding of the mechanism by which photosynthate is transferred from the mesophyll cell to the companion cell of the phloem. Two distinct pathways have been proposed for this transfer, symplastic and apoplastic transport. In symplastic transport, photosynthate remains in the symplasm and passes through the plasmodesmata into the phloem (2). Although this type of transport is important in several tissues, it is not the commonly accepted route for photosynthate transfer between mesophyll and phloem (9, 10). Instead, transfer of photosynthate probably involves the apoplastic pathway, in which release of photosynthate occurs across the mesophyll plasma membrane with subsequent uptake by the phloem companion cells.Interest in the apoplastic route has promoted a considerable effort to characterize the process by which sucrose is loaded into the phloem (3,7,(9)(10)(11)(12)(13)29). Phloem loading of sucrose has been shown to be an energy dependent process that probably involves the cotransport of protons and sucrose into the phloem. Hence, the driving force for sucrose uptake was proposed to be a proton gradient established across the plasmalemma by a vectorial ATPase (12). The process of unloading sucrose from the mesophyll into the apoplast has received less attention than phloem loading.

Dissimilar phloem loading in leaves with symplasmic or apoplasmic minor-vein configurations

“…The stimulation of sugar release also indicates PCMBS inhibition of active resorption by the mesophyll (Turgeon 1984;Van Bel et al 1986). In leguminous species, however, PCMBS appears to block the release of sugars from the mesophyll (Anderson 1983(Anderson , 1986Secor 1987;M'Batchi and Delrot 1988). The contrasting results may point to more than one mechanism of photosynthate release from the mesophyll.…”

Phloem loading in Ricinus cotyledons: sucrose pathways via the mesophyll and the apoplasm

“…The sucrose carriers in the mesophyll of green leaves and cotyledons probably serve different functions according to the respective metabolic and developmental status: while in green source leaves a possible function will be to take up sucrose leaking out of the cells (Maynard and Lucas 1982) to Control sucrose export via apoplasmic or symplasmic routes, the mesophyU cells of cotyledons depend on net uptake of external sucrose to maintain leaf growth and to store carbohydrates; and yet, in both cases, mesophyll carriers will be involved in the controlled release of sucrose into the cell-wall space during apoplasmic phloem loading (Anderson 1983;Secor 1987;M'Batchi and Delrot 1988).…”

Different phloem-loading machineries correlated with the climate

“…But disc systems still fail to determine the exact site and mechanism of release from the mesophyll symplast and the subsequent uptake by the SE/CC-complex. Leakage from discs probably includes several simultaneous processes: loss from the mesophyll, leakage from the cells engaged in photo-assimilate release from the mesophyll symplast and, perhaps, delivery from the cut veins (Anderson 1983(Anderson , 1986Kaiser & Martinoia 1985). Furthermore, the leakage solute is contaminated to some extent with the compounds in the apoplast at the start of the washing.…”