Solute Distribution in Sugar Beet Leaves in Relation to Phloem Loading and Translocation

The electrogenicity, pH-dependence, and stoichiometry of the proton-sucrose symport were examined in plasma membrane vesicles isolated from sugar beet (Beta vulgaris L. cv Great Westem) leaves. Symport mediated sucrose transport was electrogenic as demonstrated by the effect of membrane potential on ApH-dependent flux. In the absence of significant charge compensation, a low rate of sucrose transport was observed. When membrane potential was clamped at zero with symmetric potassium concentrations and valinomycin, the rate of sucrose flux was stimulated fourfold. In the presence of a negative membrane potential, transport increased six-fold. These results are consistent with electrogenic sucrose transport which results in a net flux of positive charge into the vesicles. The effect of membrane potential on the kinetics of sucrose transport was on V,,, only with no apparent change in Km. Sucrose transport rates driven by membrane potential only, i.e. in the absence of ApH, were comparable to ApH-driven flux. Both membrane potential and ApHdriven sucrose transport were used to examine proton binding to the symport and the apparent Km for H was 0.7 micromolar. The kinetics of sucrose transport as a function of proton concentration exhibited a simple hyperbolic relationship. This observation is consistent with kinetic models of ion-cotransport systems when the stoichiometry of the system, ion:substrate, is 1:1. Quantitative measurements of proton and sucrose fluxes through the symport support a 1:1 stoichiometry. The biochemical details of protoncoupled sucrose transport reported here provide further evidence in support of the chemiosmotic hypothesis of nutrient transport across the plant cell plasma membrane.

Section: Stoichiometry Of the Proton-sucrose Symportmentioning

confidence: 99%

Electrogenicity, pH-Dependence, and Stoichiometry of the Proton-Sucrose Symport

Bush

1990

“…The major portion of material exported from the leaves of higher plants consists of a relatively few sugars and sugar alcohols (20). Data from previous studies (5,6,16) indicate that phloem loading involves active uptake of sugar from the apoplast. An understanding of selectivity and other characteristics of active uptake from the free space of the various exportable sugars seems to be an important starting point for studying the control of export from leaves.…”

mentioning

confidence: 99%

Sugar Selectivity and Other Characteristics of Phloem Loading in Beta vulgaris L.

Fondy¹,

Geiger²

1977

The rte of phloem loading, its selectivity, and the disposition of labeled carbon were studied following application of 14C-labeled sugars to the free space of source leaves of sugar beet (Beta vulgaris L.). Buffered 10 mm solutions of 14C-labeled sucrose, fructose, stachyose, mannitol, 3-0-methyl glucose or L-glucose were applied to the abraded epidermis of source leaves held in the dark. Distribution of the labeled carbon from sugar taken up from the free space was studied by microdensitometry of autoradiographs. Uptake of labeled sugar from the free space, partition between mesophyll and minor veins, metabolic conversions, export and respiration were followed during the 3-hr time course studies. Rates of sugar uptake into the minor veins, flux rates through the sieve element-companion cell complex membrane and concentration ratios between free space and the interior of the minor vein phloem cells were compared for the six sugars studied for evidence of active uptake. The composition of the free space solution in leaves photosynthesizing in 14CO2 was studied by vacuum infiltration of the source leaf air spaces and removal of the solution by centgation. Labeled compounds in this solution were compared to those in an aqueous ethanol extract of the same leaf pieces.The results in sugar beet source leaves support the concept of direct, active uptake of sucrose from free space into minor veins. This is not the case for fructose, 3-0-methyl glucose, mannitol, or stachyose. The latter two sugars, which are translocated in some plants, are not loaded into the minor veins at a rate sufficient to make them a significant component of the material translocated. The rate of phloem loading is controlled in part by mesophyll metabolism, especially as it affects the availability of sucrose to the free space. Both the rate and selectivity of export are controlled by uptake from the free space into the sieve element-companion cell complex of the minor veins.Phloem loading in leaves of higher plants has an important role in the productivity of crops. Because the bulk of material *that is translocated must first undergo phloem loading, factors which affect this latter process are likely to play a significant part in determining the partitioning of assimilate between export and retention in source leaves. The major portion of material exported from the leaves of higher plants consists of a relatively few sugars and sugar alcohols (20). Data from previous studies (5,6,16) indicate that phloem loading involves active uptake of sugar from the apoplast. An understanding of selectivity and other characteristics of active uptake from the free space of the various exportable sugars seems to be an important starting point for studying the control of export from leaves. 2 Abbreviations: SA: specific activity; se-cc: sieve element-companion cell; 3-0-MG: 3-0-methyl glucose.Mechanisms which affect the availability of various sugars to the free space and their selective uptake into the phloem are likely to control export from the leav...

“…There is evidence for sucrose compartmentation in various tissues of exporting leaves (5,16,17). We have previously documented (5) the high solute potential of the sieve element-transfer cell complex in sugar beet leaves indicating substantial sucrose accumulation in these cells.…”

mentioning

confidence: 99%

“…The time course of metabolism of 14CO2-derived translocate arriving in the sink leaves of intact sugar beet plants is shown in Figure 5. Initially, all of the C entering the leaf is water-soluble (>90%) but after 5 hr, 60 to 65% of the "C has been incorporated into the starch (25-30%), protein (25-30%), lipid (5%), and residue (5%) fractions (Fig. 5A).…”

mentioning

confidence: 99%

See 1 more Smart Citation

Source and Sink Leaf Metabolism in Relation to Phloem Translocation

Giaquinta

1978

125

The import-export transitio in sugar beet leaves (Bets vu*ans) occurred at 40 to 50% leaf expansion and was characterized by loss in assimilate import and Increase in pbotosyntbesis. The metabolsm and partitoning of assimilated and transocated C were determine during leaf development and related to the traslocaton status of the leaf. The It is well established that as a young leaf matures its status changes from an importer to an exporter of photosynthetic assimilates (4, 13, 25). In many dicot species, the import-export transition occurs when the leaf is 40 to 50%o expanded and is characterized by the rapid loss in the ability to import assimilates with the concomitant onset of export function. Autoradiographic studies (4,13, 25) have shown that export initiates at the leaf apex and rapidly develops basipetally. The basipetal source development is correlated with the cessation of assimilate import. The physiological and anatomical events accompanying leaf maturation have been studied in most detail in Populus deltoides (2, 3, 13), Cucurbita pepo (25), and Beta vulgaris (4).Many studies on the import-export conversion have been to distinguish the preparatory events from the causal events ofexport. In this regard, Turgeon and Webb (24, 25) characterized the import-export conversion during leaf development in C. pepo. The basipetal acquisition of export was accompanied by an increased photosynthesis rate, basipetal development of mesophyll intercellular air spaces, synthesis of the transport sugar, stachyose, and structural maturation of the minor veins. These studies suggested that no one system in the development specifically regulated the onset of export but instead the import-export conversion resulted from the integrated maturation of many structural and physiolog- Many of the previous studies have been at the whole-plant level and aimed at elucidating the causal events for export. In this study, the leaf import-export conversion is investigated at the cellular and biochemical level not with the intent of determining the causal event(s) of export but rather to elucidate the processes of sink and source leaf physiology in relation to translocation. The metabolic correlates of leaf maturation were studied both in detached leaves and in the intact translocating sugar beet plant, B. vulgaris. Specifically, the metabolism and partitioning of assimilated and translocated C were characterized during leaf development and related to the translocation status of the leaf. Additionally, the levels ofthe transport sugar, sucrose, and the activities of various sucrose-metabolizing enzymes were determined in source and sink leaves. These results are discussed in terms of intracellular compartmentation of sugars in relation to import and export.MATERIALS AND METHODS B. vulgaris L. (monogerm hybrid, size 3) plants were grown for 8 to 10 weeks in a controlled environment under conditions described previously (8). Four experimental protocols were used in this study: (a) the relationship between leaf import capacity ...