Sucrose and Glucose Uptake into Beta vulgaris Leaf Tissues

Abstract

Concentration curves for sugar and amino acid uptake by Beta vulgaris L. leaf tissues contained both a saturable and a linear component. Similarly shaped curves were obtained for influx of sucrose, glucose, and 3-0-methyl glucose by leaf discs, whole petiole slices, petiole segments containing pith tissue only, and petiole segments containing vascular bundles, although the tissues took up the various sugars via different proportions of saturable versus linear uptake. Two millimolar p-chloromercuribenzenesulfonic acid selectively inhibited the saturable component of sucrose uptake, but had almost no effect on the linear component. Uptake of glucose and 3-0-methyl glucose remained unaffected by p-chloromercuribenzenesulfonic acid treatment. Anoxia was found to inhibit the linear component of both sucrose and 3-0-methyl glucose influx, while the saturable component remained unaffected. The linear component of sucrose uptake was also competitively inhibited by maltose, as well as being selectively promoted by certain exposures to 5 millimolar N-ethylmaleimide, 2 micrograms per milliliter cycloheximide, and high levels of mannitol acting as osmoticum. These results support the proposal that the linear component is due to a process more complex than simple, or exchange, diffusion. It would also appear that the linear transport component utilizes a separate energy source than does the saturable component of sucrose influx.Evidence for phloem loading from the apoplast was re-examined with respect to the present findings. Saturable sucrose uptake by minor vein tissues may represent retrieval of solute from the free space, which could explain the 'apoplastic loading' phenomenon.Beta leaves and leaf discs readily absorb ["Cisucrose from exogenous solutions (13,28,31). The assumption is frequently made that such uptake predominantly represents phloem loading, the process by which assimilate is accumulated in the sieve elements prior to translocation. However, it has been well-established that exogenous sucrose does not enter phloem tissues alone (1,7,27). Both Geiger (9) and Giaquinta (16)

Substrate Specificity of the H+-Sucrose Symporter on the Plasma Membrane of Sugar Beets (Beta vulgaris L.)

“…In protoplasts isolated from soybean cotyledons, uptake of sucrose was inhibited by a 100-fold excess of maltose, inulobiose and methyl-j-D-fructofuranoside (16). Maltose also inhibited sucrose uptake into sugar beet leaf discs (14) but did not alter sucrose uptake into PM vesicles isolated from sugar beet leaves (2, 3), although in this later study the maltose concentration was only 10-fold greater than sucrose.…”

Phloem Loading by the PmSUC2 Sucrose Carrier from Plantago major Occurs into Companion Cells.

“…Such retrieval mechanisms have been discussed previously as the primary or even exclusive function of plant sugar transporters (Maynard and Lucas, 1982). Second, the PmSUC2 protein may catalyze not only energy-dependent phloem loading but, under appropriate conditions in the sinks, also the export of sucrose from the se-cc complex.…”

Carbohydrate transport in discs of storage parenchyma of celery petioles

“…As the CCCP-induced inhibition of uptake had suggested the presence of SH-groups at the mannitol carrier, the efTect of two other SH-group modifiers were tested. p-Chloromercuribenzenesulphonic acid (PCMBS; 2mM; Maynard & Lucas, 1982) and Nethylmaleimide (NFM; 2mM; Maynard & Lucas, 1982) inhibited mannitol uptake by 37 and 89%, respectively. Preincubations with the SH-containing reagent DTT (5 mivi) substantially abolished inhibition by PCMBS.…”